PSI - Issue 47

Ilia Nikitin et al. / Procedia Structural Integrity 47 (2023) 617–622 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

618

2

1. Introduction The very high cycle fatigue tests (VHCF) under pure torsion loading conditions are extremely rare. The first ultrasonic testing system were built up on the longitudinal piezoelectric converters, Bathias (2004) that lead to some additional bending loading. However, the first experimental results have shown that spatial crack shape under VHCF torsion is quite complex. Most of the first results were obtained for steels because it is a common material for industrial springs working under torsion conditions. In the case of steels, the surface and subsurface crack initiation were observed. The subsurface cracks were originated from nonmetallic inclusion. The scenario for surface crack initiation was like for high cycle fatigue (HCF) loading. The morphology of VHCF torsion crack was quite complex and represented a classic ‘manufactory roof’. Later the direct torsion piezoelectric machine was presented, Nikitin (2015). The first results on two-phased titanium alloy (nonmetallic inclusion-free material) were presented. Still the subsurface crack initiations were reported. It was found that torsion fatigue crack has numerous branching during the crack propagation. Such complex scenario of VHCF torsion crack growth makes difficult to predict the fatigue life and spatial shape of crack. This paper is aimed to apply the multi-regime two-criteria model to reconstruct the spatial torsion crack path in smooth specimens under pure VHCF loading.

Nomenclature  * H   − ) , B    ( ) (

damage function Heavyside function

stress-state sensitive coefficients

eq 

equivalent stress

2. Experimental procedure and result All the VHCF tests were performed on piezoelectric fatigue testing system at 20 kHz at continuous constant amplitude loading. During the test the surface of specimen is cooled by dry compressed air. The axial VHCF tests on VT3-1 were carried out by using tension-compression system Bathias (2004) and used for determination the parameters of the multi-regime model. These results are available in work Nikitin (2016-a). The results of VHCF torsion tests were used to examinate the multi-regime two criteria model. The details of the loading cell are described in Nikitin (2015) and main results on VT3-1 titanium alloy are discussed in Nikitin (2016-b). However, the key results on fatigue crack initiation and growth mechanisms under VHCF torsion loading should be discussed. The macroscopic view of the developed VHCF torsion crack is shown on Fig.1. Based on obtained results there are two types of macroscopic cracks: (1) single 45-degree crack, Fig.1-a, and double-crossed 45-degree crack, Fig.1 b. The precise study on the early crack growth shows that in both cases the initial stage of crack propagation is being in the plane of a maximum shear stress (0 or 90 – degree).

(а) (b) Fig. 1. Single fatigue torsion crack (a) and double-cross fatigue crack (b).