In an extensive flow of studies on material fracture, fracture mechanics has successfully established engineering standards for the safety evaluation of structural components. Vital difficulties in theories have been the management of the crack-tip stress singularity and the existence of incipient crack(s). Plasticity complicates fracture theories, and understanding the microscopic process of fracture is crucial for material design. This paper aims to shed light on the role of plasticity throughout the entire fracture process, remarking mostly brittle-like fracture, both in theory and experiment. Lattice deterioration due to plastic deformation increases potential energy, a key concept in deriving fracture criteria. Studies demonstrating the maturing of strain-induced lattice defects, primarily vacancy clustering, are reviewed to play a crucial role, operating as void source in fracture as a precursor to crack initiation.

Strain localization due to microstructural inhomogeneities are remarked to characterize the material’s susceptibility to fracture. The extent of strain localization, coupled with external and local stresses, provides favorable fracture paths through crack nucleation and extension, as exhibited in fracture surface morphology. However, a single type of morphology does not specify a fracture event, and its continuous transition during crack extension suggests operation of an essentially common mechanism between seemingly different morphologies.

Lattice defects generated during plastic deformation persist into later stages, and environmental variations alter dislocation configurations, generating vacancies. As a method to assess the intrinsic material’s susceptibility, detecting the progress of lattice deterioration in response to cyclic stressing is proposed.

Pearlitic steels achieve an exceptional balance of strength and ductility through the lamellar stacking of ferrite and cementite. While this synergy enhances mechanical performance, cementite also serves as a preferential site for crack initiation, making its thickness and the extent of deformation localization caused by dislocation pile-ups critical factors in the plasticity–fracture transition. In this study, molecular dynamics simulations were performed to clarify how cementite thickness and dislocation pile-ups govern deformation and fracture. The results reveal that thinner cementite or smaller pile-ups promote dislocation emission across the interface, whereas thicker cementite and larger pile-ups facilitate crack initiation within cementite. Comparison with a conventional continuum model showed qualitative agreement but also highlighted nanoscale effects—such as core relaxation of penetrated dislocations in cementite—that are beyond continuum descriptions. These findings provide atomistic insights into the mechanisms controlling the plasticity–fracture transition in pearlitic microstructures.

The intergranular crack growth in an Fe-25Cr-1.1N austenitic steel (in wt.%) was examined by in situ scanning electron microscopy and three-dimensional tomographic reconstruction based on Xe-focused-ion beam serial sectioning. The intergranular crack growth exhibited discontinuity, crack deflection/branching along {111}, and crack tip blunting. These features could be interpreted by considering the effects of planar dislocation slip that causes stress concentration at grain boundaries and Lomer-Cottrell sessile dislocations. The models explaining the intergranular cracking and associated crack deflection were proposed based on an assumption of intense planar slip and no cross slip until near-fracture, which was observed by in situ electron channeling contrast imaging under mechanical loading in the present study. In this context, because crack tip deformation is significantly constrained in the specimen interior (plain strain condition), the dislocation-driven intergranular crack growth occurred preferentially in the specimen interior, and subsequently, surface crack propagation occurs in a ligament portion. After blunting of the main crack tip, the coalescence of the main crack and planar-slip-induced brittle crack allows further crack growth.

The effect of Si solid solution on the dislocation behaviour of body-centred cubic (BCC) steels was studied by performing neutron diffraction line profile analysis (LPA) on Fe–4wt%Si (4%Si steel) and interstitial-free (IF) steels strained to 11.7% nominal strain. Dislocation parameters were quantitatively determined: density (ρ), character (q), arrangement parameter (M^*) and crystallite size (D). Plastic deformation substantially increased dislocation density in both steels, with the increment in the 4%Si steel being more than twice that in the IF steel. In the 4%Si steel, plastic deformation also remarkably increased the screw dislocation fraction. This behaviour can be attributed to cross-slip suppression by Si, which confines the screw dislocations to their original slip planes and reduces their annihilation probability. This leads to the formation of a planar dislocation substructure characterised by spatially isolated screw dislocations, which fragment the crystal into relatively small coherent domains and high M^* values. By contrast, the active cross-slip in the IF steel promotes dislocation annihilation and develops a cell substructure with interacting dislocations concentrated in the cell walls, resulting in relatively large coherent domains and low M^* values. Overall, neutron diffraction LPA reveals that Si addition alters dislocation cross-slip behaviour and the resulting substructural development in BCC steels.

