Tensile bond strength development between liquid-bound pellets during compression

Novel experiments have been performed to measure the tensile bond strength developed between two liquid-bound pellets when they are compressed together at low strain rates. Pellets 20 mm in diameter were made from 75-μm mass-mean sized glass ballotini with water and three different viscosity silicone oils (0.01, 1 and 60 Pa s). The water-bound pellets formed bonds which were brittle and ruptured quickly when strained in tension. The silicone oil-bound pellets were plastic and stretched back a significant fraction of their original length before the bond ruptured. The peak tensile strengths and rupture energies of the bonds were proportional to the radial strain in the bond zone. A model was developed based on a cold-welding analogy to predict the peak tensile strength of the bond as a function of the strength of the bulk pellets and the extent of radial strain in the bond region. There was good agreement between the model predictions and the experimental results for radial strains less than 7%. At higher radial strains, the bond strength appeared to level off, probably because the bonds began to fail by gradually peeling apart rather than by simultaneous rupture across the whole failure plane. This simple model should help in predicting the bond strength formed when two liquid-bound agglomerates collide, which will be important in understanding and modelling granule coalescence growth behaviour. It was also observed that the compressive strength of the pellets decreased with increasing liquid viscosity, probably due to a lubrication effect reducing inter-particle friction. This contrasts with the effect of liquid viscosity seen by other workers at high strain rates, and suggests that the strength ranking of formulations with viscous binders may be strain-rate dependent.