Mixed Fracture Research Articles

Influence of heat treatment on microstructure and tensile properties of selective laser melting metastable Ti55531-0.5Nb alloy

The deposition of metastable titanium alloys using additive manufacturing techniques is of increasing interest. The as-build samples generally require further subsequent heat treatment to control the microstructure and to meet the good comprehensive performance of structural components. In this study, the Ti55531-0.5Nb alloys fabricated by selective laser melting (SLM) technology after solution treatment were cooled in different modes to investigate the effect of cooling rates on microstructural evolution and tensile properties. The results indicate that the volume fraction of primary α decreases with the increase of cooling rate, while the secondary α displays the opposite trend. The high cooling rates increase the residual stresses in the sample, providing an additional driving force for phase precipitation during the aging process. In addition, all samples with different cooling rates exhibited high elongation (>10 %). The difference is that the Ti55531-0.5Nb alloys are more sensitive to changes in elongation than ultimate tensile strength (UTS) with variations in cooling rates. The UTS and elongation of the sample with solution plus water cooling and aging (STWC+AG) are 1185 ± 9 MPa and 10.3 ± 0.3 %, respectively, while the furnace cooling and aging (STFC+AG) sample exhibits a 71.4 % improvement in elongation at the expense of the UTS of 13.8 %. Furthermore, the fracture behavior of the samples exhibited a mixed fracture mechanism dominated by ductile fracture, and the ductile fracture is more prominent with the cooling rate decreases. The microscale shear band contributing to the elongation was found in the shear lip zone for the low cooling rate.

A new approach for fracture parameters optimization of multi-fractured horizontal wells: Virtual boundary method

Horizontal well fracturing technology stands as the cornerstone for achieving large-scale commercial production in shale reservoirs. Yet, the complexity of fracture variables within horizontal wells, comprising both discrete and continuous factors, presents a formidable challenge for optimization. Existing methods struggle to holistically optimize these mixed fracture parameters, including discrete fracture number, continuous fracture location, and half-length. This study pioneers a fracture deployment optimization approach tailored for fractured horizontal wells in shale reservoirs, leveraging the innovative virtual boundary method (VBM). Initially, a virtual boundary model is developed by extending the length of the fracturing horizontal wellbore, defining the extended area as a virtual reservoir devoid of hydrocarbon. Subsequently, the net present value (NPV) is adopted as the objective function, with continuous fracture location and half-length serving as optimization variables. The modified simultaneous perturbation stochastic approximation (mSPSA) algorithm is then employed for optimization. Following optimization, the optimized fractures within the virtual area are identified and deleted via position identification, enabling the comprehensive optimization of fracture number, location, and half-length. Notably, the computational efficiency of this method surpasses existing approaches by a factor of 5.1 compared to the bisection nested SPSA optimization method. Validation through three calculation cases, encompassing homogeneous reservoir, distinct sweet spot reservoir, and strongly heterogeneous reservoir, further underscores the method's reliability. Post-optimization, NPV increases by 1.1 times, 1.4 times, and 0.34 times for the respective calculation cases compared to the initial plan. When applied to an actual shale oil reservoir, the method yields a 1.67 times increase in NPV compared to existing deployment plans. This research lays a solid theoretical foundation for the design of horizontal fracturing well construction plans in shale reservoirs.

Effects of plasma surface treatment on the bond strength of zirconia with an adhesive resin luting agent.

The purpose of this study was to evaluate the effect of the atmospheric pressure plasma treatment as a surface treatment method on the contact angle and shear bond strength (SBS) of zirconia ceramics and the failure mode between the self-adhesive resin luting agent and zirconia. The zirconia specimens were divided into eight groups based on the surface treatment method: alumina blasting, air plasma, argon plasma (AP), Katana cleaner, ozonated water, ozonated water+AP, Katana cleaner+AP, and tap water+AP. The contact angles, SBS, and fracture modes were tested. AP treatment significantly reduced the contact angle (p<0.0001). The combination of AP and other cleaning methods showed a higher bond strength and more mixed fractures. Our findings indicate that using atmospheric pressure plasma with argon gas, combined with other cleaning methods, results in a stronger bond than when using alumina blasting alone.

Physico-Mechanical Properties and Bonding Performance of Graphene-Added Orthodontic Adhesives.

