Mechanical Behaviors and Damage Mechanisms of Metallic Materials

1. Introduction and Scope

Metallic materials continue to be widely used across both traditional and advanced engineering sectors, including civil, marine, automotive, aeronautics, and aerospace, to name just a few. Given their broad range of potential applications, metallic materials must endure various combinations of mechanical and environmental conditions while in service. These may include static or dynamic loading (such as fatigue or impacts), fretting, and wear, which can occur at room, elevated, or cold temperatures and are often present in harsh environments.

When assessing the structural integrity and safety of materials used in critical load-bearing components, it is essential to first understand their mechanical properties and deformation, fracture, and damage mechanisms. Each combination of environmental and loading conditions triggers a specific mechanical response—such as elastic or plastic deformation, creep, fatigue, ratcheting, wear, or fretting—and may increase the susceptibility to particular damage mechanisms, which in some cases can lead to catastrophic failure.

The mechanical response and damage mechanisms depend on the specific type of material or alloy and its microstructure, which in turn depends on the material fabrication conditions [1]. Fabrication processes and heat treatments play a crucial role in producing microstructures with the desired mechanical properties and deformation patterns (e.g., high strength, ductile vs. brittle fracturing, creep resistance) [2]. These can be achieved, for example, by directly controlling the grain size and/or the type, distribution, and size of the precipitates and phases. Innovative manufacturing techniques—such as additive manufacturing—can also be employed to fabricate components with complex geometries. At the microstructural scale, these often exhibit unique features resulting from the high energy density during the fabrication process [3].

An in-depth understanding of the different types of mechanical behaviors and damage mechanisms of metals and alloys is, therefore, of paramount importance to achieve flawless and safe engineering design. This must extend beyond an understanding of mechanical behavior at room temperature under simple loading conditions, considering more demanding service environments, such as those involving high temperatures and harsh chemical or mechanical conditions [4].

Based on these premises, this Special Issue presents an up-to-date overview of experimental findings on the mechanical behaviors, deformation, and damage mechanisms of metallic materials under various simple and complex loading and environmental conditions. Various ferrous and non-ferrous alloys, produced using both conventional and additive manufacturing techniques, are investigated. Approaches that combine experimental outcomes with those obtained using advanced numerical techniques are also presented.

2. Contributions

Nine articles are published in this Special Issue, covering various topics related to the mechanical behavior, damage, and failure mechanisms of metals and alloys. Comprehensive analyses were performed through experimental testing, often supported by results from numerical and/or theoretical models. In most of the articles, the materials’ mechanical behavior is evaluated based on the results of hardness measurements or uniaxial tensile, constant amplitude fatigue, or fretting fatigue tests. One article also includes results from environment-assisted tests, such as those considering the hydrogen permeation and embrittlement susceptibility. The Digital Image Correlation (DIC) technique [5] was successfully employed in several studies to measure the full-field displacements and strains during testing. Microstructures were analyzed using optical and transmission electron microscopy (TEM), while fracture surfaces were investigated using scanning electron microscopy (SEM), often supported by Energy-Dispersive X-Ray Spectroscopy (EDS) for chemical characterization.

Regarding the numerical analyses, the finite element model—calibrated using experimental test data—was employed to evaluate the displacement, strain, and stress fields, which often complemented the outcomes of the experimental tests.

Most of the published articles focus on ferrous and non-ferrous alloys—such as aluminum, titanium, magnesium, and nickel–chromium—produced using conventional fabrication processes [6]. One contribution (Contribution 1) instead considers an aluminum alloy manufactured using advanced techniques, namely Laser-Based Powder Bed Fusion (L-PBF) additive manufacturing. Specifically, it investigates the effect of two distinct heat treatments—T5 and a novel rapid T6 treatment—on the rotating bending fatigue strength of hourglass-shaped cylindrical specimens made from an L-PBF AlSi10Mg alloy, which was tested at both room temperature and an elevated temperature (200 °C). The fatigue strength was estimated using the staircase method and correlated with experimental findings from residual stress measurements, as well as microstructural and fractographic analyses. The experimental findings indicated that the T5 heat treatment led to a 50% reduction in the tensile residual stresses characteristic of the as-built material, without significantly altering its ultrafine cellular structure. In contrast, the rapid T6 treatment induced microstructural changes, resulting in a composite-like structure with spheroidal Si particles embedded in an α-Al matrix, and introduced compressive residual stresses. Specimens subjected to the same heat treatment exhibited similar fatigue failure mechanisms at both room temperature and 200 °C, with no evidence of creep phenomena. In the T5 alloy, the eutectic Si network was less resistant to fatigue crack initiation than the rapid T6 alloy’s composite-like microstructure. Overall, these distinct microstructural characteristics help to explain the observed fatigue strength differences, with the rapid T6-treated alloy demonstrating better fatigue performance than the T5-treated one at both room and elevated temperatures.

