Issue 33

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 33 (2015) 335-344; DOI: 10.3221/IGF-ESIS.33.37 335 Focussed on multiaxial fatigue Surface crack growth in cylindrical hollow specimen subject to tension and torsion V. Shlyannikov, R. Yarullin, I. Ishtyryakov Kazan Scientific Center of Russian Academy of Sciences shlyannikov@mail.ru, yarullin_r@mail.ru , ivan_200999@mail.ru A BSTRACT . The subject for studies is an aluminium cylindrical hollow specimen with external axial and part circumferential semi-elliptical surface crack undergoing fatigue loads. Both the optical microscope measurements and the crack opening displacement (COD) method are used to monitor and calculate both crack depth and crack length during the tests. The variation of crack growth behaviour is studied under cyclic axial tension, pure torsion and combined tension+torsion fatigue loading. For the particular surface flaw geometries considered, the elastic and plastic in-plane and out-of-plane constraint parameters, as well as the governing parameter for stress fields in the form of I n -integral and plastic stress intensity factor, are obtained as a function of the aspect ratio, dimensionless crack length and crack depth. The combined effect of tension and torsion loading and initial surface flaw orientation on the crack growth for two type of aluminium alloys is made explicit. The experimental and numerical results of the present study provided the opportunity to explore the suggestion that fatigue crack propagation may be governed more strongly by the plastic stress intensity factor rather than the magnitude of the elastic SIFs alone. One advantage of the plastic SIF is its sensitivity to combined loading due to accounting for the plastic properties of the material. K EYWORDS . Surface crack; Tension; Torsion; Crack growth. I NTRODUCTION n order to provide operation in a safe condition, it is necessary to perform fracture mechanics assessment of a structural component under cyclic loading. The fatigue growth analysis of surface cracks is one of the most important elements for structural integrity prediction of the circular cylindrical metallic components (bars, wires, bolts, shafts, etc.) in the presence of initial and accumulated operation damages. In most cases, part-through flaws appear on the free surface of the cylinder and defects are approximately considered as semi-elliptical cracks. Multiaxial loading conditions including tension/compression, bending and torsion are typical for the circular cylindrical metallic components of engineering structures. The problem of residual fatigue life prediction of such type of structural elements is complex and the closed solution is often not available because surface flaws are three-dimensional in nature. The fatigue failure of cylindrical specimens often develops from surface flaws, and thus several analyses have been carried out to determine the stress intensity factors along the front of an edge defects and crack growth rate study on this base [1-4]. An actual surface crack may usually be replaced by an equivalent circular arc or elliptical-arc edge flaw. The elastic stress intensity factors have been published for part-circular, part-elliptical, or straight fronted cracks in a cylindrical specimen. In this paper, firstly experimental results of fatigue crack growth for a crack starting from a semi-elliptical edge notch in an cylindrical hollow specimens under axial loading with or without cyclic torsion are given. The influence of different I