This study aimed to assess the key physico-mechanical properties and bonding performance of orthodontic adhesives with graphene addition for bonding a fixed retainer. Transbond LR (3M) and Transbond LV (3M) with no graphene were set as the control groups. Graphene was added into LR and LV at concentrations of 0.01 wt%, 0.05 wt% and 0.1 wt%. The stickiness of the uncured samples (n = 5) and real-time degree of conversion (DC) of the samples (n = 3) were measured over a 24-h period using Fourier-transform infrared spectroscopy. The hardness and other mechanical parameters, including the Martens hardness (HM), indentation modulus (EIT), elastic index (ηIT) and creep (CIT), were measured (n = 5). To measure the shear bond strength (SBS), adhesive composites were applied using a mold to bond the retainer wire to the lingual surfaces of bovine incisors (n = 10). Fracture modes subsequent to the SBS test were examined under light microscopy. Statistical analysis was conducted using ANOVA and Tukey tests (α = 0.05). In the LR groups, the LR + 0.01 showed the highest SBS (12.6 ± 2.0 MPa) and HM (539.4 ± 17.9 N/mm2), while the LV + 0.05 (7.7 ± 1.1 MPa) had the highest SBS and the LV + 0.1 had the highest HM (312.4 ± 17.8 N/mm2) among the LV groups. The most frequent failure mode observed was adhesive fracture followed by mixed fracture. No statistical difference was found between the graphene-added groups and the control groups in terms of the EIT, ηIT and CIT, except that the CIT was significantly lower in the LR + 0.01 than in the control group. Graphene addition had no significant adverse effect on the stickiness and DC of both LR and LV.

Mechanical behavior of mixed-mode I-II fracture in deep laminated shale based on finite discrete element method

Fracture patterns in deep-textured shales play a pivotal role in determining the structural stability of engineering constructions and optimizing energy utilization. This study utilizes a numerical model of a semicircular bending (SCB) specimen developed through the finite discrete element method to unveil the mixed fracture mechanical characteristics inherent in deep-textured shales. The investigation delves into the influence of prefabricated crack inclination, texture inclination, and texture strength on mixed fracture toughness, with the specimen’s damage process elucidated by analyzing the evolution of the acoustic emission distribution. The findings disclose the following insights: (1) An increase in the inclination of the texture leads to a concomitant rise in the tortuosity and length of the crack, followed by subsequent decreases. Elevating the inclination of the prefabricated crack reduces the requisite load force for specimen damage and significantly mitigates crack deflection. (2) Textured shale specimens manifest four distinct damage modes: penetrating extension, deflection extension, tangential extension, and mixed extension. The reduction in texture strength amplifies the probability of mixed extension and concurrently augments the shear damage rate of the specimen. (3) Mixed fracture toughness displays a linear decrease with crack inclination and a linear increase with texture strength. (4) The toughening mechanism of textured shales is categorized into three classes: separation along the texture, fracture path deflection, and texture cracking. The preeminent controlling effects on toughening mechanisms, in descending order of strength, are texture strength > texture inclination > prefabricated crack inclination.

Effect of electromigration on microstructure and properties of CeO2 nanopartical-reinforced Sn58Bi/Cu solder joints

To mitigate the decrease in mechanical performance of Sn58Bi/Cu solder joints resulting from electromigration-induced damage. The CeO2 nanoparticles were incorporated into Sn58Bi solder by a melt-casting method, and their effects on the microstructure and properties of Sn58Bi/Cu solder joints under electromigration were investigated. The study results demonstrate that the addition of 0.125 ~ 0.5 wt% CeO2 nanoparticles refines the eutectic microstructure of Sn58Bi solder alloy. At an addition amount of 0.5 wt%, the composite solder alloy exhibits the maximum tensile strength of 68.9 MPa, which is 37% higher than that of the base solder. CeO2 nanoparticle-reinforced Sn58Bi solder can achieve excellent solderbility with Cu substrates and the joints can significantly inhibit the growth of the anodic Bi-rich layer, which is responsible for electromigration. With the extension of current stressing time, Bi-rich and Sn-rich layer are respectively formed on the anode and cathode in the joints. The intermetallic compound (IMC) layer grows asymmetrically, transitioning from a fan-shaped morphology to a flattened structure at the anode and to a thickened mountain-like morphology at the cathode. Adding the CeO2 nanoparticles helps to mitigate the decrease in mechanical performance caused by electromigration damage during current application to some extent. Over the current stressing period of 288 ~ 480 h, the fracture position shifts from the anodic IMC/Bi-rich interface to the cathodic Sn-rich/IMC interface. The fracture mechanism transitions from a brittle fracture characterized by plate-like cleavage to a ductile–brittle mixed fracture with fine dimples and cleavage.

Experimental and virtual testing of mode II and mixed mode crack propagation under dynamic loading

Under static loading, measuring experimentally the mode II and mixed mode fracture toughness of composite materials and adhesive joints is well standardised. However, under dynamic loading, no standard procedure has been defined yet. Therefore, this paper proposes an experimental methodology to measure the mode II and mixed mode interlaminar fracture toughness of composite materials and adhesive joints. The methodology is based on a modified split Hopkinson compression bar. Two different data reduction schemes are explored and compared, one based on measuring the crack length, and another based on measuring the force from the strains in the transmitted bar. The two data reduction methods provided considerably different results. By using the method based on measuring the force, the mode II and mixed mode fracture toughness for both interlaminar and adhesive joints decreased for higher strain rates, while the opposite was found with the other approach. The method based on the crack length measurement was deemed to be unreliable due to the difficulties in measuring it.

