Papers by Keyword: Crack Initiation

Abstract: Reliable fretting fatigue prediction requires rigorous evaluation of analytical methods under realistic loading conditions. This study builds upon previous research on the fretting fatigue behavior of 42CrMo4+QT steel by incorporating new experimental data from square cross-section specimens tested under axial loading with various pad geometries. The application of a non-zero tensile mean bulk load promoted localized crack initiation near the specimen edges, leading to more asymmetric crack growth in the majority of cases unlike the more symmetric behavior observed under fully reversed loading (R = –1). Finite element analysis (FEA), along with the Dang Van and Papuga QCP methods, was employed to evaluate whether this behavior could be accurately modeled. In addition, a linear-elastic fracture mechanics approach was used to model and explain these observations. Furthermore, fretting fatigue tests on 34CrNiMo6+QT steel revealed that tribological effects governed crack initiation, in contrast to the stress-driven failure observed in 42CrMo4+QT. These findings enhance understanding of fretting fatigue mechanisms and improve predictive modeling approaches.

Abstract: A significant number of high-performance engineering structures are repeatedly subjected to both thermal and mechanical loads, often in a combined fashion. However, because of the increase in the plasticity of metallic structures when they are loaded at high temperatures, the analysis become very complex. This presents a significant obstacle for the comprehension of both the growth of cracks and the thermo-mechanical fatigue performance of the material. Thermomechanical fatigue and thermal fatigue are characterized by external and internal constraining forces, respectively. The beginning and spread of thermal fatigue cracks are controlled by a variety of factors, including the modes of heating and cooling, the temperature range, the maximum temperature rates, and the holding times. The process of a crack beginning and the rate at which it spreads are both sped up when the temperature is raised. However, because of the development of powerful statistical learning algorithms as well as rapid advancements in computational power, there has been an increased adoption of machine learning approaches as well as other advanced computational analyses and numerical software for crack damage detection and damage severity. This has led to an increase in the use of these methods.

Abstract: Characteristic features of fatigue damage of 316L austenitic stainless steel cyclically strained axially in tension-compression, reversed torsion and combined axial-torsional mode were studied. All loading modes resulted in the formation of persistent slip markings (PSMs). Predominantly one slip system was activated in the case of axial and torsional loading while at biaxial loading, activation of several slip systems was involved. PSMs acted as sites of multiple fatigue crack initiation. The path of subsequent crack growth at a macroscopic scale differed considerably in dependence on loading mode and applied amplitude. The hardening-softening curves and fatigue life curves were evaluated and results were compared and discussed in terms of the type of applied loading.

Abstract: Challenging structural applications such as customized jet engine parts are increasingly fabricated by Selective Laser Melting (SLM) of Inconel 718 powder. The as-built surface quality of SLM parts is however inferior of the machined version and the fatigue behavior is negatively affected. The as-built fatigue response of SLM Inconel 718 was quantified here using three sets of directional specimens. Since the surface quality is influenced by powder characteristics, process parameters and layer-wise fabrication, fatigue results showed a directional contribution that was interpreted using metallography and fractography.

Abstract: Effect of the plating thicknesses on tensile and fatigue properties of hot-dip galvanized steel at room temperature was evaluated. The galvanized steel with thickness of 100 μm and 200 μm were prepared. Both microstructures of η-phase and δ₁-phase were similar with each other. In the comparison with the galvanized steel with thickness of 100 μm, the microstructure of ζ-phase for the galvanized steel with thickness of 200 μm was blunt columnar structure due to long immersion time. Tensile and fatigue strengths for a galvanized steel are sensitive to the microstructure of the galvanized layer. The tensile strength and the strength of fatigue limit for the galvanized steel with thickness of 200 μm were smaller than that of 100 μm. In the galvanized steel with thickness of 200 μm, the peeling at plating layer easily occurred. The exfoliated sites have the potential to become subcracks. As the result, the main crack may propagate at early cycles.

Abstract: The prediction of a crack initiation and propagation occurring on the microstructural level of heterogeneous materials can be a very demanding problem. According to the results of recent investigations, the emerging phase field approach to fracture has a strong potential in modelling the complex crack behaviour in a simple manner. In this study, recently developed phase field staggered solution scheme with the residual norm stopping criterion has been employed for the fracture analysis of heterogeneous microstructure exhibiting complex crack phenomena. The microstructural geometries based on the metallographic images of the nodular cast iron and the material properties of an academic brittle material have been used in numerical simulations where the graphite nodules have been considered as porosities. Two commonly used energy decomposition models, the spectral decomposition and the spherical-deviatoric split, and their effects on the results of the phase field modelling are investigated. Numerical results show that the proposed algorithm recovers the complicated crack path driven by the complex microstructural topology.

Abstract: Bearing fails due to the flaking failure which is caused by the subsurface cracks. The observation of the subsurface cracks is not easy beacause the cracks propagate inside the material. In order to observe the whole subsurface cracks, we performed rolling contact fatigue (RCF) tests of carburized SCM415 until over 10⁷ cycles with the single-ball RCF machine. After the RCF tests, we directly observed the subsurface cracks.

Abstract: The initiation and propagation behaviors of small fatigue crack in TC4 were investigated in the present work. Surface replication on the basis of a two-part silicon mixture and confocal laser scanning microscope were used to record and observe the small crack initiation and growth processes at room temperature in air. Results showed that surface cracks initiated from the interfaces between α and β phases. When the crack lengths were below ~200 μm, the crack growth rates exhibited large oscillations and temporary retardations due to the presence of α/β interfaces. The corner crack propagated much faster and might have shielding effect on the surface crack.

Abstract: Tension-tension fatigue tests were conducted using ultrafine-grained commercially pure Titanium (Ti) plates fabricated by multi-directional forging (MDFing). The MDFed pure Ti plates with the thickness of 1 mm were developed aiming at dental implant application. The fatigue properties of MDFed pure Ti plates were superior to those of the conventional rolled pure Ti plates. The higher fatigue strengths in MDFed plates could be attributed to the much finer grains evolved by MDFing. Fatigue crack initiated from specimen surface, when number of cycles to failure was shorter than 10⁶ cycles. In the high cycle fatigue (HCF) region, however, subsurface crack initiation with typical fish-eye feature was recognized in the MDFed pure Ti plate in spite of the thin thickness. Fractographic analyses revealed that no inclusion existed at the center of fish-eye. The subsurface crack initiation mechanism could be related to the inhomogeneity of microstructure with some coarse grains in the inner part of the plate.

Abstract: The alkali-activated slag is an alternative building material to ordinary Portland cement based materials. This type of material is effective in reducing CO₂ emissions and energy consumption. Addition of graphite powder increases its electric conductivity, hence, introducing new functionality to building materials such as self-sensing and self-heating properties. In this study, the effect of graphite filler on the crack initiation of alkali-activated slag composite is investigated. The graphite powder was added in the amount of 5, 10 and 15% with respect to the slag mass. Beam specimens with an initial stress concentrator were tested in three-point bending at the age of 28 days. The load versus crack mouth crack opening displacement (F–CMOD) diagrams were recorded during the fracture tests and subsequently evaluated using the Double-K fracture model. This model allows the quantification of two different levels of crack propagation: initiation, which corresponds to the beginning of stable crack growth, and the level of unstable crack propagation. The course of fracture tests was also monitored by acoustic emission (AE) method.