This paper presents a new approach to the subject of crack instability based on the J-integral R-curve approach to characterizing a material's resistance to fracture. The results are presented in the chronological order of their development (including Appendices I and II). First, a new nondimensional material parameter, T, the “tearing modulus,” is defined. For fully plastic (nonhardening) conditions, instability relationships are developed for various configurations, including some common test piece configurations, the surface flaw, and microflaws. Appendix I generalizes these results for the fully plastic case and Appendix II treats confined yielding cases. The results are presented for plane-strain crack-tip and slip field conditions, but may be modified for plane-stress slip fields in most cases by merely adjusting constants. Moreover, an accounted-for compliance of loading system is included in the analysis. Finally, Appendix III is a compilation of tearing modulus, T, properties of materials from the literature for convenience in comparing the other results with experience.

The effectiveness of stationary metal-to-metal seals is investigated with respect to contact pressure and length, load history, and the use of sealing compounds. Laboratory cup and cone tests were carried out, and experimental results were used to develop a sealability criterion. A sealability parameter is proposed and a critical value for it was obtained from experiments. The criterion was validated through full-scale tests of tubular connections and FEA. This criterion can be used for the comparison, qualification and future performance improvements of different metal-to-metal seals, in particular those used in the petroleum industry.

This paper presents methods for evaluating the J-integral and the tearing moduli from a single test record. Several different aspects of the problem have been combined in this work. An overview of the conditions for separability of the load into multiplicative functions of displacement and crack length as well as for the existence of η-factors is presented. The consequences of expressing J by a Merkle-Corten type formula are explored in terms of the crack increment, da, the tearing modulus, T, and J itself, including the case of growing cracks. A simple method is suggested to obtain the correct J for crack growth and T from nothing more than the test record itself; the procedure is applied to available experimental data and the results are compared with those obtained by other formulae. Additional physical interpretation is given on the Tmat-versus-Tapp stability criterion and the remaining compliance capacity CCR is defined.

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The creep crack growth behavior of a type 316 stainless steel was characterized at 594°C (II00°F) using precracked single edge notch specimens loaded in displacement rate control. The steady-state crack growth rate, da/dt, correlated with J-integral and did not correlate with C*. The creep crack growth behavior in this material and temperature is compared with our previous creep crack growth rate data on a Cr-Mo-V steel at 538°C (1000°F) and on type 304 stainless steel at 594°C in which da/dt correlated with C*. A detailed discussion is included on why in some materials creep crack growth rate correlates with J integral and in others it correlates with C*.