JOHANNES KEPLER

UNIVERSITÄT LINZ

Altenberger Straße 69

4040 Linz, Österreich

www.jku.at

DVR 0093696

Submitted by

David Mittermayr

Submitted at

Institute of Polymeric

Materials and Testing

Supervisor

Assoc. Prof. DI Dr.mont.

Jörg Fischer

Co-Supervisor

DI Paul J. Freudenthaler,

BSc

July 2022

ENVIRONMENTAL STRESS CRACKING

RESISTANCE OF HIPS UNDER CYCLIC LOADING

USING CRACKED ROUND BAR SPECIMENS

Master Thesis

To obtain the academic degree of

Diplom-Ingenieur

In the Master’s program

Polymer Technologies and Science

II

ACKNOWLEDGEMENT

I want to thank o.Univ.-Prof. Dipl.-Ing. Dr.mont. Reinhold W. Lang, head of the Institute of Poly-

meric Materials and Testing at the Johannes Kepler University Linz (JKU-IPMT) and Assoc. Prof.

DI Dr.mont. Jörg Fischer for making the presented research work possible and for enabling the

execution and writing of my master thesis with a high degree of self-determination. In addition,

they supported me with any organizational problems, which made it easier for me to conduct my

work at their institution.

I would also like to thank DI Paul J. Freudenthaler, BSc for his unrestricted support on every issue

that came up on this thesis. Especially in this area of highly specialized research, I could always

count on his help and guidance. Furthermore, I want to thank all of my colleagues at the Institute

of Polymeric Materials and Testing for support on many small challenges that came up during my

practical work and testing for my thesis.

In the context of this work, several topics have challenged me as a student of the science and

engineering fields. These challenges occurred on both sides of this thesis. On the one hand, chal-

lenges in the practical work and on the other hand, challenges within theoretical comprehension

of the research. I am happy to say that I have been educated by the problems and have learned

a lot about dealing with polymeric materials in the context of processing, characterization, and

testing. None of this would have been possible without my dear friends and family who have sup-

ported me at every turn during the last years of my studies at JKU.

This thesis was performed as part of a round robin test with the Technical Committee 4 of the

European Structural Integrity Society and I am grateful for the opportunity to work on such an

interesting topic as part of their research.

III

STATUTORY DECLARATION

I hereby declare that the thesis submitted is my own unaided work, that I have not used other than

the sources indicated, and that all direct and indirect sources are acknowledged as references.

Place, Date

Signature

IV

ABSTRACT

The objective of this thesis was to investigate two different high impact polystyrene (HIPS) poly-

mers in terms of their environmental stress cracking resistance (ESCR) in air and sunflower oil

environments. An oily or fatty environment has the probability of decreasing the mechanical long-

term stability of different polymers, including HIPS. The ESCR investigation gives information

about the fatigue crack growth of polymers starting at an initialized crack/notch until failure with

different surrounding environments while testing.

This work is part of a round robin test with the Technical Committee 4 (TC4) of the European

Structural Integrity Society (ESIS). A round robin test is a tool to evaluate the precision and un-

ambiguity of test instructions and methods. For a round robin test different institutions work by the

same manual and through the comparison of the result’s similarities or dissimilarities a valuation

of the instruction or method is possible. Therefore, the processing parameters and test parameters

need to be standardized within the participants of the round robin test.

The ESCR testing was performed with an electro-dynamic tension test system of the type Instron

ElectroPlus E3000 which applied the cyclic tension loading on the cracked round bar (CRB) spec-

imens. This system keeps track of cyclic loading forces, the displacement curves and the number

of cycles. Through the assistance of a camera system, it is possible to correlate a measurable

crack length to the number of cycles with the fatigue loading.

At the beginning, the preparation and testing parameters for the CRB specimens were examined.

This included investigations concerning the immersion time in oil prior to testing to get a saturated

testing material for a neglection of the testing time influence itself, the test frequency to investigate

the influence of a possible hysteretic heating in the material, and a blade-geometry dependence

with which the specimens were notched. For the blade-geometry dependency seven different

blades were tested with consideration of the handling behavior while processing, the availability,

optical notch visibility, and of test results at same loadings deriving different cycles to failure.

After the processing parameters were fixed, the ESCR testing comparison between the two grades

was carried out. It showed a clear result that the HIPS grade with bigger rubber particles, even

though it has lower short-term mechanical performance in tensile modulus, yield strength, and

notched impact strength, is preferrable in terms of ESCR and should be used in fatty environment

applications.

V

TABLE OF CONTENT

1 INTRODUCTION AND SCOPE ............................................................................................ 1

2 GENERAL BACKGROUND .................................................................................................. 3

2.1 European Structural Integrity Society ............................................................................ 3

2.1.1 Technical Committees ........................................................................................ 3

2.2 Round robin tests .......................................................................................................... 4

2.3 High Impact Polystyrene ................................................................................................ 4

2.3.1 Polymerization of HIPS ....................................................................................... 5

2.3.2 Morphology of HIPS ........................................................................................... 6

2.3.3 Mechanical properties ........................................................................................ 6

2.4 Environmental stress cracking resistance ...................................................................... 7

2.4.1 Slow crack growth .............................................................................................. 7

2.4.2 Fatigue ............................................................................................................... 9

2.4.3 Environmental stress cracking .......................................................................... 13

3 EXPERIMENTAL ................................................................................................................ 14

3.1 Material ....................................................................................................................... 14

3.2 Testing environment .................................................................................................... 15

3.3 Specimen preparation ................................................................................................. 16

3.3.1 Compression molding ....................................................................................... 16

3.3.2 Annealing ......................................................................................................... 18

3.3.3 Specimen machining ........................................................................................ 19

3.4 Mechanical test procedure........................................................................................... 22

3.4.1 Cyclic testing machine and in-situ measurement devices ................................. 22

3.4.2 Measurement of specimen and mounting ......................................................... 23

3.4.3 Mechanical loading conditions .......................................................................... 24

3.4.4 Post-test measurement..................................................................................... 24

4 RESULTS ........................................................................................................................... 26

4.1 Immersion tests in oil environment .............................................................................. 26

4.2 HIPS-A in air environment ........................................................................................... 27

4.2.1 Frequency dependency .................................................................................... 29

4.2.2 Blade-geometry dependence ............................................................................ 31

4.3 HIPS-A in oil environment ............................................................................................ 33

4.4 HIPS-B air vs oil .......................................................................................................... 35

4.5 Comparison between HIPS-A and HIPS-B .................................................................. 36

5 SUMMARY AND CONCLUSION ........................................................................................ 38

6 REFERENCES ................................................................................................................... 40

1

1 INTRODUCTION AND SCOPE

Combining brittle amorphous polymers with rubbers either by blending or copolymerization is a

well-known method to enhance the fracture toughness (‘rubber toughening’). This led over the

past decades to several new materials that are now widespread in practical applications. Promi-

nent examples are HIPS (high-impact polystyrene), ABS (acrylonitrile–butadiene–styrene) or MBS

(methyl methacrylate-butadiene-styrene plastic) (Pijnenburg et al., 2005).

These impact-modified two-phase system polymers consist of around 3 to 15 wt% of the rubber

polymer. Their chemical consistency and therefore their mechanical properties give them a broad

variety of applications. The durability of a rubber modified polystyrene is strongly depending on

the application environment. Prolonged contact of the polymer with an oily or fatty environment

increases the tendency of crazing. The deterioration process is beginning with crazing, which

leads to cracking and eventual failure of the structure through the simultaneous exposure to a

combination of mechanical stress (externally and/or internally) and aggressive fatty substances

(Nke-Aka & Delaney, 1996). In addition, the polymers tend to lose significant physical properties

when exposed to long-chain oils and/or fats. This is referred to as "environmental stress cracking"

(ESC) (Khodabandelou et al., 2009).

The Environmental Stress Cracking Resistance (ESCR) of polymers depends on the environment

system in question. One method to get a prediction of the susceptibility to ESC is to compare the

closeness of the solubility numbers of the material and liquid of interest (Hansen & Just, 2001).

