Deformation is the change of an object shape, brought about by temperature or force. This change normally occurs when heat or pressure is applied on the object. While this term is commonly used for thermoplastics and metals, the truth is, a number of other materials can undergo through these changes. They include, concrete, bones, and stones. Truly, the deformation phenomenon has played an essential role in the shaping our world, since its rock structure was formed over many years under very high pressure. According to how heat/pressure is applied, the material type, and its own structure, deformation can be of different types. Let’s focus on the plastic type of deformation.
In engineering and material science, plastic deformation is defined as a process that causes a permanent change in the size and shape of an object. This is actually the reverse of elastic deformation, in which the object returns to its initial size and shape, after the force causing the distortion is withdrawn.
When a pressure is applied on an object, it can result in an alteration in its size and shape. But, if pressure is below a particular limit, taught during our Physics tuition class, called ‘elastic limit’, the object regains its original dimensions after withdrawal of the force. This type of temporary change is referred to as elastic deformation. However, when the pressure applied surpasses the elastic limit, the object undergoes permanent shape and size change, known as plastic deformation.
In case the pressure remains applied on the object even after plastic deformation occurs, then its cross sectional area decreases at a specific point known as ‘necking’ and it then breaks, also known as ‘fracturing’. The plastic deformation is mainly a property of ductile materials like, steel, silver, and gold.
This is actually the most probable and important form of plastic deformation occurring in crystalline solids. In these solids, the atoms are closely packed into symmetrical structures, known as crystals. Each and every crystal is made up of atoms arranged in distinct planes or layers.
Slip deformation takes place when a crystal’s atomic layers ‘slip’ over one another when it’s subjected to tension. Each atom in a crystal plane shifts by a number of inter-atomic spaces. This is comparable to the movement of a cards’ deck, when pushed from one side. Slipping takes place when the applied unaligned forces exceed a certain limit.
The movement of a crystal’s individual planes causes the formation of ‘steps ‘, in which a plane projects from the crystal. Nevertheless, distinct planes still possess same orientation to one another, since all atoms get displaced by exactly the same distance. Furthermore, the ‘slip’ between distinct layers takes place on ‘slip planes’, which are where most atoms are concerted. This slip happens because atoms adjust position to fill in ‘dislocations’.