Fatigue failure plays an important role in engineering applications, especially when structural components experience significant cyclic thermal loading and complex force loading simultaneously. During the last decades, several post-processing techniques have been developed based on empirical investigations of experimental evidence to predict the fatigue life of materials. The work at hand postulates a conventional continuum damage theory for thermomechanical fatigue failure modeling. In particular, an implicit gradient-enhanced approach is employed to address the ill-posedness of the partial differential equation system when the damage onsets. An internal fatigue variable is phenomenologically defined based on the accumulation of viscoplasticity. In the sequel, a regularized fatigue variable is obtained to further yield the damage softening function, which straightforwardly applies to the stress, material tangent, and viscoplastic dissipation. A multi-field problem, consisting of the strain field, the temperature, and the non-local variable, is taken into consideration, leading to a fully coupled system. This numerical methodology is consistently derived and implemented into the context of the finite element method. Several representative and demonstrative examples are performed, which yield good numerical stability and agreement with experimental data. Conclusive findings and further perspectives close this article.
The modeling of concrete has always been of great interest. In this work, a yield criterion with three surfaces is proposed to capture the plastic yielding behavior under different loading conditions, including uniaxial tension/compression, biaxial tension/compression and tension-compression. Material failure, characterized by the degradation and finally the complete loss of material integrity, can be modeled by the continuum damage approach. Within finite element methods, the nonlocal enhancement of integral-type or gradient-type is often required for the well-posedness of the partial differential equation system. In this work, a localizing gradient damage model has been adopted to obtain mesh-insensitive material responses, while ruling out the unphysical broadening of the damage zone often observed in constant length scale gradient damage models. Following a consistent derivation, the plasticity-damage coupled model has been implemented into an in-house finite element framework. Several representative and demonstrative examples serve to illustrate the capability of the proposed description in concrete modeling, followed by the conclusion, where some final insights are provided.
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