Evaluation of multiple site damage in lap joint specimens.

摘要

Multiple site damage (MSD) is the occurrence of many small cracks at multiple locations such as rivet holes. Once the MSD is initiated, the strength of structure may be quickly reduced to an unacceptable level and caused catastrophic failure.; This dissertation describes research into the fatigue life of lap joint structure with MSD. The load transfer mechanism in joint structure is studied, including several types of rivets, material properties and structure geometry. The load transfer for each rivet is determined as a function of crack propagation. Two other mechanisms can affect the load transfer mechanism-interference between the rivet and rivet hole due to riveting process, and clamping pressure between the panels, were also investigated. Since these two mechanisms are complicated to analyze, and rely on the quality of the manufacture and assembly, the riveting process was well controlled by applying the rivet squeeze force through an MTS machine.; Single lap joint specimens, containing fatigue MSD in one row of rivet holes, were cycled to failure under constant amplitude loading. A modified MSD life prediction model is used to estimate fatigue life. Predictions for the growth and coalescence of individual cracks are in accordance with the experimental results. During the fatigue tests, the propagation of some small cracks at low remote stress levels were delayed due to clamping-up stress under the countersink rivet head. Marker band analysis, therefore, is utilized to detect the crack growth information under countersink rivet head and includes the delay effect into the fatigue prediction program.; The individual/relative effect of two random variables—the interference fit levels and crack length distribution at rivet holes—on MSD fatigue life is investigated. A thousand Monte Carlo simulations are performed to determine the statistical degradation in residual strength as the Wide Fatigue Damage (WFD) extends by fatigue. The fastener interference, reflected as riveting quality, determines the overall evaluation of fatigue behavior of a lap joint. The influence of both rivet installation (i.e. fastener interference) and initial WFD size on fatigue life and residual strength are presented, and indicate the relative influence of these parameters on structural integrity.