A. Crosky

Material characterisation and mechanical properties of Al2O3-Al metal matrix composites

The mechanical properties of metal matrix composites (MMCs) are critical to their potential application as structural materials. A systematic examination of the effect of particulate volume fraction on the mechanical properties of an Al2O3-Al MMC has been undertaken. The material used was a powder metallurgy processed AA 6061 matrix alloy reinforced with MICRAL-20™, a polycrystalline microsphere reinforcement consisting of a mixture of alumina and mullite. The volume fraction of the reinforcement was varied systematically from 5 to 30% in 5% intervals. The powder metallurgy composites were extruded then heat treated to the T6 condition. Extruded liquid metallurgy processed AA 6061 was used to establish the properties of the unreinforced material.

Strength improvement by fibre steering around a pin loaded hole

A fibre steering technique has been applied around boltholes in carbon fibre reinforced epoxy composite laminates to locally enhance the bearing strength of bolted joints. The procedure can precisely place dry tows of fibre on a prepreg fabric following both the tensile and compressive principal stress trajectories around the hole. The bearing test results indicate that fibre steering improved the peak load of the composite bolted joints approximately in linear proportion to fibre addition by weight. The best result achieved an increase for the peak load by a factor of 2.69. The best improvement of bearing strength was by a factor of 1.36 for a specimen reinforced by 3 k fibre tows in tensile principal stress patterns and 6 k fibre tows in compressive principal stress patterns. The bearing strength improved due to significant increase in peak load and moderate change in thickness.

Interdiffusion behaviour in aluminide-coated René 80H at 1150°C

Abstract The hot-section components in commercial aero gas turbines are typically made of a nickel-base superalloy and are protected by a diffusion-aluminide coating. At elevated temperatures this alloy/coating system is metastable and, as a consequence, microstructural changes occur. The extent to which these changes occur is critically dependent upon temperature and time. In the case of overheating, where the component temperature exceeds about 1100°C, the microstructural changes are often extensive enough to cause a decrease in both the strength and protectiveness of the alloy/coating system in a relatively short period of time. This paper reports the effects of overheating on the microstructural changes that occur in an aluminide-coated, nickel-base superalloy, Rene 80H. The overheating condition was simulated by isothermally heating at 1150°C for up to 167 h. Both the alloy/coating interdiffusion kinetics and the time-dependent phase changes resulting from interdiffusion are discussed. The interdiffusion behaviour is complex and requires the application of diffusion paths for proper interpretation.

An evaluation of failure criteria for matrix induced failure in composite materials

Abstract This paper reports on work being undertaken in the Cooperative Research Centre for Advanced Composite Structures Ltd. (CRC-ACS) to develop improved techniques for predicting the failure of composite materials. The procedures being investigated include a maximum strain criterion for fibre failure. For failure of the resin a new approach, which includes determination of the residual stresses due to manufacturing, is being trialed. This work closely parallels the new criteria proposed by Gosse and Hart-Smith [AIAA/CRC-ACS text on composite materials, submitted for publication] and we have subsequently replaced a simple stress criterion for matrix failure with their proposals based on strain invariants. The new procedures are applied to the failure of laminates in bolted joints with complex steered fibre patterns. Thermal residual stress was included to predict the matrix failure of T-section laminates under loads that open the angle between the flanges and the web. Here a transverse tension stress criterion was used.

Failure of a screw in a helicopter fuel-control unit: Was it the cause of a fatal crash?

Investigations into a fatal helicopter crash centered around the failure of a cadmium-plated, non-conforming, steel screw in a fuel-control unit. The present study, which includes a critical evaluation of previous investigations, was undertaken with the aim of getting more definitive answers as to whether the screw failed in flight due to hydrogen-embrittlement or stress-corrosion cracking, or whether the screw failed due to liquid-metal embrittlement during a post-crash fire. Previous investigations had not resulted in any consensus regarding the mode of failure or whether the failure was responsible for the crash.

Fibre placement processes for composites manufacture

Abstract Fibre placement refers to composite fabrication processes that involve the laying down of reinforcing fibres along predefined trajectories in the component. It is also referred to as directed fibre placement and fibre steering. The goal of fibre placement is to maximise the performance of a particular part by utilising the highly directional strength of fibre reinforcement. Fibre placement can be used to improve stiffness and strength of components, to locally reinforce holes and cutouts, or even to produce a part with a complex load–deflection response (e.g. bend–twist coupling). Fibre placement technology tightly integrates design (the definition of the fibre trajectories) with manufacturing technology (the fabrication of those trajectories). Several technologies are addressed in this chapter that can be used with fibre placement techniques, including automated fibre placement, tailored fibre placement, and fibre patch preforming. Analytical and optimisation techniques for defining the fibre trajectories are also considered here. The principal stress and load path trajectory methods are discussed in this chapter, as well as the use of a genetic algorithm.