Development of Planar Tri-Axial Fatigue Testing Machine for Reproducing Arbitrary Stress States

A complex stress occurs on a mechanical surface of transport machineries in service such as automobiles and railway vehicles. The authors have developed a fatigue testing machine enable to reproduce such a complex stress. The surface stress on mechanical structures can be described as in-plane structural element deformations in transverse, longitudinal and diagonal directions. The testing machine is able to reproduce stress states working on surfaces of mechanical structures by applying three independent loads to the test specimen using actuators which apply loads in the 0, 45 and 90 degree direction. The testing machine was developed with consists of a fixed frame which forms 45-degree axis and two movable frames which form 0-degree and 90-degree axes. Evaluations were conducted on the test specimen with the complex stress data obtained from an actual operating transport machinery, further it was found that the measured stress and the emulated stress corresponded with an error range of less than 10%. When additional fatigue tests were performed on a test specimen with a cylindrical projection, it was found that fatigue failure occurred at stress concentration point in the root of projection.