Other experimental methods focus on measuring time to failure and are used for PE long-term

performance testing, such as the Bell Telephone test ASTM D1693 (ASTM D1693–15, 2015), the

full notch creep test ISO 16770 (ISO/TC 61/SC 9, 2019), and the bent strip test ISO 22088 (ISO/TC

61/SC 6, 2006), in which a fixed stress or strain is applied to specimens. One of the most advanced

experimental methods is the cracked round bar test ISO 18489 (ISO/TC 138/SC 5, 2015). For this

method a cylindric polymer bar is notched circumferentially to create an initial crack from where

the crack grows through the material. This method was proposed with many others to evaluate

the long-term performance of PE using fracture mechanics. With the use of cyclic loads, it is pos-

sible to characterize the fatigue crack growth behavior and accelerate the testing times by an

immense amount of time (Pinter et al., 2007). Furthermore, with cracked round bar testing a su-

perimposed mechanical-environmental loading of polymeric materials is possible and therefore

ESCR investigation in different environments (Fischer et al., 2019).

The objective of this master thesis is to investigate two different HIPS polymers and their

environmental stress cracking resistance (ESCR) in air and oil environments. This research is part

of a round robin test with the Technical Committee 4 (TC4) of the European Structural Integrity

Society (ESIS). For this purpose, a clear structured specimen production with defined tools is

needed for a good comparability between the different round robin participants. Therefore, it is

2

necessary to investigate different blades for the notching process of the specimens and find a

favorable option. Another factor that should be taken into account is the oil absorption process of

the polymers before and/or during the testing in oil environment. Moreover, it is required to exam-

ine different test frequencies to find an optimum in testing time and different influences. The main

aim of this study is to characterize the stress cracking resistance of the two HIPS materials in air

environment as well as in oil environment and furthermore to compare the results with the material

differences and conclude in a statement of correlation between material property and ESCR.

3

2 GENERAL BACKGROUND

This chapter is about the general background for understanding this thesis. It explains terms like

the European Structural Integrity Society, Round Robin tests, High Impact Polystyrene, and envi-

ronmental stress cracking resistance.

2.1 European Structural Integrity Society

European structural integrity society (ESIS) is an international non-profit organization. Its goal is

to create and expand knowledge about all aspects of structural integrity and the dissemination of

that generated knowledge world-wide and therefore the improvement of safety and performance

of structures and components. The organizations logo is shown in Figure 2.1.

Figure 2.1: Logo of the European Structural Integrity Society (ESIS, 2016).

The organization is run by the Executive Committee which is instructed by a council of delegates.

Each delegate represents a European country affiliated to the society including the Technical

Committee chairpersons (ESIS, 2016).

The specific objective of ESIS include the following points:

• To promote research and cooperation for the prevention of failure of mechanically stressed

materials, components and structures and related phenomena.

• Development of new test methods, numerical methods, and engineering estimation tech-

niques for structural integrity valuation.

• Improvement of inspection procedures.

• Improvement of engineering designs and manufacturing processes.

• To publish and spread knowledge in form of scientific publications.

• Teaching of prospective engineers in terms of structural matter.

2.1.1 Technical Committees

To break down structural integrity matters in different aspects, ESIS assigns them to Technical

Committees (TC). These committees are ESIS’ most important part and are supported in their

4

activities by ESIS. An indicator for the strength of a TC is given by the number of members and

the quality of the scientific outputs they generate.

This work falls in the field of TC04 - polymers, polymer composites and adhesives. ESIS Technical

Committee 4 on Fracture of Polymers, Polymer Composites and Adhesives was founded in 1985

and aims to promote the application of fracture mechanics for these types of materials. This aim

is tried to be achieved by first, development of new standard test methods based on fracture me-

chanics using extensive round robin experiments and publishing of the related or generated infor-

mation on conferences and through publications in journals sponsored by ESIS and secondly, by

complementing these standards with numerical simulations and modelling. Furthermore, ESIS en-

ables a discussion of specialists with semiannual meetings and provides chances for the partici-

pants for networking and to collaborate with each other (Brunner, 2022).

2.2 Round robin tests

A round robin test is a method for external quality assurance for measurement procedures and

measurement/testing laboratories. It was mainly used in the field of analytical chemistry but can

now be found in many other technical and non-technical fields.

Basically, identical samples are examined with identical procedures on different equipment or with

different procedures. Most of the time a combination of the two methods is carried out by multiple

independent scientists performing tests with a given instruction. The comparison of the results

allows to give information about the measurement accuracy of the involved institutions or about

the quality of the given guideline.

In former days round robin tests were not popular, because they have proven to be worthless in

so many cases that many people have suggested that the methodology itself has no value. It was

a problem caused by not fully understanding the benefits of a round robin test, improper written

guidelines and inadequate statistical design and evaluation. But now the benefits derivable out of

a round robin test are being cherished and is used for evaluating the reproducibility of a test

method as well as the verification of a new analysis method (Pierson & Fay, 1959).

2.3 High Impact Polystyrene

Unmodified polystyrene (PS) is an amorphous or semicrystalline polymer with a glass transition

temperature of approximately 100 °C. Depending on the tacticity it can be in an amorphous state

(atactic) without a melting temperature or in a semicrystalline state with a melting temperature of

240 °C (isotactic) or 270 °C (syndiotactic), shown in Figure 2.2a. It is a rather hard and brittle

material with a density of 1040 - 1090 kg/m³. Polystyrene is one of the most common polymers

and is used in many ways. For example, it can be used unmodified as a packaging material, but

also can be modified through foaming to an expanded polystyrene (EPS) or extruded polystyrene

(XPS) and be used as an insulation. One other modifying possibility is to combine it with other

5

polymers like rubbers to increase the toughness. One of these rubber reinforced materials is High

impact Polystyrene (HIPS) (Kern GmbH, Germany). HIPS is a graft copolymer out of polybutadi-

ene rubber and polystyrene, with the structural formulars shown in Figure 2.2b and c. It achieves

a higher toughness through incorporation of micron sized rubber particles in the polystyrene matrix

(Domininghaus, 2012).

(a)

(b) (c)

Figure 2.2: (a) Different tacticity of PS, (b) structural formular of polystyrene and (c) structural formular of

polybutadiene (Lechner et al., 2014).

2.3.1 Polymerization of HIPS

The usual process for carrying out a HIPS polymerization is to dissolve polybutadiene rubber in

styrene monomers and solvent. The unique part of this polymerization is that two processes ap-

pear simultaneously. On the one hand, the formation of matrix material out of polystyrene polymer-

ization and on the other hand, the graft from polybutadiene and polystyrene. The necessary com-

patibility between the matrix and the rubber phase is ensured by grafting as well as sizes and

6

structure of the formed rubber particles. The size of the rubber particles and their internal structure

as well as the share of the rubber phase are dependent in a complex way on the process control

of the polymerization (Brydon et al., 1973). While polybutadiene/styrene initially forms the coherent

phase, it switches after the phase change to polystyrene/styrene in which the rubber particles are

now dispersed. Since the components are largely incompatible, during polymerization segregation

occurs, resulting in the formation of an interesting structure of the rubber particles, which in turn

contain polystyrene (Stabenow & Haaf, 1973).

2.3.2 Morphology of HIPS

HIPS has in comparison to other rubber-modified styrene polymers, like acrylonitrile butadiene

styrene copolymer (ABS), a larger rubber particle size. As a result of the multiple segregation while

polymerization, the rubber particles exhibit a foamed-up structure. Figure 2.3a shows an electron

micrograph of an ultrathin section of HIPS. For these images the rubber was contrasted with os-

mium tetroxide and stands out darkly from the light polystyrene phase. This structure explains the

toughness of HIPS, because a growing crack is slowed down by the interface between the poly-

styrene and the rubber particle. The size of the butadiene-particles prevents the crack from grow-

ing around the polybutadiene phase, therefore the crack must go through the particles with many

interfaces between polystyrene and polybutadiene which shows a drastically crack stopping char-

acteristic, illustrated in Figure 2.3b (Stabenow & Haaf, 1973).

(a)

(b)

Figure 2.3: Micrograph of an ultra-thin section of (a) HIPS where the darker phases is rubber and the

brighter phases is polystyrene and (b) HIPS where the white lines through the rubber phases

are crazes (Stabenow & Haaf, 1973).

