In this work two approaches to the description of short fatigue crack growth rate under large-scale yielding condition were comprehensively tested: (i) plastic component of the J-integral and (ii) Polák model of crack propagation. The ability to predict residual fatigue life of bodies with short initial cracks was studied for stainless steels Sanicro 25 and 304L. Despite their coarse microstructure and very different cyclic stress–strain response, the employed continuum mechanics models were found to give satisfactory results. Finite element modeling was used to determine the J-integrals and to simulate the evolution of crack front shapes, which corresponded to the real cracks observed on the fracture surfaces of the specimens. Residual fatigue lives estimated by these models were in good agreement with the number of cycles to failure of individual test specimens strained at various total strain amplitudes. Moreover, the crack growth rates of both investigated materials fell onto the same curve that was previously obtained for other steels with different properties. Such a “master curve” was achieved using the plastic part of J-integral and it has the potential of being an advantageous tool to model the fatigue crack propagation under large-scale yielding regime without a need of any additional experimental data.

The contribution describes the fatigue lifetime predictions of polyethylene notched specimens based on the theory of critical distance. The approach uses the line method, which averages the axial stress over the critical distance. The critical distance is determined from experimental data of Cracked Round Bar (CRB) specimens and specimens with a model notch. From these two sets of experimental fatigue data and corresponding axial stress distributions, the critical distance is determined. The critical distance depends on the number of cycles to failure and notch radius. For this reason, the critical distance is modified by the ratio of stress concentration factors and the modified distance is used for fatigue lifetime predictions of notched specimens with various notch radii. Using this approach significant savings in testing time can be achieved. CRB fatigue tests are commonly used tests for ranking PE pipe grades and are easily available. Adding the fatigue tests of notched specimens with a model notch, the critical parameter can be found, and fatigue lifetime predictions of various notches can be calculated.

While several studies have experimentally investigated the fatigue behavior of polymeric metamaterials, systematic methodologies for predicting their fatigue lifetime are still lacking. Therefore, this paper proposes and experimentally validates a comprehensive methodology for fatigue lifetime prediction of polymeric mechanical metamaterials. The approach is demonstrated on a metamaterial with tunable stiffness manufactured by additive manufacturing from polyamide 12 (PA12). The base PA12 material was thoroughly characterized by mechanical tests, including tension, compression, dynamic excitation, and fatigue loading. The resulting data were used to develop a material model for finite element simulations, enabling stress analysis and identification of fatigue-critical locations within the metamaterial structure. The numerical model was validated experimentally under compressive loading. The influence of printing repeatability was also investigated, demonstrating that relatively small printer-dependent geometric deviations can significantly affect structural stiffness, dominant failure locations, and fatigue lifetime. Subsequently, fatigue lifetime was estimated at the identified critical locations using four different criteria. The results show that the simplest criterion based on the first principal stress provides the most accurate fatigue lifetime prediction, which was confirmed by experimental fatigue testing of the metamaterial structure.