A discrete element model (DEM) for predicting apple damage during handling

Apples can suffer from significant mechanical injury in the form of bruise damage. The discrete element method (DEM) allows for individual particle contacts and the dynamic behaviour of a group of particles to be studied. Successfully applying this method to apples allows for future investigations into postharvest mechanical damage to be performed on a range of fruit and vegetables. A contact model that closely replicated the visco-elastic nature of apples was used and the material properties were successfully determined using a pendulum device. Bruise damage models (in the form of bruise volume, bruise area and bruise depth) were coupled to the peak impact forces available through DEM. Bruise formations resulting from multiple impacts and variable time durations between impacts were studied. A detailed multi-sphere particle shape representation along with a realistic contact point loading scheme was implemented. Overlapping bruises were studied and accounted for on a post process level. The resulting DEM model was successfully validated and extended to include run-time bruise visualisation. Qualitatively the model accurately predicted the dynamic bulk behaviour of the apples. Quantitatively, the model succeeded in predicting the contact forces experienced by apples to within 11%. The model predicted the mean bruise damage of a single apple for realistic situations within an accuracy of 47% in terms of mean bruise volume, 35% for bruise area and 30% for bruise depth.