An application of the energy release rate concept to crack growth in attachment lugs

Abstract

quently employed in aerospace and machine structures, and is commonly subjected to repeated load spectra. Furthermore, the lug joint has a relatively low fatigue strength, as cracks can be rapidly initiated in the bore of the hole where all of the load is transferred. Crack initiation is rapid, due to the very high stress concentration caused by the pin bearing load. A typical elastic gross section stress concentration factor for a lug is five or greater, compared with three for an open hole in a tension stress field. Furthermore, the crack initiation life of a lug is also reduced by fretting between the pin or bolt and the lug hole during load reversals. A typical attachment lug configuration is shown in Figure 1. A numerical method involving a variable multipoint constraint capability is used to determine the energy release rates associated with a propagating crack in straight sided, round-head and square-head attachment lugs. The method, formulated within the framework of finite element procedures, evaluates the changes in energy as a function of changing crack length along a prescribed crack path. A single idealization consisting of conventional quadrilateral finite elements is used in the computations. Results are presented for a variety of geometries and loading cases involving axial, as well as skewed, loadings. Comparisons with other existing analytical and experimental results are made for verification. The fatigue performance of attachment lugs has been extensively studied( 3 3 ) . However, fatigue Nomenclature life is actually a two-stage process, consisting of a crack initiation and crack propagation parts, and it crack length head length modulus of elasticity lug load gross stress thickness vector of generalized nodal displacement diameter radius width stress intensity factor stress concentration factor is important to be able to predict both &ages. Furthermore, recent damage tolerance specifications(4) require the prediction of the crack propagation life, TRANSVERSE LOAD DIRECTION I I L J angle between lug axis and load direction angle between lug axis and crack direction Poisson's ratio normalized stress intensity factor = K rn energy release rate strain energy stiffness influence coefficient matrix vector of generalized nodal forces potential of applied external loads total potential energy