Brian Cotterell

Fracture of fiber-reinforced materials

The fracture behaviour of fiber-reinforced materials is studied in this paper. Using a simple shear lag model, which includes friction at the debonded interface and the Poisson contraction of the fiber, the fiber-matrix debonding problem is solved. This gives the relationship between debonding load and debonded length. Interfacial friction is shown to have a significant effect on the debonding load. The fracture toughness of fiber-reinforced materials due to fiber debonding, frictional dissipation at fibre-matrix interface following debonding and other micro-fracture mechanisms is discussed with reference to strong and weak fibres. Finally, the strength and toughness of short fibre-reinforced materials are given.

Crack growth resistance curves in strain-softening materials

Abstract A theoretical model has been developed for the crack growth resistance ( K R ) curves in strain-softening materials with power law softening stress (σ)-crack opening displacement (δ) relationships. Both exact and approximate solution methods have been used to calculate K R curves for a fibre cement composite in a double-cantilever-beam (DCB) geometry. There is good agreement between these two solutions. When the crack growth is normalized with respect to the saturated softening (or fibre bridging) zone there is an almost unique K R curve which is independent of specimen geometry and initial crack length. The effects of the softening index ( n ) of the power law σ-δ relationship on the shapes, saturated softening zone lengths and plateau values of the K R curves are also studied.

A mechanical assessment of flexible optoelectronic devices

This work has demonstrated novel experimental methods and their relevant analysis to evaluate the fracture properties of thin brittle films on compliant substrates for flexible optoelectronic devices. Based on the understanding of the failure mechanisms, mechanical calculation has been provided to estimate the thin film fracture toughness and film delamination toughness. These values serve as design parameters on the device flexibility and reliability.

The formation of flakes

An understanding of the mechanics involved in flake formation provides an opportunity for deriving more behavioral information from flake and flake scar morphology. The mechanics of flake formation are directly relevant to the identification of prehistoric flaking techniques and stone tool use. In this paper we provide a model of flake formation that accounts for much of the variation in flake morphology. Flakes can form in a number of ways and despite popular belief they are not all of the conchoidal variety. The bending flake is common in use wear though it is often misidentified as a conchoidal flake. A third major type of flake, the compression flake, is a common product of bipolar impact. To account for the wide variation in flake morphology we follow a tripartite scheme of flake formation comprising initiation, propagation, and termination phases, within which different mechanisms can operate.

The fracture of brittle thin films on compliant substrates in flexible displays

One mechanical issue in flexible organic light emitting displays (OLED) is the fracture of extremely thin brittle conducting transparent oxide films deposited on thin flexible substrates. Understanding the behaviour of these films under flexed condition is essential for designer of flexible OLED. Controlled buckling experiments on the film and substrate have been designed to study the fracture of the films under both tension and compression. Fracture of the film is superficially similar in both tension and compression. However, under tension a channelling crack is formed, while under compression, the film delaminates, buckles and cracks in a tunnelling motion. The fracture toughness of the film and the delamination toughness have been estimated from these experiments. Design to maximise the flexibility of the device is discussed.

Buckling and cracking of thin films on compliant substrates under compression

It is shown that unless the substrate is at least as stiff as the film, the energy stored in the substrate contributes significantly to the energy release rate of film delamination under compression either with or without cracking. For very compliant substrates, such as polyethylene terephthalate (PET) with a indium tin oxide (ITO) film, the energy release rate allowing for the deformation of the substrate can be more than an order of magnitude greater than the value obtained neglecting the substrates deformation. The argument that buckling delaminations tunnel at the tip rather than spread sideways because of increase in mode-mixity may need modification; it is still true for stiff substrates, but for compliant substrates the average energy release rate decreases with delamination width and the limitation in buckled width may be due to this stability as much as the increase in mode-mixity.

Effect of specimen geometry on the essential work of plane stress ductile fracture

Abstract The effect of specimen geometry on the specific essential work of fracture for a low work hardening aluminium sheet metal alloy (AA2S) are investigated. Four different test geometries, viz. the deeply double edge notched PENT), centre-notched (DCNT), single edge notched (DSEN) and modified double edge notched (MDENT) tension specimens, are used to evaluate the essential works of fracture in the crack tip process zone for both crack initiation and propagation. It is shown that the specific essential work at fracture initiation ( w i ), which can be identified with J i , is independent of test peice geometries. Owing to specific problems associated with the DCNT and DSEN specimens during fracture propagation only the DENT and MDENT specimens are successful in yielding specific essential work measurements which are constant and which can be identified with J p for propagation. The experimental results given in this paper confirm that the specific essential work of plane stress ductile fracture is a fundamental material property being independent of specimen geometry.

Effects of pre-strain on plane stress ductile fracture in α-brass

The effects of pre-strain on plane stress ductile fracture in a 70/30 alpha brass Austral 207 have been studied using the deep-edge-notched tension (DENT) specimens. The amount of pre-strain varies between 5 and 35%. It is found that both the specific essential work of fracture (we) and the critical crack opening displacement (δc) decrease with increasing pre-strain. A simple theory for estimating the specific essential work of fracture in the presence of pre-strain is suggested and it gives good agreement with experimental results. Elongations to fracture in the DENT specimens are also predictable from a simple deformation analysis which considers the plastic elongations due to crack initiation, crack propagation and final stretch of a ligament that has reached a necking strain equal to that in a simple plain tension test. Micro-hardness measurements show that the strain localization is more intense near the fracture surface as the pre-strain level is increased and this is suggested to be an explanation for the low δc values obtained in pre-strained specimens.

On the effect of plastic constraint on ductile tearing in a structural steel

The effect of plastic constraint on the initiation of ductile tears in a structural steel has been studied by measuring the crack opening displacement at initiation in 3- point bend specimens with deep and shallow notches. It is shown that delta for shallow notches where plastic flow reaches the surface is about twice that for deep notches. The crack opening displacement at maximum load is shown to be proportional to the specimen size. It is also shown that sidegrooves reduce this crack opening displacement. (Author/TRRL)

Crack growth resistance curve and size effect in the fracture of cement paste

A general theory is presented for the fracture of cementitious materials. It is shown that crack growth resistance curves can be constructed for cement pastes using fracture data available in the literature. The crack growth resistance curves are used to explain the specimen size and crack length dependence of fracture toughness in cement pastes.