Brittle Fracture Research Articles

Effect of Friction on Critical Cutting Depth for Ductile–Brittle Transition in Material Removal Mechanism

Abstract The material removal process takes place due to phenomena such as plastic deformation and brittle fracture. A long continuous chip is formed when the plastic deformation dominates, whereas a fracture-induced discontinuous chip is formed when the brittle fracture dominates. The means of material removal changes at a certain cutting depth for a particular material, the so-called transition depth of cut (TDoC). This article aims to predict the TDoC while including the effect of friction between the tool and workpiece. We propose a modification to a recently developed model (Aghababaei et al., 2021, “Cutting Depth Dictates the Transition From Continuous to Segmented Chip Formation,” Phy. Rev. Lett., 127(23), pp. 235502) to incorporate the effect of friction. The model predicts a transitional depth of cut as a function of tool geometry, material properties, and friction. The model is supported by performing orthogonal cutting experiments on different polymers such as polymethyl methacrylate (PMMA), polyoxymethylene (POM), and polycarbonate (PC). The model is also compared with existing models in the literature, where an improvement in the prediction of TDoC is shown. Moreover, the effect of the friction coefficient and rake angle on the TDoC is discussed. The results show that transitional cutting depth is reduced by increasing the friction coefficient. Alternatively, the TDoC reaches its maximum at an optimum rake angle, which is a function of the specific material being cut. The model aids in accurately predicting the TDoC, a crucial factor for optimizing various material removal processes.

In Vitro Wear of Titanium Reinforced Hydroxyapatite Coatings in Simulated Body Fluid

Abstract Titanium-reinforced hydroxyapatite (Ti-HAP) coatings have been deposited over 254 SMO stainless steel using a low-velocity oxygen fuel technique. FESEM, XRD, Vickers microhardness, and shear strength tests have been performed to characterize the developed coatings. Further, in vitro wear behaviors of the coatings were evaluated using a pin-on-disc wear tester in simulated body fluid. The results reveal that 40Ti-HAP coating possesses superior wear resistance compared to 60Ti-HAP, Ti, and HAP coatings, attributed to higher surface hardness and higher shear strength values. Amongst the tested samples, the friction coefficient was lower for Ti coating, followed by 60Ti-HAP composite coating. The surface roughness was a prominent factor in reducing the friction coefficient of coating samples. Local detachment and brittle fracture were the dominant wear modes in Ti-HAP coating, whereas an additional micro-plowing wear mode was observed in pure hydroxyapatite and titanium coating.

Influence of γ‐irradiation dose on the mechanical and tribological properties of fluoroelastomer

AbstractFluoroelastomer (FKM) undergo various degrees of degradation in a gamma‐irradiated environment, leading to changes in their mechanical and tribological properties. In this paper, changes in the properties of FKM were investigated for six different doses. Fourier transform infrared‐atomic absorption spectroscopy (FTIR‐ATR) results show that FKM samples undergo dehydrofluorination and oxidation reactions during irradiation, resulting in the formation of C=C, C=O, and ‐OH functional groups. The results of the swelling test showed that the degree of cross‐linking of the FKM specimens increased with increasing irradiation dose. Mechanical test results show that the fracture mechanism of FKM specimens gradually evolves from ductile fracture to brittle fracture with the increase of irradiation dose. Its tensile strength reaches its maximum at an irradiation dose of 1000 kGy, and the modulus of elasticity becomes larger with increasing irradiation dose. The results of wear tests show that the average coefficient of friction of FKM specimens first decreases, reaches a minimum at 500 kGy, and then gradually increases. The amount of wear increases with increasing irradiation dose. The wear mechanism of FKM specimens is abrasive wear at 0–500 kGy, adhesive wear at 1000 and 2000 kGy, and fatigue wear at 3000 kGy.Highlights FKM was irradiated by gamma‐ray with a total dose of 3000 kGy. Dehydrofluorination and oxidation reactions occur during irradiation. Increased cross‐linking leads to changes in mechanical properties. The wear mechanism of FKM with different doses has been investigated.

