The document discusses two main mechanisms of plastic deformation: slip and twinning. Slip occurs when one part of a crystal moves over another along specific crystallographic planes and directions. Twinning results when a portion of a crystal takes on a symmetrical orientation to the rest of the crystal, dividing it into two mirrored regions. Factors like external loads, crystal structure, and orientation influence whether slip or twinning occurs. The document also covers different types of material failure like ductile fracture, brittle fracture, creep, and fatigue.