2.3.3 Mechanical properties

Polystyrenes are brittle plastics with elongations at break of approx. 2 %, while rubber-modified,

impact-resistant polystyrenes, on the other hand, achieve values of >20 % (Ehrenstein, 2011).

The second phase in HIPS does not only contribute to the to the toughness behavior of the mate-

7

rials, but it also modifies most other mechanical properties like the tensile modulus, which de-

creases to less than half of the modulus of the unmodified PS, illustrated in Table 2.1. All the

strengths and moduli listed in this table decrease over the factor from two to three, comparing PS

to HIPS. However, if you now compare the elongation at break and the notched Izod impact

strength (ISO/TC 61/SC 2, 2019) values HIPS outperforms PS with the factor of 3 and 10.

Table 2.1: Mechanical properties of PS and HIPS (Peters, 2006).

property PS HIPS

density [kg/m³] 1040 1040

tensile modulus [GPa] 3.14 1.56

tensile strength [MPa] 51.1 15.0

elongation at break [%] 21 65

flexural modulus [GPa] 3.54 1.68

flexural strength [MPa] 102 30

notched Izod impact strength [kJ/m] 0.021 0.221

Vicat softening temperature [°C] 105 96

hardness (Rockwell) R130 R75

Hence, even though the high values of PS outperform the properties of HIPS in moduli and

strength, if the requirement is a tough, non-brittle material, it is not the preferred choice. For these

cases HIPS is often used and shows tremendous advantages compared to PS.

2.4 Environmental stress cracking resistance

Environmental stress cracking resistance (ESCR) is an important long-term property of many plas-

tics that come into contact with various media such as cleaning agents, oils and insecticides. Such

medias can accelerate crack initiation and propagation in laboratory measurements as well as in

usage (Choi et al., 2009). This chapter deals with theory behind slow crack growth, the fatigue

effect, and the influence of surrounding media.

2.4.1 Slow crack growth

Slow crack growth (SCG) remains the most common failure mode in long-term loaded plastic

structures such as pressure pipes (Leevers, 2001). SCG is an effect caused by many parameters.

Different influencing factors like environment, temperature, mechanical stresses, and time can

cause a high stress region which leads to crazing or shear yielding (Fischer et al., 2018). The

fracture mechanics of SCG has shown that the process has two phases. The first phase is called

the induction period. It starts with stress concentrations in a defect in the material or at the tip of a

crack or craze. Hence, a material with a large number of voids and a fibrillar structure is formed in

this phase. The second phase is crack growth, during which the microfiber formed in the polymer

is snapped and it appears as a crack, illustrated in Figure 2.4 (Michler, 1992; Pan et al., 2015).

8

Figure 2.4: Schematic drawing of the crazing progress (Pinter, 1999)

Depending on the loading direction and corresponding to the possible relative movement to the

crack surface, there can be distinguished between three loading modes, illustrated in Figure 2.5:

The plain crack opening (Mode I), the in-plane shear loading (Mode II), and the out-of-plane shear

loading (Mode III) (Grellmann & Seidler, 2007).

Figure 2.5: Three different loading models in fracture mechanics (Grellmann & Seidler, 2007).

The concept of linear-elastic fracture mechanics (LEFM) specifies that the crack growth rate under

static loading is only dependent on a single parameter. The stress intensity factor (KI) describes

the stress and strain field in the near crack tip area. The index of KI is here standing for the crack

loading mode I (Grellmann & Seidler, 2007). A relation between stress intensity factor, more pre-

cisely the stress intensity factor range (∆K), and the crack growth rate under fatigue loading

(da/dN) is given by a power-law equation with two additional material constants (C and m) the

so-called Paris-Erdogan equation, shown in Equation 2.1 (Paris & Erdogan, 1963).

d𝑎

d𝑁= 𝐶 ∗ ∆𝐾𝑚 Equation 2.1

9

This power-law equation can also be used for a static loading with the Equation 2.2 where the

crack growth rate (da/dt) is now the derivative of the crack length (a) dependent of the time (t)

(Lang et al., 1997).

d𝑎

d𝑡= 𝐴 ∗ ∆𝐾𝑚 Equation 2.2

Depending on the loading conditions which are summarized by KI, the crack growth can be divided

into three regions, shown in Figure 2.6. Region I is called threshold regime where the crack has

little to no growth. Region II is the stable crack growth regime where the crack growth rate has a

power law (Paris equation) relation to KI. The last region, Region III, is the critical crack growth

regime with an unstable crack growth until failure at elevated loadings. Two important character-

istics are the limiting stress intensity range values. First, the applicable loadings above which crack

growth is taking place, KI,th. Components designed on basis of the fatigue threshold are assumed

to survive in service under cyclic loading conditions without catastrophic failure. And secondly, the

critical stress intensity factor KIc, at which a component faces an unstable crack growth and

thereby failure occurs (Lang et al., 1997; Nagarajan et al., 2017).

Figure 2.6: Three regions of the crack growth rate depending on the stress intensity factor KI (Lang, 1997;

Lang et al., 2005).

2.4.2 Fatigue

Materials and specimens can be tested/characterized in different settings. One can differentiate

the testing methods with mechanical loading states. First, you can test with a monotonic or dy-

namic loading which means the applied forces/displacements do not stop growing until failure.

Secondly, there are static characterization methods where the applied stresses build up to a set

level and stay constant over time. The third of the three most important loading conditions is a

cyclic or fatigue loading where the stresses increase and decrease with a defined frequency. The

following illustration (Figure 2.7) shows schematically the stress for these three different loading

conditions over time (Richard & Sander, 2012).

10

(a)

(b)

(c)

Figure 2.7: Stress over time curves for (a) monotonic loading, (b) static loading and (c) cyclic loading.

The loading condition influences the acceptable stress duration for a specimen directly. A mono-

tonic loading or dynamic loading can be seen as one point on the failure stress over time-to-failure

diagram, shown in Figure 2.8. Focusing on the two other loading cases in this image, one can say

that with increasing time-to-failure values the allowable failure stress decreases. Also, that mate-

rials under fatigue loading will fail sooner compared to long-term static loading at comparable

applied loading conditions.

Figure 2.8: Failure stress over time-to-failure for different loading conditions (Lang, 2021).

This acceleration of fatigue testing can be used for decreasing testing times in laboratories. The

possible acceleration is strongly dependent on the loading frequency and the load ratio R. R is

defined by the lower limit of the stresses (σmin) divided by the upper limit of the stresses (σmax),

also shown in Equation 2.3 (Hertzberg Richard W. et al., 2013; Richard & Sander, 2012).

𝑅 =𝜎𝑚𝑖𝑛

𝜎𝑚𝑎𝑥 Equation 2.3

An R value of 0 means a cyclic loading of zero stresses to maximum value. This case is the fastest

in terms of testing times. All R values above zero stand for pure tension or a pure compression

loading, but the lower cyclic stresses aren’t zero anymore. Increasing the lower loading limits lead

to longer testing times especially approaching the limit load ratio of 1, which means a static long-

time, t

stress, u

time, t

stress,

s

time, t

stress,

ma

11

term loading condition, pictured in Figure 2.9. So, the testing times are strongly dependent on the

load ratio R. This behavior can be explained through plastic deformations of the material. Even

though the absolute stresses are purely derived through tension loading, the cyclic movement and

inhomogeneities of the plastic deformations in the cross section of the specimen lead to plastic

deformations in tension and compression, which fatigue the material (Lang et al., 2005; Pinter et

al., 2002).

Figure 2.9: Schematical illustration of load ratios R approaching a static loading condition (Lang et al.,

2005).

The stress intensity factor range (∆KI) is derived through KI and the load ratio R and gives the

range of fluctuation of the stress intensity factor.

Cracked round bar specimen

Every specimen type has its own formulas for ∆KI, because of the strong dependency of ∆KI on

the geometry. The specimen configuration of the cracked round bar (CRB) specimen is shown in

Figure 2.10. This symmetrical geometry has a defined notch in the middle (marked with 1) and

threads on both sides which allows for a stable fixation within the testing machine.

12

Figure 2.10: Schematical sketch of the cracked round bar specimen with dimensions, whereas L is the

specimen length, Lmin the minimum distance between notch and clamping system, D the spec-

imen diameter, Dini the target initial ligament diameter, aini the target initial crack length and 1

the razor-sharp notch (ISO/TC 138/SC 5, 2015).