Examples of the successful use of the non-contact DIC technique to measure the full-field deformations and strains are provided by Chmelko et al. (Contribution 2), Khameneh et al. (Contribution 3), Fang et al. (Contribution 4), and Jonsson and Kajberg (Contribution 5).

Chmelko et al. (Contribution 2) investigated the trends of experimentally measured Poisson’s ratios of steel and aluminum alloys undergoing both elastic and plastic deformation. Both the engineering and true stress–strain curves were also determined. Using DIC, the longitudinal and transverse strains were measured in tensile test specimens made from different materials: a cast AlSi10Mg alloy and various steel grades, including low-carbon steels (S355, X52), a high-carbon steel (C55), a high-strength steel (42CrMo4), and a high-strength additively manufactured steel (MS1). Poisson’s ratio remained stable up to approximately 75% of the yield stress. The observed values agreed with common ones in the literature for steel (0.27–0.3) and aluminum alloys (0.34–0.36). Beyond the yield stress, during the transition from elastic to plastic deformation, Poisson’s ratio began to increase significantly. This increase became partially saturated—i.e., the rate of change was significantly reduced—at total strains above 5% and fully saturated at strains exceeding 8%. A Poisson’s ratio of ν = 0.5 (condition of constant volume) was only reached by the 42CrMo4 and C55 steels, whereas for the other steels the ratio did not exceed 0.46, or 0.44 for the AlSi10Mg alloy. The significant influence of the changes in Poisson’s ratio on the structural integrity analyses was finally demonstrated through a finite element simulation of a pressure vessel cover subjected to radial pressure. The numerical results suggest the need to incorporate refined material constitutive models that account for the evolution of Poisson’s ratio during the transition from elastic to plastic deformation.

Another example of the DIC technique in use is presented by Khameneh et al. (Contribution 3), who conducted a numerical and experimental investigation of the fracture behavior of dual-phase advanced high-strength steel (DP1180) under combined simple shear and uniaxial tension. This mixed-mode loading condition is not well represented by the conventional fracture tests (i.e., those examining the simple shear, uniaxial and biaxial tension, and plane strain) used in automotive sheet metal characterization. Combined simple shear and uniaxial tension was achieved by modifying the existing shear geometries to produce approximately linear strain paths suitable for DIC. Two planar specimen geometries, which avoided through-thickness machining and were compatible with standard test frames, were evaluated experimentally and numerically. A finite element method was used to assess the notch eccentricity’s influence on the stress states and fracture locations. The most promising geometry for each of these outcomes was selected for experimental testing on DP1180 steel. The performance of each geometry was evaluated based on the strain linearity, stress state, and fracture initiation location. The best-performing geometry was then used to recalibrate the modified Mohr–Coulomb fracture model using data from combined shear and tension tests. The recalibrated fracture model accurately predicted a valley in the fracture strain between pure shear and uniaxial tension, confirming its suitability for use with complex loading paths. Overall, this research bridges a critical gap in fracture characterization by introducing practical test geometries and validating a robust numerical model, thereby enhancing the understanding of fracture mechanisms in high-strength steels and providing a more accurate framework for predicting failure under mixed-mode loading. This is an essential advancement for automotive applications, where components often experience complex stress states.

Another study demonstrating the effective use of DIC to assess the structural integrity of welded joints in U71Mn railway rails subjected to fatigue loading is presented by Fang et al. (Contribution 4). In this study, Compact Tension specimens were extracted from three distinct locations—the head, waist, and bottom—of both the base material and the weld zone. During the fatigue tests, DIC was employed to measure full-field displacements near the crack tip in real time. A method was proposed that exploits DIC-measured data to precisely determine the crack tip’s horizontal and vertical positions, as well as accurately evaluating the plastic zone in front of the crack tip and the variation in the crack tip opening displacement (CTOD) during loading and unloading. Measuring the CTOD values under overloaded and unloaded conditions allowed the size of the elastic and plastic CTOD area to be assessed. Finally, a method was proposed to calculate the range of the effective Stress Intensity Factor for fatigue crack propagation, considering the crack tip plasticity-induced crack closure effect based on Elber’s model. Overall, this study offers a refined methodology for assessing the fatigue life of welded rails, with implications for improving the safety and durability of high-speed railway infrastructure. The integration of DIC and advanced crack closure modeling enhances the understanding of fatigue mechanisms in welded rail steels, supporting the development of more reliable maintenance and design strategies.

