Issue 41

M. Sakane et alii, Frattura ed Integrità Strutturale, 41 (2017) 16-23; DOI: 10.3221/IGF-ESIS41.03 16 Focused on Multiaxial Fatigue Cracking directions in multiaxial low cycle fatigue at high and room temperatures Masao Sakane, Takamoto Itoh Department of Mechanical Engineering, Ritsumeikan University, 1-1-1, Nojihigashi Kusatsu Shiga, 525-8577, Japan sakanem@se.ritsumei.ac.jp, itohtaka@fc.ritsumei.ac.jp A BSTRACT . Cracking direction in multiaxial low cycle fatigue is an important research subject because crack initiation and propagation behavior is a physical background for developing an estimation method of multiaxial low cycle fatigue lives. However, there are a few open questions on cracking direction in multiaxial low cycle fatigue because cracking direction in multiaxial low cycle fatigue is complex and changes depending on stress multiaxiality, strain range, notch and material. This paper overviews cracking directions in tension-torsion low cycle fatigue of low alloy steels and nickel base superalloys. Two types of cracking directions in these materials, maximum shear direction and maximum principal direction, are discussed in relation with strain multiaxiality and an existence of notch and precrack. The two cracking directions in torsion low cycle fatigue of SUS 304 stainless steel are also discussed in relation with strain range. Detailed micro crack observations are finally presented to discuss the two cracking directions in torsion low cycle fatigue of a SUS 304 unnotched specimen. K EYWORDS . Multiaxial fatigue; Crack mode; Notch; Fatigue life; Torsion. Citation: Sakane, N., Itoh, T., Cracking directions in multiaxial low cycle fatigue at high and room temperatures, , Frattura ed Integrità Strutturale, 41 (2017) 16-24. Received: 28.02.2017 Accepted: 15.04.2017 Published: 01.07.2017 Copyright: © 2017 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 wo types of cracking directions have been reported in multiaxial low cycle fatigue (LCF); Stage I cracking and Stage II cracking. Stage I cracking is the cracking on maximum shear plane (shear crack) and Stage II cracking is that on principal plane (principal crack). In a uniaxial push-pull low cycle fatigue of ductile materials, shear cracks initiate and they turn to be principal cracks as they grow, one of them forming a main crack to bring failure of specimen [1]. However, in multiaxial LCF, cracking direction is not so simple as in the uniaxial case, and cracking direction is influenced by several factors. Numerous papers have discussed influential factors on cracking direction but still some of their effects seem to be conflicting. Major factors influencing cracking direction are strain/stress multiaxiality, type of material, notch and precrack, material anisotropy, strain/stress range, oxidation and so on. T