Anthony R. Bunsell

Fine diameter ceramic fibres

Two families of small diameter ceramic fibres exist. The oxide fibres, based on alumina and silica, which were initially produced as refractory insulation have also found use as reinforcements for light metal alloys. The production of SiC based fibres made possible the development of ceramic matrix composites. Improved understanding of the mechanisms which control the high temperature behaviour of these latter fibres has led to their evolution towards a near stoichiometric composition which results in strength retention at higher temperatures and lower creep rates. The SiC fibres will however be ultimately limited by oxidation so that there is an increasing interest in complex two phase oxide fibres composed of α-alumina and mullite as candidates for the reinforcement of ceramic matrices for use at very high temperatures. These fibres show low creep rates, comparable to the SiC based fibres but are revealed to be sensitive to alkaline contamination.

Microstructure and mechanical characteristics of alpha-alumina-based fibres

AbstractThe high-temperature mechanical behaviour of alumina-based ceramic fibres has been investigated by the comparison of a dense pure alumina fibre, a porous pure alumina fibre and a zirconia-reinforced dense fibre. Tensile and creep tests have been conducted up to 1300°C in air in parallel with microstructural investigations on the as-received and tested fibres. Room-temperature behaviour of the fibres is close to that of bulk materials having the same microstructure, but the fibre form allows higher failure stresses to be attained. High-temperature deformation of the three fibres is achieved by grain-boundary sliding (

Damage accumulation processes and life prediction in unidirectional composites

\dot \varepsilon \propto \sigma ^2

Life prediction for carbon fibre filament wound composite structures

), and is accompanied by isotropic grain growth. The specific microstructures of each fibre induce differences in the creep threshold levels as a function of temperature and stress and also in creep rates and resistance to damage. Despite better resistance to creep and damage of the zirconia-reinforced fibre, alumina-based fibres are limited to applications below 1100°C. Grain boundaries are the principal cause of mechanical degradation at high temperature with these fibres.

Statistical analysis of strength distribution of alumina based single fibres accounting for fibre diameter variations

The microstructure and tensile properties of the as-received and heat-treated Nextel 720 fibre have been studied. During its fabrication the Nextel 720 fibre is pyrolysed at a temperature lower than 1400°C for a very short time which does not allow the microstructure to be stabilised. A pseudo-tetragonal metastable alumina rich mullite is formed which crystallises in the form of mosaic grains containing low angle boundaries. These mosaic grains enclose some rounded and elongated α-alumina grains. The evolution of the mullite to the stable orthorhombic symmetry is seen from 1200°C for post heat treatments lasting several hours. For longer heat treatments at 1200°C or at 1300°C and higher temperatures, the mosaic grains begin to recrystallise into single grains and the alumina rejected from the mullite contributes to the growth of elongated α-alumina grains which suggests diffusion through an intergranular liquid silicate phase. At 1400°C the mullite has the 3:2 composition and after 24 h the growth of the elongated α-alumina grains leads to a reduction of the room temperature tensile strengths. The α-alumina creation coupled to the phase transformation and dissolution of mullite leads to an increase of the Youngs modulus after heat treatments from 1200°C.

Fibre Break Failure Processes in Unidirectional Composites. Part 1: Failure and Critical Damage State Induced by Increasing Tensile Loading

A parallel has been made between the design and the failure of some composite structures, such as pressure vessels, and that of a unidirectional carbon fibre-reinforced epoxy resin. This type of composite has many characteristics that make it attractive, however, long-term damage accumulation in it, when under load, is not understood in any quantitative manner. It is therefore necessary to identify the processes involved in the degradation of the composite when subjected to long-term loading. Acoustic emission (AE) has been used to detect damage in the composite material and to relate the microstructural damage processes to the activity recorded from composite specimens through the development of a detailed analytical model of the damage processes involved. A multiscale model beginning at the scale of individual fibres and the surrounding matrix has been developed by taking into account the accumulated effects of individual fibre breaks on the behaviour of the whole composite and therefore explains the AE detected during the tests.

Prediction of the long term behavior of synthetic mooring lines

The micro/macro structural evolution of polyamide 66 fibres during tensile loading and failure initiation have been studied by coupling multi-scale measurements such as wide angle X-ray diffraction (WAXD) with profile fitting, birefringence, differential scanning calorimetry (DSC), micro-Raman spectroscopy and mechanical testing at two strain rates. A large dependence of mechanical properties, including strain rate effects, on the behaviour of both oriented and random amorphous regions has been shown with important contributions from the isotropic amorphous domains. The results indicate the presence of compressive residual stresses beneath the skin, showing a skin/core sub-structure. Statistical fracture treatments have been applied using a time-dependent weakest link Weibull model, in order to give an evaluation of the dispersion of defects. Local damage was taken into account using a crack growth propagation law as a function of stress intensity factor near the defect. The paper shows how both experimental and theoretical fracture toughness results are in quite good agreement.

Stochastic factors controlling the failure of carbon/epoxy composites

The processes governing the failure of filament wound composite structures have been examined. It is shown that the fibres controlling the failure of such a structure, when it is internally pressurised, can be considered to be subjected only to tensile loads. A multi-scale model has been developed which considers the effects on the scale of the elastic fibres and includes the effects of the viscoelastic matrix as well as debonding around fibre failures. The intact fibres neighbouring fibre-breaks are subjected to an increase in stress and a higher probability of failure than elsewhere in the composite. During a monotonic failure test, the initially random fibre failures are seen to begin to coalesce in a way governed by the stochastic nature of the fibre-breaks and this eventually leads to failure. Under prolonged loading, relaxation of the matrix around fibre-breaks causes overloads in the neighbouring fibres to evolve and induce delayed fibre-breaks, which eventually lead to instability in the structure. The model allows these processes to be considered in calculating the behaviour of the whole composite structure.