Pavel Hutař

Laser powder bed fusion (L-PBF) imprints a dense cellular dislocation architecture that fundamentally controls deformation-induced martensitic transformation in metastable austenitic stainless steels. Here, the role of this sub-grain structure is isolated in L-PBF processed 304L stainless steel by comparing as-built and partially recovered state with identical grain structure, texture and chemical microsegregation. In-situ neutron diffraction, magnetic induction and electron microscopy investigations show that dislocation cell walls drive early local stress accumulation, activate multiple slip systems and promote shear band intersections, triggering pronounced martensite nucleation. Partial recovery by annealing, suppresses these processes, delaying the transformation kinetics. Dislocation architecture thus emerges as a key microstructural design parameter for tailoring mechanical behavior in additively manufactured steels.

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.

Uniaxial creep tests in tension of as-cast nickel-based B1914 and INC738LC superalloys were carried out at testing temperatures of 800 and 950°C and at the applied stresses ranging from 100 to 700 MPa. Creep tests were followed by microstructural and fractographic investigations using scanning and transmission electron microscopy. The experimentally determined creep deformation and fracture characteristics of both superalloys were analysed by linking them to the identified creep degradation mechanisms, in terms of empirical formulas for the creep life assessment, and by the creep damage tolerance factor λ_CDF as an important outcome of the continuum damage (CDM) approach. It was found that the experimental data obey the Monkman-Grant relationship and imply a close link between the creep deformation and fracture processes. Fractographic investigation revealed that, under testing conditions, the creep fracture is a brittle mostly mixed transgranular and intergranular mode, resulting in relatively low values of the fracture strain.

The problem of crack propagation from internal defects in thermoplastic cylindrical bearing elements is addressed in this paper. The crack propagation in these elements takes place under mixed-mode conditions—i.e., all three possible loading modes (tensile opening mode I and shear opening modes II and III) of the crack are combined together. Moreover, their mutual relation changes during the rotation of the element. The dependency of the stress intensity factors on the crack length was described by general parametric equations. The model was then modified by adding a void to simulate the presence of a manufacturing defect. It was found that the influence of the void on the stress intensity factor values is quite high, but it fades with crack propagating further from the void. The effect of the friction between the crack faces was find negligible on stress intensity factor values. The results presented in this paper can be directly used for the calculation of bearing elements lifetime without complicated finite element simulations.

Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left with the crack open for the same time as that needed for application of the underload cycles, revealed no retardation effect. The results can improve our understanding of environmental effects on fatigue crack propagation and understanding the differences between the results of laboratory testing and the fatigue lives of components in service.

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.

Cast polycrystalline superalloys are widely used for critical components in aerospace and automotive industries, such as turbine blades or turbocharges. Therefore, their fatigue endurance belongs to one of the most essential mechanical characteristics. Full-scale testing of such components involves great technical difficulties and requires significant experimental effort. The present study evaluates the effects of microstructural parameters with respect to representative fatigue testing of a cast turbine blade by separately cast specimens. For that purpose, the cast polycrystalline MAR-M 247 Ni-based superalloy was investigated in the following conditions: (i) specimens extracted from a real gas turbine blade; specimens separately cast into the mould with (ii) top or (iii) bottom filling systems. Obtained diverse microstructures allowed us to assess the effect of grain size, porosity, and texture on fatigue performance. The tests were held at a symmetrical loading regime at temperature 800 °C in laboratory air. The results indicate that the level of porosity is a dominant structural parameter determining the fatigue endurance, while grain size and texture effects were of minor importance contributing mainly to fatigue life scatter.

Lifetime of plastic pipes can be estimated by integration of a power law describing the crack kinetics. However, this procedure requires an FEM (finite element method) calculation of the possible crack propagation in the pipe to obtain stress intensity factor dependency on the crack length. It is very important for the simulation to consider every possible load that is acting on the pipe. This contribution deals with FEM modelling of a pipe that is loaded by internal pressure, residual stresses and soil loads. Comparison of the factors and pipe lifetime estimation is carried out.