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A general instability condition of solid mechanics, valid for a nonlinear structural component in an otherwise elastic system, is shown to be inherent in the T-modulus concept of instability. This condition involves the slope of the descending part of the load-displacement curve of the nonlinear component. It is shown that this slope can be used to evaluate T appl at instability; Tappl can then be used to give a simple estimation of Tmat without the need for exact crack length determination. In this paper such evaluations for large panel specimens and smaller bend and CT specimens are reported for three structural steels. The obtained data are in good agreement with conventionally determined data. However T-evaluations for tension-type specimens give values which are higher by a factor of two than those calculated for bendtype specimens. This indicates that T is not geometry independent, but that T from bend and CT specimens gives a lower bound. A method is discussed of analyzing the instability of a large structure with a cracked, ductile component using the slope of the load-displacement curve and the general instability criteria of mechanics without recourse to fracture parameters.

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