Editorial for the Special Issue “Fatigue and Fracture of Crystalline Metal Structures”

1. Introduction

Fatigue and fracture continue to represent some of the most important degradation and failure mechanisms affecting engineering structures and components. It has been estimated that fatigue-related failures account for a significant proportion of in-service structural failures and generate substantial economic losses worldwide due to maintenance costs, production interruptions, and unexpected breakdowns. The challenge is particularly relevant for crystalline metallic materials, which remain indispensable in aerospace, railway, automotive, energy, civil engineering, and manufacturing applications owing to their excellent combination of mechanical properties, processability, and durability.

The fatigue and fracture behaviour of crystalline metallic materials are governed by a complex interplay among loading conditions, stress concentrations, manufacturing processes, defects, environmental effects, and microstructural characteristics. Damage initiation and propagation are strongly influenced by the underlying crystal structure, grain morphology, phase distribution, inclusions, residual stresses, and local plasticity mechanisms. As engineering systems are increasingly required to operate under more demanding loading conditions while simultaneously meeting stringent weight, safety, and sustainability requirements, the development of reliable methodologies for fatigue-life prediction and structural integrity assessment remains a major scientific and technological challenge.

Recent advances in experimental characterisation techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and focused ion beam (FIB) analyses, have significantly improved our understanding of fatigue damage mechanisms at multiple length scales. In parallel, substantial progress has been achieved in computational modelling through crystal plasticity, finite element methods, molecular dynamics simulations, phase-field modelling, machine learning approaches, and probabilistic reliability assessment. These developments have enabled researchers to establish stronger links between microstructural features and macroscopic fatigue and fracture responses, leading to more physically based predictive frameworks.

The Special Issue Fatigue and Fracture of Crystalline Metal Structures, co-edited by Dr. Xiangnan Pan and Prof. Abílio M.P. de Jesus, was conceived as a forum for the dissemination of recent advances in the understanding, modelling, characterisation, and prediction of fatigue and fracture phenomena in crystalline metallic materials and structures. The scope of the Special Issue includes experimental studies, theoretical developments, numerical simulations, multiscale modelling approaches, reliability assessment methodologies, additive manufacturing applications, microstructure characterisation, and engineering case studies. Particular attention is devoted to the relationships between microstructure evolution, crack initiation, crack propagation, and structural integrity under monotonic, cyclic, impact, and vibration loading conditions.

The Special Issue attracted high-quality contributions addressing experimental, numerical, theoretical, and application-oriented investigations of fatigue and fracture phenomena in metallic materials and structures. Particular attention was devoted to studies involving fatigue crack growth, microstructure characterisation, additive manufacturing, reliability assessment, multiaxial fatigue, fracture mechanics, and advanced materials design.

The Special Issue comprises eleven papers, including ten research papers and one review paper, published between 2024 and 2026. Collectively, these contributions illustrate recent advances in fatigue crack growth modelling, microstructure-sensitive fatigue behaviour, very-high-cycle fatigue, additive manufacturing, structural reliability, and material optimisation. Taken together, the published papers provide a representative overview of current research trends and emerging challenges in the field of fatigue and fracture of crystalline metallic materials and structures. The following sections summarise the main scientific contributions and discuss their implications for future developments in fatigue and fracture research.

2. Overview and Analysis of the Contributions

2.1. Fatigue Crack Growth Modelling and Fracture Mechanics

A dominant theme within this Special Issue is the numerical analysis and prediction of fatigue crack growth in metallic materials. Fageehi and Alshoaibi [1] conducted a comprehensive parametric investigation of fatigue crack propagation under positive and negative stress ratios using SMART crack growth technology implemented within the ANSYS environment. Their work demonstrated the significant influence of stress ratio and hole positioning on crack trajectories and fatigue life predictions.

In a subsequent contribution, the same authors [2] examined fatigue crack growth under compressive loading conditions and at negative stress ratios. Their results highlighted the importance of crack-closure phenomena and reversed plasticity effects for accurately representing crack-tip mechanics under complex cyclic loading.

The influence of material parameters on fatigue crack propagation was investigated through a comparative numerical study involving Inconel 718, Ti-6Al-4V, Al7075-T6, and ASTM A514 steel [3]. The results showed substantial differences in fatigue crack growth resistance among these alloys and provided useful guidelines for material selection in fatigue-sensitive applications.

A fourth contribution by the same research team focused on curvilinear fatigue crack growth in the Haynes 230 superalloy under mixed-mode loading conditions [4]. Using experimentally validated finite element simulations, the authors assessed crack path evolution and plastic zone development while discussing the limits of applicability of linear elastic fracture mechanics.

Together, these four papers demonstrate the progressive evolution of computational fracture mechanics toward increasingly realistic crack growth predictions and highlight the importance of integrating advanced numerical methods with experimental observations.

2.2. Microstructure-Controlled Fatigue Behaviour in Railway Components

Two papers focused on railway applications, in which fatigue performance directly affects operational safety and reliability. Gao et al. [5] investigated the microstructure and mechanical properties of high-speed train wheels, comparing the wheel rim and web regions. Their work revealed significant differences arising from manufacturing and heat-treatment processes, demonstrating how localised microstructural modifications influence strength, ductility, and fatigue resistance.

The same group subsequently studied fatigue crack propagation in induction-hardened S38C axle steel with a gradient microstructure [6]. Their results demonstrated that the hardened surface layer exhibited superior crack growth resistance, while microstructural transitions and residual stress effects significantly influenced crack propagation behaviour.

