Issue 33

F. Fremy et alii, Frattura ed Integrità Strutturale, 33 (2015) 397-403; DOI: 10.3221/IGF-ESIS.33.44 398   1 n n n n eq I II III K K K K          (2) Corrections are required to account for the closure effect, or for the in-phase or out-of-phase nature of the fatigue cycles. As a matter of fact, most papers devoted to fatigue crack growth under out-of-phase or sequential mixed mode loading conditions indicate a detrimental effect of the mode-mixity variation during the fatigue cycle and do underline the role of crack tip plasticity [1-7]. A set of experiments was therefore conducted in order to characterize specifically the importance of the load path effect on mixed mode fatigue crack growth, all the other effects being as far as possible kept the same. In these experiments, the stress intensity factor ranges and mean values are the same for each mode and each load path. A static mode I load is always applied so as to limit the effect of crack closure. The tests were conducted with different load paths, yet both "in phase" or both "out of phase", in the sense that the extremes values of the stress intensity factors in each mode are attained simultaneously or not. E XPERIMENTS Material & Experimental setup he tested material is an AISI 316 L austenitic stainless steel. This material is employed in power plants to produce various components such as pumps, mixing tees and taps because of its excellent resistance to corrosion, its good formability and ductility. The elastic-plastic behavior of this material has been extensively studied in uniaxial and multiaxial conditions [8]. The experiments were conducted on the multiaxial servo-hydraulic testing machine ASTREE, available at LMT-Cachan. Six actuators are used simultaneously to perform the tests (Fig. 1). Three pairs of actuators are used to load the specimen along three orthogonal axes and to keep fixed the intersection of the three loading axes fixed. Each horizontal loading axis is load controlled. Figure 1 : Six actuators servo-hydraulics testing machine ASTREE, experimental set-up. A cruciform specimen was used for the experiments (Fig. 2). A centered 30 mm long slit is machined in the specimen (Fig. 2), the slit plane being inclined at 45° with the loading axes of the specimen. Linear elastic finite element analyses were conducted in order to determine the relations between the loads FX, FY and FZ applied along the three axes of the specimen (Fig. 2) and the stress intensity factors KI, KII and KIII at mid-thickness for a coplanar crack propagating from the slit. A mode I stress intensity factor is obtained by applying the same load along the two in-plane axes of the specimen (FX=FY). A mode II stress intensity factor is obtained by applying FX=-FY. A mode III stress intensity factor is obtained by applying an out-of-plane load FZ. The mode I+II+III loading cycles used in the experiments do always include a positive mean value of KI, for three main reasons. First of all, a positive mean value of KI allows limiting the crack closure effects. Second, the mean value of KI was determined so that the in-plane loads FX and FY will always remain positive during cycling so as to avoid any buckling of the specimen. Third, when an out-of-plane load FZ is applied onto the specimen, it induces a bending of the T