AXIAL CYCLIC BEHAVIOR AND FATIGUE AFTER TORSIONAL PRE-HARDENING FOR A 304L SS

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AXIAL CYCLIC BEHAVIOR AND FATIGUE AFTER TORSIONAL PRE-HARDENING FOR A 304L SS

Adel Belattar, Lakhdar Taleb, Said Taheri

To cite this version:

Adel Belattar, Lakhdar Taleb, Said Taheri. AXIAL CYCLIC BEHAVIOR AND FATIGUE AFTER TORSIONAL PRE-HARDENING FOR A 304L SS. 18th International Symposium on Plasticity and its current applications - Plasticity’2012, Jan 2012, San Juan, PR, United States., Jan 2012, San Juan, PR, United States. pp.3. �hal-01096673�

AXIAL CYCLIC BEHAVIOR AND FATIGUE AFTER TORSIONAL PRE-HARDENING FOR A 304L SS

Adel Belattar*, Lakhdar Taleb*, Said Taheri**

*INSA, GPM, CNRS UMR 6634, BP 08 avenue de l’université, 76801 St. Etienne du Rouvray Cedex, France, [email protected] [email protected]

** LaMSID EDF-CNRS 2832, 1 Avenue du Général de Gaulle, 92141 Clamart Cedex, France, said.taheri@edf

ABSTRACT- At room temperature, shear strain control cycles were applied in four loading sequences with increasing/decreasing Von Mises equivalent amplitudes on the same specimen made up of 304L SS. Stress-strain responses show that cyclic curve is not unique. The one obtained in a second sequence is located above that obtained after the first sequence. However, the difference is smaller after the following sequences. Fatigue life is significantly reduced by the previous pre-hardening, such effect is reduced when the applied amplitude during the fatigue tests is high.

INTRODUCTION: To determine the required parameters for the reliable and accurate prediction of fatigue life, the effect of prior straining on the subsequent cyclic behavior and fatigue, with or without mean stress and strain, must be considered. This work is a continuation of a previous research performed by Kpodekon and al. [2010]; where, fatigue tests were executed under strain or stress control on virgin and prehardened specimens using monotonic and cyclic pre-hardening. In this previous study, four configurations of the last point of the pre-hardening in the axial stress-strain loops were considered. The mean stress may be positive or negative depending on the location of these breakpoints. The obtained results show that fatigue life is strongly influenced by prehardening; it appears beneficial when the fatigue test is executed under stress control but detrimental in strain-control. It appears for both prehardening modes that the presence of mean stress in the fatigue cycles has a significant effect on fatigue life. Therefore, our objective here is to study the effect of a pre-hardening in the shear direction on the fatigue in the axial direction at zero shear stress and strain. The comparison with the axial pre-hardening will be given later in a full length paper.

PROCEDURES, RESULTS AND DISCUSSION: The cyclic behavior is investigated

through cyclic shear strain control experiments carried out in four loading sequences with

increasing/decreasing amplitude on the same specimen. The first sequence is composed

of (applied cycles/equivalent strain amplitude): (50/0.2%)  (40/0.3%)

(20/0.5%)  (20/1%)  (10/1.5%). The second sequence is a progressive come back to

the origin: (20/1%)  (20/0.5%)  (30/0.4%)  (40/0.3%)  (50/0.2%). The third and

the fourth sequences are the same as the first and the second ones respectively. The effect

of pre-hardening on fatigue is studied through fully-reversed axial strain controlled tests

conducted at different amplitudes (0.22%, 0.36% and 0.5%) on specimens initially

subjected to the four sequences described above. The chosen pre-hardening process has

the main advantage to ensure a progressive come back to the origin which ensure zero mean stress and strain during the subsequent fatigue tests. The fatigue life of pre- hardened specimens is compared to the one of specimens without previous loading history.

