Watch the video clips of the bubble raft undergoing compressive, tensile and shear deformation. It may help you to watch each clip several times.
At small strains, the arrangement of the bubbles does not change. This is elastic deformation of the raft. The bubbles change shape and move slightly apart in an effort to maintain the lowest energy close-packed configuration.
At larger stains, plastic deformation occurs. The bubble raft rearranges by dislocation motion. Notice how dislocation motion occurs along three directions in the raft. These are the close-packed directions, along which the distance between bubble centres is smallest.
In the different loading conditions, dislocations tend to move mainly along different sets of directions. In each case, the direction along which dislocations generally move is that with the highest resolved shear stress . Dislocations may nucleate near a different type of crystalline defect, such as a grain boundary, solute atom or vacancy.