Issue 38

C. Gourdin et alii, Frattura ed Integrità Strutturale, 38 (2016) 170-176; DOI: 10.3221/IGF-ESIS.38.23 171 I NTRODUCTION AND AIM he problem of multiaxial fatigue is a major concern and has been extensively studied in the literature. More or less innovative experimental means have been developed. However, some references which deal with the multiaxial aspect in steels show, blatantly, the aggravating effect on multiaxiality and in particular of biaxiality on the fatigue curves. The service lives are significantly reduced [1-6]. Unfortunately, there is no experimental data available concerning fatigue strength for the austenitic stainless steels subjected to multiaxial loadings, which are used for power plants components. In order to obtain fatigue strength data under multiaxial loading, biaxial test means were developed at LISN. The particularity of this equipment is to consider only isothermal equibiaxial mechanical loadings, which are both in phase and proportional. It will be possible to conclude from the tests conducted on the specimen “FABIME2” whether the austenitic stainless steel material is sensitive or not to the biaxial state loading in the high cycle fatigue regime. On the other hand, tests undertaken by Poncelet et al. [6] on 304L austenitic stainless steel cruciform specimens have concluded that equibiaxial stress state is not detrimental compared with uniaxial fatigue. Another conclusion made by these authors is the penalizing effect of the mean stress. T HE EXPERIMENTAL DEVICE he objective of this new experimental fatigue test is to dissociate the effect of the mean stress and equibiaxial state loading. Indeed, we try to obtain a negative load ratio in order to get the same results as the uniaxial data and eliminate the residual strain. In this study, equibiaxial state loading generated from fatigue will be considered. It will be used to optimize the geometry of a disk specimen refined in its center. It is used as a circumferentially embedded diaphragm with an applied pressure on both sides in order to obtain an equivalent strain in each loading direction in the plane (Fig. 1). Figure 1 : Principle of the new experimental fatigue test. The experimental device called “FABIME 2” is divided into four parts [7]: - Fatigue cell (Fig. 2) which contains the spherical bending specimen - Pressure generating system until 100 bars - Electrical enclosure - Homemade software developed under LABVIEW that provides control and acquisition data during the tests Two half-shells allow the positioning of the spherical bending specimen. Seal and embedment are realized by bolting these two parts. Maximum experimental conditions are 100 bars for the pressure and 90°C for the temperature. An alternative differential pressure between the two sides of the spherical specimen is applied during the fatigue test. T T