Issue 38

T. Inoue et alii, Frattura ed Integrità Strutturale, 38 (2016) 259-265; DOI: 10.3221/IGF-ESIS.38.35 259 Focussed on Multiaxial Fatigue and Fracture Random non-proportional fatigue tests with planar tri-axial fatigue testing machine T. Inoue, R. Nagao, N. Takeda Hitachi, Ltd., Research & Development Group, Center for Technology Innovation – Mechanical Engineering, 832-2 Horiguchi, Ibaraki, 312-0034 Japan takeshi.inoue.sb@hitachi.com, riichi.nagao.ac@hitachi.com , norio.takeda.uf@hitachi.com A BSTRACT . Complex stresses, which occur on the mechanical surfaces of transport machinery in service, bring a drastic degradation in fatigue life. However, it is hard to reproduce such complex stress states for evaluating the fatigue life with conventional multiaxial fatigue machines. We have developed a fatigue testing machine that enables reproduction of such complex stresses. The testing machine can reproduce arbitrary in-plane stress states by applying three independent loads to the test specimen using actuators which apply loads in the 0, 45, and 90 degree directions. The reproduction was tested with complex stress data obtained from the actual operation of transport machinery. As a result, it was found that the reproduced stress corresponded to the measured stress with an error range of less than 10 %. Then, we made a comparison between measured fatigue lives under random non-proportional loading conditions and predicted fatigue lives. It was found that predicted fatigue lives with  cr , stress on critical plane, were over a factor of 10 against measured fatigue lives. On the other hand, predicted fatigue lives with  ma , stress in consideration of a non-proportional level evaluated by using amplitude and direction of principal stress, were within a factor of 3 against measured fatigue lives. K EYWORDS . Fatigue; Non-proportional; Random; Plane stress; Fatigue life prediction; Testing machine. Citation: Inoue, T., Nagao, R., Takeda, N., Random non-proportional fatigue tests with planar tri-axial fatigue testing machine, Frattura ed Integrità Strutturale, 38 (2016) 259-265. Received: 26.07.2016 Accepted: 28.08.2016 Published: 01.10.2016 Copyright: © 2016 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION he main objective of this study is to develop a fatigue testing machine that enables reproduction of complex stresses occurring on the mechanical surfaces of transport machinery in service such as automobiles and railway vehicles. It is well-known that non-proportional loading generating such complex stresses brings a drastic degradation in fatigue life [1, 2]. Methods for prediction of fatigue life in consideration of life degradation have been proposed using fatigue testing results with two types of fatigue testing machines, a planar cross axial testing machine and an axial and torsion testing T