The most important parameters for the polyethylene specimens according to the ISO 18489

(ISO/TC 138/SC 5, 2015) can be listed as follows:

▪ L ........ specimen length: 80 – 100 mm

▪ Lmin ..... minimum distance between notch and clamping system: 20 mm

▪ D ........ specimen diameter: 14 mm

▪ aini ...... target initial crack length: 1.5 +0.15/-0.0 mm

The tension forces to apply on the specimens are depending on the desired stress level ∆σ*, the

load ratio R, and the diameter of the ligament as shown in Figure 2.10 according to the ISO 18489

(ISO/TC 138/SC 5, 2015). The stress intensity factor range for a CRB specimen can be calculated

by using the formulas according to Benthem and Koiter (Benthem & Koiter, 1973). The stress

intensity factor range can be determined by using the maximum stress intensity factor KI,max and

the load ratio R according to Equation 2.4. The non-dimensional correlation function f(b/r) for de-

termination of ∆KI,max can be solved with Equation 2.6, where b is the ligament radius derived

through the radius of the specimen r and the crack length a with Equation 2.7. Using the non-

dimensional correlation function, the maximum force, and geometry parameters, the maximum

stress intensity factor can be calculated according to Equation 2.5. These formulas make it possi-

ble to create a diagram, where a linear correlation between applied stress range and cycles to

failure is visible. An example of this correlation is shown in Figure 2.11.

∆𝐾𝐼 = 𝐾𝐼,𝑚𝑎𝑥 ∗ (1 − 𝑅) Equation 2.4

𝐾𝐼,𝑚𝑎𝑥 =𝐹𝑚𝑎𝑥

𝜋 ∙ 𝑏²∙ √

𝜋 ∙ 𝑎 ∙ 𝑏

𝑟∙ 𝑓 (

𝑏

𝑟) Equation 2.5

13

𝑓 (𝑏

𝑅) =

1

2∙ (1 +

1

2∙ (

𝑏

𝑟) +

3

8∙ (

𝑏

𝑟)

2

− 0,363 ∙ (𝑏

𝑟)

3

+ 0,731 ∙ (𝑏

𝑟)

4

) Equation 2.6

𝑏 = 𝑟 − 𝑎 Equation 2.7

Figure 2.11: Examplary diagram of the linear correlation of the applied stress range with the cycles to fail-

ure in a double logarithmic diagram (ISO/TC 138/SC 5 General properties of pipes, 2015).

2.4.3 Environmental stress cracking

Environmental stress cracking (ESC) is an important long-term effect on many polymers in contact

with different media. Through the influence of an environment surrounding the polymer, failure

may occur sooner or later. Environments like detergents, oils, and other reagents can have a huge

influence on the lifetime of polymer products (Choi et al., 2009). Amorphous polymers tend to be

more influenced by different surroundings, compared to semi-crystalline polymers, because of

their loose structure and higher free volume which makes it easier for fluids to permeate into the

polymer. This behavior is due to increased disentanglement of molecules under the influence of

the surrounding liquid and to the scission of chains. Around of 15 % of all failures of polymer

components are evoked through ESC (Altstaedt et al., 2004). Another aspect for the increased

stress cracking in different environments is the reduction of energy necessary for creating a new

surface. The surrounding liquid is wetting the surface of the polymer and thus promoting the for-

mation of voids, which is likely to be a main driving force in the early stages of cracking (Mark,

2004).

The resistance against ESC is called environmental stress cracking resistance (ESCR) and is the

ability of a material to withstand a surrounding environment without an increase of their stress

cracking speed. Extended exposure of polymers to liquid chemicals accelerates the crazing pro-

cess, which makes it possible to initiate crazes at stresses much lower than for example in air.

Crazes in components often lead to cracks and eventually to failure (Li, 2005).

14

3 EXPERIMENTAL

This chapter is about the testing conditions of this thesis. It includes the tested material as well as

the specimen production and preparation. Furthermore, the mechanical test procedure as well as

the evaluation are covered.

3.1 Material

Two materials were investigated in this thesis. Both were HIPS, but with different properties and

morphologies. The first material was the EDISTIR® ICE R 830D and is called HIPS-A in this thesis

and the second material was the EDISTIR® R 850E and is called HIPS-B. While HIPS-A is an

environmental stress cracking resistant thermoforming grade due to larger rubber particles in the

polystyrene matrix, HIPS-B is a general-purpose thermoforming HIPS grade optimized for sheet

extrusion and thermoforming applications, shown in Figure 3.1.

(a)

(b)

Figure 3.1: Transmission electron micrograph of (a) HIPS-A and (b) HIPS-B (Andena et al., 2013).

15

Interesting to see are the mechanical parameters of HIPS-A and HIPS-B, listed in Table 3.1.

HIPS-B is ~30 % stiffer and shows a ~30 % higher yield strength, whereas HIPS-A outperforms

HIPS-B in terms of strain at brake, impact strength and ESCR according to ISO 22088-3 (ISO/TC

61/SC 6, 2006).

Table 3.1: Physical and mechanical properties of the two HIPS materials (Andena et al., 2013).

Property condition normative unit materials

HIPS-A HIPS-B

Density ISO 1183 g/cm³ 1 .04 1 .04

Melt flow rate 200 °C - 5 kg ISO 1133 g/10 min 3 4

Glass transition temperature (PS matrix)

DMA 1 Hz, 1 K/min °C 108 110

Yield stress 50 mm/min Mpa 16 21

Break stress 50 mm/min ISO 527 Mpa 23 .5 24

Break strain 50 mm/min % 70 60

Modulus 1 mm/min Mpa 1450 1850

Flexural strength 2 mm/min ISO 178 Mpa 35 38

Izod impact strength

(notched)

+23 °C

+23 °C

-30 °C

- 3.2 mm

- 4 mm

- 4 mm

ISO 180/4A

J/m

kJ/m²

kJ/m²

130

10

8

125

10

6

.5

Deflection temperature 1.8 Mpa – 120 °C/h ASTM D 648 °C 82 85

ESCR 0.5% def. – 50 min-23 °C ISO 22088-3 εr/ε0% 94 26

3.2 Testing environment

Tests were carried out in two different testing environments. First, tests were conducted in air with

a temperature of 23 °C and a relative humidity of 50 %. The second environment was sunflower

oil from the brand Gíglío Oro Girasole (Carapelli Firenze S.p.A, ITA) with a temperature of 23 °C

as well. For this testing environment a containment was necessary (shown in Figure 3.12b) and

was filled with the oil illustrated in Figure 3.2.

Figure 3.2: Sunflower oil from the brand Gíglío Oro Girasole (Carapelli Firenze S.p.A, ITA) used for the oil

environment.

16

The condition of the oil was repeatedly checked, and it was replaced when particles, which were

introduced by the specimen, were found in it. Apart from that, the oil had no apparent reason to

be replaced within the few weeks of testing.

3.3 Specimen preparation

The production of CRB specimens included steps in the following order: compression molding,

annealing, cutting into bars, and lathing into finished specimen. The specimens used for the ESCR

tests were cracked round bar (CRB) specimen as shown in Figure 2.10. The used threads were

M14 metric thread with a pitch of 1.25 mm which allowed for a stable fixation within the testing

machine. The parameters standardized for polyethylene, listed in chapter 2.4.2, were also used

for the HIPS-specimen with a specimen length of 100 mm.

3.3.1 Compression molding

Both materials got delivered in form of pellets, so the first step for producing specimens was to

press plates. For a plate-forming process heat and pressure are necessary and with the help of

the hydraulic press of the type LZT-OK-50-L (Langzauner GmbH, Austria), shown in Figure 3.3,

this was possible. For the right forming process a dipping edge tool was used, meaning that one

plate immerses into the other creating a closed cavity, where the bottom form plate was equipped

with a temperature sensor for an exact temperature control of the polymer. The upper part had a

milled undercut, so that after cooling the solid plate would be pulled out of the cavity by the opening

of the mold, illustrated in Figure 3.4a and b.

Figure 3.3: Hydraulic press with control and cooling unit LZT-OK-50-L (Langzauner GmbH, Austria).

