Abstract
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.
Highlights
Two 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 and Stage II cracking is that on principal plane
The reason of the transition of cracking direction depending on strain range has not been well understood so that this paper discusses this topic from micro crack observations on specimens fatigued in torsion
Summary
Two types of cracking directions have been reported in multiaxial low cycle fatigue (LCF); Stage I cracking and Stage II cracking. 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]. Major factors influencing cracking direction are strain/stress multiaxiality, type of material, notch and precrack, material anisotropy, strain/stress range, oxidation and so on. The factors that this paper overviews are stress/strain multiaxiality, notch and precrack and stress/strain range. This paper discusses the strain range dependency of cracking directions in torsion LCF in more detail. The reason of the transition of cracking direction depending on strain range has not been well understood so that this paper discusses this topic from micro crack observations on specimens fatigued in torsion. Micro crack 2a < 0.1 mm Figure 1: Crack observation in tension-torsion LCF of SUS304 stainless steel at 923K
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