An efficient multi-scale method for failure mechanism analysis of SiCf/Ti composites with experimental validation

This study focuses on developing and validating a high-precision concurrent multi-scale finite element model considering the progressive damage model (PDM) and the cohesive zone model (CZM) for predicting the damage evolution of SiCf/Ti composites. At the macro-scale, a constitutive model describing the anisotropic damage criterion of SiCf/Ti composites was implemented by means of a user-defined subroutine (VUMAT). Meanwhile, based on the submodelling, a representative volume element (RVE) model was developed to systematically analyze the failure modes of the SiC fibers, the matrix, and the interface, respectively. Specifically, the crack initiation and propagation in the stress concentration zones were further discussed. Simultaneously, experimental methods were employed to verify the feasibility of the current model. During the uniaxial compression, the brittle fracture of the SiC fibers and ductile fracture of the matrix are the predominant failure modes of SiCf/Ti composites, with fiber fracture being the dominant factor. The crack initiates at the fibers, propagates rapidly to the fiber-matrix interface, and then extends into the matrix. Moreover, the fracture locations are suscepticle to stress concentrations, leading to the crack initiation and propagation. The predicted results show that the local damage has a significant effect on the failure mechanism of SiCf/Ti composites and this multi-scale model provides a scientific reference for the design and optimization of composites.

Mechanical and microstructure characterisation of 2.5D C/C-SiC composites applied for the brake disc of high-speed train

Carbon fibre reinforced silicon carbide (C/C-SiC) composite materials have attracted increasing attention in brake system of high-speed trains, due to their low density, high specific strength, thermal stability, and frictional wear performance. This study aims to explore the mechanical properties of the 2.5D C/C-SiC composites, in order to characterise its potential application to the friction braking system of high-speed trains. To comprehensively evaluate the mechanical performance of the 2.5D C/C-SiC composites, the chemical vapor infiltration (CVI) method was adopted to prepare novel samples with high densification and low content of residual Si. The mechanical properties and failure mechanisms of the composites were investigated under various loading conditions, including tension, compression, bending, and shear tests. The experimental results indicated that the 2.5D C/C-SiC composites exhibited superior mechanical properties, with average in-plane tensile strength, compressive strength, bending strength, and interlaminar shear strength reaching 127.67 MPa, 326.92 MPa, 355.74 MPa, and 9.77 MPa, respectively, which are 2–8 times higher than the mechanical properties of C/C-SiC composites in existing publications. Under different loading conditions, the composites demonstrated characteristics of ductile fracture, pseudo-plastic compression fracture, pseudo-plastic bending fracture, and brittle shear fracture. Both high fibre content and the formation of SiC structure were advantageous for enhancing the load-bearing performance of C/C-SiC composites. The outstanding mechanical properties of the 2.5D C/C-SiC composite render the brake disc highly resistant to deformation and cracking under high stress, thereby enhancing its durability and establishing it as the ideal material for the next generation of high-speed train brake discs.

In Vitro Assessment of a New Block Design for Implant Crowns with Functional Gradient Fabricated with Resin Composite and Zirconia Insert.

This study aims to evaluate and compare the mechanical resistance, fatigue behavior and fracture behavior of different CAD/CAM materials for implant crowns. Eighty-eight implant crowns cemented-screwed with four sample groups: two monolithic G1 Zirconia (control) and G3 composite and two bi-layered G2 customized zirconia/composite and G4 prefabricated zirconia/composite. All static and dynamic mechanical tests were conducted at 37 °C under wet conditions. The fractographic evaluation of deformed and/or fractured samples was evaluated via electron microscopy. Statistical analysis was conducted using Wallis tests, which were performed depending on the variables, with a confidence interval of 95%, (p < 0.05). The Maximum Fracture Strength values displayed by the four groups of samples showed no statistically significant differences. The crown-abutment material combination influenced the failure mode of the restoration, transitioning from a fatigue fracture type located at the abutment-analog connection for monolithic materials (G1 and G3) to a brittle fracture located in the crown for bi-layered materials (G2 and G4). The use of layered crown materials with functional gradients appears to protect the crown/abutment connection area by partially absorbing the applied mechanical loads. This prevents catastrophic mechanical failures, avoiding long chairside time to solve these kinds of complications.

