Compression facing wrinkling of composite sandwich structures

Abstract

A thorough investigation was conducted of face wrinkling failures of sandwich columns under compression and beams in three- and four-point bending and cantilever beams under end loading. The beams were made of unidirectional carbon/epoxy facings and aluminum honeycomb and closed-cell PVC foam cores. The constituent materials were fully characterized. Face wrinkling failures were observed in sandwich columns and beams with foam cores, but not in those with honeycomb cores. Several wrinkling failure loads were measured and compared favorably with an early expression of Hoff and Mautner for the case of columns and long beams where the core is in the linear elastic range. However, for short span beams, core failure precedes face wrinkling. Core yielding and stiffness loss reduce core support of the facing and precipitate facing wrinkling at a lower stress. For this case a modified Hoff and Mautner expression, where the values of the reduced core Young’s and shear moduli in the through-the-thickness direction are introduced, appears more appropriate. It was concluded that failure by wrinkling is prevalent in the case of low through-the-thickness stiffness and long beam spans. In other cases, other failure modes including shear core failure, compressive facing failure, face sheet debonding or indentation failure may occur.

Introduction

Sandwich construction consisting of two strong, stiff, thin facings and a soft lightweight thicker core is a highly efficient way of carrying structural loads. Commonly used materials for facings are composite laminates and metals, while cores are made of metallic and non-metallic honeycombs, cellular foams, balsa wood or trusses. The facings carry almost all of the bending and in-plane loads and the core helps to stabilize the facings and carries the through-the-thickness shear loads. Sandwich structures exhibit complex failure mechanisms including face sheet compressive failure, adhesive bond failure, indentation failure, core failure and facing wrinkling.

Wrinkling of sandwich beams subjected to compression or bending is defined as a localized short-wave length buckling of the compression facing. Wrinkling may be viewed as buckling of the compression facing supported by an elastic or elastoplastic continuum, the core. It is a common failure mode of sandwich beams, leading to loss in the beam stiffness. The wrinkling phenomenon is characterized by the interaction between the core and the facing of the sandwich construction. Thus, the critical wrinkling load is a function of the stiffnesses of the core and facing, the geometry of the problem and the applied loading.

A large number of theoretical and experimental investigations on sandwich construction wrinkling have been published in the literature. A general review of failure modes in composite sandwich beams was given by Daniel et al. (2001). Some of the early works were presented and compiled by Plantema (1966) and Allen (1969). Hoff and Mautner (1945) tested sandwich panels in compression and observed that failure occurred according to symmetric and skew symmetric (antisymmetric) wrinkling. They presented a strain energy theory and gave an approximate formula for the wrinkling stress. From the experimental and theoretical results they concluded that the critical stress is independent of the geometrical dimensions of the panels and depends only on the elastic moduli of the core and facing materials. Benson and Mayers (1967) developed a unified theory for the study of both general instability and facing wrinkling simultaneously for sandwich plates with isotropic facings and orthotropic cores. This theory was extended by Hadi and Matthews (2000) to solve the problem of wrinkling of anisotropic sandwich panels.

Pearce and Webber (1973) studied the overall buckling and facing wrinkling of sandwich panels with carbon fiber reinforced plastic facings and honeycomb cores subjected to uniaxial compression. The experimental wrinkling loads were compared with theoretical predictions which assume that the facings are orthotropic (Pearce and Webber, 1972). Webber et al. (1976) developed the theory further for the case of unbalanced laminated cross-ply facings in which coupling terms as well as the effect of the adhesive layer between facings and core were taken into account. The theory was extended for flexural wrinkling of honeycomb sandwich beams by Gutierrez and Webber (1980). This theory was further extended by Ditcher and Webber (1982) to include the nonlinear behavior of the facings. The modification involves the use of appropriate tangent moduli in the facing equations, instead of the initial linear moduli. The wrinkling load and the facing strains were calculated by a double iteration scheme. A variational formulation for the interactive overall and localized buckling behavior of compressed sandwich structures was developed by Hunt and Wadee (1998). The analysis was extended to include orthotropic cores by Wadee and Hunt (1998). Niu and Talreja (1999) presented a model for the analysis of wrinkling behavior of sandwich panels in compression. They gave a single expression for the wrinkling stress for the single-sided face wrinkling, and the in-phase and out-of-phase wrinkling. They showed that the wrinkling stress in all three cases is almost the same for short wavelength wrinkling. Furthermore, they discussed the existing wrinkling models and concluded that the Winkler model and the two-parameter model underestimate or overestimate the wrinkling stress, respectively, when the ratio of the core to facing thicknesses is large. An analytical solution for the determination of the critical wrinkling load of a sandwich plate with orthotropic facings and thick transversely isotropic cores was developed by Vonach and Rammerstorfer, 2000a, Vonach and Rammerstorfer, 2000b. A finite element analysis for the study of the post-buckling behavior of isotropic sandwich shells was performed by Stiftinger and Rammerstorfer (1997). An analytical model that leads to a single explicit equation for the critical wrinkling load of sandwich plates with isotropic face layers and thick orthotropic cores was developed by Vonach and Rammerstorfer, 2000a, Vonach and Rammerstorfer, 2000b. It was found that for highly orthotropic cores (e.g., honeycombs) the wrinkling load depends strongly on the in-plane stiffness of the core.

In a series of recent publications Daniel and coworkers (Daniel and Abot, 2000; Daniel et al., 2001; Gdoutos et al., 2001a, Gdoutos et al., 2001b, Gdoutos et al., 2002a, Gdoutos et al., 2002b) studied the mechanical behavior and failure of composite sandwich beams subjected to combined shear and bending. The beams were made of unidirectional carbon/epoxy facings and aluminum honeycomb or closed-cell PVC foam cores.

In the present work face wrinkling failures of sandwich columns under compression and beams in three- and four-point bending and cantilever beams under end loading were studied. Wrinkling failures were observed in sandwich beams with foam cores, but not in those with honeycomb cores. Several wrinkling failure loads were measured and compared with existing analytical solutions.