The Dorn maximum load criterion and the Swift criterion for plastic tensile instability of a flat sheet stretched by in-plane forces are discussed. Both hypotheses are applicable when an initial nonuniformity of a sheet material is considered but that one is valid which is satisfied first. Experimental results in biaxial stretching of aluminum sheet specimens showed general agreement with both criteria; however, the Dorn maximum load criterion more accurately described the condition for necking when both principal strain rates on the plane of the sheet were positive. The significance and the calculation of the strain hardening parameter n = (ε/σ)(dσ/dε) are discussed, and the effect of prior deformation of the material on instability is analyzed.

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