Deformation Of Solids

The shape of an object is the geometrical description of the part of the space occupied by the object, as determined by its external boundaries. A change in shape due to the application of force is known as deformation. Even small forces are known to cause some deformation. Objects under the actions of external forces undergo deformation. This may be squashing, twisting, ripping, or pulling apart the object. In Physics, two terms describe the force on objects undergoing deformation: stress and strain.

Stress and Strain

The quantity that describes the magnitude of forces that cause deformation is known as stress. It is defined as force per unit. Stress is of various types as follows:

  • Tensile Stress – When forces pull an object and cause its elongation such as the stretching of a rubber band
  • Compressive Stress – When the forces result in the compression of an object
  • Bulk Stress – When an object is squeezed from all sides like the submarine in the depth of the ocean
  • Shear Stress – A type of stress where the deforming stress acts tangentially to the object’s surface

The quantity that describes the deformation undergone is known as strain. The strain is given as a fractional change in either the length, geometry or volume. It is a dimensionless number. The greater the stress, the greater the strain. The proportionality constant in this relation is known as the elastic modulus. The relation between stress and strain is given as follows:

Stress = Elastic Modulus × Strain

When an object has a large value of elastic modulus, the effect of stress is small. Likewise, a small elastic modulus means that stress produces noticeable deformation. For example, stress on a rubber band produces a larger strain as compared to a steel band of the same size. The elastic modulus of a rubber band is smaller than the elastic modulus of the steel band. Elastic moduli of different materials are measured based on physical conditions, such as varying temperature, and collected in engineering data tables for reference. These tables are valuable references for industry and for anyone involved in engineering or construction.

Definitions:

  • Stress is the measure of the force required to cause deformation.
  • A strain is the measure of the degree of deformation.
  • The ratio of stress to strain is known as elastic modulus. Elastic modulus defines the amount of force required per unit deformation.

Types of Deformation

Deformation can be of two types as follows:

  • Permanent Deformation – Also known as plastic deformation, it is irreversible. It is a type of deformation that stays even after the removal of applied forces.
  • Temporary Deformation – Also known as elastic deformation, it is reversible. It is a type of deformation that disappears after the removal of applied forces.

When a material is subjected to applied forces, the material experiences elastic deformation followed by plastic deformation. The transition from elastic to plastic state is characterized by the yield strength of the material. The plastic deformation mechanism is different for crystalline and amorphous materials. In crystalline materials, deformation is accomplished through a process known as the slip that involves the movement of dislocations. While, in amorphous materials, deformation takes place by the sliding of atoms and ions without any directionality.