Mechanical Properties and Stress–Strain Response
Stiffness, yielding, strength, ductility, hardness and toughness in engineering tests.
The curve is a compressed story
An engineering stress–strain curve records elastic response, yielding, work hardening, necking and fracture under a specific loading rate, specimen geometry and environment.
Do not confuse the terms
Stiffness is resistance to elastic deformation and is related to modulus. Strength is resistance to permanent deformation or failure. Ductility is the capacity for plastic strain. Toughness is energy absorbed before fracture. Hardness measures resistance to local indentation or scratching and often correlates with strength within a material family.
Engineering and true values
Engineering stress uses the original area; true stress uses the instantaneous area. After necking, engineering stress falls even while the local true stress may continue rising.
Data needs context
Properties vary with orientation, temperature, strain rate, surface condition, specimen size and microstructure. A handbook value is not a design allowable.
Takeaway
Read mechanical data as the result of a defined experiment, not as an eternal number attached to a material name.