Effect of Sn content on microstructure and mechanical properties of Mg-2Zn-1Ca-xSn /Al composite plate

This article explores the effects of different amounts of Sn (1,2,5 wt%) added to magnesium matrix on the microstructure and formability of Mg-2Zn-1Ca-xSn/Al composite plates. The second phase analysis results indicate that Sn element has a significant inhibitory effect on Ca2Mg6Zn3 phase. As the Sn element increases, the Ca2Mg6Zn3 phase gradually disappears, but the number and size of CaMgSn phase increase significantly (5–8μm). The tensile results at room temperature indicate that the composite plate has the highest strength but lower elongation when the Sn content is 2 wt%. It is mainly attributed to the dispersed distribution of small-sized second phases (<1μm), which cause pinning effect on dislocations and lead to dislocation accumulation. Excessive Sn (5 wt%) can promote dynamic recrystallization, resulting in equiaaxial grain distribution. The average Schmidt factor (0.22) of the basal slip is concurrently increased, makes the base slip system easier to start at room temperature. The Mg-2Zn-1Ca-5Sn alloy exhibits obvious softening, leading to a significant reduction in hardness difference between the Mg-2Zn-1Ca-5Sn alloy and the Al alloy. The enhanced feature also facilitates improved coordinated deformation capability and serves to prolong the occurrence of interface delamination in deformation. The Mg/Al composite plate exhibits excellent plastic deformation ability when the Sn content is 5 wt%, the average performance values were 207.52 MPa (UTS), 145.48 MPa (YS) and 17.45 %(EL), respectively. The fracture behavior of the composite plate undergoes a shift from brittle fracture to a combination of ductile and brittle fracture modes, occurring simultaneously. In summary, the incorporation of an optimal amount of Sn content can enhance the plastic formability of Mg/Al composite plates.

Assessment of fracture toughness in fibrous concrete via Brazilian test: Experimental study with low-clinker cementitious binder

As the predominant and cost-effective material in the construction industry, concrete is susceptible to tension weaknesses, leading to significant flaws such as cracking and brittle fracturing. Recent advancements in fibrous concrete have emerged as a solution to mitigate these issues. Incorporating steel fibers in concrete has emerged as a promising solution to improve crack resistance and structural integrity. This study focuses on developing eco-friendly concrete using a low-clinker cementitious binder and short steel fibers to enhance fracture toughness and mitigate the environmental impact by effectively utilizing lime sludge. Concrete specimens were prepared with three binder types: treated lime sludge (TLS) at 15 % and 30 % and calcined clay (CC) at 15 % and 30 %, replacing conventional clinker. Short steel fibers were added at 1.5 % by volume to enhance mechanical properties. Fracture toughness was evaluated using notched Brazilian disc specimens, assessing mode I, II, and mixed-mode (I/II) fracture at multiple notch orientations (β = 0°, 15°, 28.83°, 45°, 60°, 75°, and 90°). Microstructural analysis during strength development was performed using scanning electron microscopy and X-ray diffraction. The findings revealed that specimens containing 15 % TLS, 30 % CC, and 1.5 % steel fibers exhibited the highest fracture toughness. Mode II fracture toughness exceeded mode I, indicating improved resistance to crack propagation. The addition of fibers to the specimens under mode II demonstrated improved fracture toughness, ranging from 0.44 to 0.53 MPa·m^1/2 compared to the corresponding non-fibrous specimens. The fibrous specimens showed significantly higher ultimate loads at β = 90° compared to β = 0°, indicating superior crack resistance and structural integrity under perpendicular loading conditions. The Brazilian disc specimens demonstrated variability in fracture toughness across different loading orientations, highlighting their suitability for mixed-mode fracture assessment.