For a given loading sequence, the shearing cyclic curve represents the evolution of Von Mises equivalent stress amplitude obtained at the steady state (or at the last cycle) versus the Von Mises equivalent strain amplitude. Cyclic curve for the second sequence (BC) was higher than that of the first sequence (AB) (Fig. 1). This observation confirms the results obtained previously (Colin and al. [2009], Taheri [1996]) performed in tension- compression. However, we want through the application of another sequence loading to see the evolution of the cyclic behavior. The results show that the difference between the third and the fourth cyclic curves with the second one is relatively small. Therefore, it seems that the behavior of the material has a tendency to stabilize. Cyclic strain hardening obtained during sequential cycles is analyzed in terms of kinematic and isotropic hardening (see Fig. 2). The procedure adopted (Taleb, Hauet [2009]) is described in Fig. 3. The hardening obtained was mainly of the kinematic type (X);

especially for the higher strain amplitudes. The isotropic component (R) presented a quasi-linear evolution versus the number of cycles; it appeared to be independent of the loading path. It should also be noted that the saturation levels of the kinematic variable for each strain amplitudes obtained during the second sequence (High-Low amplitudes) is higher than the one recorded in the first (High-Low amplitudes). The difference is about 40 Mpa for 0.2% stain amplitude.

Fatigue results test (Fig. 3) showed that prehardening significantly reduced the fatigue life of the material, which was approximately 144000 cycles in the absence of prehardening. Furthermore, It seems that the increase in the fatigue amplitude decrease the effect of pre-hardening. Life reduction rate is about 52% in case of 0.22%

fatigue strain amplitude however, it is of the order of 37% and 27% in the case of 0.36% and 0.5% respectively. The pre-hardened specimens loaded at 0.22% and 0.36% fatigue strain amplitude represent a cyclical softening to feature. Secondary hardening has been observed on virgin specimens loaded at 0.5% and at 0.36%. It is also observed on pre-hardened specimen loaded at 0.5%.

Obtained fatigue life will be compared to the case where both pre-hardening and fatigue tests are carried out in the same loading direction. Such studies with microstructural investigations are in progress and will be presented in a future full length paper.

CONCLUSIONS: After an initial sequence of preloading, the 304L SS displayed a

cyclic hardening. However, as the loading was continued, the hardening tended to

stabilize. The strain hardening obtained during the cycles was mainly of the kinematic

type. The isotropic component presented a quasi-linear evolution. Torsional pre-

hardening reduces significantly the fatigue life; such affect seems smaller when the strain

amplitude applied during the fatigue tests is higher.

Fig. 1. Shearing cyclic curves corresponding to each applied loading

sequence

Fig. 2. Contributions of the kinematic and isotropic components of the cyclic shear strain

hardening

Fig. 3. Identification procedure for

strain hardening components. Fig. 4. Stress amplitude variation of fatigue tests.

REFERENCES:

Colin, J., Fatemi, A. and Taheri, S. 2010, “Fatigue behavior of stainless steel 304l, including strain hardening, pre-straining, and mean stress effects”. J. Eng. Mater.

Technol. 132/021008-13.

Kpodekon, C., Taleb, L., Taheri, S. 2010, “Effect of pre-deformation on the fatigue and isotropic/kinematic cyclic hardening of 304L anstenitic stainless steel”. Submitted to Inter. J. Pressure Vessels and Piping.

Taheri, S. 1996, Multiaxial and Fatigue Design, ESIS 21. London 283-299.

Taleb, L., Hauet, A. 2009, "Multiscale experimental investigations about the cyclic behavior of the 304L SS". Int. J. Plast. 25, 1359-1385.

0 100 200 300 400 500

0 0,5 1 1,5

√3 *σθz (MPa)

γθz / √3 (%)

Sequence 1 (A-B) Sequence 2 (B-C) Sequence 3 (C-D) Sequence 4 (D-E) A

B

C

D

E

0 100 200 300

0 100 200 300 400 500 600

strain-hardening parameters (Mpa)

Number of cycles X- Kinematic R- isotropic

-100 0 100 200 300

0 0,004

Stress (MPa)

Plastic strain

σ^max

σ^offset

Offset = 0.0001

X 2(

σ

y offset

R   

 

 2

max

2

max offset

X  



y : is the initial elastic limit

155 210 265 320 375

100 1000 10000 100000 1000000

Stress Amplitude (Mpa)

Number of cycles Ref 0.5%

CPH 1.5% Shear + F 0.5%

Ref 0.36%

CPH 1.5% Shear + F 0.36%

Ref 0.22%

CPH 1.5% Shear + F 0.22%

Ref = Reference.

CPH = Cyclic Pre-Hardening.

F = Fatigue