Issue 38

C. Gourdin et alii, Frattura ed Integrità Strutturale, 38 (2016) 170-176; DOI: 10.3221/IGF-ESIS.38.23 175 The proposal approach to determine the level of the equivalent strain for each FABIME2 test is as follows: • Determination of the value of the radial strain corresponding to the imposed deflection from the strain-deflection calibration curve obtained in the previous part of this paper. With a similar mechanical behavior, the calibration curve can be used for the two materials (Fig. 3). • Determination of the von Mises or TRESCA equivalent strain from the relation between the radial strain and the equivalent strain (von Mises or TRESCA). This relation has been determined by elasto-plastic calculation of the fatigue test. Theses elastic-plastic behavior computations are used to determine the “real” value of the Poisson’s ratio by taking into account the elastic and plastic part. This method has been applied to the equi-biaxial fatigue tests presented earlier. The corresponding fatigue life curves are compared to that under uniaxial loading in Fig. 5. It appears that there is also no impact of equi-biaxial fatigue for the two types of materials, considering both von Mises and TRESCA equivalent strains. C ONCLUSIONS his paper is focusing on the study of the impact on the equibiaxiality on the fatigue curves. A new experimental FABIME2 device has been developed at LISN. Two calibration tests (with strain gauges and image correlation) were used to obtain the relationship between the imposed deflection and the radial strain on the FABIME2 specimen. Biaxial fatigue tests are carried out on two austenitic stainless steels: 316L THY and 304L CLI for different values of the maximum value of deflection, and with a load ratio equal to -1. The interpretation of the experimental results requires the use of an appropriate definition of equivalent strain. In nuclear industry, two kinds of definition are used: von Mises and TRESCA equivalent strains. The results obtained during the experimental campaigns carried out in the context of our study and for two austenitic stainless steels submitted to equibiaxial loadings show that crack initiation have a low impact on the fatigue life, which remains in the field covered by the design curve defined and used in the codification. This FABIME2 device allowed the study of the impact of fatigue life on equibiaxial loadings and crack propagation in austenitic stainless steel. So, the device has the capability to study other different aggravating factors like surface roughness, mean stress or strain, residual stress, pre-hardening. A new device based on FABIME2 is under development for the study of the impact of the environmental effect. This device will study the impact of the equibiaxial loadings with a primary water environment PWR (300°C with a permanent pressure of 140 bars). R EFERENCES [1] Fissolo, A., et al., Crack Initiation under thermal fatigue: an overview of CEA experience, Part 1 : thermal fatigue appears to be more damaging than uniaxial isothermal fatigue, Int. Journal of Fatigue, 31(3) (2009) 587-600. [2] Fissolo, A., et al., Crack Initiation under thermal fatigue: an overview of CEA experience, Part 2 : Application of various criteria to biaxial thermal fatigue tests and a first proposal to improve the estimation of the thermal fatigue, Int. Journal of Fatigue, 31(7) (2009) 1196-1210. [3] Lefebvre, D.F., Hydrostatic Pressure effect on Life Prediction in Biaxial Low-cycle fatigue”, Biaxial and Multiaxial Fatigue, EGF 3, (1989). [4] Itoh, T., Sakane, M., et al., A design procedure for assessing low cycle fatigue life under proportional and non- proportional loading, Int. Journal of Fatigue, 28 (2006). [5] Parsons, M. W., Pascoe, K. J., Development of a biaxial fatigue testing rig, Journal of Strain Analysis, 10(1) (1975) 1-3. [6] Poncelet, M., et al., Biaxial High Cycle Fatigue of a type 304L stainless steel: Cyclic strains and crack initiation detection by digital image correlation, European Journal of Mechanics A/Solids, (2010). [7] Gourdin, C., Fissolo, A., Balestreri, F., Crack initiation under an equibiaxial fatigue, development of a particular equibiaxial fatigue device, Transactions of SMIRT 21, New Delhi, India, (2011). [8] Bradai, S., et al., Crack Initiation under Equibiaxial Fatigue, Development of a particular Equibiaxial Fatigue Device. PVP2013-97200, Paris, France (2013). T