Abstract
Pipelines and piping components may be exposed to extreme loading conditions, for instance earthquakes and hurricanes. In such conditions, they undergo severe plastic strains, which may locally reach the fracture limits due to either monotonic loading or ultra-low cycle fatigue (ULCF). Aiming to investigate the failure process and strain evolution of pipes enduring ULCF, a lab-scale ULCF test on an X65 steel pipeline component is simulated with finite element models, and experimental data are used to validate various material modeling assumptions. The paper focuses on plastic material modeling and compares different models for plastic anisotropy in combination with various hardening models, including isotropic, linear kinematic and combined hardening models. Both isotropic and anisotropic assumptions for plastic yielding are considered. As pipes pose difficulty for the measurement of plastic properties in mechanical testing, we calibrate an anisotropic yield locus using advanced multi-scale simulation based on texture measurements. Moreover, the importance of the anisotropy gradient across thickness is studied in detail for this thick-walled pipeline steel. It is found that the usage of a combined hardening model is essential to accurately predict the number of the cycles until failure, as well as the strain evolution during the fatigue test. The advanced hardening modeling featuring kinematic hardening has a substantially higher impact on result accuracy compared to the yield locus assumption for the studied ULCF test. Cyclic tension-compression testing is conducted to calibrate the kinematic hardening models. Additionally, plastic anisotropy and its gradient across the thickness play a notable, yet secondary role. Based on this research, it is advised to focus on improvements in strain hardening characteristics in future developments of pipeline steel with enhanced earthquake resistance.
Highlights
The finite element (FE) method has been widely used in the design and implementation of metal forming to predict the distribution of stress and strain in the formed part
The axial strain at this so-called reference position has been used to detect the onset of buckling in both experiment and simulation
Compared to isotropic and linear kinematic hardening assumptions, the prediction of buckling is delayed with two bending cycles, resulting in eight or 8.5 cycles in total for the ultra-low cycle fatigue (ULCF) test setup under consideration, whereas eight cycles are experimentally found
Summary
The finite element (FE) method has been widely used in the design and implementation of metal forming to predict the distribution of stress and strain in the formed part. Metals 2017, 7, 140 the constitutive plasticity modelling significantly influences the attainable accuracy in FE simulations. Isotropic hardening is often assumed in numerical simulations of sheet metal forming processes. It has the advantage of simplicity, but only approximates real material behavior, as for instance, it is unable to present the Bauschinger effect, which is encountered in many metals. The Bauschinger effect is characterized by a reduced yield stress upon load reversal after initial plastic loading. To enable modeling of the Bauschinger effect, the linear kinematic hardening rule has been proposed [1], which translates the yield locus by the kinematic stress tensor or back stress tensor with a single constant hardening modulus. Combined hardening models have been introduced, which integrate isotropic and kinematic hardening, e.g., as illustrated in
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