A physically-based model for the notched strength of woven quasi-isotropic CFRP laminates

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Abstract

Damage growth and fracture at circular holes in quasi-isotropic CFRP laminates loaded in tension has been studied. The laminates were based on two weave types and of three different thicknesses. A notch edge damage analysis was performed on a layer by layer basis after decomposition of the matrix. It was shown that near the notch edge, damage in the form of fibre tow failure occurred in the tows orientated at 0°, whilst the tows oriented at 45° within the damage area remained intact to loads just below the maximum load. The well known semi-empirical point and average stress criteria resulted in accurate notched strength predictions with the average stress criterion being better than the point stress criterion. A recently developed critical damage growth model, requiring as input parameters the unnotched strength and fracture toughness of the laminate which were measured from independent experiments, yielded accurate notched strength predictions, while being easy to implement. The resilience of the critical damage growth model was investigated and it was found that a variation in both the input parameters of the model by 10% result, even in the worst case, in a 10% variation in the notched strength predicted by the model.

Introduction

Over the years an extensive literature has evolved concerned with developing methods to predict the notched strength of laminated composites. Most of the early work focused on continuous (non-woven) systems, as reviewed in Ref. [1], while more recently there has been a growing interest in the behaviour of woven fabric based composites [2], [3], [4].

Predicting damage and fracture processes in notched composite coupons is a challenging problem, since the damage processes that develop at a stress raiser under load lead to a local redistribution of stress which is difficult to model analytically. There are, however, some situations in which the damage can be represented in a simple way that appears to capture the essential physics. In recent work [5], [6], we have shown that one such situation is the fracture of notched woven GFRP laminates, in which matrix cracking and fibre tow fracture adjacent to the notch are the main damage mechanisms before ultimate load and fracture. This observation led to the application of a simple model for the failure, based on the evolution and subsequent catastrophic propagation of a crack. The input parameters required for the model are the unnotched laminate strength and the laminate toughness; the latter was measured using a single edge notch fracture mechanics test. The model was applied to a range of GFRP woven laminates of different weave types (plane weave, eight harness satin), lay-ups (cross-ply and quasi-isotropic) and thickness with circular holes and elliptical cut-outs. Good predictions of notched strength resulted. The present work is concerned with investigating the applicability of the approach to CFRP laminates based on woven fabric reinforcement.

A total of six different quasi-isotropic CFRP laminates have been investigated—two weave types and three laminate thicknesses. Data are presented for the base-line mechanical properties and the strength as a function of notch size. The fracture model developed in previous work is shown to describe the notched strength data satisfactorily. The underlying assumption of the model, that damage proceeds stably from the notch tip, has been verified by identifying tow fractures near the notch tip in laminates unloaded prior to catastrophic fracture.

Section snippets

Material preparation

There were six different quasi-isotropic carbon fibre reinforced polymer (CFRP) composite laminates investigated in this study; they were based on either plain weave (PW) or five harness satin weave (5HS) reinforcement and three different thickness of laminate were investigated for each weave type.

Both the PW and 5HS weave laminates were manufactured from Primco pre-pregs, which were made using the Toray T300 high strength carbon fibre. The pre-pregs had an approximate weight of 200 g/m2 and

Laminate thickness and fibre volume fraction measurements

The laminate thickness and fibre volume fractions (determined using acid digestion) are shown in Table 2. The laminate thickness is an overall average for the laminate and the fibre volume fraction values given are for an average of three samples for each laminate. It should be noted that the laminate thickness varied slightly across each laminate. From the results it can be seen that the plain weave laminates had a higher fibre volume fraction than the five harness satin laminates. This is

Model principles

The models considered in the present study were the Whitney-Nuismer point and average stress criteria [11] and a critical damage growth model applied previously to notched woven fabric composites [5], [12]. Details of the Whitney-Nuismer model are not repeated here, but the relevant expressions for the critical damage growth model are summarised, briefly, below.

The analysis is based upon the stable growth and subsequent catastrophic failure of a damage zone at the edge of the circular hole. The

Concluding remarks

A range of mechanical property data (stiffness, unnotched strength, notched strength and toughness) has been obtained for a quasi-isotropic CFRP laminate based on woven fabric reinforcement. Fracture from circular holes has been studied. It has been shown that tow fracture occurs at the hole prior to catastrophic fracture. The notched strength data can be adequately described by the well-known Whitney-Nuismer point and average stress criteria models, or by a damage and fracture model which is

Acknowledgements

The authors would like to acknowledge support from the EPSRC and Rolls-Royce plc and to thank our colleague, Mr Reg Whattingham, for invaluable technical assistance.

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