Thermal shocks during cold rolling significantly reduce the service lives of work rolls. This study clarified the metallurgical changes that govern thermal crack initiation under rapid heating and cooling. Thermal shock studies were conducted using 1.0 wt% C steel with tempered martensite to replicate the physical phenomena and examine the associated phase transformations and stress evolution. Additionally, thermomechanical simulations using a fast Fourier transform were performed, enabling the stress contributions of different metallurgical phenomena to be decoupled and incorporating phase transformations to predict the stress evolution during thermal shock. The surface-tempered martensite underwent reverse transformation to austenite during heating due to frictional heat and then transformed into fresh martensite during cooling. Residual stress analysis indicated that the maximum tensile stress occurred close to the interface between the tempered and fresh martensite, and numerical predictions of the stress evolution during martensitic transformation were consistent with the experimental measurements, validating the model. The dominant tensile stress was due to volume expansion during austenite-to-martensite transformation rather than tempering shrinkage, explaining thermal crack initiation. These findings provide insights into the formation of thermal cracks and offer guidelines for use in improving the durabilities of work rolls in cold-rolling operations.

Mechanical modeling for fatigue crack extension mechanisms can be categorized into three types: Plastic Deformation mode of Fatigue Crack Growth (PD-FCG), Damage Accumulation mode of Fatigue Crack Growth (DA-FCG), and Damage Accumulation mode of Fatigue Crack Propagation (DA-FCP). The modeling based on these mechanisms allow for more rational fatigue design than conventional fracture mechanics-based methods without considering the mechanism. However, although some mechanisms have been already proposed, such methods have been proposed only for PD-FCG. This study focuses on DA-FCG, which is influenced by micro-structural effects. Fatigue tests were combined with digital image correlation (DIC) analysis to evaluate localized shear plastic strain. A parameter, Delta gamma xy,DIC, was introduced as the mechanical driving force for DA-FCG crack growth by averaging Delta gamma xy over the plastic zone estimated via continuum mechanics. Furthermore, the Taylor factor M tau of the shear load-a material index representing resistance to crack growth along shear-driven paths-was introduced. Moreover Delta gamma xy,FEM obtained from EP-FEM and M tau controlled Delta gamma xy,DIC obtained from DIC were introduced. A correlation between M tau and Delta gamma xy,DIC was confirmed, indicating their relevance as material properties. Based on these findings, a method for predicting DA-FCG behavior using Delta gamma xy,FEM and M tau was proposed, offering a framework for microstructure-informed fatigue strength prediction.

Hydrogen-related fracture behavior in ultra-high-strength martensitic steels is strongly influenced by the stress concentration associated with defect geometry. However, a systematic comparison between notch-root-controlled and crack-tip-controlled stress states, particularly using precracked specimens that better represent crack-like defects in service, remains limited. This study systematically compared notched and precracked specimens of a 2 GPa-class martensitic steel to clarify how defect geometry controls hydrogen-related fracture behavior through notch-root stress concentration and crack-tip stress intensity. Slow-strain-rate tensile tests, combined with scanning electron microscopy and electron backscatter diffraction analyses, were performed to correlate fracture morphology with local plastic strain accumulation under hydrogen-charged conditions. Hydrogen exposure significantly reduced the tensile strength of both specimen geometries. In the notched specimens, the strength decreased from 2276 to 1854 MPa, whereas in the precracked specimens, it decreased more severely from 1489 to 577 MPa, indicating a higher susceptibility of crack-tip-controlled configurations to hydrogen embrittlement. Distinct fracture features were observed depending on the type of stress concentration. The notched specimens exhibited mixed fracture features involving intergranular (IG) fracture and shear-band-associated ductile fracture. In contrast, the precracked specimens showed dominant IG fracture at the crack tip, followed by localized transgranular interlath features and increasing ductile dimpling with crack extension, consistent with elevated crack-tip plastic strain revealed by kernel average misorientation analysis. These findings demonstrate that hydrogen-related fracture behavior in ultra-high-strength martensitic steel is strongly controlled by defect-geometry-dependent local stress and strain fields, with important implications for structural integrity assessment in hydrogen environments.

