Issue 38

T. Inoue et alii, Frattura ed Integrità Strutturale, 38 (2016) 259-265; DOI: 10.3221/IGF-ESIS.38.35 260 machine [3, 4]. However, it is hard to reproduce such complex stress states with conventional multiaxial fatigue testing machines. In this study, we have developed a fatigue testing machine that enables generation of arbitrary in-plane stress states which occur on a mechanical surface where fatigue crack initiates. Also, fatigue tests under random non-proportional loading conditions were performed with the advanced tasting machine. E XPERIMENTAL M ETHOD Testing Machine he advanced testing machine has three structural features. First, the advanced testing machine consists of mechanical structures for generating three independent loads in the 0, 45, and 90 degree directions using actuators which apply loads. For generating arbitrary in-plane stress states, namely,  x ,  y and  xy in the xy plane, on the test specimen, three independent loads in the same plane are required. Stress states generated by conventional multiaxial fatigue testing machines are shown in Fig. 1.  1 ,  2 , and  1 in Fig. 1 are the maximum principal stress, the minimum principal stress, and the maximum principal stress direction in the xy plane, respectively. Since conventional machines consist of mechanical structures for applying two independent loads to the test specimen, stress states to be generated are limited.  1 generated by the planar cross axial testing machine is limited to 0 or 90 degrees as shown in Fig. 1 (a). When the axial stress direction is x ,  y generated by the axial and torsion testing machine is limited to 0 as shown in Fig. 1 (b). On the other hand, in the advanced testing machine, the structure for applying the load in the 45 degree direction is added to the planar cross axial fatigue testing machine.  1 ,  2 , and  1 generated by the advanced testing machine are therefore described in the following expression:     2 2 0 90 45 0 90 45 1 2 2 2              (1) 1 45 1 0 90 1 tan 2              (2) where  0 ,  90, and  45 are stresses generated by loads in the 0, 90, and 45 degree directions, respectively. The limit of  1 is eliminated for the advanced testing machine. In other words, this testing machine could generate arbitrary in-plane stress states.    1  2 ･  x >  y or  x <  y ･  x =  y  1 (  2 ) =  x (  y )  1 =0 and  /2    1  2 ･  y =0 2 2 )2(1 4 2 1 2 xy x x        (  x ,  xy ) (  y ,-  xy ) 2  1       y x y y x x if if        1       y x x y x y if if        2       y x y x if if       2 0 1                         0 2 2 tan 2 1 0 2 tan 2 1 1 1 1 x x xy x x xy if if         (a) Planar cross axial testing machine. (b) Axial and torsion testing machine. Figure 1 : Stress states under Mohr’s stress circle generated with each testing machine. T