The equation describing Robinson’s life fraction hypothesis has been derived from a generalized equation based on chemical reaction-rate theory. It is shown what further assumptions are needed in order to make this hypothesis valid. For design considerations the following equation is suggested  
j=1mtjtajk
where tj = time spent at Tj, σj; taj = time needed to rupture, etc., at Tj, σj; k = empirical parameter, smaller than 1. The selection of a numerical value for k is based on an analysis of how conventional creep data comply with various assumptions made in the derivation of the equation; T = temperature; and σ = stress.
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