Development of Instrumented Composite Cylinders

Development of Instrumented Composite Cylinders

H. Hernández-Moreno1, a, F. Collombet2, b, B. Douchin2, c, P. Davies3, d, J. L. González-Velázquez4, e

1Instituto Politécnico Nacional, ESIME Unidad Ticomán, Av. Ticomán No. 600, Col. San José Ticomán, 07340, México D. F., México

2Laboratoire de Génie Mécanique de Toulouse, research team PRO²COM, IUT Paul Sabatier, 133c avenue de Rangueil, BP 67701, 31077 Toulouse Cedex 4, France.

3IFREMER Materials & Structures group, Brest Centre, BP70, 29280 Plouzané, France.

4Instituto Politécnico Nacional, ESIQIE, Av. IPN, Col. Lindavista, 07300, México D.F., México

a [email protected], b [email protected], c [email protected] d [email protected], e [email protected]

Keywords: composite materials, filament winding, Bragg gratings, smart materials.

Abstract. This paper describes a technique to fabricate instrumented composite cylinders made by

the filament winding process and based on the insertion of Bragg grating sensors contained in an

optical fiber, so that real time monitoring of the temperature and strain response can be made. The

main advantage of this method is that, for the first time, the strain response of the wound layers as a

function of time and temperature can be monitored during fabrication without alteration of the

composite structure, since the sensor is a filament itself. The sensor data reveals the existence of

several phenomena related to the manufacturing process and material response. The results show

that during the curing stage of the fabrication of the composite cylinders there is a dimensional

contraction without change in temperature which indicates that a material transformation occurs,

and also that at the last stage there is a residual compressive strain which may affect the in-service

behavior of the composite cylinder.

Introduction

In marine and oceanographic research, cylindrical composite vessels are used to transport

instruments into deep water; these are generally made of composite material fabricated by the

filament winding process. In deep water, the vessels are under high compressive pressures that can

cause failure before expected if there are residual stresses and strains in the vessel structure, usually

as a result of the fabrication process. In addition, residual strains may also generate defects in the

material after fabrication, such as internal delaminations [1] and fiber distortion [2]. Up to date,

there is no reported experimental investigation on the nature and magnitude of these residual

strains, nor there is knowledge of what fabrication stage produces these strains in filament wound

composite cylinders using embedded sensors. Other studies using strain gages have required the

cutting of small instrumented coupon in order to measure strains before and after cutting [3].

Recently, the advance of optical fiber based strain sensors has opened the possibility to insert

sensors into the composite materials without altering the host material behavior. These intelligent

sensors offer an opportunity of not only recording the material condition during and after

fabrication (in the form of strain and temperature evolution), but also of continuous monitoring the

material performance and soundness once the instrumented piece is set in service. Embedded

sensors also offer the possibility of giving information of the internal material behavior, which is

not possible with strain gages and other techniques, since they all are superficial.

One type of embedded sensors which is widely used as strain and temperature sensor is the Bragg

grating [4], but its application in filament wound vessels and pipes has been limited to in service

monitoring [5]. To insert and use embedded optical sensors in filament wound cylinders during the

fabrication process (winding and curing) is of great interest, but represents a technological

challenge, due to the fact that the composite cylinder is rotating [6].

Materials Science Forum Vol. 560 (2007) pp. 59-66online at http://www.scientific.net© (2007) Trans Tech Publications, Switzerland

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This paper describes a newly developed technique to instrument filament wound composite

cylinders with the ability of the embedded sensors to follow strain and temperature changes during

fabrication and demonstrates its usefulness in the detection of mechanical and physical phenomena

that occur during fabrication.

Bragg Gratings. A Bragg grating sensor is an optical fiber that has a periodic variation of the

refractive index in the fiber core. When a luminescent signal arrives to the grating, most of the

signal is transmitted and only a narrow band of the spectrum is reflected. This reflected signal has a

peak around a characteristic wavelength; and the wavelength variation is lineally proportional to

strain and temperature. The simplified behavior of the sensor can be expressed by Eq. 1:

ε∆λ

λ∆bTa

B

B += (1)

where ∆λB represents wavelength variation, λB is the grating characteristic wavelength, a is the

thermo-optic coefficient, b is the strain-optical coefficient, ∆T is temperature variation, and ε is the

strain along the optical fiber axis.