Since there are several types of flaws in metallic parts modeled by laser powder bed fusion (LPBF), it is expected that their strength will be evaluated by linear elastic fracture mechanics (LEFM), but its validity has only been verified by limited experimental data. Therefore, this study was conducted for verification via finite element analysis (FEA) and experimentation. The FEA results of a cracked composite material with a softer surface layer and a harder solid interior indicated that a considerable amount of strain was attributed to the plastic strain formed far away from the cracked region by crack-induced triaxiality and a crack tip plastic zone 6% smaller than that calculated by LEFM, which satisfied the small-scale yielding (SSY) condition. In addition, several AZX912 Mg alloy specimens of different sizes and the same fabrication parameters were produced by LPBF and subjected to tensile testing. The results showed that increasing the specimen size increased the number of defects on the fracture surface and considerably decreased the ultimate tensile strength. Furthermore, a different crack extension process was evident when a specific condition was satisfied. As a result, the as-built LPBF products with small flaws exhibited quasi-brittle fracture behavior, the SSY condition was determined empirically and LEFM could be used to increase the reliability of the LPBF process.

Fatigue crack initiation and subsequent crack propagation behaviour in as-quenched low-carbon low-alloy steel were examined using a rotating-bending fatigue test and electron backscatter diffraction analysis to clarify the relationship between the fatigue limit and the microstructural heterogeneity of martensite. The as-quenched low-carbon low-alloy steel exhibited a low fatigue limit relative to its ultimate tensile strengths. The fatigue fracture was originated from slip deformation due to dislocation glide in the matrix. Furthermore, a tensile test revealed a low elastic limit in the steel, which can be explained by the movement of high-density mobile dislocations introduced during the transformation. These findings suggest that the low fatigue limit of as-quenched low-carbon low-alloy steel is due to its low elastic limit. Fatigue cracks initiated at prior austenite grain boundaries (PAGBs), at packet boundaries, and parallel to the block boundaries. These crack initiations were triggered by the preferential activation of slip systems parallel to the habit plane in the coarse martensite, which was nucleated at the PAGBs in the early stage of transformation and satisfied the Kurdjumov-Sachs orientation relationship (K-S OR), with not only its own parent austenite grain but also the adjacent austenite grain (i.e. the double K-S OR). Additionally, the initiated cracks were arrested at the fatigue limit. This is probably due to plasticity-induced crack closure stemming from the significant plastic deformation of the early transformed coarse martensite.

This study integrates Linear Elastic Fracture Mechanics (LEFM) parameters into Elastic-Plastic Fracture Mechanics (EPFM), employing Imaginary Crack Tip Opening Displacement (I-CTOD) as a unifying metric. Generalized non-dimensional parameters based on I-CTOD enable unified analysis across small-scale yielding (SSY) and large-scale yielding (LSY) domains. Elastic-plastic finite element simulations validate these parameters, evaluating crack length effects on the critical stress intensity factor (KC) and establishing a theoretical basis for SSY. The proposed SSY condition is compared with empirical formulae, highlighting limitations and refining LSY guidelines. The I-CTOD enhances crack resistance characterization, expanding LEFM’s applicability to diverse fracture mechanics scenarios.

This study conducts compressive fatigue tests with an extended notch on a strain-localized material for quantitative evaluation of damage during compression fatigue and the corresponding effect of loading history on subsequent tensile fatigue limits. Hence, fatigue crack "growth" and "propagation" of two types are found in damage accumulation (DA) mode. The former features several simultaneous multi-crack initiations and independent extensions. Contrarily, the latter features coalescence between the main and secondary cracks. Moreover, the near-crack-tip mechanics causing crack extension and non-propagation in the respective fatigue crack extension types are discussed. Furthermore, a method for subsequent tensile fatigue limit prediction considering the compression fatigue effect is proposed by studying the non-propagating crack length, Vickers hardness, and residual stress in the DA mode during compression fatigue, corresponding to Murakami-Endo's equation parameters for a mechanically small crack. Thus, this study is anticipated to hold great significance for understanding fatigue damage caused by different load blocks and improving Miner's rule.

