Papers by Author: Philip J. Withers

Abstract: Metal matrix composites comprising a magnesium matrix and Mg₂Si/MgO dispersoids obtained by hot pressing silica nanoparticle agglomerates and metal powder in a Degussa press were characterized. Two powder mixtures having weight proportions of Mg:SiO₂ of 10:0.3 and 10:1 were identically sintered. Their microstructures were characterized by optical microscopy and X-ray diffraction. The size and distribution of the Mg₂Si and MgO dispersoids formed in situ were assessed as a function of the original nanosilica content. The behaviour of the composites under compression testing was assessed in 3D by X-ray microtomography using 225kV Nikon X-tek and 150kV Xradia MicroXCT scanners. This provided insights into composite strengthening mechanisms and matrix particle decohesion.

Abstract: This paper deals with the use of X-ray tomography and Digital Volume Correlation in order to study the mechanical behaviour of a low density polymeric auxetic foam. First, the metrological performances of the procedure are assessed using rigid body translation. Then, tensile test results are analyzed for two load steps and comments on the deformation process of the foam are given with a specific view to Poisson’s effect.

Abstract: The microstructure of novel AZ91 magnesium matrix composite reinforced with glassy carbon particles has been characterized in this study. The composite was produced by hot pressing a mixture of glassy carbon particles and metal powder. Beforehand, the particles were coated with SiO2 by a sol-gel method. Metris X-tek and Xradia MicroXCT laboratory X-ray tomography systems were used to obtain 3D microstructural information (particle-size distribution, volume fraction of particles, phase formation and defects in the form of clusters of particles and pores). These 3D features were validated by 2D scanning electron microscopy (SEM) methods.

Abstract: This paper investigates the variation of residual stress with depth and radial location in a nickel base superalloy, RR1000, introduced by face finish turning. X-ray diffraction stress measurement has revealed that the hoop stress at the surface becomes less tensile towards the centre of the face, whilst the level of radial sub-surface compression increases. The unstrained lattice spacing d₀ and the diffraction peak width (FWHM) were used to make inferences regarding the thermal excursion and the plastic work, respectively. It was found the increase in the compressive stress from the outer towards the inner radius was associated with an increase in thermal excursion.

Abstract: This paper examines the extent to which mechanical shot peening (MSP), ultrasonic impact treatment (UIT) and laser shock peening (LSP) can affect the tensile residual stresses in the fusion zone caused by welding for a 10mm multi-pass 'V' groove weld within a 20 mm thick 304L stainless steel plate. Stresses are measured by deep hole drilling, neutron diffraction and incremental center hole drilling. For the UIT and LSP treated samples, the tensile stresses present in the as-welded plate are reversed to compressive stresses to a depth in excess of 2-4mm. For MSP the affected depth is much less (~0.5mm). The depth of these compressive stresses is similar to those measured in 20 mm thick parent plate test coupons.

Abstract: A 3D model for intergranular thermal stresses in coarse polycrystalline alumina has been derived using Diffraction Contrast Tomography. Larger tensile thermal strains develop when the (0001) pole of adjacent grains lies closer to the grain boundary normal. This agrees with observations of cracked boundaries, obtained through digital image correlation of in-situ observations in fine alumina.

Abstract: Measuring residual stress at the sub-micron scale imposes experimental challenges. We propose a new technique, namely the incremental micro-hole-drilling method (IµHM), for measurement of residual stress profiles as a function of depth with high spatial definition. Like its macroscale counterpart, it is applicable to either crystalline or amorphous materials, but at the sub-micron scale. Our method involves micro-hole milling using the focused ion beam of a dual beam FEGSEM/FIB microscope. The surface displacements are tracked by digital image correlation of SEM images recorded during milling. The displacement fields mapped around the whole are used to reconstruct the variation of the in-plane stress tensor as a function of depth. In this way the multi-axial state of residual stress has been characterised around drilled holes of 2 microns or so, enabling the profiling of the stress variation at the sub-micron scale to a depth of 2 microns. Here we demonstrate the efficacy of this method by measuring the stresses in a surface-severe-plastically-deformed (S2PD) Zr50Cu40Al10 bulk metallic glass (in atomic percent, at.%) sample after failure under four-point-bending-fatigue.

Abstract: Simple experimental tests of fatigue life are often insufficient to characterise fatigue behaviour. Fatigue crack growth in polycrystalline metals is governed by a number of interacting mechanical effects at the crack tip, such as the deformation inside a plastic zone and contact between the crack faces over part of the loading cycle. Typically, results tend to be interpreted in terms of an empirical fatigue law such as the Paris equation, which in itself fails to generalise to different load ratios or multiaxial load cases. While extensions to this equation have been used, these are mostly empirical and do little to enhance understanding of the fatigue growth mechanisms. Recently, the use of diffraction to characterise crack tip stress effects has become increasingly popular. In this paper, we consider the opportunities and the difficulties associated with making such measurements by neutron and synchrotron diffraction. In particular we examine grain size effects, plane stress/plane strain issues, optimisation of the gauge geometry, measurement of the plastic zone and crack closure effects.

Abstract: Uniaxial deformed fcc metal samples have been studied by diffraction peak profile analysis. A method that can explain changes in broadening of different peaks by use of a Taylor model has been investigated. It was found that the method qualitatively describes the changes in broadening in nickel and stainless steel samples. It is argued that the differences between predictions and measurements are a feature of how the different samples deform at the microstructural scale.

Abstract: The current study investigates the effect of foreign object damage (FOD) on the pre-existing compressive residual stress field associated with laser shock peening (LSP) and its evolution upon combined LCF/HCF cycling. FOD was introduced onto an aerofoil-shaped specimen that had been previously LSP treated through ballistic impacts at angles of 0° and 45° to the leading edge. It is shown that the FOD notch created by 45° impact was asymmetric in shape and smaller in depth compared to that created at 0° impact. Significant through thickness compression was introduced parallel to the leading edge as a result of the LSP process. The residual strain distribution was mapped around the FOD notch by synchrotron X-ray radiation. The results show predominantly compressive stresses ahead of the notch, being greater for the 0 compared to 45 impact. No significant stress relaxation was observed after a combined (1000 HCF cycles superimposed on 1 LCF cycle) cycle.