Strain-based design (SBD) is used to complement conventional allowable stress design for pipelines operated in environments with potentially large ground movements such as those found in permafrost and seismically active regions. Reliable and accurate prediction of tensile strain capacity (TSC) plays a critical role in strain-based design. As reported previously over the past 6+ years, a comprehensive experimental and numerical program was undertaken to characterize the TSC of welded pipelines, develop a finite element analysis (FEA) approach and equations capable of predicting TSC, and establish a strain-based engineering critical assessment (SBECA) methodology. The previous FEA model and TSC equations were validated against about 50 full-scale pipe strain capacity tests and are accurate within the validated variable ranges. In the current paper, enhancements of the previous model and equations are described. The enhancements include incorporation of advanced damage mechanics modeling into TSC prediction, development of a new TSC equation, expansion of variable ranges and functionality upgrades. The new model and equation are applicable over larger ranges of material properties and flaw sizes. The new FEA model is also used to establish surface flaw interaction rules for SBD. The new FEA model is validated against more than 40 full-scale non-pressurized and pressurized tests and underpins the development of the new TSC equation. The equation is validated against a total of 93 full-scale tests (FST). In addition to the enhancements, sample applications of the TSC model and equation are presented in the paper, for example, an investigation of the effects on strain capacity of Lüders strain and ductile tearing. Challenges in predicting TSC are also addressed.
a b s t r a c tPins are commonly used to join members of mechanical mechanisms. In order to maintain the integrity of the joint and prevent failure, there must be sufficient material of adequate strength around the pin hole to sustain the bearing and tear out loads from the pin connection. In this work, a multi-scale materials simulation model based on finite elements (FE) is developed for design and evaluation of materials for such applications. We specifically examine several constitutive models for simulating the elasto-plastic behavior of the plate material while maintaining computational efficiency. Here, models are developed for two plate materials: copper (Cu) and α-uranium (α-U), with vastly different plastic behaviors owing to their crystal structures and crystallographic textures. For Cu, digital image correlation (DIC) tests are carried out during loading of the plate/pin assembly to characterize the strain distributions in the critical hole/pin area. The corresponding FE simulations are carried out using a combination of constitutive laws involving a fine-scale polycrystal plasticity calculation, a J2 flow theory, or a combination of both. We show that the FE model using the fine-scale polycrystal plasticity constitutive law successfully captures the DIC strain fields in the hole region at different plate displacements. Surprisingly, use of the more computationally efficient J2 plasticity model also produces reasonable results in comparison with the measurements and the fine-scale constitutive law. An interesting finding is that combining fine-scale constitutive laws in the region surrounding the hole and continuum J2 theory elsewhere gives the worst agreement. It also precariously produces non-conservative estimates for the hole opening with applied displacement. These results on Cu helped subsequent simulations on α-U, where use of the fine-scale polycrystal simulation is fundamental considering the highly plastic anisotropic response of this complicated material. We demonstrate that in the α-U plates, the localized deformation in the hole region is highly dependent on the direction of displacement.
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