17

(a)

(b)

Figure 3.4: Dipping edge tool for hydraulic press: (a) bottom half with temperature sensor and (b) upper

half.

First, the hydraulic press was equipped with the two forming plates and the bottom plate was filled

with 290 g of the respective material pellets, as seen in Figure 3.5. The second step was the

pressing step itself with different pressure levels, delay times, heating, and cooling steps. There-

fore, it started with a pressure less closing of the tool, afterwards the forming plates were heated

up to 200 °C with only the tare weight of the upper tooling half on it. Holding the temperature for

15 minutes assured a homogenous temperature in the softened material. The next step was to

increase the pressure up to 4 MPa and initialize the cool down after a short holding time. Reaching

the 40 °C mark, the machine released any pressure on the tool and opened up. The exact param-

eters can be seen in Table 3.2.

(a) (b)

Figure 3.5: (a) Bottom form plate with HIPS pellets mounted in the hydraulic press and (b) upper form

plate mounted in the hydraulic press.

18

After the compression molding was finished, the HIPS plate stuck to the upper plate, shown in

Figure 3.6a. The finished plate could be removed by pushing it sideways off the plate with a rubber

hammer. After deburring the finished plate was between 15.6 mm and 15.8 mm thick with a width

and height of 120 mm x 150 mm, pictured in Figure 3.6b.

Table 3.2: Process steps for compression molding of HIPS plates.

Closing of tool without pressure 50 Max. speed [mm/s] 3 Min. speed [mm/s]

Heating up without pressure 200 Temp. [°C] 10 Gradient [°C/min]

Holding temperature without pressure 15 Minutes [min]

Pressing pressure 4 Stress [MPa]

Cooling down 40 Temp. [°C] 4 Gradient [°C/min]

Pressure release and opening tool

(a)

(b)

Figure 3.6: (a) Sticking HIPS plate to the upper plate after pressing and (b) finished HIPS-A plate.

3.3.2 Annealing

To minimize the chances of any residual stresses and inhomogeneities in the material an anneal-

ing step was performed with the finished plates. This made the usage of an oven necessary. Using

the FED 53 (Binder GmbH, Germany), displayed in Figure 3.7, the plates were heated up to 90 °C

in 30 minutes, maintained there for an hour followed by a slow cooling phase of 10 hours, which

was realized by turning off the oven and letting the plates cool down overnight in the closed oven.

19

Figure 3.7: Dry and convection oven FED 53 (Binder GmbH, Germany).

3.3.3 Specimen machining

To get the specimen with the defined geometry two machines/processes were needed. To use the

lathe, it was necessary to first cut the plate into bars with a quadratic cross section. For this pro-

cess a CS 70 EB circular saw (Festtool GmbH, Germany) with a vacuum unit was used. It was

possible to get 8 bars from one plate, but both outer bars were sorted out to avoid possible mar-

ginal influences. The next step to get the rotational symmetric specimens was the lathing of the

quadratic bars using an Emcomat 14-D (EMCO GmbH, Austria), pictured in Figure 3.8.

Figure 3.8: Lathe Emcomat 14-D (EMCO GmbH, Austria).

20

With the help of a four-jaw chuck, it was possible to clamp the cuboid-shaped bars. Through drilling

a small centering hole in the samples and using a centering mandrel (displayed in Figure 3.9b), it

was possible to only clamp them on a small area, so that the whole length could be machined

without reclamping. This method made it possible to machine the whole surface to a diameter of

14 mm, using the lathe chisel tool shown in Figure 3.9a. Using the automatic drive with a slow

feed speed assured a smooth surface. After that the crack notch was cut in in the middle of the

specimen. For this step seven different blade types, shown in Figure 3.10, were used with different

geometrical features and methods of cutting which can be seen in Table 3.3. The blades were

mounted in different special blade holder tools, illustrated in Figure 3.9a. The notch cutting was

also done by the automatic drive of the lathe machine with a feed speed of 0.03 mm/rotation. The

advantage of machining a specimen in one single clamping is that the rotational axis for the sur-

face and the notch are the same without any eccentricities which can appear due to reclamping.

The next steps were cutting of the specimen to the right length and cutting threads onto both ends

with consideration of at least 20 mm distance of the thread end to the notch using the thread tap

visible in Figure 3.9c.

(a)

(b)

(c)

Figure 3.9: Lathing tools: (a) for surface machining and notch cutting (from right to left: parting tool, lathe

chisel, blade holder for razor and industrial blades, blade holder for Stanley blade, blade holder

for chipping blade), (b) centering mandrel and drill, (c) thread tap.

21

(a)

(b)

(c)

(d)

Figure 3.10: (a) Razor blade of the model Astra Superior Platinum (Procter & Gamble, UK) with a thick-

ness of 0.1 mm, (b) schematical sketch of a chipping blade modified from a trapezoid Stanley

blade of the type 1992N (Stanley Black & Decker Inc., USA), (c) schematical illustration of an

industrial blade with different thicknesses of 0.1, 0.2, 0.3 and 0.4 mm (Martor KG, Germany),

(d) regular trapezoid Stanley blade of the type 1992N (Stanley Black & Decker Inc., USA).

Table 3.3: List of the seven different blades used for notching of CRB specimens.

thickness number of facets cutting method

[mm] [-]

Astra razor blade 0.10 1 Material displacement

Chipping blade 0.65 1 Chipping

Industrial blade

0.10 1 Material displacement

0.20 1 Material displacement

0.30 1 Material displacement

0.40 2 Material displacement

Stanley blade (NO. 1992N) 0.65 1 Material displacement

The finished specimens were stored in air at test conditions of 23 °C and 50 % relative humidity

for a minimum of 24 hours after manufacturing. Since tests were carried out in two different envi-

ronments, specimens for testing in sunflower oil environment were immersed in the oil for a pre-

determined time prior to testing in a container at 23 °C and again for one hour in the test setup to

make sure the specimens are completely wetted in oil. Therefore, the desired testing condition in

oil is met from the beginning for all tests and testing time dependent immersion effects are avoided.

The necessary immersion time prior to testing was evaluated through measuring the mass in-

crease of round bar specimens over the time.

22

3.4 Mechanical test procedure

This chapter covers the test procedure for the CRB - fatigue testing. It comprises the cyclic testing

machine, in-situ measurement devices, the measurements prior to testing, specimen mounting,

test loadings, post testing measurements, and data treatment.

3.4.1 Cyclic testing machine and in-situ measurement devices

The device used for the cyclic tensile testing was an Instron ElectroPuls E3000 (Illinois Tool Works

Inc., USA) illustrated in Figure 3.11a. The E3000 is a linear-tension dynamic test instrument, which

means it can perform tensile/compression movements at various speeds and frequencies. The

loading capacity of this all-electric machine is up to ±3 kN with a frequency of up to 100 Hz. For

image acquisition during the testing an industrial camera of the type HXG40 (Baumer Electric AG,

Switzerland) was used. It was combined with a Sigma 50mm f/2.8 EX DG Makro lens (K.K. Sigma,

Japan). Both can be seen in Figure 3.11. The industrial camera HXG40 uses a monochromatic

CMOS sensor with an acquisition frame rate of up to 56 fps and a resolution of 2048 x 2048 pixels.

(a)

(b)

(c)

Figure 3.11: (a) Instron EletcroPlus E3000 (Illinois Tool Works Inc, 2021), (b) industrial camera for image

acquisition (Baumer Electric AG, 2022), (c) Sigma 50mm f/2.8 EX DG Makro (Sigma GmbH,

2022).

23

3.4.2 Measurement of specimen and mounting

Before testing, the diameter of the specimen was recorded on both sides of the notch at ~2 mm

distance to the notch. The average of the measurements was used for further calculations. Deter-

minations of the diameter directly on the notch would have given to high values because of the

material displacement which forms during notching. For mounting the specimen, the two threads

were used and screwed into the clamping system, shown in Figure 3.12a. Depending on whether

the testing was in air or oil environment additional setup material was needed. Tests in oil required

a containment in which the testing could be done. A glass bulb and fixation which was also used

in previous publications for tests in hydrocarbons (Schoeffl et al., 2014), pictured in Figure 3.12b,

was used. Independent on the test setup any twisting and bending of the specimen was avoided,

so that there is no damage in the material prior to testing.

