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Sulzer-type engines are widely used in ships given their excellent fuel-saving, low-noise, and low-vibration properties. Here, we evaluated the low-cycle fatigue (LCF) properties of SNCM 420H steel after heat-tempering. LCF was studied at both $400^{\circ }\mathrm{\text{C}}$ and $800^{\circ }\mathrm{\text{C}}$, at strains of 0.8, 0.9, 1.2 and 1.5%. We used a triangular waveform for LCF testing to find fracture. The fatigue life of SNCM 420H at $800^{\circ }\mathrm{\text{C}}$ was about 60% less than that at $400^{\circ }\mathrm{\text{C}}$.

Fatigue analysis in metallic frame structures can be challenging due to associated computational costs; if localized plasticity is involved, then the approach of three-dimensional (3D) continuum plasticity models for direct computation of stresses will be infeasible for the analysis of cyclic loading that would need to be modeled in medium- to high-cycle fatigue and vibratory fatigue applications. This difficulty is particularly accentuated in architected structures, for which high-resolution 3D finite element analysis (FEA) would be prohibitively expensive. In this work, we propose an alternative approach based on the use of novel elasto-plastic frame model with continuous flow (i.e. no sharp yield function) for modeling 3D frame and lattice structures. Rather than splitting the strains (as is done in classical plasticity) we split the deformation measures, extension, curvature and twist, into elastic and plastic components and postulate a rate type evolution rule for the plastic variables in terms of the stress resultants (axial force, bending moment, and torque). The combination of structural models together with the use of elasto-plastic operator split to solve the resulting boundary value problem allows for much faster determination of localized plasticity than continuum models can provide. The use of a continuous transition from elastic to rate-independent plasticity (as opposed to an abrupt change with classical plasticity models) allows us to capture localized microplasticity and determine resulting fatigue progression using a cycle-count-free, plastic work-based approach, formulated in terms of the curvatures and resultants. We demonstrate that (a) the model is able able to reproduce the response of 3D FEA with very few elements and (b) the model has the ability to rapidly predict the fatigue life under variable amplitude combined loading with relatively few frame elements.

The experimental campaign carried out at the European Laboratory for Structural Assessment of the Joint Research Centre at Ispra in a two-storey steel frame is presented herein. The experimental activity is part of the Environmental and Climate SteelQuake Project, for the assessment of the low-cycle fatigue behaviour of steel beam-to-column welded connections of beams noncomposite with the slab in the location of the plastic hinges. The description of the mock-up, the testing apparatus and the relative instrumentation for measuring displacements, forces and deformations is presented. Pseudodynamic and cyclic tests were performed and the corresponding results are presented at the structure and connection levels in terms of the absolute and accumulated plastic ductilities in relation to the failure events of the welded connections. The importance of the panel zone in contributing towards the total rotation of the connection is highlighted.

In comparison with the state-of-the-art Ni-based superalloys, refractory multiprincipal element alloys (MPEAs) exhibit considerably higher strengths at temperatures above 1600${}^{\circ }$C, which can be a significant potential required in the high demand for aerospace applications. However, the atomic-scale work-hardening behavior of such important materials during low-cycle loading remain unknown. Here, using molecular dynamics simulations, we study the low-cycle fatigue of nanocrystalline refractory multiprincipal element alloy with different grain sizes, to reveal the cyclic deformation, work hardening and damage mechanism. As a result, an extensive grain growth is observed during the cyclic deformation, thus driving the dynamic Hall–Petch strengthening mechanism. For the model with large grain size, the glide of partial dislocations with screw structure can be responsible for the deformation behavior under cyclic loading, and at small grain size the grain growth-coordinated deformation twinning can control the plastic process. The deformation twin boundaries generated during the cyclic loading show high stability, while the remaining high-angle grain boundaries are highly unstable. The initial softening followed by hardening depends upon the dislocation density and grain size. In particular, atomic-scale element segregation occurs after cyclic loading. This study gives a cyclic deformation micromechanism, and thus accelerates the design and development of superior fatigue-resistant refractory multiprincipal element alloy over a wide temperature range.