The fatigue properties of martensitic steels are highly microstructure-dependent, preventing the use of fatigue design criteria for ordinary steels, thus hindering their widespread application. This study focused on fatigue crack extension mode-type and, proposed a material index representing the steels and a new evaluation of fatigue strength. For this purpose, rotating bending fatigue tests were conducted on three types of 18% Ni bcc martensitic steels with different carbon contents. The fatigue crack extension behavior was analyzed by replica observation on the specimen surface, and the relationship among the local plastic strain, microstructure, and crack initiation/extension was analyzed by electron backscatter diffraction (EBSD) on fatigue-interrupted specimens. Moreover, fractographic observation analyzed the fatigue crack extension mode-type transition. As a result, the damage-accumulation mode of fatigue crack growth type (DA-FCG) and the DA mode of fatigue crack propagation type (DA-FCP) appeared as new fatigue extension mode types, with DA-FCP dominating the fatigue limit. In this type, the block size in the microstructure is proposed as the material index of martensitic steel, and the fatigue limit equation is presented in conjunction with the material index.

Strain localization (SL) in metals manifests in various forms, including dislocation slip bands, Portevin-Le Chatelier (PLC) bands, dislocation pile-ups at grain boundaries, and shear bands. These phenomena contribute to strain hardening and softening, significantly influencing crack behavior. Although a physics-based crystal plasticity finite element method (CPFEM) model incorporating SL can simulate these localized deformation mechanisms, its suitability and accuracy in predicting stress and strain distribution around the crack tip remain uncertain. Furthermore, the necessity of employing CPFEM over the conventional elastic-plastic finite element method (EPFEM) for fatigue crack behavior prediction remains a subject of investigation. To address this, we analyzed the plastic strain distribution around the notch tip using a physics-based CPFEM model incorporating SL, complemented by an in situ tensile test on a notched specimen. The role of CPFEM in fatigue crack behavior prediction is evaluated by comparing its strain distribution results with those obtained from EPFEM around the notch tip. The findings indicate that the physics-based CPFEM model incorporating SL reliably predicts plastic strain distribution around the notch tip. Moreover, the model successfully captures SL phenomena arising from dislocation slip, PLC effects, shear band formation, and grain boundary interactions. Additionally, CPFEM is essential for accurately predicting damage accumulation fatigue crack propagation (DA-FCP).

This study re-examined fatigue tests on Mg alloys and a carbon steel, measuring the threshold stress intensity factor range (Delta Kth) for various sizes of small cracks. Effective Delta Kth values were measured and the presence of non-propagating cracks were observed at the Delta Kth, indicating key fatigue mechanisms. A re-proposed engineering formula for Delta Kth was validated for Mg alloys and steels, based on experimental fatigue results and micro-meso plastic characteristics of stationary cracks. The formula's applicability to light metals with specific strain-hardening exponents was discussed and modified to finalize a quantitative formula for the size effect of small cracks on fatigue limit properties, validated by past Delta Kth data about other light metals.

The fatigue characteristics of a strain-localized material specimen with an inclined notch, subjected to cyclic tension-compression under plane strain conditions, have not been fully elucidated. An unloading elastic compliance test and electron backscattering diffraction analyses were conducted near a fatigue crack tip on inclined notched specimens of JIS-SM490YB hot-rolled steel. Crack extension caused a characteristic change in the local mean strain from zero to a positive value, corresponding to the transition from local plasticity induced by a notch to that induced by a fatigue crack. The sudden increase in local elastic compliance and negative crack opening load were the mechanical indicators of the damage accumulation mode of the fatigue crack extension mode-type.

The degree of stress concentration in structures is of critical importance in safety designs. Many stress concen-tration factors for various shapes of notches with defined their geometry have been published, and if not pub-lished, the stress concentration factors can be obtained by FEM. However, when a complex, non-reproducible, and diverse notch exists on the surface of an actual machine that has undergone processing or is in a corrosive environment, it is usually more rational to use an immediate approximate solution rather than a rigorous but time-consuming analytical solution. There are two methods proposed for obtaining such approximate solutions. Although physical and mathematical approximate methods are separately used to estimate stress concentration factors in a specific-shaped notch, the application limits to arbitrarily sized and shaped notches are not yet clarified. In this study, we first extended the double-notch concept to the "parent-child notch" concept in the mathematical method, and then conducted finite element modelling for typical and complex notches. Further, we introduced the two representative lengths in the stress distribution induced by the parent notch from the me-chanical consideration, and we deductively and inductively elucidated the application limits of the physical and mathematical approximations for an arbitrary parent-child notch. It was finally found that the stress concen-trations of an arbitrary parent-child notch could be estimated only with the physical or mathematical approx-imation. Furthermore, a method for predicting fatigue limits of real structures subjected to cyclic loading was also proposed based on the present results.

