Perpustakaan Digital ITB

Military aircraft are subjected to complex operational loading, while structural discontinuities such as rivet holes create local stress concentrations that can accelerate the propagation of pre-existing flaws. The wing-rib lower flange is a critical structural region containing multiple fastener holes and is therefore susceptible to fatigue-crack propagation. This study develops a numerical procedure based on linear elastic fracture mechanics (LEFM) to analyse multiple-crack propagation and the transition between different crack geometries. The load-range exceedance data were derived from Flight Data Recorder (FDR) measurements obtained during trainer-aircraft operations. The data were discretized into twelve blocks and converted into a Paris–Walker equivalent constant-amplitude load. The resulting equivalent load factor was 1.3743 g, corresponding to a nominal maximum stress of 36.250 MPa. Stress-intensity factors were determined using compounded geometry correction factors, while crack propagation was calculated using a thresholded Paris law. The model represents the transition from three collinear cracks at adjacent rivet holes to a single eccentric crack. Three asymmetric initial-flaw cases and load sensitivities from 1 to 9.2 g were evaluated. Under the equivalent loading, only the 1.27 mm primary flaw was initially active, while the 0.127 mm flaws remained below the crack-growth threshold. Initial-flaw location affected the governing crack tip and propagation duration, with Case A producing the shortest valid endpoint. Increasing load caused a nonlinear reduction in propagation cycles. The model is limited to deterministic, two-dimensional Mode-I loading and excludes load-sequence effects, crack closure, fastener contact, three-dimensional crack geometry, and experimental validation.