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Stress–strain curve for brittle materials compared to ductile materials. Some common characteristics among the stress–strain curves can be distinguished with various groups of materials and, on this basis, to divide materials into two broad categories; namely, the ductile materials and the brittle materials. [1]: 51
English: Stress-strain curves for brittle and ductile materials. Brittle materials fracture at low strains and absorb little energy. Conversely, ductile materials fail after significant plastic strain (deformation) and absorb more energy. Note that in this idealized example, the yield and ultimate tensile stresses are the same for both ...
The work-hardened steel bar has a large enough number of dislocations that the strain field interaction prevents all plastic deformation. Subsequent deformation requires a stress that varies linearly with the strain observed, the slope of the graph of stress vs. strain is the modulus of elasticity, as usual.
The Considère construction for prediction of the onset of necking, expressed as the gradient of the (true) stress-strain curve falling to the true stress, for a material conforming to the Ludwik-Hollomon relationship, with the parameter values shown. The condition can also be expressed in terms of the nominal strain:
Ductile deformation is typically characterized by diffuse deformation (i.e. lacking a discrete fault plane) and on a stress-strain plot is accompanied by steady state sliding at failure, compared to the sharp stress drop observed in experiments during brittle failure.
Toughness is related to the area under the stress–strain curve. In order to be tough, a material must be both strong and ductile. For example, brittle materials (like ceramics) that are strong but with limited ductility are not tough; conversely, very ductile materials with low strengths are also not tough. To be tough, a material should ...
The reversal point is the maximum stress on the engineering stress–strain curve, and the engineering stress coordinate of this point is the ultimate tensile strength, given by point 1. Ultimate tensile strength is not used in the design of ductile static members because design practices dictate the use of the yield stress. It is, however ...
The actual (true) strain in the neck at the point of fracture bears no direct relation to the raw number obtained from the nominal stress-strain curve; the true strain in the neck is often considerably higher. Also, the true stress at the point of fracture is usually higher than the apparent value according to the plot.