In their article, Jonsson and Kajberg (Contribution 5) combine DIC with numerical techniques. Precisely, they propose a novel approach to evaluating automotive components’ crashworthiness using a combination of high-speed imaging, a 3D Digital Image Correlation (3D-DIC) technique, and finite element analysis. The proposed approach enabled the full-field continuous strain monitoring of component deformation throughout a crash test, allowing crack initiation and propagation to be detected and correlated with the load and intrusion history in this experiment. The approach was applied to assess the energy absorption and deformation behavior of crash boxes made from two high-strength steels—a third-generation TRIP-aided bainitic steel and a boron steel—during dynamic axial compression tests [7]. The crash performance was evaluated using a crash index and the decrease in it as the key metrics. Unlike traditional methods requiring multiple specimens, this approach enabled reliable evaluation using only one or a few components. A finite element model, incorporating a rate-dependent simplified Johnson–Cook constitutive model and a phenomenological damage model based on a modified Mohr–Coulomb one with a von Mises yield function—both calibrated using experimental test data—showed good agreement with the experimental results in terms of the deformation and energy absorption, though the crack formation was overestimated. The study concludes that the proposed method is effective for crashworthiness assessment and that incorporating fracture toughness as an intrinsic material property could improve the crack prediction accuracy in future models. This integrated experimental–numerical framework offers a more efficient and insightful way to evaluate advanced high-strength steels for automotive safety applications.

Experimental studies on non-ferrous alloys are presented by Tang et al. (Contribution 6), Mao et al. (Contribution 7), and Liu et al. (Contribution 8). In their study, Tang et al. (Contribution 6) investigated the fretting fatigue behavior of an Inconel 690 alloy used in steam generator heat transfer tubes. These are critical components in nuclear power plants and are subjected to vibration-induced contact with support plates and anti-vibration bars. Conventional fatigue analysis methods often simplify these contact interactions, potentially underestimating the resulting damage. Fretting fatigue experiments were conducted using a purpose-built test device. These included high-cycle fretting fatigue tests on tube specimens in air at room temperature and low-cycle fretting fatigue tests on sheet specimens in a high-temperature water environment. The fatigue test results were compared with the mean and design S–N curves provided by the ASME code, Sections III and VIII [8]. The Smith–Watson–Topper (SWT) model was employed as a fatigue life prediction tool. The results demonstrated that fretting significantly reduced the fatigue strength, particularly under ultra-high-cycle conditions, where the standard ASME design fatigue curve failed to accurately represent the material’s performance. This highlights the necessity of downwardly modifying fatigue curves to maintain safe design margins. By integrating experimental testing with simulation analysis, the study offers a more precise evaluation of the fatigue life in nuclear components, underscoring the critical importance of accounting for fretting effects in structural integrity assessments.

Two contributions focus more specifically on the mechanical properties and deformation behavior of non-ferrous alloys. Mao et al. (Contribution 7) studied the biaxial deformation behavior of AZ31 magnesium alloy hot-rolled sheets, using cruciform specimens subjected to biaxial tension with various stress ratios between the rolling and diagonal directions (45° between the normal and transverse directions). The use of different stress ratios enabled a detailed analysis of the roles of twinning, dislocation slips, and texture evolution, where the primary slip modes in magnesium alloys included basal, prismatic, and pyramidal slips. The microstructure and texture of both undeformed and deformed specimens were analyzed using the Electron Backscatter Diffraction (EBSD) technique. The experimental results showed that twinning contributed minimally to plastic deformation and conformed to Schmid’s law. The activation of twinning produced a twin texture component, with the c-axes of the hexagonal close-packed crystal structures predominantly at 45° between the normal and transverse directions. Under a stress ratio of 4:1 (rolling direction to diagonal direction), deformation was primarily accommodated by prismatic and basal slips, with basal slips becoming increasingly dominant at lower stress ratios. This study’s findings advance the understanding of deformation mechanisms in magnesium alloys under complex loading conditions, which is essential for enhancing their formability and structural performance in lightweight engineering applications.

