A Solution for the Initial Postbuckling and Growth Behavior of Internal Delaminations (original) (raw)

34th Structures, Structural Dynamics and Materials Conference, 1993

Abstract

This paper presents a closed form solution for the initial postbuckling and growth behavior of delaminations in plates. The solution is derived with no restrictive assumptions on the delamination thickness and plate length, i.e. the usual thin film assumptions are relaxed. A perturbation procedure is used, based on an asymptotic expansion of the load and deformation quantities in terms of the distortion parameter of the delaminated layer, the latter being considered a compressive elastica. In addition to determining the load and mid-point delamination deflection as a function of the applied compressive displacement the analysis produces closed form expressions for the energy-release rate and the mixity ratio (i.e. Mode I1 vs Mode I) at the delamination tip. A higher Mode I component is found to be present during the initial postbuckling phase for delaminations of increasing ratio of delamination thickness over plate thickness, h/T (i.e. delaminations further away from the surface). Moreover, the energy release rate corresponding to the same applied strain is larger for a higher h/T ratio. The reduced growth resistance of these configurations is verifed by experimental results on unidirectional composite specimens with internal delaminations. Introduction Delaminations or interlayer cracks are developed as a result of imperfections in production technology or due to service loads which may include impact by foreign objects. As a consequence, structural elements with delaminations under compression suffer a degradation of their stiffness and buckling strength and potential loss of integrity from possible growth of the interlayer crack. Besides strength, delaminations can influence other performance characteristics, such as the energy absorption capacity of composite beam systems’. Delamination buckling in plates under compression has received considerable at tention and numerous contributions have addressed related issues in both onedimensional and two-dimensional However, although the critical point can be fairly well determined and has been extensively studied, limited work has focused on the postbuckling behavior, which ultimately governs the growth characteristics of the delamination. The one configuration most thoroughly studied is the one-dimensional delamination, consisting of a delamination in an infinitely thick plate. In this model2, which has also been called “thin film” model, the unbuckled (base) plate is assumed to be subject t o a uniform compressive strain. Closed form expressions for the energy release rate from the thin film model were derived by Chai et a12 by using the strain energy expressions before and after delamination buckling. The other configuration most extensively studied is the axisymmetric counterpart to the one-dimensional delamination, i.e. a circular delamination in a perfectly rigid supporting plate. The latter relates also to the socalled blister test used to determine adhesive and cohesive material properties. For this configuration, Evans and Hutchinson5 derived a formula for the energy release rate by using an asymptotically valid solution to the system of governing equations for small buckling deflections. Results for the energy release rate of a circular delamination were also given by Chai7 and calculated through a path-independent integral approach by Yin’. In the same context, Stordkers and Anderssong derived general potential energy theorems and associated bounds for composite plates within the kinematical assumptions usually attributed to von Karman, and studied in detail the efficiency of different analytical and numerical means for this circular delamination case. For delaminations in plates that cannot fullfil the “thin film” model assumptions, both the critical load and the post-critical behavior are expected to deviate from the predictions of Chai et al’. To this extent, Simitses et a13 studied the citical load for a delamination of arbitrary thickness and size in a finite plate. Their results showed indeed a range of critical load vs thin film load ratios, depending on delamination and base plate dimensions, as well as base plate end fixity (simplysupported vs clamped). Concerning the post-critical behavior of delaminations of arbitrary size, Kardomateas” provided a formulation for studying the postbuckling behavior by using elastica theory for representing the deflections of the buckled layer; this work resulted in a system of nonlinear equations rather than closed form expressions. In this paper, the initial postbuckling behavior of delaminated composites (with no restrictive assumptions on the delamination dimensions) is studied by using a perturbation procedure based on an asymptotic. Copyright 01993 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. 2679 expansion of the load and deformation quantities in terms of the distortion parameter of the delaminated layer, the latter being considered a compressive elastica.…

G. Kardomateas hasn't uploaded this paper.

Let G. know you want this paper to be uploaded.

Ask for this paper to be uploaded.