These studies emphasise the fundamental role of microstructure optimisation in improving fatigue resistance and extending service life in transportation systems.

2.3. Additive Manufacturing and Very-High-Cycle Fatigue

Additive manufacturing is increasingly recognised as an enabling technology for advanced metallic structures. However, fatigue behaviour remains one of the principal barriers to broader industrial deployment.

Liu et al. [7] studied the rotating bending fatigue behaviour of AlSi10Mg manufactured by powder bed fusion laser beam technology using different layer thicknesses. Although both manufacturing conditions produced similar microstructures, significant differences in defect populations resulted in notable variations in fatigue strength, particularly in the very-high-cycle fatigue regime.

The review article by Gao et al. [8] addressed very-high-cycle fatigue behaviour in laser powder bed-fused Ti-6Al-4V alloys. The authors comprehensively summarised the effects of porosity, metallurgical defects, microstructural features, and post-processing treatments on fatigue performance, and proposed strategies to enhance fatigue resistance in additively manufactured components.

Taken together, these papers highlight the continued importance of defect control and microstructural optimisation for achieving reliable fatigue performance in additively manufactured metals.

2.4. Fatigue Life Prediction, Reliability Assessment, and Materials Design

The remaining papers address broader aspects of fatigue life assessment, reliability modelling, and materials optimisation.

Tao et al. [9] proposed a novel equivalent strain energy damage model for multiaxial fatigue lifetime estimation under variable-amplitude loading. Their formulation integrated critical-plane concepts with energy-based approaches and showed favourable agreement with experimental fatigue data.

Wang et al. [10] developed a reliability assessment methodology for turbine blade–disc dovetail joints used in aero-engines. By combining probabilistic fatigue-life modelling, Kriging surrogate models, and Copula-based reliability methods, the authors demonstrated how failure correlations influence structural reliability predictions.

The influence of grain refinement on mechanical performance was studied by Wang et al. [11], who examined the effects of Al-Ti-B-Er additions on an ultrahigh-strength aluminium alloy. Their work showed that appropriate grain refinement simultaneously improves strength and ductility through microstructural modification.

Collectively, these studies highlight the growing integration of materials science, fatigue modelling, and reliability engineering in the pursuit of safer and more durable metallic structures.

3. Research Trends and Scientific Challenges

Several important trends emerge from the papers published in this Special Issue. First, numerical simulation continues to play an increasingly important role in fatigue and fracture research. Four contributions [1,2,3,4] employed advanced finite element crack growth methodologies capable of addressing complex geometries, mixed-mode loading, and realistic stress ratio effects.

Second, microstructure-sensitive approaches remain fundamentally important. Studies on railway wheels and axles [5,6], additively manufactured materials [7,8], and grain-refined aluminium alloys [11] all demonstrate the critical influence of microstructural features on fatigue behaviour.

Third, the fatigue performance of additively manufactured metals remains strongly controlled by defects and processing-induced heterogeneities [7,8]. Understanding and mitigating these limitations will continue to be a major research priority.

Finally, there is growing interest in reliability-based methodologies and advanced fatigue life prediction approaches [9,10]. As engineering systems become increasingly complex, uncertainty quantification and probabilistic design methods will become even more important.

Despite these advances, significant scientific challenges remain. Future work should focus on establishing stronger quantitative relationships between microstructure, defect populations, loading conditions, and structural lifetime, ideally through integrated multiscale and data-driven frameworks. In addition, the continued development of advanced computational methodologies to model fatigue crack initiation and propagation across multiple length scales remains an important research priority. Among these emerging approaches, phase-field fatigue modelling has attracted considerable attention for its ability to simulate complex crack evolution, including nucleation, propagation, branching, and coalescence, without requiring predefined crack paths. Recent studies combining phase-field modelling and experimental investigations have demonstrated promising capabilities for analysing fatigue damage in additively manufactured metallic materials, opening new opportunities for physically based fatigue life prediction and structural integrity assessment [12]. The integration of such advanced numerical frameworks with microstructural characterisation techniques, machine learning algorithms, and reliability-based methodologies is expected to play an increasingly important role in the development of next-generation predictive tools for fatigue and fracture analysis.

4. Conclusions

The eleven papers published in this Special Issue provide a comprehensive overview of current developments in fatigue and fracture of crystalline metallic materials and structures. The contributions encompass fatigue crack growth modelling, microstructure-sensitive behaviour, additive manufacturing, very-high-cycle fatigue, multiaxial fatigue assessment, reliability engineering, and advanced materials design. Together, they illustrate the multidisciplinary nature of contemporary fatigue and fracture research and demonstrate how experimental investigations, numerical simulations, microstructural characterisation, and probabilistic methodologies can be combined to improve structural integrity assessment.

As Guest Editor, and on behalf of the Guest Editors, I would like to express my sincere appreciation to all authors for their valuable contributions and to all reviewers for their rigorous evaluations and insightful comments, which have significantly enhanced the quality of the published papers. I also gratefully acknowledge the editorial team of Crystals for their professional assistance and continuous support throughout the development and publication of this Special Issue. We hope that the papers collected in this Special Issue will serve as a valuable reference for researchers and engineers, stimulate further scientific advances in fatigue and fracture of crystalline metallic materials, and encourage new research collaborations within this important field.