Deformation in Materials: Understanding its Importance and Types

Deformation in materials refers to the physical changes in their shape or size caused by the application of an external force. This concept is fundamental to understanding the mechanical behavior of materials, as it determines how they respond to applied loads and affects their strength, durability, and applicability in different contexts. There are two main types of deformation in materials: elastic deformation and plastic deformation, each with distinct characteristics and behaviors.

Definition of deformation

Deformation is the change in the shape or size of a material due to the application of an external force. This force can be tension (stretching), compression (squeezing), shear (sliding) or by thermal expansion. Deformation can be temporary, where the material returns to its original shape once the force is removed (elastic), or permanent, where the material retains the new shape even after removing the force (plastic).

Elastic deformation

Elastic deformation is when a material temporarily deforms in response to an applied force but can return to its original shape once the force is removed. That is, the material exhibits a recovery capacity and does not undergo permanent deformation.

This can be observed in elastic bands. You stretch them, but when you stop applying force, the band returns to its original length. An important concept in objects is the 'elastic limit.' The elastic limit is a property of an object that indicates the maximum level of elastic deformation a material can experience just before breaking or deforming permanently. In our previous example, if we continuously apply tension to the elastic band, there will come a point when it can no longer deform and will proceed to break. All objects are elastic to some degree. Some more than others; therefore, all objects have an elastic limit. An elastic band is much more elastic than a layer of epoxy paint, of course; however, the layer of epoxy paint, though imperceptible to the human eye, also deforms elastically at the microscopic level. The difference is that this deformation is extremely smaller than that experienced by the elastic band. So small that our vision cannot detect it.

Plastic deformation

Plastic deformation occurs when a material undergoes permanent deformation after applying a force, even if this force is relatively small. In this case, the material cannot return to its original shape and retains the deformation even after removing the force.

This type of deformation occurs when objects are subjected to a stress that exceeds their elastic limit. When the object exceeds its elastic limit, it has two options: it either breaks or deforms permanently, also called plastically. Objects that break are considered brittle; those that deform are considered tough. Supermarket plastic bags are somewhat tough materials; we can stretch them and see that they have a degree of elasticity, as for small stresses the material returns to its original shape, but if you increase the force, you can cause the bag to remain stretched permanently: that is plastic deformation. Of course, if we keep stretching it further, it will eventually break. Materials that exhibit a high degree of plastic deformation are excellent for providing safety and protection to the user, as they give a warning before breaking. If one sees a material that has been plastically deformed, one can deduce that its structure has been compromised and that it should be used with caution or replaced.

Deformations from a molecular perspective

At the molecular level, a molecular structure being elastically deformed does not change its shape but stretches or compresses as a whole. Molecules are composed of chemical and physical bonds. All bonds have a degree of flexibility; meaning they can rotate around a certain axis. So, if we have two molecules joined by a chemical bond that can rotate, let's say, 30 degrees, when stretched, the molecule will rotate on its axis, and the system of two molecules will elongate. If we have an object composed of millions of these molecules, and each set of molecules is stretching, the body as a whole will stretch or deform elastically. This deformation is elastic, and therefore, the two molecules should return to their original position when the tension ceases. This can be understood by imagining the molecule rotating the 30 degrees in the same way a spring stretches when we apply force, but the only reason this happens is because a force is being applied. Both the spring and the elastic molecule want to return to their original position. When we stretch a spring, we feel a force pulling back to its original position. The same happens with elastic molecules.

In contrast to elastic deformation, in plastic deformation, the molecular structure changes shape. In our example of the two molecules that could rotate 30 degrees relative to each other, if we reach their elastic limit, the stretched molecule will have reached the 30 degrees and will not be able to stretch further. What happens if we keep stretching it? It breaks, or the molecule bonds stably to another molecular structure. When the latter happens, it is said that the set has deformed plastically. Now, when we stop applying force, the molecule will not return to the other molecule but will stay with the new one. Plastic deformation typically occurs with materials whose structure is bonded by physical bonds such as hydrogen bonds or Van der Waals forces because these bonds facilitate bonding and unbonding without a large energy investment. Normally, a chemical bond requires certain conditions to form.