Failure of tapered composites under static and fatigue tension loading

Abstract

One glass fiber-epoxy and two carbon fiber-epoxy rapidly tapered sections with dropped 0- and ±45-deg plies have been designed, manufactured, and tested. Under static loading, close to net section tensile strength was achieved. The glass fiber taper also achieved net section strength under tension fatigue loading, but the carbon fiber tapers showed a significant reduction in fatigue strength due to delamination from the ends of the terminating 0-deg plies. The most significant factor controlling strength is the number of plies dropped together. Unidirectional dropped plies are much more susceptible to delamination than ±45-deg plies. Satisfactory strength predictions can be made for both static and fatigue tension loading based on net section stresses and a simple equation for the strain energy release rate. This approach looks promising as the basis for a general method for designing tapered composites. APERED sections involving the progressive dropping of plies are prone to delamination, especially under fatigue loading. Such tapered sections are very commonly used to achieve changes in thickness. A typical example is a helicopter rotor blade, where increased thickness is necessary near the root. Normally a short tapered transition region is desirable. However, due to the lack of reliable methods for predicting delamination, a very gradual taper is usually used. A number of authors have investigated static strength of tapered laminates.18 However, there is no consensus on how to predict failure. Various approaches have been proposed based on maximum stresses,2 stresses averaged over a distance,6 stresses evaluated at a characteristic distance,8 and strain energy release rates.5 Several investigations on tension fatigue of tapered specimens have been published^11 The problem of predicting fatigue strength has not received very much attention, although in the latter study a method was proposed for predicting onset of unstable delamination based on strain energy release rate analysis.11 A program of research has been undertaken at Bristol University to investigate failure in tapered composites under static and fatigue loading. A series of tests on very simple symmetric specimens have enabled failure mechanisms to be studied.12 Much of the work has been on unidirectional material, avoiding some of the potential problems with edge effects. It has been found that the dominant failure mechanism for these specimens is delamination initiating above and below the ends of the terminating plies and propagating into the thick section along the interface between continuous and discontinuous plies. The results have indicated that the most important parameter affecting failure is the strain energy release rate associated with the terminating plies. Based on this work, a method has been proposed for predicting static strength of tapered specimens. A simple equation is used for calculating the strain energy release rate. This is then compared with a value of fracture energy deduced from a tension test on a constant thickness unidirectional specimen with the central plies cut across the complete width. Satisfactory correlation has been obtained for tapered unidirectional glass fiber-epoxy specimens under static tension and compression loading.13