(a)

(b)

1. notched specimen

2. thread clamping

3. fixed/static clamping

4. double-walled oil filled glass bulb

5. glass bulb fixation

6. drain valve

7. force applying/moveable clamping

8. camera lens

9. flashlight LED panel

Figure 3.12: (a) Clamping system with internal thread on the left side and a ball head mount on the right

side and (b) test setup for CRB measurements in oil on the Instron E3000.

24

3.4.3 Mechanical loading conditions

The aim of the testing was a result of brittle failure by slow crack growth within a reasonable testing

time. Therefore, the load forces were chosen to be between 250 to 700 N depending on the spec-

imen material and test environment. The chosen forces resulted in a stress range of between 2.6

and 7.3 MPa. For a geometrically optimal specimen with the fixed R-ratio of 0.1 during the testing,

a stress intensity factor range between 0.12 to 0.35 MPa·m0.5 was calculated. After the mounting

and conditioning of the specimen for 1 h within the test setup, the cyclic load was applied. Most of

the tests were carried out with a frequency of 10 Hz, but for analyzing the influence of the fre-

quency a spectrum of 1, 2, 5, 10, 20 Hz were also used. Any static loading prior to testing was

avoided as this may cause creep in the notch of the specimen. With beginning of the test, the

number of cycles were tracked for data acquisition. The cyclic loading was applied constantly until

the test piece failure. During the testing the camera system kept track of the crack length. For this

tracking the camera software was calibrated on the outer diameter of the specimen and took pic-

tures at every fixed number of cycles. The interval between two pictures was shortened with grow-

ing crack length to get a detailed image of the crack growth and to avoid collecting high amounts

of data in the threshold regime were the crack length stays constant.

3.4.4 Post-test measurement

The measurement of the notch depth of the razor blade notch is in the untested specimen difficult,

therefore the initial crack length (aini), is measured on the fracture surface of the already tested

and hence broken specimen using a stereomicroscope system of the type SZX16 (Olympus K.K.,

Japan), illustrated in Figure 3.13.

Figure 3.13: SZX16 stereomicroscope (Olympus K.K., Japan).

25

The razor blade notch surface structure was distinguishable from the fatigue crack grown fracture

surface. An example for this optical difference can be seen in Figure 3.14a. The visible outer ring

was cut with the blade and the inner part was formed under fatigue crack growth. Under usage of

the microscope and different lighting configurations, it was possible to determine the initial crack

length. The inner diameter value which was used in calculations was collected by measuring the

ligament diameter at least three times with the software Olympus Stream Motion 2.5 (Olympus

Soft Imaging Solutions GmbH, Germany) and averaging it as shown in Figure 3.14b.

(a)

(b)

Figure 3.14: Stereomicroscopy images of the CRB specimen after testing of the (a) whole cracked surface

and (b) cracked surface with measurement of the initial ligament diameter.

26

4 RESULTS

To evaluate the ESCR of two materials, CRB specimens were tested using different test condi-

tions. The conditions varied in terms of loading range, frequency, loading range, used blade to

produce the initial crack of the specimen and test environment. The results of these tests are

described in the following chapters, starting with immersion tests in oil.

4.1 Immersion tests in oil environment

For the purpose of eliminating all time-dependent oil absorption processes while testing, the

weight increase of specimens in oil environment was observed. In Figure 4.1 two weight increase

curves are shown over time. The curve of the first test was generated from measuring three sam-

ples of HIPS-A immersed in oil under testing conditions (23 °C, environmental pressure) averaged

to a single point data for different testing times. The second test was a single specimen at the

same conditions but tested more often in the first few hours to gain a higher test resolution. The

immersion time tests were conducted for up to 221 hours at which two of the three samples from

test 1 were taken out and stored in a closed air environment, visible on the right-hand side in

Figure 4.1. The third specimen was used for testing an even higher immersion time, which led to

no more weight increase and is not shown in the graph. In the first few hours the most oil was

absorbed which led to a weight increase of roughly 0.2 %. At this weight increase a plateau was

reached with an immersion time of around 50 hours. The weight measurements in air showed no

significant weight decrease over storage time. Therefore, it was decided to use an immersion time

of two days (48 hours) prior to testing for each specimen and a possible storage of already condi-

tioned specimens in a closed air environment of at least 12 days.

27

Figure 4.1: Weight increase of HIPS-A specimens immersed in oil and weight change of a formerly in oil

immersed specimen in a closed air environment.

4.2 HIPS-A in air environment

The first results with CRB specimens were gathered by testing HIPS-A with standardized condi-

tions of a frequency of 10 Hz and a crack notch produced with a normal razor blade. The Figure

4.2 illustrates the number of cycles to failure for different stress intensity factor ranges. The double-

logarithmic scaled graphic is fitted with a linear curve for all measured points which is monoto-

nously decreasing meaning a clear tendency of lower cycle numbers for higher loadings. The

number of cycles was between 3.97x104 and 2.41x106 which yields to testing times between

1.1 hour and 2.8 days for test forces Fmax between 450 N and 700 N, respectively. These forces

result in a load range of 4.26 to 6.63 MPa which is higher than used load ranges for rubbers but

lower than the ones for many other polymers visible in Table 4.1. The post-testing measurements

of the ligament diameter were used to calculate the corrected ∆KI which showed only small differ-

ences to the assumed ∆KI (ligament diameter of 11 mm), because of the little deviations of the

manufactured initial ligament diameter and the targeted one, shown in Figure 4.3 as well as in

Figure 4.2. From now on, all ∆KI values shown or mentioned in this thesis are already corrected

by the post-test measurements.

0 50 100 150 200

0.000

0.025

0.050

0.075

0.100

0.125

0.150

0.175

0.200

0.225

0.250

we

igh

t d

iffe

ren

ce

[%

]

immersion time [h]

test 1

test 2

0 100 200 300

storage time [h]

test 1

weight increase weight decrease

28

Figure 4.2: HIPS-A stress intensity factor ranges vs. cycles with razor blade notching.

Figure 4.3: Corrected compared to assumed stress intensity factor ranges vs. cycles of HIPS-A with ra-

zor blade notching.

104 105 106 107

0.10

0.15

0.20

0.25

0.30

0.35

0.40

razor blade

DK

I [M

Pa

·m0

.5]

cycles [-]

HIPS-A

10 Hz

R = 0.1

air, 23 °C

104 105 106 107

0.10

0.15

0.20

0.25

0.30

0.35

0.40

assumed

corrected

DK

I [M

Pa

·m0

.5]

cycles [-]

HIPS-A

10 Hz

R = 0.1

air, 23 °C

razor blade notched

29

Table 4.1: Testing parameters for the CRB fatigue tests of different materials (Arbeiter et al.,

2015).

material load-range [MPa]

polypropylene 13 to 16

polybutadiene 10 to 13

unplasticized polyvinylchloride 7.5 to 28

polyamide 12 24 to 34

polyoxymethylene 26 to 44

hydrogenated nitrile rubber 1.8 to 2.3

Table 4.2: HIPS-A stress intensity factor range correction with post-testing measurement.

maximum

force

specimen

diameter

ligament

diameter

assumed

∆KI

corrected

∆KI

number of

cycles

[N] [mm] [mm] [MPa·m0.5] [MPa·m0.5] [-]

450 13.99 11.01 0.224 0.223 2409212

480 14.02 11.03 0.239 0.238 387548

500 14.01 11.04 0.249 0.247 266979

550 13.99 11.04 0.274 0.271 139845

600 14.00 10.99 0.299 0.299 79469

700 14.00 11.02 0.348 0.347 39748

4.2.1 Frequency dependency

In the next step the frequency influence was tested on the material HIPS-A. For this dependency

evaluation, different test frequencies between 1 Hz and 20 Hz were applied on two specimens per

frequency. The results can be seen in Figure 4.4 with the linear fit for the standardized 10 Hz

measurements for comparison. All test specimens were produced with a razor blade notch and all

tests were caried out with the same maximum force of 550 N, with exception of the different max-

imum forces of the 10 Hz points for the linear fit. Except for one of the two 20 Hz measuring points,

lower frequencies result in lower cycle times and vice versa. Two different effects must be consid-

ered. On the one hand, hysteretic heating of the specimen with a growing probability for higher

frequencies leading to lower cycle numbers. On the other hand, the testing time which increases

with lower frequencies, illustrated in Figure 4.5. Focusing on the testing time, the averaged 1 Hz

measurements did last 10 times longer than the averaged 20 Hz measurements. This time differ-

ence in a tensile loaded stress state can lead to a higher share in creep deformations and therefore

influence the material durability while testing. The main concern of the hysteretic heating could not

be detected and therefore the standardized frequency of 10 Hz was used for all other tests.

