SUMMARY
A mechanistic model of impact attrition has been developed in the previous IFPRI programme (Ghadiri and Zhang, 1992). The model was tested on ionic crystals that were known to fail under semi-brittle mode, and showed that the theoretical predictions agreed reasonably well with the experimental results. In the current programme the work is extended to glassy polymers since they represent a category of materials that is completely different from ionic crystals. Polymethylmethacrylate (PMMA) was selected as a model material because it is the most common of the glassy polymers, it is readily available, and has a wide variety of applications. The PMMA particles used in the experiments were cubic extrudates, provided by ICI, in the size range 2.36-2.80 mm.
The primary objectives of the work are as follows:
1. To identify the failure mode of PMMA particles under impact loading, by carrying out single particle impact testing, high speed photography of the process of impact, and scanning electron microscopy (SEM) of the impact damage.
2. To determine the velocities at which transitions occur from plastic deformation to chipping and from chipping to fragmentation.
3. To assess the impact attrition propensity of polymethylmethacrylate by evaluating gravimetrically the mass loss per impact as a function of impact velocity.
4. To analyse the data by comparison with the predictions of the model of impact attrition developed previously.
Single particle impact testing and high speed photography of the impact event show that PMMA fails in the semi-brittle mode under the prevailing high strain rates. However, PMMA shows a significant amount of ductility under quasi-static conditions as reflected in the relatively large size of the plastic zone when compared with the dimensions of the specimen.
The results of single particle impact tests reveal two distinct transition velocities. The first marks the onset of chipping and is about 25 m/s, while the second marks the onset of fragmentation and lies around 89 m/s. The above transition velocities apply only to the particle size used, i.e. 2.36-2.80 mm. However, the threshold velocities for other particle sizes have been estimated, based on the minimum load required for initiating various types of crack. For example, when the particle size is around 1 mm, the impact velocities for the onset of chipping and fragmentation are estimated to be approximately IS6 m/s and 556 t/s, respectively. These velocities are more relevant to comminution than attrition processes.
The SEM photographs reveal the occurence of extensive plastic deformation beneath the impact site. The morphology of the impact site indicates that chipping is responsible for material loss.
The results of the impact attrition experiments show that attrition becomes appreciable only at very high velocities. In the range of impact velocities of interest to attrition, i.e. up to about 30 r&s, the attrition rate is very small. Within the chipping regime, the attrition rate is proportional to the impact velocity to the power of 234. If the fragmentation regime is taken into account, the power of velocity can reach up to 5.69. The attrition behaviour of PMMA does not follow the trends predicted by the model of impact attrition developed previously. The exact reasons are not clear at present, but hit is considered that the lateral crack propagation in PMMA may not follow closely the model of lateral crack extension used previously. Formation of subsurface lateral cracks in PMMA requires further detailed investigations. In conclusion, PMMA appears to be an attrition-resistant material at least up to moderately high velocities.