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Strain is the fractional deformation of an object under stress, expressed as the change in dimension divided by the initial dimension. Unlike stress, which is the applied force per unit area, strain is dimensionless and describes the object's response to that force. Strain quantifies how much an object actually deforms when stress is applied.

The four main types of strain are tensile strain (length increase), compressive strain (length decrease), bulk strain (volume change from all-direction forces), and shear strain (deformation along a plane when force is parallel to the surface). Each type corresponds to different stress applications and describes distinct deformation patterns in materials.

Elastic modulus is the proportionality constant between stress and strain for small deformations, calculated as the ratio of stress to strain. Materials with large elastic moduli resist deformation, while those with small moduli deform easily under the same stress. For example, steel has a much larger elastic modulus than rubber, so rubber deforms more under identical stress.

Young's modulus is the elastic modulus for tensile or compressive stress, measuring a material's resistance to length changes. It is calculated as the ratio of tensile stress to tensile strain and applies when forces pull or push along the object's length. Young's modulus is commonly used to characterize how stiff materials are under tension or compression.

Bulk modulus measures resistance to volume change under pressure from all directions, calculated as volumetric stress divided by volumetric strain. Shear modulus measures resistance to shape change when force is parallel to a surface, calculated as shear stress divided by shear strain. Both are elastic moduli but apply to different types of deformation.

Elastic modulus is measured in pascals because strain is dimensionless. Since elastic modulus equals stress divided by strain, and stress is measured in pascals, the unit of elastic modulus is also pascals. This allows direct comparison of elastic moduli across different materials and stress types.

For small deformations, stress and strain are directly proportional, with elastic modulus as the constant of proportionality. However, for larger deformations, this linear relationship breaks down. The proportionality only holds when stress is sufficiently low; beyond that threshold, the relationship becomes nonlinear and problem solving on stress and strain requires more complex analysis.