30

Figure 4.4: HIPS-A stress intensity factor ranges tested with different test frequencies with 550 N and the

linear fit for the standardized 10 Hz measurements shown over cycle numbers.

Figure 4.5: HIPS-A stress intensity factor ranges tested with different test frequencies with 550 N and the

linear fit for the standardized 10 Hz measurements shown over time.

104 105 106 107

0.10

0.15

0.20

0.25

0.30

0.35

0.40

razor blade

1 Hz

2 Hz

5 Hz

10 Hz

20 Hz

DK

I [M

Pa

·m0

.5]

cycles [-]

HIPS-A

R = 0.1

air, 23 °C

razor blade notched

103 104 105 106

0.10

0.15

0.20

0.25

0.30

0.35

0.40

razor blade

1 Hz

2 Hz

5 Hz

10 Hz

20 Hz

DK

I [M

Pa

·m0

.5]

time [s]

HIPS-A

R = 0.1

air, 23 °C

razor blade notched

31

4.2.2 Blade-geometry dependence

Further investigations took focus on the influence of the used blade for generating the crack notch.

For this purpose, seven different blades in three different clamping systems were used in the

production of the specimens. An outcome of notching with the different blades can be seen in

Figure 4.6. The notch of the razor blade affected the visibility of the crack tip, while the industrial

blade with the same thickness of 0.1 mm exposed the crack much better. With thicker blades more

material had to be transferred out of the notch and was displaced creating a locally larger diameter

on the specimen near the notch. This behavior is not favorable because it does influence the

geometry of the specimen near notch creating a difference in terms of stresses. The comparison

of the various industrial blades and the worst case in this test, the Stanley blade with a thickness

of 0.65 mm, shows this most clearly. The material displacement effect can be avoided by using

the chipping blade, which made one of the best visible crack tips, because of its chip lifting cutting

method.

Figure 4.6: Microscopic image of cracks derived from notching with different blades in HIPS-A. From left

to right: (1) razor blade, (2) chipping blade, (3) 0.1 mm, (4) 0.2 mm, (5) 0.3 mm, and (6) 0.4 mm

industrial blade, and (7) Stanley blade.

The fatigue testing was carried out with the same maximum force of 550 N creating a similar stress

state with a ΔKI of around 0.27 MPa·m0.5 only variating through slightly different specimen/notch

geometries, shown in Figure 4.7. The highest cycle numbers of around 106 were obtained with the

thickest blades (0.4 mm industrial blade and Stanley blade) with the highest material displacement

while notching. The goal in the production of the crack notch is, in addition to the exact geometry,

a crack tip that is as sharp as possible. Higher cycle numbers to failure at the same loading con-

ditions can indicate a blunted crack tip which decreases the notch effect on the stresses and is

therefore not favorable. Specimen produced through the notching with the industrial blade of

0.1 mm to 0.3 mm lasted around 4·105 cycles before failing which is a high decrease comparing

to the 106 cycles of the 0.4 mm industrial blade. The lowest cycles to failure were achieved by the

razor blade and chipping blade notched specimens. With roughly 1.4·105 cycles to failure the razor

blade and the chipping blade proofed favorable for specimen production. Also shown in Figure 4.7

are the measured cycles needed to initiate crack growth. These cycle numbers were gathered

through a one-sided optical measurement. As this measurement does not observe the complete

cracked round bar circumference, it cannot be guaranteed that the crack growth did not already

start at an unobserved part of the specimen. However, this threshold cycles until initiation is useful

information about the crack growth behavior of the notches produced by the different blades. A

higher share of crack growing cycles means a lower share of initiating cycles and therefore a

32

sharper crack. For most of the blades, the crack growing cycle percentage was around 15 % of

the total cycles to failure, with exception of three blades. First, the razor blade which 18 % share

of crack growing cycles. Second, the 0.4 mm industrial blade with 12 % share and the Stanley

blade with a spontaneous failure with a not measurable crack growth on the observed specimen

side. The razor and the chipping blade had both their advantages, such as the specimen notched

with the chipping blade survived the same number of cycles with a lower ΔKI, indicating a sharper

crack tip compared to the razor blade notching. This stress difference was only generated by the

different initial crack length of the specimens, due to the difficult handling of the chipping blade

during the notching process. It was much easier to hit the desired initial crack length with the razor

blade. Through this balance of advantages both blades were investigated further. All other blades,

like the industrial blades and the Stanley blade, were not further investigated in this study.

Figure 4.7: HIPS-A stress intensity factor ranges vs. cycles tested with different blades for notching of

the initial crack with 550 N and the linear fit for the standardized razor blade crack for com-

parison.

Single point data were not sufficient for a detailed study of the differences between the razor blade

and the chipping blade. Therefore, different maximum force levels of 450 N, 500 N, 550 N, and

600 N were used to test specimens produced with the chipping blade and compared to results of

razor blade notched specimens. The results can be seen in Figure 4.8, with the four points of the

chipping blade notched specimens marked in red and the razor blade notched specimens in black.

For both measurement series a linear fit was calculated and displayed in the graphic. Even though

104 105 106 107

0.10

0.15

0.20

0.25

0.30

0.35

0.40

cycles to failure initiation

razor blade

chipping blade

industrial blade 0.1 mm

industrial blade 0.2 mm

industrial blade 0.3 mm

industrial blade 0.4 mm

stanley blade 0.65 mm

DK

I [M

Pa

·m0

.5]

cycles [-]

HIPS-A

10 Hz

R = 0.1

air, 23 °C

33

the chipping blade specimens achieved lower cycle counts on average than the razor blade spec-

imens at the same stress level, the slope of the linear fits were nearly the same. Meaning that the

differences between both functions were practically independent on the applied stresses with only

insignificantly lower cycle numbers for chipping blade notched specimens, which indicates a fa-

vorable sharper crack tip, were observed.

Figure 4.8: HIPS-A multipoint stress intensity factor ranges vs. cycles of razor blade and chipping blade

notched specimens with standardized test conditions in air environment.

4.3 HIPS-A in oil environment

For an accurate evaluation of the best blade for notching the samples not only tests in air, but also

tests in oil environment were necessary. For this purpose, both razor blade and chipping blade

were used to generate specimens and test them in oil environment. The chosen maximum forces

for the cyclic loading were in the range of 300 N to 600 N. The outcomes of these tests are illus-

trated in Figure 4.9. The trend is much like the tests in air environment with only slight difference

in the cycle numbers at the same stresses and a quite similar slope of the two linear fits. The

biggest difference compared to air environment was that now the razor blade notched specimens

achieved insignificantly lower cyclic numbers at similar stresses. As before in air, the results are

not distinctive due to the low differences and compared to those high deviations from the linear

correlation. The standard deviation or rather reproducibility is influenced by many different factors,

as for instance the measurement inaccuracies and material imperfections. The tests may also

have been influenced by locally inhomogeneous oil pressures through the cyclic deformation of

104 105 106 107

0.10

0.15

0.20

0.25

0.30

0.35

0.40

razor blade

chipping blade

DK

I [M

Pa

·m0

.5]

cycles [-]

HIPS-A

10 Hz

R = 0.1

air, 23 °C

34

the specimen creating a pumping mechanism. This pumping mechanism could be different for the

chipped blade notch and the pushed razor blade notch and therefore lead to differences in testing

performance. The major advantage of the good visible crack length of the chipping blade notching

was outperformed by the ordinary razor blade notching, because of its easier handling, better

availability, easier clamping system of the blade and the fact that it is preferred by the ISO 18489

(ISO/TC 138/SC 5, 2015) standard. If in-situ measurement of the actual crack length during the

test is not desired, the visibility of the crack notch is irrelevant and the chipping blade unnecessary.

