Ultimate strength refers to the maximum stress a material can withstand before failure, regardless of whether the failure occurs due to fracture or significant plastic deformation. It represents the peak stress value on a stress-strain diagram and is a key property in material selection and mechanical design.
When discussing ultimate strength, the most common reference is ultimate tensile strength (UTS), which applies specifically to materials subjected to tensile loading. UTS is the maximum stress a material can endure before breaking when stretched and is expressed as:
UTS = maximum force / original cross-sectional area
Since UTS is based on engineering stress, it does not account for changes in cross-sectional area due to necking, making it different from the true stress at failure. In ductile materials, such as metals, the UTS occurs after yielding but before fracture, as plastic deformation continues until the material can no longer support the applied load. In brittle materials, such as ceramics, the UTS and fracture stress are often nearly the same, as they fail with little to no plastic deformation.
Ultimate strength can also refer to ultimate compressive strength (for materials under compression) or ultimate shear strength (for materials under shear loading), though UTS is the most commonly cited in material property charts. In engineering design, safety factors ensure that materials operate well below their UTS to prevent failure under expected loading conditions.