Trans RINA, Vol 152, Part B2, Intl J Small Craft Tech, 2010 Jul-Dec FAILURE CHARACTERISATION OF L-BEND CURVED COMPOSITE LAMINATES


R Raju, B G Prusty, D W Kelly and G D Peng, University of New South Wales, Australia D Lyons, EMP Composites, Australia (DOI No: 10.3940/rina.ijsct.2010.b2.107)


SUMMARY


One of the critical failures in curved composites is the delamination which can significantly reduce the strength and integrity of the structure. This paper is aimed to predict the delamination initiation and progression in a curved composite laminate. Experimental and numerical analysis are presented. Interlaminar shear and tensile stresses acting at the interface of the layers are computed and assessed using strength based criterion. 3D Finite Element Analysis using MSC.MARC is conducted and experimental investigation is performed to validate the numerical model. The curved panels are fixed at the base & pulled at the other end; as a result, multiple delamination occurred in the laminate bend due to induced interlaminar tensile/shear stresses. The effect of stacking sequence, laminate thickness and radius of the curvature is studied. A four channel acoustic emission equipment is used to characterise the laminate failure initiation and progression. Good agreement is obtained between the experimental and numerical results.


1. INTRODUCTION


Marine structures generally use Glass Fibre Reinforced Polymer (GFRP) composites for the added benefits over metal structures


of low cost and weight. These


composites generally contain curved shapes such as T- Joints, top hat stiffeners, L-bends, angle bracket, a co- cored web, etc., to accommodate the design and aesthetic features [1, 2, 3]. The damage mechanism of the curved composites is complex and is necessary to analyse the residual life and strength of the structure. Studies conducted in the past focused on the unidirectional fibres or cross-plied fibres alone. But in marine applications, short fibre composites play an important role; Chopped Strand Mat (CSM) is used commonly in conjunction with cross ply and/or unidirectional layers. This paper focuses on the delamination behaviour in laminated composites with the inclusion of CSM layer along with Cross ply- Double bias (DB - oriented ±450 to loading axis) and Unidirectional (UD - oriented 00 to loading axis) layers.


1.1 LITERATURE REVIEW


Premature failures of curved composites have occurred in the past, mainly due to the lack of appreciation for the interlaminar tensile stresses, which may be less than 3% of the in-plane strength of a composite


laminate.


Generally, interlaminar tensile and compressive stresses are developed when curved composite laminates are subjected to flexural loading in the plane of curvature, resulting in induced interlaminar failures [4]. The interlaminar tensile strength is a function of the basic laminate constituents, type of resin & reinforcement, stacking sequence, manufacturing process, fabrication quality, aging, type of loading [5, 6, 7]. Many methods have been suggested to arrest the interlaminar failure such as using tougher matrix polymers, using interleaf layers, through thickness


reinforcements, improved


fibre/matrix strength, optimisation of fabrication, etc. [8,9].


Of the various failures of composite materials,


delamination is one of the predominant modes of failure of curved laminates. Initial failure through delamination may go undetectable as they are frequently embedded between the layers within the composite structure. In some cases, the delamination effect on the structure may cause a dramatic loss of residual strength of up to 60% [10]. The problem of interlaminar tensile/compressive or shear stresses was highlighted at the bonded joints and attachments of marine composite structures [11]. Failure of joints in FRP marine structures was initially noted by Smith [12,13]. Wisnom [14, 15] was one of the primary investigators to study the delamination failure through experimental, analytical and numerical approaches to detect interlaminar failure and flexural strength of FRP composite


structures. Lekhnitskii’s equations are


applicable only for either pure bending or edge loading, as they cannot sustain the circumferential force without radial restraints. Hence Shenoi & Wang [1] developed equations based on elasticity theory for delamination and flexural strength of curved composites. The effects of key variables


such as stacking sequence, radius of


curvature and thickness of skin of sandwich beam on stress distribution within a curved layered beam and sandwich beam were also studied. Raju et al [16] studied the failure of the curved bends of top hat stiffeners for three different layups similar to the layups studied in this study. A progressive failure methodology


for the


laminated stiffened and unstiffened panels is presented using arbitrarily oriented stiffener formulation [17]. The formulation was based on the linear, arbitrarily oriented stiffener formulation of Prusty [18] for the analyses of laminated hat stiffened plates and shells.


In a loaded structure, elastic energy is produced and stored in the structure. When this elastic energy exceeds the critical value, a crack or defect is produced and rapid release of elastic energy is observed. This rapid release of energy is generally termed as acoustic emission (AE) [19]. The examination of AE is a very successful tool for the sensitive detection and location of active damages in


©2010: The Royal Institution of Naval Architects


B-93


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