Welcome back!
Today is all about the Physics of Destruction. I'm going to discuss different types of deformation as a result of different types of forces and stress vs. strain.
In material sciences and engineering, Deformation is defined as the change in an object's shape, size or structure as a result of applied forces or changes in temperature. For now, I'm only going to discuss forces. There are different types of applied forces that can cause deformation:
Tensile Forces - These types of forces are pulling forces, or tension forces. These are forces that pull in opposite directions of the material and are intended to stretch or lengthen the object. For example, consider a rubber band. If you wrap one finger on both ends of the rubber band and pull in opposite directions with your fingers, you would see the rubber band start to stretch! That would be a Tensile Force!
Compressive Forces - These types of forces are pushing forces. These are forces that push inward towards the material and are intended to squeeze or shorten the object. For example, consider an empty soda can. Sometime people like to show off their strength by putting their hands on both ends of the can and pushing inward to crush the can. This would be a Compressive Force!
Shear Force - This is a type of unaligned force that pushes one part of a material in one direction and another part of that material in the opposite direction. For example, consider a deck of cards. When you push the top of a deck of cards in one direction, and the bottom of the deck in the opposite direction, you will see the deck move in two opposite directions. Shear forces can cause cracking or tearing of the material.
And here are other concepts related to deformation that result from forces, but they aren't forces themselves:
Bending (Flexure) - The behavior of a structural material that is subjected to some force perpendicular to its longitudinal axis. (see picture below)
Torsion - Also known as twisting, as a result of torque. (see picture below)
Now, I also mentioned how there were different types of deformation. The three different kinds of deformation include:
Elastic Deformations - These are reversible deformations, meaning that as soon as the applied force of this material is removed, the material will return to its original shape. For example, consider a couch pillow. When you push your foot down on the pillow a fair amount, and then lift it up after some time, most pillows will go back to their original form.
Plastic Deformations - These are irreversible deformations, meaning that these materials will not resume their original shape after an applied force is removed. This is the 2nd stage of deformation. Before an object can undergo Plastic Deformation, it MUST first undergo Elastic Deformation, even if only by a small amount! Consider a piece of metal. If you apply a decent amount of force to the object, you will notice that it won't return to its original shape. It'll be reshaped and restructured.
Fractures - These are also irreversible deformations, except these deformations result in the material breaking and being split into two or more different pieces as a result of excessive stress. This is the 3rd and final phase of Deformation, where the material first has to undergo Elastic Deformation and then Plastic Deformation, THEN fracturing. Consider breaking a stick for a bonfire. When you bend a big, rigid branch far enough, it will usually snap into two different pieces! That would be a Fracture!
And finally, I wanted to clarify the different between stress and strain.
Stress = the applied force per unit area of a material. This is a vector, since it has a direction!
σ = F / A where σ = stress, F = force applied, and A= cross sectional area of the object.
Strain = the deformation of a material or object due to excessive stress beyond that object's maximal stress load. Too much stress causes strain! Strain is what causes the different types of deformation, as seen in the Stress-Strain chart below.
Stress-Strain Model
Received from https://en.wikipedia.org/wiki/Deformation_(engineering)
Bending structural beam
Retrieved from https://en.wikipedia.org/wiki/Bending
Torsion
Retrieved from https://en.wikipedia.org/wiki/Torsion_(mechanics)