This study investigates the strengthening mechanisms in duplex stainless steel (DSS), a dual-phase alloy comprising ferrite (α) and austenite (γ), produced by friction stir welding (FSW) using in situ neutron diffraction during tensile testing. Two welding conditions were applied: a lower peak temperature (FSW300) and a higher peak temperature (FSW600). Electron backscatter diffraction confirmed significant grain refinement in both phases, with γ grains reduced below 1 µm in FSW300. In situ neutron diffraction provided phase-resolved stress data during deformation, enabling direct evaluation of the load-sharing behavior of α and γ. Tensile testing revealed that both FSW conditions increased yield and tensile strength while reducing uniform and total elongation compared with the base metal (BM); however, FSW300 retained greater total elongation than FSW600, attributed to less severe local elongation loss. Neutron diffraction results revealed that γ acted as the harder phase in the BM, whereas α became the harder phase in the FSWed specimens. Phase stress analysis indicated that α is more sensitive to grain refinement strengthening than γ, shifting the dominant contribution to strength and work-hardening from γ in the BM to α in the welded specimens. Although stacking fault formation in γ was more pronounced in the ultrafine-grained microstructures, work-hardening capability of γ decreased, while α showed enhanced texture development and dislocation accumulation. These findings demonstrate that low-temperature FSW improves DSS strength primarily through α refinement, offering insights for designing stronger dual-phase alloy joints.

The strength at 600°C in fire-resistant steel exhibits a significant dependence on strain rate, likely attributed to a shift in deformation mechanism from slip deformation to grain boundary sliding (GBS) with increasing temperature. In this study, GBS was observed using a grid method, and the strain distribution introduced by a high-temperature tensile test in a carbon steel was visualized. The characteristics of the strain distribution in the grains composing the grain boundary where GBS occurred were discussed. Although the test temperature was 500°C, high-temperature tensile tests with strain rates of 10⁻³ and 10⁻⁵ s⁻¹ orders were successfully conducted. Strain rate dependence of strength was comparably small at 500°C. The discontinuous slide of the grid line at the grain boundary was observed after the tensile tests at 500°C, regardless of strain rate, indicating that GBS occurred. The strain was distributed inhomogeneously during the high-temperature tensile tests. The grain boundary generating GBS lay 45° from the tensile direction and consisted of a pair of high- and low-strain grains. The m’ value, which represents the ease of slip transfer between adjacent grains, was low at the grain boundaries generating GBS. This suggests that GBS was induced by pile-up dislocations at the grain boundary.

Strain-induced lattice defects that form during the incubation stage of hydrogen embrittlement fracture in the plastic region were quantified and their relationship with mechanical properties and fracture morphologies was investigated. Pure iron was subjected to plastic strain by tensile testing at various strain rates and hydrogen charging conditions. After charging tracer hydrogen as a probe for detecting lattice defects under conditions that reached equilibrium, specimens were quickly cooled with liquid nitrogen to prevent hydrogen desorption, and total tracer hydrogen was detected using low-temperature thermal desorption spectroscopy (L-TDS), which is capable of continuously elevating the temperature and subsequently performing measurement from that temperature. Dislocation density was not affected by the strain rate or hydrogen content. However, the vacancy concentration increased in the presence of hydrogen and displayed strain rate dependence even at the same strain level. A comparison of the mechanical properties with/without hydrogen showed that the flow stress with hydrogen increased with a decreasing strain rate compared with that without hydrogen, i.e., dislocation mobility decreased. It was established that strain-induced vacancies, which were excessively generated in the presence of hydrogen and formed complexes with it, were responsible for reducing dislocation mobility. Furthermore, fractures, albeit predominantly quasi-cleavage ones, along the {001} plane, which is the cleavage plane in body centered cubic iron, were present on the fracture surfaces, and their proportion increased with decreasing dislocation mobility. This suggests that vacancy-hydrogen complexes contribute to cleavage fracture by inhibiting dislocation motion.

