Summary
This report describes the work carried out in the year 1989 on the project ‘Impact Attrition of Particulate Solids’ supported by a grant from IFPRI. It contains a brief summary of our previous work outlining the features which require further investigation, a literature survey on the relation between fracture mechanics and attrition, and some preliminary work on the effect of hardness on the formation of cracks by quasi-static compression of a corner of cubic particles.
The main objective of this work is to investigate the mechanisms of attrition of particulate solids, and therefore particular attention is paid to the initiation and propagation of cracks and their morphology. Ionic crystals with cubic habit such as NaCl, KC1 and MgO have been used so far as model granular materials because their physical and structural properties are well-characterised. It has been shown previously that the impact attrition of these crystals takes place mainly through localised loading on the corners and edges. This leads to plastic deformation of the impact site, followed by the formation of diagonal cracks, and detachment of platelets from the face adjacent the impact site. It is the last feature which is particularly responsible for the formation of debris in the attrition process, and whose mechanism is under investigation in the current work.
To ascertain whether the formation of platelets in impact attrition of particulate solids is due to strain rate hardening, single melt-grown crystals of KCl, NaCl and MgO were subjected to quasi-static compression of their corners against a hard flat surface. These crystals have different values of hardness, so that the effect of hardness can be investigated independent of dynamic effects which may be present under High strain rates.
The results show that in all cases the material response is elastic/plastic; compression of a corner leads to plastic deformation followed by initiation and propagation of cracks from the plastic zone. The deformation stress is found to be lower than the Vickers hardness.
Two types of crack morphology are observed: {110190 and subsurface lateral cracks. UO)90 cracks are found on all the three crystal types. These cracks are similar to both the quasi-static indentation fracture by a sharp indenter and impact fracture. This is expected as these cracks are formed by dislocation activity on {110145 slip planes, a, common feature in all the three materials. Subsurface lateral cracks are found in MgO crystals only, i.e. in the hardest of the three materials tested. This supports the idea that the strain-rate hardening mechanism is responsible for the formation of the platelets as observed previously in the impact fracture of the softer NaCl crystals, It is therefore necessary to investigate the stress field in the region of the plastically deformed zone, with particular attention to the role of hoop tensile stresses in initiating the cracks. The most appropriate way to analyse the stress field in view of the anisotropy of the material is by Finite Element Analysis. Therefore our plan for the current year includes a theoretical analysis of the stress/strain state by the above numerical technique. This will be complemented by experimental work on quasi-static indentation, and on impact by a hard projectile in order to validate the theoretical work.