Investigation on 3D fatigue crack propagation in surface-cracked specimens

Abstract

In the present work the fatigue crack growth in AISI304 specimens is investigated experimentally. In 3D finite element analysis the virtual crack closure technique is applied to calculate distributions and variations of the stress intensity factor along the surface crack front. It is confirmed that the stress intensity factor along the surface crack front varies non-uniformly with crack growth. Crack growth rate is proportional to the stress intensity factor distribution in the 3D cracked specimen. The fatigue crack growth in surface cracked specimens can be described by the Forman model identified in conventional compact tension specimens. For crack growth in the free specimen surface the arc length seems more suitable to quantify crack progress. Geometry and loading configuration of the surface cracked specimen seem to not affect the fatigue crack growth substantially.Critical cracks exist mainly on surfaces of mechanical components in engineering. To predict fatigue crack growth life, characterization and verification of a surface crack through the conventional fracture mechanics specimen, such as compact tension (CT), are of interesting. From fracture analysis it is known that the stress field in a 3D surface crack differs from the conventional plane strain tip field. The stress intensity factor (SIF) depends additionally on crack front curvature, crack edge to free surface, and structure configuration. Especially, SIF may vary non-proportionally with crack growth. Predictability and accuracy of the fracture mechanics to 3D surface crack are still interesting issue for many mechanical parts under complex loading conditions.In the present work, surface crack in circular rod of stainless steel AISI304 is studied experimentally. The crack growth rate (da/dN ) versus stress intensity factor (∆K) relation with different load ratios (R) was firstly obtained from standard CT specimens. Fatigue crack growth in the CT specimens can be described by the Forman model. 1 To investigate surface crack growth, the axial tension rods are fabricated and pre-cracked. Fatigue crack growth tests on the rods were carried out at different load ratios. Backtracking method 2,3 was attempted for monitoring crack shape evolution. To obtain more accurate SIFs, beach marks on the crack surface were represented by fitting elliptical arc curves. 3D finite element meshes are generated in compliance with the crack configurations. The SIF along the whole crack front was calculated via the virtual crack closure technique (VCCT). 4-7 The relation of crack shape evolution with the SIF distribution along the crack front was discussed. Finally, the Forman model obtained from the standard CT specimen is used to predict the surface crack evolution in tensile rods.A 3D crack front can be represented by an elliptical a) Corresponding author. Email: h.yuan@uni-wuppertal.de arc crack front 8-16 as shown in Fig. 1. Here a denotes the depth of the surface crack. The elliptical crack can be described by the semi-axes, a and b. For a given crack depth, the ...