Critical strains and necking phenomena for different steel sheet specimens under uniaxial loading

When defining ultimate loads or failure criteria for shells, vessels and containment liners, for instance, reference will be made to critical strains of the material under consideration.In general the critical strains are derived from uniaxial tensile test results obtained with rods.The actual deformation behaviour of sheets (plate and shell type structures), however, differs considerably from the uniaxial rod behaviour.From various tests – along with theoretical investigations – it was found that for membrane or even bending stressed sheets higher ultimate strains are reached.The typical strain behaviour caused by increasing load starts with uniform strain distribution over the specimen length and finally ends with localized necking.As “critical strains” the well known uniform elongation strain and the ultimate strain are defined.For sheets in addition the quasi uniform elongation strain has been introduced in this paper.Whereas for rods under tension localized necking is directly following the uniform straining, tests with sheets show a so-called diffuse necking behaviour before localized necking starts.As a consequence the deformation capacity of steel sheets turns out to be significantly higher than relying on the data from standard tension tests with rods.

► Critical strains for steel sheets under uniaxial loading are introduced and confirmed by experiments: uniform elongation strain: diffuse necking starts; quasi uniform elongation strain: localized necking starts; ultimate strain: fracture occurs.
► For tensile sheets the deformation capacity is found to be considerably larger than for tensile rods.