Effect of lubricants during the rolling process on the mixed mode fracture behavior of the as-rolled material

Friction is one of the major factors in energy consumption and metal depreciation during various mechanical processes. Lubrication is often used during the process of forming metals to cope with friction and its consequences. Mineral lubricants are employed in the rolling process to decrease the rolling force and enhance the quality of the sheets. This research aims to compare the performance of mineral lubricants such as crude oil, gas oil, and CM405A to enhance the production efficiency of the aluminum forming process, especially in the rolling process. The ring compression test was utilized to evaluate the influence of lubrication on the friction coefficient which was extracted through calibration curves. Then, the effect of friction coefficient was experimentally examined during the forming process through rolling. Al 1050 was rolled by the mentioned lubricants. The pre-cracked rolled samples were then tested by fracture test at different loading angles of 0 (pure mode-I), 45(mixed-mode of I and II), and 90(pure mode-II). J-R curves were extracted using elastic–plastic fracture theory based on material characteristics. The fracture tests indicated maximum resistance to crack growth occurred in the sample prepared with CM405A lubricant. In contrast, the lowest crack growth resistance was recorded in the case where crude oil was employed as a lubricant. In the end, the SEM images of the fractured surfaces were explored for a deeper understanding of the fracture behavior. Although a combination of brittle and ductile fracture mechanisms can be observed on the fractured surfaces, cleavage was the dominant phenomenon and fracture mechanisms.

Mechanical properties of Si(1−x)–C(x): strength and stiffness of materials using LAMMPS molecular dynamics simulation

This study investigated the mechanical properties (elastic modulus, tensile strength, yield strength, and toughness) of different percent C of silicon carbide (SiC) using molecular dynamics simulations via the large-scale atomic/molecular massively parallel simulator (LAMMPS) with the uniaxial tensile test at four strain rates: 0.1, 0.5, 1.0, and 5.0 m s−1, using the Tersoff potential. The simulation uses 20 × 20 × 20 atoms (108.6 Å × 108.6 Å × 108.6 Å) of the diamond cubic structure of Si, then carbon atoms were placed randomly at 5% intervals from 0–50 percent C. Results show improved mechanical properties when increasing percent C until peaking at 25%, before decreasing. This is caused by the shortest bond length at 25 percent C from the increase of Si=C using the radial distribution function analysis. Increasing the strain rate generally improves the mechanical properties of the material. The deformation mechanism shows that increasing (decreasing) strain rate generally results in multiple (lesser) failure points with a ductile (brittle) fracture mode.

Relationship between microstructure and mechanical properties of V-N microalloyed high strength ship plate steel

V-N microalloying treatment is an important way to improve the service performance of non-quenched and tempered ship plate steel. Herein, the influence of V(C, N) on the evolution of microstructure and improvement of mechanical properties was studied. In addition, the relationship between microstructure and mechanical properties of V-N microalloyed high strength ship plate steel was revealed. The results showed that the composite addition of V and N not only formed a fine dispersed precipitated phase, but more importantly, significantly refined the ferrite/pearlite microstructure, promoted the formation of intragranular acicular ferrite, increased the proportion of high angle grain boundaries, and decreased the kernel average misorientation value. The optimization of microstructure brought about by V-N microalloying achieved synchronous improvement of strength and cryogenic toughness. The impact energy of V-N microalloying ship plate steel increased from 97 J of V-N-free ship plate steel to 239 J at −40 °C, and the impact fracture mode changed from brittle quasi-cleavage fracture to microvoid coalescence fracture with a large number of equiaxial dimples.

