Procedia Engineering 66 ( 2013 ) 669 – 675

Available online at www.sciencedirect.com

1877-7058 © 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of CETIM

doi: 10.1016/j.proeng.2013.12.119

ScienceDirect

5th

Fatigue Design Conference, Fatigue Design 2013

Experimental facility for high cycle thermal fatigue tests using

laser shocks

L. Vincenta*, M. Poncelet

b, S. Roux

b, F. Hild

b, and D. Farcage

c

a CEA SACLAY, DEN, DMN, SRMA, 91191 Gif sur Yvette cedex, France b LMT CACHAN, ENS Cachan / CNRS / UPMC / PRES UniverSud Paris

61 avenue du Président Wilson, 94235 Cachan cedex, France c CEA SACLAY, DEN, DPC, SEARS, 91191 Gif sur Yvette cedex, France

Abstract

A new thermal fatigue testing facility is proposed in which thermal shocks are conducted with a pulsed laser beam. The

temperature field on the surface of the specimen is measured with an infrared (IR) camera. Moreover, a speckle is created on this

surface prior to applying the thermal shocks and an estimation of strain variations is proposed using Digital Image Correlation

(DIC) techniques. This experimental strain variation is compared to the prediction obtained by Finite Element (FE) analyses

conducted in previous experimental studies.

© 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of CETIM, Direction de l'Agence de Programme.

Keywords: Thermal fatigue, High cycle fatigue, Laser shock, Full-field measurement

1. Introduction

Thermal fatigue due to the turbulent mixing of two fluids at different temperatures is one of the damage

mechanisms that occur in pipes of nuclear power plants. Specific experimental campaigns have been performed on

devoted facilities designed to reproduce the typical thermomechanical loading of real components [1-6]. In such

facilities, the temperature is only measured on a limited number of points using thermocouples brazed in holes at a

prescribed distance from the location of the maximum variation of temperature. The estimation of the equivalent

* Corresponding author. Tel.: +33-1-69-08-46-39; fax: +33-1-69-08-71-67.

E-mail address: ludovic.vincent@cea.fr

© 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

Selection and peer-review under responsibility of CETIM

Open access under CC BY-NC-ND license.

670 L. Vincent et al. / Procedia Engineering 66 ( 2013 ) 669 – 675

strain variation in the region of crack initiation is then performed through a numerical chain of thermomechanical

simulations. This estimation is eventually compared to the strain fatigue curve of the material and it turned out that,

in many cases, the prediction of crack initiation in thermal fatigue is non-conservative [7].

Several hypotheses are proposed to explain such a discrepancy. Among them, a potentially bad estimate of the

temperature variations in the crack initiation area cannot be excluded and constitutes a strong motivation to revisit

the test design.

A new test is proposed where thermal shocks are generated by a pulsed laser beam and the thermal field on the

surface of the specimen is measured with an infrared (IR) camera. Moreover, a speckle pattern is created on this

surface prior to applying the thermal shocks and a measurement of strain variation is proposed using Digital Image

Correlation (DIC). The experimental strain variation is compared to the predictions obtained by Finite Element (FE)

analyses relayed to previous experimental studies.

2. Experimental set-up

A continuum Ytterbium fibre laser (IPG Photonics, YLR-LP) is used to prescribe thermal shocks on the centre of

one face of a plate made of 304L austenitic stainless steel (length = 50 mm, width = 50 mm, thickness = 10 mm).

Due to the relatively low absorptivity of the sample surface (i.e., proportion of incident laser energy absorbed by the

surface), the beam is tilted in the vertical plan and a calorimeter is used to capture the reflected energy (Fig. 1.(a)).

On the horizontal plane, an IR camera (JADE (Cedip), 320 × 256 pixel definition, with a 50-mm lens and a 20-mm

extension ring) is focused on the impacted zone of the sample with a black-body (HGH DCN 1000N) in reflection.

A visible camera (BASLER ac2040, 2048 × 2048 pixel definition) is focused on the region of interest, normal to the

sample surface, with a macro lens of magnification ×1 (Canon EF 100mm f/2.8).

