What does Poisson's ratio describe?

Poisson's ratio is a fundamental concept in materials science and engineering that quantifies how materials deform in response to applied loads. Specifically, it describes the ratio of the change in lateral dimensions (the width or diameter) to the change in longitudinal dimensions (the length) when a material is subjected to uniaxial stress, like stretching or compressing.

When a material is stretched, it tends to elongate in the direction of the applied force and simultaneously contract in the directions perpendicular to that force. Poisson's ratio provides the numerical relationship between these two strain measures, expressed mathematically as the negative ratio of lateral strain to longitudinal strain. This value is often denoted by the symbol ν (nu).

Understanding Poisson's ratio is crucial for predicting how materials will behave under various loading conditions, which impacts design and analysis in civil and structural engineering. For example, materials with a high Poisson's ratio tend to exhibit significant lateral contraction when stretched, whereas materials with a low ratio may have less lateral change.

Thus, the correct answer reflects a key aspect of how materials respond to stress, providing insight into their mechanical behavior and informing appropriate engineering choices.