Structural Shear Joints: Analyses, Properties and Design for Repeat Loading
4 Fatigue of Shear Joints
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Fatigue, the loss of load carrying capacity with repeated loading, proceeds in three stages: (i) crack initiation, (ii) stable crack growth and (iii) fracture. In the crack initiation stage, the repeated cycles of tensile stress and plastic strain progressively damage the microstructure of the material in isolated locations. The accumulated damage ultimately produces a fully formed, microscopic crack. The per-cycle damage increases either with the local stress amplitude or the mean stress. For this reason cracks initiate at the sites of stress concentration, or sites combining high stresses and intense fretting. Locations of reported crack initiation sites, important for NDE, are identified in Figure 4.1 [34,51–54]. In the second, or crack growth stage, the crack extends by small, regular, sub-micron and micron size increments with each repeated loading cycle. The growth trajectory is normal to the direction of cyclic tension. The growth stage ends with the stress cycle that produces a crack length just short of the critical value corresponding with crack instability. The fracture stage proceeds during the next and terminal cycle which produces rapid, unstable extension of the crack, and the rupture of the remaining bridges of unbroken material of either the panel or the fastener.
The three stages of the fatigue process are generic for metallic materials and structural components. However, the fatigue failure of structural shear joints can be more complex because it results from mechanical contact at multiple locations between the panels and fasteners and between the panels themselves. The finite element models featured in this text treat the intricacies of mechanical contact. But, the finite element models featured in this text do not contain cracks and so do not model crack growth or fracture. The reader is referred to references [52,55–57] for detailed finite element and fracture mechanics analyses of growth.