(a) (b)

Fig. 1. Configuration of the tests. (a) Vertical plane. (b) Horizontal plane.

2.1. Sample Surface

In order to be able to detect crack initiation when thermal fatigue tests are performed, the surface of the sample

was polished prior to the deposition of a speckle pattern necessary for the DIC technique. Two types of speckle

patterns have been tested, namely, a white acrylic paint speckle and a laser engraving of a rectangular grid (200 om

671 L. Vincent et al. / Procedia Engineering 66 ( 2013 ) 669 – 675

pitch with a thickness of the grid lines of about 60 om, see Fig. 2.), with no clear difference due to the type of

speckle used on the measurement results. Only the results obtained with laser engraving are presented herein.

The depth of the material affected by the engraving has been estimated to be equal to 4 om at most. Depending

on the power of the engraving laser and on the number of laser scans, different thicknesses of oxide layer were

obtained, resulting in different values of global emissivity and absorptivity of the surface. Four configurations were

tested with a global emissivity ranging from g"= 0.18 to g"= 0.35 (Fig. 2.). These values of global emissivity are

determined using the black-body (BB) in reflection of the IR camera with respect to the sample surface. Several

temperatures of the BB can be prescribed while the temperature of the sample is supposed unchanged and then,

using the simple relationship

* + BlackBodySpecimen h++ BBgghh /-? 1(1)

where h is the flux recorded by the IR camera, the value of the specimen emissivity g is computed provided a full

calibration of the camera was performed previously. This calibration step is performed with the BB in front of the IR

camera and gives access to a direct relationship between the Digital Levels (DL) of the camera, the temperature of

the BB and the integration time of the camera [8].

Fig. 2. Laser engraving of the surface

2.2. Laser

The 100-W continuous laser beam is controlled by a function generator to prescribe a 100-ms pulse every second.

The spatial energy distribution of the beam can be qualified as Gaussian with a cut-off radius of about 1.8 mm on

the surface of the sample in the horizontal direction and 2.3 mm in the vertical direction due to the tilt angle of the

beam.

2.3. Acquisition

Infrared images are recorded in half-ff frame format (160 × 128 pixel definition) at 500 Hz or at 360 Hz frequency

when the IR camera is triggered to capture images at the same instants of time as those of the visible camera (half-ff

frame, 1024 × 1024 pixel definition). This last acquisition mode is used to make easier the detection of the reference

image used in the DIC technique.

200om

672 L. Vincent et al. / Procedia Engineering 66 ( 2013 ) 669 – 675

3. Results

3.1. Experimental temperature and displacement fields

Typical temperature and displacement fields obtained at the end of a thermal shock are shown in Fig. 3. and in

Fig. 4.

The displacement field of Fig. 4 is obtained using a Mechanics-Aided Digital Image Correlation code [9]

developed to reduce noisy patterns usually obtained with small zones of interest in standard local DIC.

The time evolution of the average and maximum temperatures measured on a disk of approximately 10-pixel

diameter and located in the central zone of the thermal shock is plotted in Fig. 5 for the three different surface

finishes, ordered from the less oxidized to the most one. The values of emissivity used to plot these data are added to

the subfigures and one can observe, as expected, that the emissivity is an increasing function of the absorptivity of

the surface. In Fig. 5 it is observed that the temperature variations induced by the thermal shocks are very stable, as

required to run thermal fatigue tests at constant amplitude loadings.

Fig. 3. Typical Temperature field (in °C) at the end of a thermal shock, engraving number: 4.

(abscissa: horizontal pixel number with 1 pixel … 80 om, ordinate: vertical pixel number with 1 pixel … 75 om)

(a) (b)

Fig. 4. Typical displacement field (in pixel) at the end of a thermal shock, engraving number: 4

(abscissa and ordinate: pixel numbers with 1 pixel … 5.5 om)

y

z z

673 L. Vincent et al. / Procedia Engineering 66 ( 2013 ) 669 – 675

(a) (b)

(c)

Fig. 5. Change of the average and maximum temperatures on a disk 10 pixels in diameter located in the central zone of the thermal shock. Three

types of surface finishes and the same incident laser power are considered.