By using finite element method for crystal plasticity, we investigated the accumulation behavior of dislocation and atomic vacancies introduced by non-uniform deformation in pure iron polycrystals. Dislocation density was calculated from the increment of plastic shear strain and spatial gradient of the slip systems for SS and GN dislocation densities. Vacancy density was calculated from the edge component of SS dislocation density and the incremental plastic shear strain by expanding the theory of Essmann and Mughrabi, in which atomic vacancies are released by the annihilation of edge dislocations, for each slip system. The cyclic loading analysis was performed under strain-controlled with 10 cycles between a tensile process up to 0.5% nominal strain and a compressive process down to 0%. For comparison, a monotonic loading analysis was also performed. The macroscopic mechanical responses were significantly different under the two conditions, and the work hardening rate under cyclic loading was less than half that under monotonic loading. The localization of plastic strain was more pronounced in the cyclic loading deformation than in the monotonic one. The low work hardening rate for cyclic loading deformation was attributed to the low accumulation of GN dislocations due to the relaxation of the plastic shear strain gradient caused by the load reversal. The average vacancy density was twice higher for monotonic loading deformation than for cyclic loading deformation. On the other hand, the maximum value of vacancy density was almost the same in both conditions, indicating that the cyclic loading deformation was more localized.

This study investigates the effect of Ca addition on the brittle-to-ductile transition (BDT) and grain boundary decohesion by S segregation in as-quenched low-carbon fully martensitic steel. Temperature dependence of the impact absorbed energy was examined in two kinds of steels with different Ca content (Ca-added steel and Ca-free steel). The BDT temperature of the fully martensitic steel was significantly decreased with the Ca addition. The temperature dependence of the 0.2% proof stress was measured to discuss the decrease in the BDT temperature based on shielding theory. The temperature dependence of 0.2% proof stress was comparable between the two steels, indicating that Ca addition did not affect the dislocation mobility regardless of the Ca content. Observations of brittle fracture surface revealed that intergranular fracture was prominent in the Ca-free steel, whereas it was suppressed in the Ca-added steel. Auger electron spectroscopy further revealed that S was segregated at prior austenite grain boundaries in the Ca-free steel. These results suggest that the improvement in low-temperature toughness in the Ca-added steel is attributed to the increase in surface energy for intergranular fracture, resulting from the suppression of S segregation by Ca addition.

We employed the high-resolution digital image correlation study to investigate the plastic deformation of low-carbon lath martensite. The strain localization bands were mainly categorized into two types: boundary slips and intra-block deformation. Misorientation angle and inclination angle with respect to loading direction primarily determines the slip activation at boundaries. The competitive relationship between the activation of in-lath-plane and out-of-lath-plane slip systems follows the Schmid effect. The in-lath-plane slip systems were only activated in blocks with high in-lath Schmid factor (SF) value. The out-of-lath-plane slip systems were activated only when its SF is much higher than the maximum in-lath SF, offsetting the effect of the higher critical resolved shear stress (CRSS) for in-lath-plane slip systems. Moreover, the block morphology also affects the slip activation behaviour: in-lath-plane slip systems in columnar blocks were preferentially activated due to both crystallographic dynamics and strain accommodation. In contrast, out-of-plane slip systems were only observed in equiaxed blocks where the boundary effect is weakened.

The formation and evolution of plastic strain concentration zones and surface damage introduced by tensile testing of dual-phase steel were investigated by tracking precise markers on the specimen surface using electron beam lithography and a focused ion beam (FIB). Following tensile tests at a stress approaching the tensile strength, areas of concentrated plastic deformation and extremely dark surface cracks (surface damage) appeared on the specimen surfaces. Regions of strain concentration with equivalent plastic strain exceeding 0.15 were identified from the displacement of precise markers. These regions appeared within the ferrite, near the ferrite-martensite interface, and within the martensite. Within the ferrite and at the ferrite-martensite boundary, strain concentration due solely to plastic deformation was observed, rather than displacement due to surface damage. The Taylor factor for ferrite grains within these strain concentration areas tended to be lower than those for ferrite grains within the surface damage regions. Furthermore, after we applied the new markers via the FIB and performed additional tensile testing until fracture, new plastic deformation concentration zones appeared within the ferrite, and the Taylor factor of the ferrite was relatively low. The Taylor factor is determined solely by the crystallographic orientation of the ferrite and the tensile deformation conditions. The effects of the ferrite crystallographic orientation on the ductile fracture process were also examined.