Adaptive topology optimization for enhancing resistance to brittle fracture using the phase field model

Computational cost is one of the challenges in the field of fracture resistance topology optimization. An efficient topology optimization approach is proposed for enhancing resistance to structural fracture based on the adaptive isogeometric–meshfree method. The mesh can be adaptively refined in the computational and design domains simultaneously to capture the fracture and structural boundary delicately, which significantly reduces the degree of freedom and improves the efficiency of the topology optimization problems related to crack propagation. The proposed approach naturally inherits the advantages of both the computer-aided design and continuity of the higher-order basis functions, where the geometry and the analysis models in the process of topology optimization are integrated. The problem is formulated to maximize the absorbed energy throughout the crack process using the solid isotropic material with penalization method under volume constraints. The phase field method is used for modeling crack propagation, thereby eliminating the need of an explicit representation of the crack surface and complex tracking procedures, while a delicate mesh is still required. With this approach, the external work required during the fracture process is maximized, and the crack growth is delayed remarkably. Several representative examples are demonstrated to verify the effectiveness of the present approach in fracture-resistance-enhanced topology optimization, and the computational cost shows remarkable improvement.

Geochemical, Mineralogical, and Fluid Processes in the Brittle‐Plastic Transition of Continental Crust

AbstractThe brittle‐plastic transition (BPT), the strongest part of the crust, is critical to continental geodynamics but is poorly understood relative to simpler crust above and below. It is typically represented as a depth transition from brittle/frictional to plastic/viscous deformation controlled by temperature and pressure. Footwalls of low‐angle normal faults (LANFs) exhumed through the BPT provide rock records that challenge this view. Three well‐studied LANF footwalls are reviewed. All record geochemical, mineralogical and fluid‐related controls on embrittlement, not just monotonic P‐T decrease. Two quartz‐rich examples record embrittlement at unexpectedly high T (≥450–500°C) that was modulated by wetting characteristics of fluids. One had an inverted BPT: brittle fracture beneath contemporaneous mylonites. In another study, a brittle LANF grew from plastic mylonites due to mineralogic changes that strengthened parts, causing initial frictional slip and cataclasis on weak planes that ultimately linked. In all, geologically abrupt small‐scale processes controlled behavior at kilometer scales. Similar processes likely affect other tectonic settings and seismic cycles. Such processes offer fertile research opportunities in continental geodynamics; they will be increasingly tractable as computational abilities improve. Adaptive, multi‐scale approaches including the effects of fluid‐rock geochemistry and mineralogical changes on rock strength and deformation are needed. Thoughtful modeling approaches may yield key insights into the positive and negative feedbacks that are likely. Discontinuous deformation is probably needed explicitly along with exploration of initial and boundary conditions.

Experimental study on the influence of non-penetrating crack spatial distribution on brittle fracture process

To gain insights into the spatial distribution of non-penetrating cracks during the rock fracture process, a comprehensive uniaxial compression test is conducted on cubic gypsum specimens (100 mm × 100 mm × 100 mm) containing two non-penetrating cracks. The two pre-formed cracks are rectangular, with dimensions of 25 mm length, 2 mm width, and depths of 80 mm and 35 mm on adjacent sides of the specimen. The depth of the 80 mm crack can be adjusted from 0° to 150° in increments of 30°, while the other is fixed at a 45° angle. The results show that the spatial distribution of non-penetrating cracks can significantly influence the strength of the specimen. Initially, the strength of the specimen exhibits an upward trend and subsequently declines as the pre-crack inclination angle of the main rupture plane increases, ultimately reaching its pinnacle at 90°. The total percentage of tensile cracks in specimens with different inclinations are found to be 57%, 57%, 63%, 77%, 68%, and 61%, respectively. This change aligns seamlessly with the fluctuation in specimen strength as influenced by the angle of inclination. Non-penetrating cracks can also induce spalling on the specimen surface and give rise to anti-wing cracks, thereby exacerbating the spalling on the specimen surface. The inclinations of non-penetrating cracks can inevitably exert a certain influence on the propagation of neighboring non-penetrating cracks. Additionally, the macro-scale shear fracture of the specimen often occurs on the side of the non-penetrating crack that is deeper. The curved tensile fracture surface formed by the extension of the non-penetrating crack bears resemblance to the non-penetrating region in its ability to somewhat restrain the propagation of new cracks. Even under uniaxial compression, the spalling surface of the specimen containing spatial non-penetrating cracks frequently exhibits fracture characteristics belonging to I-III mode fracture, while its interior may display characteristics belonging to I-II-III mode fracture. These findings hold significant implications for comprehending and elucidating the genuine fracture process and three-dimensional fracture theory of rocks.

