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.
Closed form stress intensity factor (K1) expressions are presented for cracks in pipes subjected to a variety of loading conditions. The loadings considered are: 1) axial tension, 2) remotely applied bending moment, and 3) internal pressure. Expressions are presented for circumferential and axial cracks, and include both part-through and through-wall crack geometries. The closed form K1 expressions are valid for pipe radius to wall thickness ratio between 5 and 20.
A method of evaluating the J-integral for a circumferentially cracked pipe in bending is proposed. The method allows a J-resistance curve to be evaluated directly from the load-displacement record obtained in a pipe fracture experiment. This method also permits an analysis for fracture instability in a circumferential crack growth using a J-resistance curve and the tearing modulus parameter. The influence of the system compliance on fracture instability is discussed in conjunction with the latter application. The results suggest that a compliant piping system containing a crack can exhibit ductile fracture instability after some stable crack growth. The importance of using a J-resistance curve that is consistent with the type of constraint for a given application is emphasized.
An initial experimental investigation was conducted to confirm the theory of tearing instability developed in previous work. A simple testing program was selected which employed 3-point bend specimens with various crack size to specimen depth ratios and an additional spring bar to easily adjust effective specimen span (or loading system compliance).
All stable-unstable behaviors observed in the tests are in good agreement with those predicted by the theory. Thus the present study demonstrates the appropriateness of the tearing instability analysis, presenting guidelines for its further development.
The possibility of a pipe fracture emanating from a stress corrosion crack in the heat-affected zones of girth-welds in Type 304 stainless steel pipes was investigated. The J-resistance curve—tearing modulus parameter for the prediction of crack initiation, stable growth and fracture instability—was employed. To evaluate the analysis, a pipe fracture experiment was performed using a spring-loaded four-point bending system that simulated an 8.8-m (29-ft) long section of unsupported 102-mm- (4-in-) dia pipe. An initial through-wall crack of length equal to 104 mm (4.1 in.) was used. Fracture instability was predicted to occur between 15.2 and 22.1 mm (0.60 and 0.87 in.) of stable crack growth at each tip. In the actual experiment, the onset of fracture instability occurred beyond maximum load at an average stable crack growth of 11.7 to 19 mm (0.463 to 0.750 in.) at each tip.
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