3.2. Finite Element simulations

Three dimensional FE simulations are carried out first to simulate the evolution of the temperature field, and

second to compute the change of the induced stress and strain fields. The thermal parameters are chosen from data

used in the French design code for Nuclear power plants [10]. The mechanical behavior is described by a non-linear

kinematic hardening identified on push-pull cyclic tests at 165 and 320 °C. This law has already been used to

estimate the strain variations in other thermal fatigue experiments [11].

The objective of this section is to evaluate the complete experimental chain starting from the estimate of the

surface emissivity, followed by the measurement of temperature and displacement fields. The same variation of

temperature as that measured experimentally in the central zone of the surface sample is obtained on the simulation

by adjusting the level of surface absorptivity of the material and the standard deviations of the Gaussian profile of

the incident laser energy in both vertical and horizontal directions (Fig. 6).

The changes of the principal strains (simulated and experimentally measured) are compared in Fig. 7a). The

normal to the sample surface is labeled X for FE results, while the vertical and horizontal axes (see Fig. 4) are

respectively denoted by Z and Y.

It is observed that the level of total strain measured in the principal directions by DIC is quite well reproduced by

the simulations in the vertical direction. The simulations however underestimate the strain variations in the

horizontal direction. The origin of such discrepancy is still under investigation. It might for instance come from a

wrong parallax correction applied to the images captured by the IR camera, which is tilted with respect to the normal

674 L. Vincent et al. / Procedia Engineering 66 ( 2013 ) 669 – 675

of the sample (but not the visible camera). An asymmetry due to the tilt of the laser beam is however obtained by the

simulation even though the difference between the principal strains in the plane of the sample is not as large as that

measured experimentally.

In Fig. 7 b the hysteresis loops of the mechanical behavior of the material are plotted and it is worth noticing that

the largest plastic strain variation is in the Z (vertical) direction, contrary to what is obtained for the total strains (see

Fig. 7 a). The thermal strains are isotropic and they fluctuate in the center of the plate between 0.4% at the end of

one laser shock and 0.05% just before the next one. Since the planar mechanical strain is negative during a positive

thermal shock and with a variation less than that of thermal strains, the total strains at the end of the thermal shock

are thus minimal in the direction of maximum mechanical strain amplitude and/or stress amplitude.

Fig. 6. Comparison between experimental and numerical temperature profiles at the end of a thermal shock after identification of the surface

absorptivity and the standard deviations of the Gaussian laser power density.

(a) (b)

Fig. 7. (a) Change of strain components in the central zone of the most oxidized sample (Fig. 5). Dashed lines stand for 3D FE simulations and

solid lines for experimental results (DIC). (b) Mechanical behavior for FE results.

-0.10%

0.00%

0.10%

0.20%

0.30%

0.40%

0.50%

0.60%

0.70%

0.80%

0.90%

0000 0.2. 0.4. 0.6. 0.8. 111

Tota

l S

tra

in

Time (s)

EYY, FE

EZZ, FE

EXX, FE

Eps_yy, MA-DIC

Eps_zz, MA-DIC

( )(a)

675 L. Vincent et al. / Procedia Engineering 66 ( 2013 ) 669 – 675

4. Conclusion

A new device based on a pulsed laser is used to apply thermal fatigue loadings on a plate made of 304L austenitic

stainless steel. Different conditions of laser engraving have been performed to produce the grey level contrast

necessary for the DIC technique. Temperature and displacement fields are measured on the surface of the sample

using infrared and visible cameras. After estimating the global emissivity of the different surfaces tested, the change

of temperature is measured during the cycles and a first comparison of strain variations with finite element

simulations leads to encouraging results. The next step of the study (in progress) will consist of performing thermal

shocks with a top-hat power density profile for the laser beam to increase the volume homogeneously affected by

the thermal shock, and therefore to prevent as much as possible any size effect on future fatigue results.

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