Chapter 6: Mechanical Properties of Metals

Poisson's ratio describes the coupling between axial and lateral strains, while anelasticity explains the time-dependent recovery of elastic deformation observed particularly in polymers. The stress-strain curve serves as a comprehensive map of material behavior, revealing the linear elastic region, the yield point where permanent deformation initiates (typically identified using the 0.002 strain offset method), and the plastic region where strain hardening occurs. Key design parameters including tensile strength, ductility (measured as percent elongation or reduction in area), resilience (elastic energy absorption capacity), and toughness (total energy absorption before fracture) characterize how metals respond to loading. The distinction between ductile and brittle materials emerges from their stress-strain signatures, with true stress and true strain providing more accurate descriptions of deformation beyond the onset of necking. Strain-hardening behavior is quantified through the hardening exponent and strength coefficient, reflecting how materials strengthen during plastic deformation. The chapter expands beyond uniaxial tension to examine compressive, shear, and torsional loading modes, noting both similarities and distinctions in deformation response. Hardness measurement techniques including Rockwell, Brinell, Vickers, and Knoop tests assess resistance to localized plastic deformation, with established correlations linking hardness to tensile strength particularly useful for steels. Engineering practice requires accounting for property variability arising from testing conditions, material inhomogeneities, and processing differences through statistical analysis and conservative safety factors. Understanding these mechanical properties enables engineers to predict material performance, establish working stress limits, and design structures that operate safely within material capabilities.