J.F. Williams

An analysis of delamination behaviour

Abstract Laminated composites can be damaged by foreign object impacts that produce a subsurface delamination. Under certain loading conditions these delaminations can grow and ultimately produce structural failure. In this study it was found that delamination growth results from crack propagation that depends on the stress field at the crack front produced by buckling of the delaminated section. This result was confirmed by three independent analyses. First was the development of an analytical model based on buckling and classical fracture mechanics. Second was the development and solution of a finite element model of the crack front. Third was an experimental study of laminated aluminum sheets with a well-defined delamination.

Increasing the fatigue life of cracked fastener holes using bonded repairs

Abstract Recent experimental work has shown that the use of a suitable bonded insert can significantly reduce the rate of fatigue crack propagation for initially uncracked geometries, and that this approach may be superior to other life enhancement schemes [7]. Some initial experimental work on three-dimensional cracked geometries was also reported in a previous paper which demonstrated that significant fatigue life extension could be achieved for this type of problem. This paper deals with the fatigue testing of specimens containing three-dimensional cracks at the edge of a fastener hole, repaired with a bonded sleeve and/or a bonded composite patch. It is shown that significant increases in fatigue life can be achieved using a bonded sleeve alone. However, the additional use of an externally bonded patch produces an increase in the fatigue life of the specimens of between 1 to 2 orders of magnitude.

Thermomechanical analysis of composite specimens

Abstract This paper examines, experimentally and numerically, the use of thermal emission measurements to determine the surface stresses for a class of problems involving advanced fibre composite materials. Attention is focused on the use of this method for problems associated with repair technology, bonded joints and impact damage. This approach is found to hold great promise both as a method for detecting damage and for assessing its severity.

Analysis of cold-expansion for cracked and uncracked fastener holes

Abstract Finite element analyses of cracked and uncracked cold-worked fastener holes are carried out, and the behaviour of various damage parameters is studied. Following this, the experimental analysis of a microgridded cracked fastener hole is undertaken, in which the crack tip deformation during the cold-expansion process and subsequent remote loading is studied.

Characterisation of pure and mixed mode fracture in composite laminates

Abstract The extensive use of advanced fibre composite materials for aircraft construction has necessitated the development of a damage tolerance methodology for aircraft components. Such a methodology would facilitate the design of more efficient and reliable composite structures and their maintenance in service. Therefore, it is necessary to characterize and understand the complex failure modes of fibre composites, including the influence of temperature, moisture and various defects arising from manufacture or service conditions. This paper briefly discusses the present status of some approaches to the experimental characterization of pure and mixed-mode fracture of composite laminates.

An estimate of the residual stress distribution in the vicinity of a propagating fatigue crack

Abstract Experiments have been performed on a series of 2024-T351 Aluminum alloy uniaxial, tension specimens loaded under R = 0 conditions to grow a low cycle fatigue crack to a predetermined length. One half of the specimens were given a 33% single pulse overload at the conclusion of the test. All specimens were sectioned along the plane of the crack and measurements made of the residual displacements occurring at this location due to the relief of the internal stresses produced during crack propagation and overload. The data was used as the boundary condition in a Displacement Boundary Value Problem solving an Airy Stress Function for the residual stress distribution in the vicinity of the crack. The results indicate a significant degree of tensile stress ahead of the crack tip and compressive stresses both at the crack tip and in the wake of the crack reaching magnitudes as high as 36% of the yield stress. The results are consistent in both the overload and no overload cases however the magnitude and extent of the compressive stress region appears to be increased by the action of the overload. These results are consistent with the concept of crack closure as proposed by Elber.

Assessment of the effect of impact damage in composites: Some problems and answers

In order to provide through-life support for structural components, fabricated from advanced composite materials, it is desirable to establish a damage tolerance methodology. The availability of this methodology would greatly aid the design of safe, efficient composite structures as well as their management in service. There are, however, many difficulties in meeting this objective, including the multiplicity of failure modes in the composites, the numerous types of potentially significant defects which may arise during manufacture or in service and the sensitivity to moisture and temperature. This paper discusses the analysis and testing of impact damage composite laminates. Selected experimental studies in the literature are briefly discussed. It is shown that uniaxial S−N curves for damaged laminates have a generic shape with a pronounced threshold level. Indeed, it is clear that further research is needed to understand the physical reasons for such a threshold value.

Thermomechanical behaviour of composites

This paper derives the governing equations for the thermomechanical behaviour of composites. When the basic equations for the thermoelastic behaviour of solids were first derived in the nineteenth century several approximations were made. The effect of these assumptions are discussed and illustrated by the results of a simple laboratory test. The implications of this work on the analysis of impact damaged laminates are then discussed.

A numerical study of MSD in aluminium alloys

Abstract This paper uses the finite element alternating technique to study the effect of multiple cracks, i.e. multi-site damage (MSD), in an aluminium alloy plate. This problem arose as a result of MSD seen in the aft bulkhead of F/A-18 aircraft in service with the Royal Australian Air force (RAAF). For the particular problem under investigation, it is shown that the maximum value of the stress intensity factor can be directly estimated from a knowledge of the solution for a single crack. As a result of this investigation, it is also shown that compliance measurements may provide a useful tool for assessing the structural integrity of the bulkhead.

Application of the T∗-integral and S criteria in finite element analysis of impact damage at fastener holes in graphite/epoxy laminates under compression

This paper illustrates the application of two fracture criteria, Atluris T∗-integral and Sihs strain energy density factor S for estimating the residual strength of an impact damaged fastener hole in a composite laminate. Finite element analyses are performed, and the magnitude and distribution of T∗ and S are determined around the delamination. It is found that the profiles of these distributions are extremely complex and sensitive to the modelling of the stress fields close to the delamination. The effects of local closure, shear moduli and cracktip singularity are investigated. It is also shown that the representation of the stress singularity in the finite element model has a strong effect on the distribution of T∗ and S. The distribution of both T∗ and S is such that three local maxima occur and are situated at the same locations around the delamination. These locations approximately coincide with the points of maximum growth as revealed by ultrasonic C-scan of the damage growth of several specimens. The T∗-integral is shown to be more sensitive to the values of interlaminar shear moduli than S. The results of the analyses suggest that both the T∗-integral and S criteria may be successfully employed in the prediction of residual strength. However, S is better at predicting the direction of damage growth provided the stress fields near the delamination can be accurately modelled.