Tensile test results obtained from millimetre- and micrometre-scale specimens were correlated using crystal plasticity analysis to examine the microstructural factors dominating the mechanical properties of the as-built maraging steel produced by the laser powder bed fusion (LPBF) method for repairing die-casting tools. Micrometre-scale tensile tests revealed that the mechanical properties of single prior austenite grain (PAG) structures are dominated by the deformation of the coarse block according to Schmid’s law rather than by habit-plane-orientation-dependent slip deformation. This is owing to the low aspect ratio of the lath structure in the maraging steel produced by the LPBF method. In the millimetre-scale tensile specimens consisting of multiple PAGs in the gauge section, anisotropy of the ultimate tensile strength and elongation-to-failure was not observed, which was attributed to the high energy density in LPBF process. It was revealed that the specimen with the loading direction (LD) parallel to the build direction exhibited earlier work softening than the specimen with the LD perpendicular to the build direction, regardless of the energy density. This anisotropy was examined using crystal plasticity analysis with material parameters obtained through the fitting analysis of micrometre-scale specimens. The analysis results indicated that the anisotropic work-hardening behaviour related to the build direction was due to differences in the overall Schmid factor and the degree of lattice rotation, both of which stemmed from the texture. As the anisotropic mechanical properties observed in this study were insignificant, the application of the high-energy-density LPBF method to maraging steel is useful for repairing mechanical components.

The damage initiation and evolution behaviors of ferrite-martensite dual phase (DP), transformation-induced plasticity (TRIP)-aided dual-phase (TDP), quenched and tempered (QT), and TRIP-aided martensitic (TM) steels during tensile deformation were investigated. Voids were initiated at the phase boundaries and inside the martensite in the DP and TDP steels, whereas fine voids were observed at the prior austenite, packet, and block boundaries in the QT and TM steels. In the DP and TDP steels, the size of the voids remarkably increased with the plastic strain, even though the number of voids increased slightly. By contrast, the QT and TM steels exhibited a drastic increase in the number of voids, whereas a slight increase in the size of the voids was observed. The voids in the TM steel hardly extended as the plastic strain increased unlike those in the QT steel. The extent of voids in the DP and TDP steels might be attributed to stress and plastic strain partitioning between the different phases during tensile deformation. In addition, the promotion of void initiation and suppression of void growth might be attributed to the fine and uniform martensite matrix in the QT and TM steels. The suppression of void initiation in the TDP steel and void growth in the TM steel might be attributed to the stress and plastic strain relaxations at the void initiation site and the vicinity of voids owing to the effective martensitic transformation of retained austenite.

This study investigates the ductile fracture mechanisms in as-quenched (As-Q) and quenched-and-tempered (Q-T) dual-phase steels. Using in-situ tensile testing combined with synchrotron X-ray tomography, we performed four-dimensional quantitative analysis of void evolution to reveal how heat treatment fundamentally alters damage behavior. The high-strength, low-ductility As-Q steel exhibited premature failure. Its significant phase-hardness mismatch between ferrite and martensite phases induced continuous and widespread void nucleation via martensite cracking after reaching the ultimate tensile strength. The subsequent rapid, isotropic growth and coalescence of these numerous voids led to early fracture. In contrast, tempering the Q-T steel reduced the hardness mismatch among phases, yielding superior ductility with lower strength. The Q-T steel, in particular, exhibited a two-stage damage process. Throughout most of the deformation, voids grew anisotropically by elongating along the tensile axis. This stable growth, however, gave way to a drastic change just before fracture, characterized by a rapid proliferation of voids oriented perpendicular to the loading direction. This final phase is attributed to damage in the martensite, suggesting the quantitative and qualitative differences in void evolution between the As-Q and Q-T steels.

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.

The strain distribution developed during high-cycle fatigue bending tests exceeding 10⁶ cycles was visualized in 780 MPa high-strength steels with various microstructures using the digital image correlation (DIC) method for the secondary electron images of the specimen surface. The tensile stress was approximately identical among the steels with ferrite (F), bainite (B), and ferrite + pearlite (FP) microstructures. Microcracks were detected in the B and FP steels after the fatigue bending test, whereas no cracks were present in the F steel even after 10⁶ cycles. The strain distribution developed in the high-cycle fatigue bending test was successfully visualized using the DIC method for the first time. The strain was inhomogeneously distributed for up to 10² cycles even under an applied stress less than the yield stress. The average strain along the loading direction was approximately zero regardless of the number of cycles for all specimens. The standard deviation calculated from the strain histogram continuously increased with an increasing number of cycles for all steels. This suggests that strain gradually accumulated during the fatigue test. Microcracks tended to nucleate in high-strain regions. The plane of the microcrack was the slip plane, and its Schmid factor was high. Therefore, the microcracks were generated through intrusion and extrusion mechanisms with local slip deformation at the specimen surface.

