What does plastic strain refer to?

Plastic strain refers to the irreversible deformation of a material when it is subjected to stress beyond its elastic limit. In the context of materials science and engineering, this means that once a material has undergone plastic strain, it will not return to its original shape or size when the applied stress is removed. This behavior is critical in understanding how different materials respond to various loads and stresses, especially in structural applications.

When a material is stressed within its elastic limits, it experiences reversible deformation, returning to its original shape once the load is removed. However, once the stress exceeds this threshold, the material undergoes plastic deformation, resulting in permanent change.

The other choices relate to different concepts. Reversible deformation describes elastic behavior, which does not apply to plastic strain. Ineffective strain under low loads does not accurately capture the behavior of materials when they are subjected to forces; it simplifies a more complex interaction. Lastly, describing strain occurring without stress factors deviates from the principle that strain is a response to applied stress. Thus, the focus on irreversible deformation distinctly characterizes plastic strain and explains why it is the correct answer.