A numerical study on compressive behavior of composite plates with multiple circular delaminations considering delamination propagation

Abstract

The compressive behavior of multiply delaminated composite plates was numerically evaluated considering both structural instability and delamination growth in order to clarify the reason for the significant reduction in the compressive strength of composite laminates that results from an impact. Embedded circular delaminations were placed at regular intervals in the thickness direction at the plate center. A three-dimensional block element was used to discretize the laminates, and a cohesive element was placed at the same planes as the delaminations. The cohesive element had a bilinear relationship between traction and relative displacement, and the element failed when the energy stored in it exceeded a threshold, that is, the critical energy release rate of the interface. Through the present numerical study we found that the applied load dropped after slight increase of the load when the delaminations started growing, and the delamination growth in the buckled laminate governed the compressive strength of the damaged composite laminates.

Introduction

Composite laminates are used particularly for aerospace structures due to their high specific strength and stiffness. However, their toughness is not sufficient compared to conventional metal materials such as aluminum alloys. When composite laminates are used as a structural material, it is very important to evaluate their over-all performance using appropriate methods and considering their various damage-related properties. The compression after impact (CAI) test [1], [2], [3] provides design data for the compression strength of composite laminates for aerospace structures. The problem of compression after impact is one of the most important issues in the design of composite structures because the decreased compressive strength resulting from the low velocity impact of a foreign object is often a critical concern despite its vague mechanical meaning.

Since delamination due to impact is considered to significantly reduce the compressive strength of composite laminates, the effect of delamination on compressive behavior has been well studied. One-dimensional beam-type models of delaminated laminates have been studied by many researchers, including Suemasu et al. [4], [5], and the relationship between delamination and compressive behavior has been reported. The research on two-dimensional plate problem was initially limited to special cases as two-dimensional modeling of a delaminated plate requires a large numerical effort, particularly when delamination crack stability is considered. However, advances in computer capacity have resulted in numerous investigations of the buckling and postbuckling behavior of two-dimensional plates with a delamination [6], [7], [8], [9], [10], [11], [12], [13], [14]. The main cause of reduced compressive strength due to impact is believed to be the existence of multiple delaminations. Effect of one or multiple delamination(s) on the compressive buckling and/or postbuckling behavior of composite rectangular plates was investigated by Suemasu et al. [15] through a buckling analysis using the Rayleigh-Ritz method together with experimental observations. The observations showed that the growth of delamination cracks can trigger the final failure of a delaminated plate in the postbuckling process. Postbuckling analysis of plates with same-size multiple delaminations was done using finite element analysis [16]. The energy release rate distribution obtained along the delaminations was consistent with the observed damage growth. The effect of the size difference of multiple delaminations on compressive behavior was numerically and experimentally investigated [17] and the behavior of different-size multiple delaminations was found to be quite different from that of same-size multiple delaminations. Zhou and Rivera did a series of experiments on the compressive strength reduction of composite laminates due to impact damage and carefully reviewed related researches [18].

This paper describes the numerical investigation of the compressive behavior of composite plates with multiple circular delaminations considering their growth to clarify the failure mechanisms of composite plates that suffer impact damage. A cohesive element of the bilinear traction-relative displacement relationship was used to simulate the growth of multiple delaminations [19], [20], [21], [22].