Assessment of hydrogen-induced material degradation using electromagnetic testing technology

The production, transport and use of green hydrogen are key issues for the efficient use of renewable energies. Hydrogen is able to compensate for both temporal and spatial fluctuations that naturally occur in the production and consumption of renewable energies, regardless of weather conditions, and thus can act as an efficient energy storage medium. The use of existing gas pipelines in a dynamically operated pipeline network offers particular potential for the transport of green hydrogen with regard to sustainability, resource conservation and economic efficiency. In this context, dynamic means that pressure fluctuations are permissible to a certain extent in the operation of gas pipelines. However, a hydrogen-induced degradation of the mechanical material properties can lead to brittle fracture and manifest itself in sudden material failure. Such failure must be excluded in safetyrelevant infrastructures. Hence, it would be beneficial to detect fatigued or degraded areas in the material at an early stage, especially in the context of possible increased crack growth rates associated with hydrogen embrittlement. In addition to fatigue data, which are crucial in the calculation and design of gas pipelines, non-destructive inspections are particularly important in subsequent operation to monitor the current condition of the pipelines. Non-destructive testing focuses primarily on defects such as cracks that have already developed or on visible corrosion attack. The fatigue condition, i.e., the probability of crack initiation and crack propagation is hard to evaluate. However, this can be addressed by an electromagnetic test method capable of evaluating the current state of the material. In the presented paper, an electromagnetic test method is presented, which is able to detect and evaluate hydrogen-induced material degradation. In subsequent stages, such a method can then be used as an electromagnetic monitoring system in fatigue tests, thus providing new potential for condition assessment.

Strained behavior of monolayer HfS2, HfSe2, and HfTe2 from molecular dynamic simulations

We have employed the classical molecular dynamics (MD) simulations to investigate the mechanical properties of the HfX2 (X=S, Se, Te) monolayers (MLs) --the emerging nano-materials possessing highly attractive for functional electronic and optoelectronic systems. The studied materials have exhibited isotropic mechanical properties in armchair and zigzag directions. Both the stress-strain curves and the structural evolution around the fracture point have indicated that materials have undergone a brittle fracture once up to the ultimate stress. At the same temperature, HfS2 ML exhibits a higher Young's modulus than HfSe2, yet HfSe2 possesses slightly larger fracture strain, fracture stress, and toughness compared to HfS2. Among the three materials, HfTe2 displays the poorest mechanical performance. The calculated results may be attributed to materials’ distinct microstructures and bond lengths. It’s found that temperature can obviously affect the mechanical properties of materials. For example, as the temperature increases, the calculations show that the fracture strain, fracture stress and toughness have obviously decreased. However, the Young’s modulus decreases slightly and linearly with the increasing temperature. These results suggest temperature can induce the mechanical degradation.

Flexoelectric influence on crack propagation in ferroelectric single crystals: a phase-field approach

Ferroelectric materials, known for their inherent brittleness, are prone to brittle fracture. This limitation not only curtails the materials’ operational lifespan but also impinges on the reliability of the associated devices. Notably, the pronounced strain gradient present at the crack tip necessitates consideration of the flexoelectric effect—the interaction between strain gradient and electric polarization—in the fracture mechanics of ferroelectric materials. This study introduces a phase-field model incorporating the flexoelectric effect to elucidate its role on crack growth and domain evolution in ferroelectric single crystals. Our findings demonstrate that both crack trajectory and domain switching phenomena at the crack’s forefront are substantially influenced by the magnitude and sign of the flexoelectric coefficient, as well as the initial polarization direction. Depending on the computational scenarios, the flexoelectric effect can either exacerbate or impede crack propagation. Through meticulous examination of the mechanical field distributions and their temporal progression, we have uncovered the underlying mechanisms by which the flexoelectric effect governs crack propagation in ferroelectric single crystals. These insights pave the way for improving the fracture resistance and thereby enhancing the reliability of ferroelectric devices.