To separate the strain and temperature responses from the spectrum several methods have been

described [7]. In the present work, the method consisted of the thermal calibration of each sensor

before being embedded into the material in order to determine the wavelength variation as a

function of temperature by means of a Bragg grating thermal response equation. The strain response

can then be obtained by substracting the thermal component from the overall response. According

to this method, it is necessary to have a temperature reference in the same place as the optical

grating, which was accomplished by means of a thermocouple. The thermocouple response is

introduced in the Bragg grating thermal response equation and the strain is obtained using Eq. 2:

[ ])T(b

reading

B

λλλ

ε −=1

where λreading is the wavelength reading directly from the measurement instrument, and λ(T) is the

Bragg grating thermal response equation evaluated at the temperature at which λreading is performed

using the thermocouple.

Cylinder Geometry and Filament Winding Description

Cylinder Geometry. In the present work, several cylinders made of glass – epoxy composite were

fabricated. Specimen dimensions were: 25 mm internal diameter, 4.4 mm wall thickness and 350

mm overall length. The specimen shape and dimensions are shown in Fig. 1. The number of

filament winding layers is 7, with a filament winding fiber orientation of ± 55°. The resin is a

mixture of araldite LY 5052 and hardener HY 5052. This type of specimen was designed for

posterior testing under external pressure. The specimens have reinforced zones at both ends with a

winding angle of 90°. After the curing cycle both ends of the cylinder are machined to obtain flat

surfaces necessary for a hermetical seal during the external pressure testing.

Fabrication Process. The filament winding configuration consisted of one fiber strand that was

impregnated with resin and wound on the surface of a rigid mandrel. The mandrel had a cylindrical

shape and the winding machine had numeric control. A schematic of this process and a picture of

the winding machine are shown in Fig. 2.

After the winding stage, the resin is still in the liquid state and must undergo a curing process, in

which the specimen must be rotating to avoid a nonuniform resin distribution. For this purpose, the

mandrel-piece assembly is placed in a turning spindle, Fig. 3(a), and introduced into an oven with a

preset thermal cycle, Fig. 3(b), for resin polymerization.

(2)

Advanced Structural Materials III60

Figure 1. Geometry and dimensions of the specimen in its final form with machined ends.

(a) (b)

Figure 2. (a) Composite cylinder fabrication process and (b) winding machine used in this research.

(a) (b)

Figure 3. (a) Specimen and mandrel mounted in a turning spindle. (b) Thermal curing cycle.

Adapted Tooling and Sensor Placement

As explained in the previous section, the specimen and the mandrel are in rotation during the

winding and curing stages. To place the embedded sensors, two devises were designed; one is a

divided mandrel, adapted to let fiber terminals to exit the specimen through the inner wall at the

specimen half-length, as seen in Fig. 4(a). The second device is an interface between the sensor

terminals in the rotating setup and the static measurement instruments; this is shown in Fig. 4(b).

As explained before, in order to measure strains from the Bragg grating it is necessary to know

the temperature; for this, a thermocouple had to be placed in the specimen, with the location shown

in Fig. 4(c). To connect the static and rotary parts of the experimental setup, a dual fiber optical

rotary joint for optical signals and a slip ring assembly for thermocouples (both adapted to form a

connector system) were installed. This connection became a part of one of the mandrel flasks. This

system can operate in both the winding and the curing stages.

In the winding stage, the static cables leave the connector system and the mandrel by the end

attached to the machine spindle chuck, which is mounted on the machine hollow shaft. Static cables

Materials Science Forum Vol. 560 61

are directed through the hollow shaft, so they can leave the machine and be connected to the

measurement instruments. In the curing stage, the connector system static segment is attached to the

turning spindle. Detailed information of this design can be found in Hernandez –Moreno et al. [6].

(a) (b) (c)

Figure 4. (a) Divided mandrel with connector system. (b) Connector system assembly installed on

the filament winding machine. (c) Relative location of thermocouple and Bragg gratings.

Bragg gratings and thermocouples were placed manually during winding, one Bragg grating and

one thermocouple in the axial direction and one grating and one thermocouple in the circumferential

direction, placed between the first and the second layer. Five instrumented specimens were made,

and the strain and temperature evolution was recorded during the entire fabrication process.