The fatigue properties of martensitic steels tend to differ significantly from those of low- and medium-strength steels. Even if the hardness is the same, the fatigue strength varies substantially depending on the microstructure. Thus, for using martensitic steel rationally and developing materials with high fatigue strength, it is desirable to establish a fatigue strength evaluation method that can consider the microstructural effects on fatigue properties. For this purpose, 18% Ni bcc martensitic steel was used as a model metal to conduct rotating-bending fatigue tests in this study. The fatigue crack behavior on the smooth specimen surface was examined, and the fracture surface morphology on both sides of the broken specimen and the microplastic strain distribution near the fatigue crack tip were analyzed. The results clarified the mechanism underlying fatigue crack extension (FCE). The analytical results revealed three types of FCE mechanisms dependent on the stress amplitude, stress state (i.e., plane stress or plane strain), and martensitic microstructure. Considering these mechanisms, a unique stress-life curve was predicted, and a particular fatigue crack shape and fractography were confirmed. The material indices were speculated to be the representative properties of martensitic steels.

Precipitation-hardened steel exhibits specific characteristics due to the punching process that deteriorates its strength. To investigate these characteristics, fatigue tests were conducted with artificial micro defects on the punching processed surface that imitate the flaws caused by the process. The authors found that changes in material properties and the flaws change the fatigue strength properties. Specifically, the fatigue crack extends in the damage accumulation mode, which does not have the non-propagation mechanism and reduces the crack non-propagation limit. Furthermore, the damage accumulation mode fatigue crack propagation changes the fatigue limit's physical meaning from the fatigue crack non-propagation limit to the initiation limit. Finally, processing conditions that improve the fatigue strength characteristics were proposed.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Huynh Thanh Thuong, Shigeru Hamada, Kaneaki Tsuzaki, Hiroshi Noguchi; Crack growth behavior in air and hydrogen of iron-3% silicon single-crystal thin sheet. AIP Conf. Proc. 21 February 2023; 2482 (1): 120002. https://doi.org/10.1063/5.0110547 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search

To quantify the material and mechanical effects on the damage accumulation (DA) mode of fatigue crack propagation (FCP), a crystal plasticity finite element method was used in a polycrystalline copper specimen. Thus, the plastic normal strain localization ahead of the notch root, which is correlated with DA-FCP, was analyzed. As a result, an equation of the critical grain size formulated by the Schmid factor, misorientation angle, and plastic zone size is proposed to characterize the critical occurrence condition for the DA-FCP. Moreover, a method for predicting the DA-FCP region size is proposed.

The transition point of the crack length to a microstructurally small crack is necessary for predicting crack extension behavior. The conventional criterion is the crack length smaller than the grain size and is a probabilistic criterion. This criterion does not consider the metal's crystallite plasticity property discrepancy (CPPD). However, CPPD determines crack extension behavior for recently developed advanced materials with complex microstructures. Therefore, the authors propose a deterministic judgment method for microstructurally small cracks considering crystallite plasticity based on a physics-based crystal plasticity finite element model with strain localization. The proposed method is based on the difference value of the crack tip opening displacement varying with the ratio between the crack length and grain size with the change in the grain orientation ahead of the crack tip. A case study for copper is performed, and the results show that the novel method can be applied to define the microstructurally small crack.

In this study, the current status of linear and nonlinear fracture mechanics was surveyed, and the conditions required for elastic-plastic fracture mechanics (EPFM) as the next step of fracture mechanics were listed. Then, the imaginary crack tip opening displacement (I-CTOD), expressed by the integral of the plastic strain in a twodimensional plastic zone, was proposed as a driving force of EPFM. Elastic-plastic FEMs of cracked solids in various work-hardened materials were performed under both the plane stress and plane strain states to understand the singular field characteristics of elastic and plastic strains near a crack tip. The I-CTOD was found to represent the intensity of the plastic strain singular field, the elastic strain singular field, and the imaginary plastic deformation field near the crack tip in elastic-plastic solids under both the plane stress and plane strain states. This I-CTOD is promising as a mechanical driving force parameter of EPFM because the analyzable and visualizable characteristics of the behavior and its clear relationship to actual material behavior make it easy to develop prediction methods for the actual crack behavior.