Translaminar fracture toughness testing of composites: A review

Abstract

A comprehensive review of techniques for the experimental characterisation of the fracture toughness associated with the translaminar (fibre-breaking) failure modes of continuously reinforced laminated composites is presented. The collection of work relating to tensile failure reveals a varied approach in terms of specimen configuration, size and data reduction, despite the existence of an ASTM standard. Best practices are identified and suggestions for extending the scope of the current standard are made. Works on compressive failure are found to be less comprehensive. Measurement of the toughness associated with initiation of the failure mode in isolation has been achieved, but this review finds that significant research steps need to be taken before a resistance curve can be fully characterised.

Introduction

The desire for efficient design, coupled with an ever-increasing understanding of composite failure, is moving industry towards a more damage tolerant approach to design with composites. Methods for predicting the onset and subsequent propagation of damage in composite components are, therefore, highly desirable.

There are several approaches to model damage propagation, such as using cohesive elements [1], smeared crack models [2] or X-FEM [3]. Each of these requires the fracture toughness for the failure mode they are simulating. Development of experimental techniques for accurate characterisation of the toughness is, therefore, critical.

The failure modes exhibited by laminated composites can be divided into delamination, intralaminar fracture and translaminar fracture. Delamination has been under extensive investigation for several years. Round-robin exercises [4], [5], [6], [7] have paved the way to the standardised procedures for measurement of mode I [8], [9], [10], mode II [10] and mixed mode I/II [11] fracture toughness. The state of the art of interlaminar fracture toughness testing methods has been comprehensively reviewed by several authors over the past years (e.g. [12], [13], [14], [15], [16]).

A significant number of published studies have been dedicated to translaminar fracture toughness measurement; however, these are often isolated pieces of work with some dating back to the late 70’s. Whilst the importance of translaminar fracture toughness measurement was recognised many years ago, it has received relatively little attention from the scientific community until now. This is at least partially due to (i) a lack of confidence in composites resulting in them not being used in primary structures where this type of characterisation is useful, and (ii) the lack of modelling capabilities which can use the parameters effectively.

This has now changed: large composite primary structures can be found on the latest aircraft, for example, and finite element analysis (FEA) is a common tool used for design. It is envisaged that the fracture toughnesses associated with the translaminar failure modes will play an increasingly important role over the coming years; therefore a review of the current literature is now opportune. This review draws together the conclusions of these studies, and presents a discussion to serve as a platform from which further work in the field can move forward. Work concerning characterisation of multidirectional laminates, as well as the individual translaminar ply failure modes, is presented in sections specific to specimen configuration. Methods of data reduction are noted and important experimental observations are highlighted.