• Luke Rollings

Student thesis: Phd


Metal Matrix Composites (MMCs) have been proposed as an alternative to traditional metals in structural components for aerospace applications due to higher specific strength and improved fatigue resistance compared to monolithic metal counterparts. While Ti-based composites have been used in components such as blisks and side stays(Withers, 2010; Kyle-Henney et al., 2012), the introduction of aluminium composites reinforced with long-fibre SiC has been proposed due to their light weight, workability and relatively low melting temperature – a property valued for space-faring craft to reduce waste left in orbit at end-of-life by allowing safe burn-up upon atmospheric re-entry. The aim of this project is to understand the behaviour of continuous unidirectional Aluminium/Silicon Carbide composite, specifically aluminium alloy 6061 reinforced with SM3256 fibres produced by TISICS UK Ltd. This thesis covers the use of micromechanical testing and in-situ synchrotron imaging and diffraction to discern the interfacial behaviour of this novel composite. Fibre push-out testing was carried out on thin slices of Al/SiCf to compare the effects of heat treatment and fibre coating on the interface. Using a microhardness tester (MHT), the loads at which fibre movement occurred were used to calculate the frictional shear stress at the fibre-matrix interface. As-received AA6061 reinforced with coated SiC fibres was found to slide with frictional shear of 46 ±3 MPa. Uncoated fibre reinforcement increased the frictional shear stress, while the effect of commercial T6 heat treatment of the composite was dependant on the cooling rate – water cooling presented similar results to the as-received, coated fibre sample, whereas air-cooled samples had significantly lower values of frictional shear. While push out testing is relatively simple to undertake, the test has a number of limitations, namely broad results over the full length of a fibre, and no detail of the debonding and failure mechanisms. To overcome these shortcomings novel single fibre fragmentation tests of AA6061/SM3256 single-fibre samples were undertaken in-situ on the stage of a synchrotron X-ray beamline. Using X-Ray Diffraction (XRD) and radiographic imaging allowed micron-scale resolution of both the mechanical effects on a fibre supporting a matrix under load, and cracking behaviour and morphology within the composite to be observed. It was seen that this combination of fibre and matrix fails under “stick-slip” mechanics. The interfacial shear strength (ISS) of the composite was calculated as 94 ±10 MPa, the frictional shear measured was 15 ±5 MPa. In-situ Radiographs and post-mortem Scanning Electron Microscope (SEM) images showed failure within the SiC/C interfacial coating, resulting in debris at the interface. Further investigation into the effects of dynamic load were carried out, using Al/SiCf fatigued under tension-tension (T-T) to grow cracks through the material. These were observed by 3D X-Ray Computed Tomography (XCT). The high level of damage caused to the samples showed a great deal of delamination, with cracks growing along the interface in the fibre directions along with bifurcated matrix cracks. From the results gathered during this investigation, the behaviour this novel Al/SiCf system have been quantified and evaluated into the following conclusions. After fibre failure, the fibre-matrix interface fails at a threshold shear stress, after which the fibres undergo frictional sliding within the matrix. Damaged interfacial material can reduce the sliding stress, while no interfacial material can increase sliding stress. Furthermore, the failure mechanisms lead to large cracks along the interface during fatigue, limiting transverse crack growth however greatly enhancing axial crack growth. While these results show Al/SiCf is a promising candidate for structural components in air- and space-faring craft, further research must be conducted into the fibre coating
Date of Award1 Aug 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPhilip Withers (Supervisor) & Matthew Roy (Supervisor)


  • Fatigue
  • X-Ray Computed Tomography
  • X-Ray Diffraction
  • Fibre Pushout
  • Fibre Fragmentation
  • Aluminium
  • Space Launch
  • Metal Matrix Composites
  • Silicon Carbide

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