Figure 4.9: HIPS-A multipoint stress intensity factor ranges vs. cycles of razor blade and chipping blade

notched specimens with standardized test conditions in oil environment.

For the comparison of HIPS-A in the two different environments, tests were carried out with applied

maximum forces between 300 N to 700 N and as a result the number of cycles laid between

4.0*104 and 2.4*106. By comparing the results of HIPS-A in the two different environments, shown

in Figure 4.10, it can be seen that the oil environment enhanced the crack growing speed or rather

shortens the testing time. Furthermore, the oil crack growth speed enhancement intensified with

higher cycle numbers. At high loads, the number of cycles to failure almost halved compared to

the air environment and at lower loads, and therefore longer testing times, this ratio increased. In

other words, the in oil tested specimens withstood 10 % lower stress intensity factor ranges at the

same cycle numbers at low cycles (short tests) and 20 % at high cycles (long tests). The linear

interpolations reflect this behavior with a higher slope for in oil tested specimens.

104 105 106 107

0.10

0.15

0.20

0.25

0.30

0.35

0.40

oil

chipping blade

DK

I [M

Pa

·m0

.5]

cycles [-]

HIPS-A

10 Hz

R = 0.1

oil, 23 °C

35

Figure 4.10: HIPS-A stress intensity factor ranges vs. cycles with standardized test conditions in air and oil

environment.

4.4 HIPS-B air vs oil

The second material, HIPS-B, was tested with the before evaluated test conditions and therefore

with a razor blade notch, 10 Hz test frequency and within the two test environments air and oil.

The maximum applied force ranged from 250 N up to 500 N which resulted in stress intensity

factor ranges of around 0.25 MPa·m0.5 and below. The number of cycles until failure laid between

1.8*104 to 6.0*106, meaning testing times between 0.5 hours to 7 days, illustrated in Figure 4.11.

There was a clear enhancement of the crack growth speed in oil environment. Specimens in oil

failed with the same level of stresses 5 to 13 times faster than specimens tested in air environment.

This behavior again intensified with higher numbers of cycles/lower stresses visible through the

higher slope of the linear regression of the oil tests. The conclusion for HIPS-B is a drastically

reduced ESCR under cyclic loading in an oil environment.

104 105 106 107

0.10

0.15

0.20

0.25

0.30

0.35

0.40

air

oil

DK

I [M

Pa

·m0

.5]

cycles [-]

HIPS-A

10 Hz

R = 0.1

23 °C

razor blade notched

36

Figure 4.11: HIPS-B stress intensity factor ranges vs. cycles with standardized test conditions in air and oil

environment.

4.5 Comparison between HIPS-A and HIPS-B

Comparing both HIPS materials, HIPS-A outperforms HIPS-B in terms of SCG resistance in both

air and oil environment, shown in Figure 4.12. Even though HIPS-B is the stiffer and stronger

(higher yield stresses) material, it endures at the same stress levels nearly 10 times less cycles

until failure than HIPS-A. The slope of the linear regression for both materials in air is nearly the

same, so the differences in cycles to failure do not aggravate for either higher or lower stress

levels. Even the by the oil weakened HIPS-A material withstands higher loads than HIPS-B in air.

Only after cycle numbers above 2.5*106 does the HIPS-B in air regression line lay above the

regression of HIPS-A in oil. The worst SCG of the 4 different combinations of materials and envi-

ronments has HIPS-B in oil. It is also clearly visible, that the environment has more influence on

HIPS-B than on HIPS-A. HIPS-A does not only have a higher resistance against slow crack

growth, but also its environmental stress cracking resistance against oil is significantly higher. This

difference is given through the larger rubber particle size in HIPS-A compared to HIPS-B. This

result concurs with the data given in Figure 4.12 and indicate a clear preferable choice of the

materials in a fatty environment.

104 105 106 107

0.10

0.15

0.20

0.25

0.30

0.35

0.40

air

oil

DK

I [M

Pa

·m0

.5]

cycles [-]

HIPS-B

10 Hz

R = 0.1

23 °C

razor blade notched

37

Figure 4.12: HIPS-A and HIPS-B stress intensity factor ranges vs. cycles with standardized test conditions

in air and oil environment.

104 105 106 107

0.10

0.15

0.20

0.25

0.30

0.35

0.40

HIPS-A air

HIPS-A oil

HIPS-B air

HIPS-B oil

DK

I [M

Pa

·m0

.5]

cycles [-]

10 Hz

R = 0.1

23 °C

razor blade notched

38

5 SUMMARY AND CONCLUSION

The aim of this work was the development of a uniform manufacturing process of cracked round

bar (CRB) specimens for a fatigue crack growth characterization in different environments and the

evaluation of the environmental stress cracking resistance (ESCR) of two different high-impact

polystyrenes (HIPS) in air and sunflower oil environments. The measurements were carried out

according to the ISO 18489 (ISO/TC 138/SC 5, 2015) which is a guideline for polyethylene testing,

but different parameters were reviewed to fit for HIPS.

First of all, the absorption behavior of HIPS immersed in oil needed to be investigated. Therefore,

smooth round bars with CRB geometries were immersed in oil and the weight increase was meas-

ured. The weight increased about roughly 0.2 % in the first 48 h and reached a plateau there

without any further significant weight increase. Furthermore, the storage possibility was investi-

gated to be able to immerse many specimens at once. Measurements of the smooth round bars

showed no significant weight decrease within a testing time of roughly 310 h. As a result of these

measurements an immersion time prior to testing of 48 h was chosen with a possible storage time

of at least 12 days.

For a perfect CRB specimen, the ligament diameter needs to be 11 mm. Because this is hardly

achievable in a human operated manufacturing process, a correction of data needed to be made

after testing. After failure the cracked surface was measured via a microscope to get the initially

ligament diameter and the data were corrected.

The next step was to examine the test frequency influence on the material. Measurements were

carried out with frequencies of 1, 2, 5, 10 and 20 Hz. Tests at lower frequencies led to lower

numbers of cycles to failure. This behavior can be explained by the testing time influence on the

material. Tests at lower frequencies take far longer to achieve the same cyclic numbers than high

frequency tests. Therefore, the creep share in deformations is much larger which influences the

material. The main issue of hysteretic heating at higher frequencies could not be measured and

as a result of this the 10 Hz of the ISO 18489 (ISO/TC 138/SC 5, 2015) were used to measure

HIPS specimens.

A huge influence on the results are the blades used for notching of the CRB specimens. Therefore,

seven different blades with different thicknesses and geometries were used to generate HIPS-A

specimens which were tested in air with the same loading and showed the lowest values of cycles

to failure for a normal razor blade and a special chipping blade. Higher values in cycles to failure

indicate a blunted crack tip which is not favorable. After testing of the two preferable blades with

HIPS-A in air and in oil, it was chosen to produce specimens with the razor blade because of the

easier handling and availability for all round robin participants and no significant difference in re-

sults.

39

The ESCR measurements of both HIPS materials (HIPS-A and HIPS-B) as the final and main

point of this thesis could now be tackled. The measurements in air environment showed for HIPS-

A for maximum forces between 300 N to 700 N which resulted in number of cycles to failure of

between 4.0*104 and 2.4*106. In comparison to these values, the test in oil environment showed

that the in oil tested specimens withstood 10 % lower stress intensity factor ranges at the same

cycle numbers at a low cycle number range (short tests), and 20 % at a high cycle number range

(long tests). HIPS-B showed a similar behavior between air and oil tested specimens, but with a

larger influence of the media and at lower loading levels. With applied maximum forces between

250 N and 500 N, the number of cycles until failure laid between 1.8*104 to 6.0*106. Specimens in

oil failed with the same level of stresses about 5 to 13 times faster than specimens tested in air

environment. This behavior again intensified with higher cycle numbers/lower stresses.

Comparing both materials in the two different environments showed a clear performance cham-

pion. HIPS-A outperformed HIPS-B in every way in the ESCR measurements. HIPS-B endures at

the same stress levels nearly 10 times less cycles until failure than HIPS-A in air. This difference

was even bigger with tests in oil. Therefore, HIPS-A is the by far favorable material for applications

in an oil environment.

40

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