Pipe collapse limits are controlled by circumferential compressive material response. In addition to yield strength and elastic modulus, elastic-plastic transition and plastic collapse performance of thick-wall pipes also depends on the character of yielding and post-yield hardening. Accurate characterization of all these properties is necessary to obtain a reliable estimate of collapse load. Common standardized material test methods provide convenient means to acquire basic mechanical properties (i.e., yield strength, elastic modulus and elongation) under laboratory conditions (i.e., room temperature and relatively rapid loading). However, these test methods include specimen preparation, such as pipe-wall straightening, and rapid strain rates that are known to impact material response, particularly in the yield transition and post-yield regimes that are important to elastic-plastic collapse. Therefore, these common laboratory techniques are useful for providing an index of material properties, but their simplified methodologies can have a significant impact on the accuracy of collapse performance estimates.
This paper describes a circumferential compressive material testing technique, developed to complement strain-based design in the energy industry, used to demonstrate differences in pipe material response measured from circumferential compressive tests and standard axial tensile tests. This technique avoids straightening the pipe wall by plastic deformation that leads to artificial rounding of the measured stress-strain yield behavior. Strain controlled loading is used to reveal yield behaviors that may be impacted by changing strain rates under stroke and load control testing.
Accurate circumferential compressive material characterization improves the identification of yield and anisotropic behaviors (tension-compression and axial-circumferential) that arise from material processing, pipe manufacturing and subsequent loading. The impact of the material response is illustrated in a numerical pipe collapse simulation that directly incorporates the measured stress-strain behavior. The impact of yield strength, stress-strain yield shape and post-yield hardening are explored. Using the measured stress-strain behavior and collapse simulation results, the sensitivity of collapse load predictions to material behavior is discussed and the requirement for accurate circumferential compressive and in-situ material characterization is demonstrated.