Fatigue of thin periodic triangular lattice plates

A method for predicting the fatigue life of triangular lattices is proposed in this paper by considering fatigue properties of single lattice struts. Fatigue tests of different sizes of lattice plates of aluminium alloy, and tests of single struts with different maximum fluctuating loads, have been conducted to validate this method. It is found that the struts in a triangular lattice break near to strut intersections, where stress and strain concentrations occur. Similar crack propagation paths were observed in different lattice plate specimens: the cracks grew at a 30° angle to the initial edge crack in the upper half of lattice plate. The mixed-mode fatigue crack propagation rate was also studied and expressed using an effective stress intensity factor. A size effect on the crack growth rate of triangular lattice plates was also observed: a fatigue crack will propagate slightly quicker in larger triangular plates than in smaller ones.

3D Numerical Study on how the Local Effective Stress Intensity Factor Range Can Explain the Fatigue Crack Front Shape

The plasticity-induced crack closure of through-thickness cracks, artificially obtained from short cracks grown in CT specimens of 304L austenitic stainless steel, is numerically simulated using finite elements. Crack advance is incremented step by step, by applying constant ΔK amplitude so as to limit the loading history influence to that of crack length and crack wake. The calculation of the effective stress intensity factor range, ΔKeff, along curved shaped crack fronts simulating real crack fronts, are compared to calculation previously performed for through-thickness straight cracks. The results for the curved crack fronts support that the front curvature is associated to constant ΔKeffamplitude, thus assumed to be the propagation driving force of the crack all along its front.

The Relation Between the Peak Stress for a Blunt Flaw and the Stress Intensity Factor for an Equivalent Crack

The paper presents arguments to support the view that the often quoted relation σp = Lim[μKI/(πρ)1/2] as ρ (flaw root radius of curvature) tends to zero, with σp being the peak flaw-tip stress, KI, the effective stress intensity factor, and μ = 2, is strictly valid only for the case of an infinitely deep parabolic flaw. The importance of the parameter μ is highlighted with regards to the problem of fracture initiation at the root of a blunt flaw.