D.A. Gorham

The measurement of particle rebound characteristics

Abstract Accurate measurements have been made of the rebound behaviour of 5 mm aluminium oxide spheres impacting a thick soda–lime glass anvil (fully elastic response) and an aluminium alloy anvil (involving some plastic deformation). Speed, angle and rotation before and after impact have been measured with a strobe and single-frame digital camera, for a range of impact angles from normal to very near glancing. High levels of reproducibility and precision have been achieved by careful attention to all aspects of the measurement, including the mechanical and optical systems, illumination, electronic control, and computer-based image measurement. Rebound parameters from the elastic and plastic impacts are analysed and compared in the context of the numerical work of Maw et al. for near-normal angles when sticking, micro-slip and tangential compliance are the major factors, and of rigid body sliding for more oblique angles.

AN EXPERIMENTAL STUDY OF THE ELASTIC REBOUND OF SPHERES

Abstract Accurate measurements have been made of the impact and rebound behaviour of 5-mm aluminium oxide spheres impacting a thick soda–lime glass anvil, for impact angles from normal to very near glancing incidence. Speed, angle and rotation before and after impact have been measured with a strobe and single-frame digital camera. Reproducibility and precision are considerably better than in any previously published work, and have been achieved by careful attention to all aspects of the experiment, including the mechanical and optical systems, illumination, electronic control, computer-based image measurement, and the geometry and condition of the impacting surfaces. All aspects of the rebound dynamics of the elastic spheres, including the motion of the centre of mass and of the contact patch, the spin and the partition of energy, are fully described by the measured variation of the normal and tangential restitution coefficients over the range of impact angles. These measurements show very close agreement with the numerical work of Maw et al. which takes into account the effects of sticking, microslip and tangential compliance. For impacts with a greater obliqueness than about 30° from the normal, the results also agree closely with the classical theory of rigid body sliding.

Accurate measurement of particle impact parameters

The aim of this paper is to describe an experimental facility to measure the low velocity impact behaviour of spherical particles with high accuracy. Measurements have been made of particle rotation, normal restitution coefficient to within of glancing incidence, and tangential restitution coefficient to within of normal impact, with very low scatter. The results are accurate enough to be used for quantitative comparison to theoretical studies. Achievement of a high level of precision and reproducibility has involved detailed attention to all aspects of the experiment design, construction, control and computer-based image measurement.

A study of solid particle failure under normal and oblique impact

Systematic data is presented on the failure probability of 5.15 mm aluminium oxide spheres over a wide range of impact speed and angle. The high number of tests made, with 100 particles individually tested per data point, was necessary to average out the variations in individual particle properties and to make the results consistent. This has enabled very close curve fits to be made to the data, and the parameters from these fits provide a comprehensive and reliable summary of the material performance. The results show that the probability of particle failure varies only slightly from normal impact to about 50°. Detailed examination of damaged particles has identified four primary forms of fracture which are common to normal impact and static compression. These fractures initiate from a subsurface cone of compression damage, and develop along meridian planes. An additional form of fracture found with oblique impact appears to be a result of tangential loading causing enhanced tension in the surface.

Impact breakage of fertiliser granules

Abstract Systematic data are presented for the single impact failure of 3.2, 5.3, and 7.2 mm fertiliser granules over a wide range of impact speeds and angles. The probability of failure was found to change only slowly between 90° (normal) impact and 50°, but decreased rapidly below 50°. The probability of failure increased with increasing size of granules. The effect of impact velocity on the mean, median and the proportion of the largest-sized fragments were examined. Two distinct forms of normal impact damage were identified, corresponding to low and high impact velocities, and the mechanisms of failure are discussed.

The fracture of glass spheres

Abstract Impact and compression tests have been carried out on soda–lime glass spheres in the diameter range of 0.4–12.7 mm. The paper describes the forms of failure and their variation with diameter and with impact velocity. At the lowest velocities, fractures are mainly due to a brittle–elastic response, with typical Hertzian ring and cone crack systems. At higher velocities or loads, inelastic deformation (densification, flow, or intense local fracturing and crushing) under the impact site leads to characteristic patterns of fragmentation arising from radial, lateral and median cracks.

An empirical method for correcting dispersion in pressure bar measurements of impact stress

The dispersion of stress waves in pressure bars distorts a propagating stress pulse, and hence limits the accuracy with which dynamic stress can be measured. This dispersion is caused by a frequency dependence of phase velocity, and previous methods of dispersion correction have adjusted the phase shift of dispersed frequency components using a theoretical bar characteristic. However, the effectiveness of these schemes with more than a small amount of dispersive distortion is very limited. A new method of dispersion correction described in this paper is based on a bar phase characteristic which is derived from measured stress pulses arising from the impact of very small spheres. Examples are given of reconstructing impact stress from a range of particle sizes, showing that the method is very effective even with large amounts of dispersive distortion.

Indentation fracture of glass and mechanisms of material removal

Blunt indentation experiments at high loads have been carried out on soda-lime glass plates using detailed in situ observation. The development of failure over a range of loads is described, showing a number of material removal mechanisms that arise from the propagation of primary and secondary cone cracks. Various inelastic deformation processes are also involved in the development of failure under these conditions. The observed mechanisms of material removal imply a velocity exponent in erosion tests that is rather higher than that seen when lateral cracks are the dominant cause of material loss.

Observation of blunt indentation at high loads

Abstracts are not published in this journal

Unloading fractures in indented glass

Abstract The failure sequence of thick soda—lime glass plates under indentation from tungsten carbide spheres at high loads is described. Failure shows a combination of features normally associated with either blunt or sharp indentation, and most occurs during unloading. In particular the effects of irreversible processes of flow, densification, fracture and interfacial slip during loading on the initiation and propagation of failure during unloading are described and discussed.