Results and Discussion

It is important to emphasize that the main goal of the work reported here was the design and

construction of an instrumented composite cylinder in which the temperature and strain could be

monitored during the fabrication process and subsequently during service. This goal was fully

achieved and the following paragraphs describe the results obtained and their meaning. Another

paper will give a detailed analysis of the experimental results and its scientific interpretation as well

as the results of the external pressure tests.

The results of temperature and strain monitoring during the fabrication process are described in

three ways: strain as a function of time, strain versus temperature, and circumferential strain

compared to axial strain. To avoid excessive extension of this paper, the results of a single test are

reported with the understanding that they are representative of all of the tests done. The

characteristic process segments are identified by numbers indicating the start and finish points, and

points related to material events are identified by letters, as described in Table 1. Figure 5 shows the

thermocouple temperature vs. time data. During the winding (segment 1-2), the temperature shows

an increment of about 7 °C, which is related to ambient temperature change during this stage. In

segment 3-4, the temperature remains almost constant, but after point 4, where the curing cycle

starts with a warm up segment, a temperature increase is registered (segment 4-5). Then, the

temperature stabilizes (segment 5-6) and finally there is a cooling (segment 6-7). It is observed that

the thermal experimental response is coincident with the preset thermal cycle in the oven (see Fig.

3(b)). This gives confidence to the strain results, since it showed that the data acquisition system

worked properly.

Strain vs. Time. It is clear that material events were not recorded in the temperature vs. time plot,

since they are related to strain or thermo-elastic changes. For this reason, strain versus time


Table 1. Process Segments and Material Event Identification

Process Segments

Identification Segment Description

1-2 Winding

3-4 Waiting phase between winding and curing cycle

4-5 Heating

5-6 Constant temperature curing phase

6-7 Cooling

Material Event

Identification Event Description and Occurring Transformation

a Point from which elastic response and thermo-elastic behavior

begin to appear during heating

b

Point where axial strain shows a fast decrease and may indicate a

transformation. From this point on there is no elastic response nor

thermo-elastic behavior

c

Point at which a transformation occurs from semi-liquid or liquid

state into the solid state. Material shows again elastic and thermo-

elastic behavior

Figure 5. Thermocouple temperature vs. time.

and circumferential strain versus axial strain analyses are needed. Figure 6 shows variations in

strain between 0 and 600 µε for the circumferential strain and from 0 to - 400 µε for the axial strain

during the winding phase (segment 1-2). This is attributed to optical fiber manipulation during this

stage and other subsequent variations due to the winding operation. It must be remembered that in

this segment the resin is still in the liquid state and the strain variations can not be result of the

mechanical behavior of the material. Between the end of the winding stage and the start of the

curing cycle (segment 3-4), both strains show no variation. During the polymerization cycle, the

circumferential strain behavior is related only to temperature variations; this conclusion arises when

comparing the change with time of circumferential strain data and temperature; a similar variation

of both data sets is observed. These results also indicate that a relation is established between strain

and temperature at the start of the heating segment, since it can be observed in Fig. 6 that strain

changes follows the temperature changes. This means that polymerization starts somewhere at the

end of the winding stage and the beginning of the curing cycle.

Axial strain behaves in a different way to circumferential strain. It can be seen in Fig. 6 that axial

strain reacts to the heating ramp as the curing cycle begins, but there is a point where this

temperature coupled behavior stops (point b). From this point, even if the temperature is still

increasing, the axial strain gradually decreases until the end of the constant temperature segment


Figure 6. Bragg grating strain vs. time.

(near point 6), and at this point the axial strain is about zero. In the cooling segment, both strains

react proportionally to the temperature, as expected, because the polymerization is completed and

the resin behaves completely as a solid. During this segment there is an event where a change in

both strains occurs; this probably corresponds to the moment where mandrel and specimen separate.

At the end of the fabrication process the strain responses are: axial strain about -600 µe and

circumferential strain about 100 µε. These values can be considered as the residual strains generated

in the material during the fabrication process.

Figure 7. Bragg grating axial strain vs. temperature.

