ANS:C - stresses beyond the elastic limit.

Permanent set in a material is produced by subjecting it to stresses beyond the elastic limit. Explanation: Permanent set, also known as plastic deformation, occurs when a material is subjected to stresses beyond its elastic limit. Elastic deformation refers to the reversible deformation of a material under stress, where the material returns to its original shape after the stress is removed. However, if the applied stress exceeds the material's elastic limit, it undergoes plastic deformation, resulting in a permanent change in shape or size. Here's how each option relates to permanent set:

1. Triaxial loading: Triaxial loading involves applying stresses in three dimensions simultaneously. While triaxial loading can induce plastic deformation, it is not necessarily required to produce permanent set. Plastic deformation can occur under uniaxial (one-dimensional) loading as well, as long as the stress exceeds the material's elastic limit.
2. Fatigue loading: Fatigue loading involves cyclic loading and unloading of a material, typically at stresses below the material's ultimate strength. While fatigue loading can lead to failure over time due to cumulative damage, it primarily affects the material's fatigue life rather than causing permanent set.
3. Stresses beyond the elastic limit: This is the correct option. Permanent set occurs when the applied stresses exceed the material's elastic limit, causing it to deform plastically and undergo a permanent change in shape or size.
4. Tensile & compressive loads simultaneously: While subjecting a material to both tensile and compressive loads simultaneously can induce complex stress states, permanent set is primarily associated with stresses beyond the material's elastic limit, regardless of whether they are tensile, compressive, or a combination of both.