Matthew R. Arthington

Cross-section reconstruction during uniaxial loading

The inelastic response of materials to applied uniaxial loading is typically measured using tensile or compressive specimens of an initially circular cross-section. Under deformation, this cross-section may become elliptical due to anisotropic material behaviour. An optical technique for measuring the elliptical deformation of anisotropic, homogeneous cylindrical specimens undergoing uniaxial deformation is presented. It enables the quantification of anisotropic deformation in situ and provides data for material characterization. Three or more silhouette views of a specimen are obtained using multiple cameras or mirrored views. The positions of the edges are computed using a sub-pixel edge detection method, and 3D tangent rays from the camera through these positions are calculated. These bounding tangents are used as the basis for an elliptical fit by least squares at cross-sections along the length of the specimen. Stochastic error estimates are performed by simulation of the experiment. Error estimates, for the experimental set-up used, are also calculated by reconstructing elliptical prisms of precisely measured dimensions. Example reconstructions from specimens of rolled titanium deformed plastically in tension at quasi-static (7 × 10-4 s-1) and high strain rates (3 × 103 s-1) are presented.

Measurement and control of variable geometry during ring rolling

Ring-rolling is an industrial forming process for producing high-strength seamless metal rings up to 6m diameter. Thick-walled cylindrical rings of material, typically metallic alloys, are compressed between two or more internal and external rollers and rotated until a target geometry, often outer diameter, is achieved. A common plant configuration is that of a pair of radially acting rollers and a pair of axial rollers, the radial-axial ring rolling (RARR) machine. The most commonly produced product geometries have an axisymmetric cross-section profile. However, during the forming process the cross section is changed significantly as it passes through each pair of rollers. This irregular shape hinders geometry state measurement and this complicates modelling and control of the process. Recent developments in sensing capabilities offer high resolution measurement of ring geometry during forming. In this work, we present advances in these sensing techniques, a numerical method for storing and predicting the rings geometrical state and control laws to achieve a non-axisymmetric cross-section profile in rolled rings using existing RARR plant hardware.

An Error Analysis into the Use of Regular Targets and Target Detection in Image Analysis for Impact Engineering

This study concentrates on the use of corners targets for photogrammetry in impact engineering. An example of high speed experimentation is presented and the associated difficulties are discussed. The relevant corner detection methods that have been implemented and developed are investigated and their accuracy assessed. This study focuses solely upon the effect of blurring on the accuracy of the detection methods; it is part of a much wider investigation into the use and accuracy of different targets and target detection methods for photogrammetry in impact engineering. A set of tests has been performed and the errors between the true position of the corner and the detected position are compared.

Increasing data from high rate characterization experiments using optical reconstruction

Uniaxial characterization experiments in tension and compression are widely used to evaluate the mechanical response of materials to applied deformation over a wide range of strain rates. By taking photographs of the specimen as it deforms, it is possible to more fully characterize materials by reconstructing the specimen shape during the deformation process. This allows us to better evaluate the stress and strain in the specimen during deformation, and also provides a more rigorous validation of material models. For many materials, specimens with initially circular cross sections will evolve to a different shape; in particular, hcp metals become elliptical. We have developed a technique whereby images of a specimen during deformation (for example, from a high speed camera) from three different angles are used to reconstruct elliptical cross sections during an experiment. The technique has been applied to tensile Hopkinson bar experiments and Taylor Impact experiments on Zirconium. This paper presents an ...

Optical surface profile tracking for high-resolution strain measurement

Characterization of the mechanical behaviour of materials often requires accurate determination of specimen strain. This paper presents an optical method whereby the surface displacements of a cylindrical specimen undergoing uniaxial loading are measured by tracking the profile of its surface texture. These profiles are obtained through sub-pixel edge detection from a digital image. High-resolution digital imaging with sub-pixel edge detection is used to measure the radial position of edges in images of the specimen. Edges are compared in consecutive images to track the positions of zero-crossings and local maxima of the surface roughness. A method for dealing with cases in which the mean wavelength of the surface texture is shorter than the displacement between frames is presented. False matches can still occur and are removed by modal average proximity filtering in sub-regions spanning short axial-lengths and several frames. The new method of finding specimen displacement is found to yield high data densities (in our case 32 independent measurements per mm). The data compare favourably with measurements made using a laser extensometer: surface roughness tracking produces a drop in accuracy of ±25 μm whilst increasing the spatial data density. A further advantage of the technique is that no specimen preparation is required. The data are also used to calculate strain, values of which are validated against strains inferred from the change in cross-sectional area.

Characterising the Effects of Strain Rate, Crystallographic Texture and Direction of Loading on the Mechanical Behaviour of Ti-6Al-4V

Abstract A cross-rolled plate of the industrially important titanium alloy, Ti-6Al-4V, has been microstructurally and mechanically characterised using a range of different experimental techniques. The microstructure of the material has been studied using backscatter electron (BSE) microscopy and electron backscatter diffraction (EBSD), with the crystallographic orientation data from the EBSD used to reconstruct the orientation distribution function of the dominant α phase. The mechanical behaviour of the material has been investigated at quasi-static and high strain rates in the three orthogonal material orientations in both tension and compression. A novel in situ optical measurement technique has been used to measure the geometry of the specimens during both quasi-static and high strain rate mechanical testing, improving the accuracy of the mechanical testing results and providing unprecedented information about the evolving geometries of the specimens. The macroscopic stress–strain response and the evolution of specimen cross-sectional profiles have been qualitatively linked to the macroscopic crystallographic texture in the plate.