The effect of grain size on fracturing
mediaposted on 16.06.2021, 10:26 by Emma FinchEmma Finch
These experiments investigate the effect of grain size and relative strength on fracture development during compression using a Discrete Element Model.
The media contains ~24,000 elements with radii ranging between 0.25 and 0.5 of a unit. Grains are coloured according to their breaking strain which is between 0.025 and 0.05 of their initial separation (see figure 3, Finch, Hardy, Gawthorpe, 2004 and figure 2, Pichel et al. 2017, doi below). The breaking strain is the distance beyond which a bond is broken between a pair of elements. Grain sizes are defined at the start of the experiment where an element is chosen at random. Its neighbours within a defined distance (R) are then assigned a similar breaking strain with minor variation (<0.01%) to provide slight strength variability within grains. In these movies, R is 0.5-2.5 unit in 0.5 unit intervals. Elements coloured white have a breaking strain of 0.025 unit and for those coloured blue it is 0.05 unit.
The development of fractures is shown by colouring an element according to the number of broken bonds it has with its initial neighbours from white (0) to brown (8). As grain size increases it has greater control on the initiation of fractures where the majority of the larger grains maintain their shape at the expense of weaker ones which fail more readily. It is possible to observe this and stronger grains being rotated within the media in the models with larger initial grain sizes (for examples see the bottom right corner of bond_grain_1.5.gif and bond_grain_2.0.gif).
Movies that compare between the R = 0.5,1.5 and 2.5 units for both bonds and grain sizes are shown in the final figures.