I know that the toughness is defined as the mean value of energies consumed per unit volume up to failure, i.e., the area under the stress–strain curve up to the
ultimate failure and possibly the ductility is defined with respect to the initial slope of softening branch of stress-strain curve(of course you can improve my definitions). But I want to know what is their difference in concept?
Hi,
Toughness is the amount of energy that can be absorbed by a material before it breaks. Ductility is the amount of deformation that can occur (until it breaks) when a material is put under stress. The two are different because the rate of strain (deformation/unit of time) can determine the type of failure - ductile (deformation) or brittle (cracking). At low strain rates ductile failure (sometimes called rupture) occurs more often. Ductility is relevant in situations where strain rate is low - bending, forging, drawing, rolling, and other manufacturing processes. Temperature is also relevant because the activation energy required to facilitate ductile flow follows an Arrhenius equation. Toughness is relevant in high strain-rate situations - catastrophic failures (because there is no shape change prior to failure-no ductility), impact, ballistic shells, smashing, explosions. The material fails in a brittle mode (it is incorrect to sat that the material is brittle-depends on strain rate) because there is no time for ductile flow to occur. The goal of impact-resistant materials is to absorb as much energy as possible - this could be by crack formation (surface energy), phase changes (transformation toughening), therefore energy is the appropriate measure.
Toughness is energy of crack growth for brittle material, ductility is energy of plastic deformation for a ductile material
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