A Study of Free Edge Stresses and Delamination Initiation in Fibre Reinforced Composite Laminates

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Near free edges of fibre reinforced composite laminates modelled with homogeneous layers, the presence of high gradient interlaminar stresses has been found theoretically. Over a span of nearly 50 years, various methods, including numerical and analytical have been employed for analysis of the high gradient stresses which sometimes exist to an extent of a singularity. The analysis of such free edge stresses is found to be computationally expensive. However, since the interlaminar stresses have been found to play a role in initiating delamination in composite laminates, it becomes imperative to get an estimate of the interlaminar stresses near the free edge to be able to predict delamination initiation through suitable criteria.

The abrupt material discontinuity between dissimilar layers in a homogeneously modelled laminate is believed to be a reason for the appearance of high gradients or singularities theoretically. The mitigation of material discontinuities at such interfaces could be done by modelling the laminate heterogeneously so that the interface is modelled by matrix material and hence, a discontinuity of material could be avoided. This idea of modelling the layers of laminate with explicit definition of fibre and matrix materials has inspired the thesis project with a view to investigate the difference in free edge stresses between the two models and to further draw a correlation between the stresses from the two models to predict delamination initiation.

In previously published literature, the use of average interlaminar stresses up to a certain characteristic distance from the free edge through criterion for delamination initiation prediction is reported. Further it is also found that a single averaging distance can be proposed for a combination of laminates made of the same material. This encourages the idea of determination of averaging distance for a material and find its application on a laminate susceptible to delamination initiation. In the course of the current work, a correlation is proposed between the stresses of homogeneous and heterogeneous model [0/90]s cross-ply laminate of equivalent stiffness made of T300/934 material to determine the averaging distance and apply the determined averaging distance on [±25/90]s laminate to predict delamination initiation due to high gradient free edge stresses.

An averaging distance of 0.125 mm is determined for [±25/90]s laminate made of T300/934 material to find average stresses for incorporation into criterion for delamination initiation. It is found that the determined averaging distance predicts delamination initiation successfully for experimentally reported delamination initiation strain range for the [±25/90]s laminate. Further, the convergence of average of high gradient interlaminar stress is found to be computationally more efficient than the convergence of high gradient interlaminar stress themselves. This indicates that use of average of high gradient interlaminar stresses is an efficient means for predicting free edge stress induced delamination initiation.