Chapter 8: The Tension Test

The Tension Test begins by establishing the engineering stress-strain curve to define fundamental properties such as the modulus of elasticity, yield strength determined via the offset method, ultimate tensile strength, and ductility measures including percentage elongation and reduction of area. The text differentiates between resilience, the capacity to absorb energy elastically, and toughness, the total energy absorbed prior to fracture. A critical distinction is made between engineering values and true stress-true strain relationships, introducing the flow curve and the power-law equation characterized by the strain-hardening exponent and strength coefficient. The discussion advances to the mechanics of instability, mathematically defining the onset of necking where the rate of strain hardening balances geometric softening, and applies the Bridgman correction to account for triaxial stresses generated within the necked region. The summary further explores how external variables such as strain rate and temperature impact flow stress, detailing concepts like strain-rate sensitivity, superplasticity, and the Zener-Hollomon parameter within constitutive equations. It also examines thermally activated deformation mechanisms involving dislocation barriers, the influence of testing machine stiffness (hard versus soft machines) on observed properties, and stress-relaxation behaviors. Concluding sections address the notch tensile test for assessing sensitivity to stress concentrations and review the effects of metallurgical factors, including carbon content, microstructural variations in steel (such as pearlite, martensite, and spheroidite), and anisotropy caused by crystallographic texture or mechanical fibering.