Strain vs. Temperature. Axial strain as a function of temperature is presented in Fig. 7. In this

figure it can be seen that during winding (segment 1-2), the strain evolution is independent of

temperature, and the variations in strain have no thermal origin, but are related to optical fiber

insertion and the liquid state of the resin. When heating begins (point 4), there is no strain –

temperature coupling. This can be explained by the fact that resin is still in liquid state and the

optical fiber is not constrained by the resin. From point a, a thermo–elastic behavior is evident, and

at point b, even if the temperature is still increasing, the axial strains diminish. This occurs when the

temperature reaches 58 °C which is the gelTg of the resin used (that is, the temperature at which the

vitrification and gelification curves intersect in a Time-Temperature-Transformation diagram for a

thermoset resin) [8].

At point c the thermo-elastic behavior is reestablished, and at the end of the constant temperature

segment the axial strain is zero; it means that between point b and c there is relaxation of the axial

strain. During the cooling segment a complete thermo-elastic behavior is seen, where the cooling

down response is divided into two segments, each one having a slightly different slope; the division

may correspond to the moment where mandrel and cylinder mechanically behave in an independent

way. The behavior of the circumferential strain is similar to that of the axial strain but the difference


between the heating ramp and the cooling ramp is smaller than for the axial strain, as can be seen in

Fig. 8.

Figure 8. Bragg grating circumferential strain vs. temperature

Circumferential vs. Axial Strain. This analysis shows an elastic behavior of the material. Figure

9 shows the circumferential strain as compared to the axial strain; this plot allows identification of

the events related to the thermo-elastic response (points a, b and c, see Fig. 9). After the beginning

of heating (point 4) it is clearly observed that the behavior is linear from point a to point b. At point

b there occurrs a dramatic change, and the proportionality between the strains is no longer present;

however, at point c this proportional behavior is reestablished.

Figure 9. Bragg grating circumferential strain vs. axial strain.

Strain Behavior During Winding. To analyze in detail what happens during winding, plots of

the strain response during this segment are presented in Figs. 10(a) and 10(b). Several events can be

detected. It is first seen a constant strain (both axial and circumferential) response at the beginning

of the winding operation, which may correspond to the period where the first layer was wound,

because at this time the fiber optic sensors have not been placed yet. Next a regular undulated

response is seen where each halfwave can be related to the placement of one layer in the composite

cylinder. This behavior is observed in both axial and circumferential responses. In Figs. 10(a) and

10(b) the strains in each halfwave and the layer to which they are related are indicated (7 halfwaves

in total); at the end there is a flat response which corresponds to the period where the cylinder end

reinforcements are wound.


(a) (b)

Figure 10. Bragg grating strain evolution during winding. (a) Axial strain vs. time,

(b) circumferential strain vs. time.

Conclusions

The design and construction of a embedded system for monitoring temperature and strain during the

fabrication of fiber wound composite cylinders was successfully achieved. Data from sensors were

acquired proving the successful operation of the system.

The temperature and strain readings could be related to both process characteristics and material

responses.

Three important material transformations were detected: the beginning of a thermo-elastic

response in the warm up stage; the beginning of a material transformation which is coincident with

the gelTg of the resin used; and the beginning of full polymerization.

Residual strains were quantified, showing a contraction in the axial direction and an expansion in

the circumferential direction.

References

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marines composites - Validation expérimentale. Actes du cinquième colloque national en calcul des

structures, Giens (France), 15-18 mai 2001, 789.

[2] L.G. Stringer, R.J. Hayman, M.J. Hinton, R.A. Badcock and M.R. Wisnom: in Proceedings of

the Twelfth International Conference on Composite Materials (France 1999).

[3] P. Casari, F. Jacquemin and P. Davies: Composites A. Vol. 37-2 (2004), p. 337.

[4] N. Takeda: Ann. Chim. Sci. Mat. Vol. 25-7 (2000), p. 545.

[5] J. Degrieck, W. De Waele and P. Verleysen: NDTE Int. Vol 34-4 (2001), p. 289.

[6] H. Hernandez-Moreno, B. Douchin, F. Collombet and P. Davies: in 11th European Conference

on Composite Materials (Greece 2004).

[7] Y. Zhao and Y. Liao: Opt. Lasers Eng. Vol 41-1 (2004), p. 1.

[8] P E. Bourban, L. Carlsson, J. P. Mercier and J.A.E. Månson : Traité de Matériaux, Vol. 15

Matériaux Composites à Matrice Organique (Presses Polytechniques et Universitaires Romandes,

Switzerland 2004).


Development of Instrumented Composite Cylinders

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