This study investigates the effects of copper (Cu) addition on the bending fatigue strength of lath martensite, focusing on dislocation structures, Cu precipitates, and crack initiation sites. Steel specimens with varying Cu contents of 0, 1, 2, and 4 mass% were prepared and tempered at 523 K or 723 K. Fatigue tests revealed that Cu addition significantly enhances fatigue strength, particularly at 723 K, where the 4Cu steel (4 mass% Cu) exhibited superior performance compared to the 0Cu steel (<0.01 mass% Cu). Microstructural analysis by SEM and TEM showed that, after fatigue testing, the Cu precipitates at grain boundaries had undergone plastic deformation, indicating local stress relief at the grain boundaries. This stress relaxation effectively suppressed crack initiation at the grain boundaries and shifted the sites of crack initiation to the grain interior. Intragranular Cu precipitates were found to pin dislocations, delaying crack initiation within grains. It is also speculated that retardation of dislocation alignment during tempering, which is caused by Cu in solid solution and as fine precipitates, may further delay crack initiation, although direct confirmation of this effect is still required. At 523 K, Cu addition did not significantly improve fatigue performance, presumably due to the absence of Cu precipitates at grain boundaries. These findings suggest a dual mechanism of grain boundary strengthening by plastically deformable Cu precipitates and matrix pinning by intragranular Cu precipitates, which would explain the observed improvement in the crack initiation life and fatigue limit of Cu-added lath martensite.

The effects of stress, plastic strain, and hydrogen on hydrogen embrittlement fracture of hole-expanded transformation-induced plasticity-aided martensitic steel were investigated. The hydrogen embrittlement properties were evaluated by means of cathodic hydrogen charging to the hole-expanded specimen. The residual stress and plastic strain distributions in the hole-expanded specimens were analyzed using finite element analysis. The hydrogen content was measured using a thermal desorption spectrometer. Hydrogen embrittlement cracking occurred approximately 3 mm from the hole edge in the radial direction. As the crack propagated, it diverged in the circumferential direction. The fracture morphology primarily consisted of a mixture of intergranular and quasi-cleavage fractures. The tensile stress in the circumferential direction at the position where the hydrogen embrittlement crack was initiated was the highest, and the tensile stress in the radial direction and hydrostatic stress were also high. The hydrogen content in the vicinity of the hole edge of the hole-expanded specimen was the highest owing to the large amount of plastic strain applied by hole punching and hole expanding whereas the hydrogen content at the positions where the hydrogen embrittlement crack was propagated was not very high. Thus, the highest tensile stress in the circumferential direction is the controlling factor in the location of the crack initiation site and the direction of the initial crack and its growth during the initial phase. The high hydrostatic stress that causes hydrogen accumulation could also assist the crack initiation.

In this study, the effect of hydrogen on the spot-welded tensile shear strength of transformation-induced plasticity (TRIP)-aided martensitic (TM) steel sheet was investigated. The tensile shear tests were carried out on an Instron-type universal testing machine using the tensile shear specimen which was spot-welded at the lapped portion of 30×30 mm² using specimens with dimensions of width of 30 mm and length of 170 mm at crosshead speeds of 0.5–100 mm/min without and with hydrogen. The results were summarized as follows.

(1) The ultrahigh-strength TM steel without and with hydrogen charging possessed an excellent tensile shear stress (τ_f) in comparison with the hot-stamped (HS1) steel. This might be attributed to the TRIP effect of the TM steel which exhibits volume fraction of retained austenite of 1.52 vol% and low absorbed hydrogen concentration compared with that of the HS1 steel.

(2) The τ_f decreased with decreasing the deformation speed in the TM and HS1 steels with hydrogen, whereas the τ_f was hardly changed by the crosshead speed in the HS1 steel without hydrogen. The decrease in τ_f at slow strain rate might be caused by the occurrence of hydrogen diffusion to crack initiation site and crack tip to accelerate hydrogen embrittlement crack propagation.

(3) The hydrogen embrittlement crack was initiated at heat affected zone (HAZ) due to the hydrogen diffusion and hydrogen concentration at the HAZ which is softer than its surroundings, so the deformation is concentrated and HAZ becomes the origin of fracture, resulting in the stress concentration.

The warm V-bendabilities and hydrogen embrittlement properties of ultrahigh-strength Quenching and Partitioning (QP)-Transformation-Induced Plasticity (TRIP) steel sheets were investigated to apply the QP-TRIP steel sheets for automotive structural parts manufactured by cold or warm press forming. V-bending tests were carried out at a crosshead speed of 1 mm/min at V-bending temperatures of 25, 100 and 150°C u