Behaviors of a cracked lapped joint under mixed mode loading

Abstract

Fatigue failure of steel connections is a common failure mechanism, especially for structures which sustain heavy cyclic loads like steel bridges. In this paper, lapped bolted joints were modeled numerically to study the effect of a crack on the ultimate response of the joint. The site of crack initiation was located under different mixed mode loading in single and multiple bolts joints. The effects of axial to transverse loading ratio or load mixity (LM=Fx/Fy), friction coefficient (μ), and bolt diameter were analyzed. For a single bolt pin-joint, by increasing LM, the crack initiation site angle (γ) increased up to a certain value at which it became constant (γf) independent of LM. This value γf depended only on the coefficient of friction and the bolt hole diameter. Stress intensity factor and crack path of a propagated crack emanating from the predicted crack initiation site were analyzed in the lapped joints under either mode I or mixed mode loading. It was found that, for multiple-bolt joints, loaded with mixed mode loading, the crack path remained approximately horizontal like that for mode I loading. For pin joints, the crack path remained at the direction of the crack initiation. The numerical model developed was validated using existing experimental results for the initial stiffness of the bolted joint and using theoretical prediction of the stress intensity factor. A parametric study for different bolt diameters and numbers was developed to study the behavior of these connections under double and single side cracks. It was found that the crack advancement in a specific bolt hole may cause crack to initiate in other bolt holes, due to the increase of the stress concentration factor (SCF), Kt.