Fatigue crack growth in the intermediate range can be described by the Paris law of linear elastic fracture mechanics. However, if gross plastic deformation is involved, the crack growth rate is often underestimated by LEFM. In the present study the increased crack growth rate due to plastic deformation is determined by using the J-R curve. This model was found to give very good results over a wide range of high crack growth rates for three different geometries of two different steels. A way to predict instability under controlled cyclic load is discussed and is proved to work well in practice, Instability under displacement control is also discussed.

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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.

The stability of a cracked, nonlinear component connected to an elastic structure and subjected to an arbitrary number of controlled loads or displacements, is discussed. First this is done with a general instability criterion expressed in terms of loads and displacements of the cracked part. Then general expressions for the tearing modulus are given for different loading situations. Finally the given equations are applied to SEN specimens under bending and tension.

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