Controllable stress-adsorbed layers in Ti/TiN/TiAlN coatings: Mechanical performance and micropillar compression

Solid particle erosion often compromises the durability of turbine engine blades, particularly those used in offshore and desert environments. Surface protective coatings have emerged as a promising solution to significantly enhance the erosion resistance of compressor blades. In this study, TiAlN multilayer coatings were deposited onto Ti-6Al-4V substrates using a vacuum arc deposition technique. The article explored the strategic incorporation of TiN-Ti-TiN stress-adsorbed layers (SALs) within TiAlN-based coatings. It also investigated the microscale mechanisms leading to coating failure under conditions of micropillar compression. The findings revealed that the presence of metal/ceramic interfaces significantly improved the adhesion strength, crack resistance, erosion resistance and fracture toughness of the coatings. The multilayer deformation behavior of the coatings was primarily determined by the plastic flow of the softer metal layers. The SALs played a crucial role in reducing the elastic strain energy by generating nanotwins, coherent interfaces, and high-density dislocations during deformation, thereby improving the coatings' plastic deformation capability. Moreover, the implementation of dual-cycle SALs successfully reduced radial crack propagation, facilitating a transition from brittle fracture to a combined brittle-ductile fracture mechanism. This study highlights the critical role of metal/ceramic interfaces and SALs in improving the erosion resistance and co-deformability of TiAlN-based hard coatings.

The Brittle Fracture of Iron and Steel and the Sharp Upper Yield Point Are Caused by Cementite Grain Boundary Walls

Brittle fractures of iron and steel above twinning temperatures are caused by cementite grain boundary wall cracks. These were revealed by an Atomic Force Microscope (AFM). At temperatures below the ductile–brittle transition (DBT), cracks must propagate longitudinally within cementite walls until the stress is sufficiently high for the cracks to propagate across ferrite grains. Calculations using these concepts correctly predict the stress and temperature at the DBT required for fractures to occur. At temperatures above the DBT for hypoeutectoid ferritic steels, dislocations must be emitted across the walls transversely for plastic deformation to continue. This is responsible for the upper yield point at the elastic limit in these steels followed by a large drop in stress to the lower yield point. Here, the walls completely surround all of the grains. Where the walls are segmented, such as in iron, dislocations can pass around the walls, resulting in a gradual change from elastic to plastic deformation. The Cottrell atmosphere theory of yielding is not supported experimentally. It was the best available until later experiments, including those using the AFM, were performed. Methods are presented here giving yield strength versus temperature and also the parameters for the Hall–Petch and Griffith equations.

Effects of retained austenite and martensite microstructure on fatigue crack propagation in quenched and tempered high carbon steels

Commercial high-carbon 52,100 steel was subjected to varying quenching and tempering heat treatments to develop plate martensite and retained austenite (RA). The mode I fatigue crack growth (FCG) behavior of quenched and tempered microstructures with varying amount of RA and degree of martensite tempering was investigated. The interaction between the fatigue cracks and the surrounding microstructure was characterized using electron microscopy/backscatter diffraction and quantitative fractography. Higher initial amounts of RA and greater degree of martensite tempering improved FCG resistance of high-carbon plate-martensite austenite microstructures. Stress-assisted RA to martensite transformation was observed in the vicinity of the fatigue cracks in the microstructure heat treated to have an elevated amount of RA. The fatigue fracture surfaces of the different microstructures exhibited varying combinations of intergranular (IG) fracture, mixed-ductile brittle (MDB) and transgranular (TG: cleavage, quasi-cleavage, and ductile striations) fracture as a function of stress intensity range at the crack tip. Microstructure conditions showing greater fractions of MDB and TG fatigue fracture had better FCG resistance. The presence of a higher initial RA content suppressed brittle IG fracture and promoted TG fracture, potentially due to transformation–toughening associated with RA to martensite transformation or transformation induced micro-crack coalescence near the fatigue crack.