Additionally, Liu et al. (Contribution 8) analyzed the mechanical properties of a Ti-5111 near-α titanium alloy with three different microstructures—equiaxed, bimodal, and lamellar—obtained using distinct heat treatments, which controlled the number of α and β phases. Its tensile properties were evaluated, and the microstructure evolution at room temperature was analyzed using optical and transmission electron microscopy (TEM). The fracture surfaces were finally analyzed using scanning electron microscopy (SEM). The Ti-5111 alloy with an equiaxed microstructure demonstrated moderate strength and plasticity, whereas that with a lamellar microstructure exhibited poor mechanical performance. The moderate strength of the equiaxed microstructure was attributed to the presence of a dispersed, short, needle-like α phase, which contributed to its mechanical strength but hindered plastic deformation. This dual effect resulted in mechanical properties that were less than optimal. In contrast, the inferior mechanical properties of the lamellar microstructure were explained by the thicker lamellar α and β phases, the presence of the α phase at grain boundaries, and the parallel arrangement of the α_s phase, which was a short, needle-like secondary α phase. Compared to the previously mentioned microstructures, the bimodal one had superior strength and plasticity. Its high mechanical strength was attributed to a small grain size and many α_s/β phase boundaries; the deformation of these two phases also explained the remarkable plasticity observed in the bimodal microstructure. The three microstructures also exhibited distinct fracture mechanisms: ductile fractures for the bimodal microstructure, cleavage fractures for the lamellar one, and mixed ductile and cleavage fractures for the equiaxed microstructure.

Lima dos Santos et al. (Contribution 9) present research that also considers the environmental effects on material behavior. Precisely, they investigate the hydrogen embrittlement behavior of an API 5L X80 steel used in the oil and gas industry, focusing on the role of the centerline segregation region (CSR), a typical feature of continuously hot-rolled thick plates. The CSR exhibits increased hardness due to the chemical segregation of carbides and alloying elements, as well as high concentrations of sulfur and phosphorus—all factors that contribute to its detrimental effect on steel’s resistance to hydrogen embrittlement. The study considered two steel plates with different thicknesses. Microstructural characterization was performed using optical microscopy, while SEM and Energy-Dispersive X-Ray Spectroscopy (EDS) were employed to examine the CSR and analyze the samples’ fracture surfaces to assess their susceptibility to hydrogen embrittlement. Mechanical characterization was conducted through Vickers microhardness testing. The hydrogen permeation and embrittlement susceptibility were also investigated using slow-strain-rate tensile tests (SSRT). Increased hardness was observed in the CSR of the thicker plate, where the CSR also played an important role in determining the hydrogen embrittlement resistance due to its high-hardness microconstituents and higher percentages of carbon, manganese, sulfur, and phosphorus. The presence of hydrogen significantly decreased the cohesive strength at grain boundaries, thus promoting crack initiation at sites where localized brittle particles were present. Moreover, hydrogen redistribution under applied stress further influenced the fracture process according to the hydrogen-enhanced localized plasticity model. Hydrogen reduced the elastic interactions between dislocations, thus lowering the critical shear stress required for dislocation movement. Localized fractures occurred in specific susceptible areas where the critical hydrogen concentration was attained.

3. Conclusions and Outlook

The contributions summarized in the previous section offer a comprehensive overview of recent advancements in research on metals and alloys’ mechanical behavior and failure mechanisms. By combining theoretical, numerical, and experimental approaches, these studies provide valuable insights into material performance under complex loading and environmental conditions. The integration of DIC with numerical simulations proved particularly effective in enhancing crack analysis and crashworthiness evaluations. Investigations into non-ferrous alloys and microstructural effects further deepened the understanding of deformation and fracture mechanisms. Notably, the inclusion of environmental factors—such as hydrogen embrittlement—in these analyses underlines the importance of considering the real-world service conditions. Looking ahead, future research should focus on expanding the use of in situ monitoring techniques and leveraging data-driven methods to improve predictive modeling. Additionally, extending these investigations to non-conventional materials, such as those produced through additive manufacturing, represents a promising direction for future research. These efforts will be essential for developing safer, more durable materials and structures for critical applications in transportation, energy, infrastructure, and different environments.