Fracture surface microstructure and new fracture mechanism in the pearlite structure

The fracture behaviours of pearlite structures in several typical carbon steels were investigated by scanning electron microscopy (SEM). Microstructural analysis indicated that the fracture of pearlite is a mixed ductile-brittle fracture, and brittle fracture has cleavage fracture characteristics, which often occur inside the pearlite structure. The fracture surface can exhibit any orientation relationship with the ferrite/cementite interface plane in the pearlite structure. The two phases in the pearlite structure (ferrite (α-Fe) and cementite (θ-Fe3C)) are fine grains rather than single-crystal platelets. The cleavage fracture mechanism in any direction of pearlite structure occurs due to the fine grain substructure.

Comparative study on microstructure evolution, mechanical properties, and wear behavior of TiC and B4C single-reinforced and hybrid-reinforced Al–Mg–Si alloys by vacuum hot-press sintering

To explore the variances between single-reinforced and hybrid-reinforced Al–Mg–Si alloys with TiC and B4C, 10 wt.%-(TiC + B4C)/6061Al, 10 wt.%-B4C/6061Al, 10 wt.%-TiC/6061Al and 6061Al were prepared via wet mixing for 8 h followed by vacuum hot-press sintering at 580 °C and 30 MPa for 2 h. Microstructure evolution, mechanical properties, and wear behavior were investigated in this study. The results show that unlike single reinforcement of TiC, single reinforcement of B4C and hybrid reinforcement of TiC and B4C altered the distribution of Si and facilitated the in-situ formation of SiC. Both single reinforcement and hybrid reinforcement resulted in mixed fracture characteristics of ductile fracture and cleavage fracture, while hybrid reinforcement of TiC and B4C demonstrated superior strength and plasticity matching. At equivalent particle mass fractions, single reinforcement of TiC was more conducive to inducing recrystallization during sintering, followed by single reinforcement of B4C, and hybrid reinforcement of TiC and B4C had the least effect. At equivalent particle mass fractions, single reinforcement of B4C had the most significant effect on improving the wear resistance, followed by hybrid reinforcement of TiC and B4C, and single reinforcement of TiC had the least effect. All particle addition methods demonstrated a mixed wear mechanism involving abrasive wear, adhesive wear, and spalling wear. This study provides valuable insights into the development of aluminum matrix composites.

Finite–Discrete Element Method Simulation Study on Development of Water-Conducting Fractures in Fault-Bearing Roof under Repeated Mining of Extra-Thick Coal Seams

The formation of water-conducting fractures in overlying strata caused by underground coal mining not only leads to roof water inrush disasters, but also water-conducting fractures penetrate the aquifer, resulting in the occurrence of a mine-water-inrush disaster and the loss of water resources. It destroys the sustainability of surface water and underground aquifers. This phenomenon is particularly significant in extra-thick coal seams and fault-bearing areas. Numerical simulation is an effective method to predict the failure range of mining overburden rock with low cost and high efficiency. The key to its accuracy lies in a reasonable constitutive model and simulation program. In this study, considering that the three parts of penetrating cracks, non-penetrating cracks, and intact rock blocks are often formed after rock failure, the contact state criterion and shear friction relationship of discrete rock blocks and the mixed fracture displacement–damage–load relationship are established, respectively. Combined with the Mohr–Coulomb criterion, the constitutive model of mining rock mass deformation–discrete block motion and interaction is formed. On this basis, according to the engineering geological conditions of Yushupo Coal Mine, a numerical model for the development of water-conducting cracks in the roof with faults under repeated mining of extra-thick coal seams is established. The results show the following: The constitutive relation of the continuous deformation–discrete block interaction of overlying strata and the corresponding finite element–discrete element FDEM numerical program and VUSDFLD multi-coal seam continuous mining subroutine can numerically realize the formation process of faults and water flowing fractures in overlying strata under continuous mining of extra-thick multi-coal seams. The toughness of sand mudstone is low, and the fracture will be further developed under the repeated disturbance of multi-thick coal seam mining. Finally, it is stabilized at 216–226 m, and the ratio of fracture height to mining thickness is 14.1. When the working face advances to the fault, the stress concentration occurs in the fault and its overlying rock, which leads to the local fracture of the roof rock mass and the formation of cracks. The fault group makes this phenomenon more obvious. The results have been preliminarily applied and tested in Ningwu mining area, which provides theoretical support for further development of roof water disaster control under the condition of an extra-thick coal seam and avoids the loss of water resources in surface water and underground aquifers.

Comparative Study of the Mechanical Properties and Fracture Mechanism of Ti-5111 Alloys with Three Typical Microstructures

In this work, Ti-5111 alloys with equiaxed, bimodal and lamellar microstructures were prepared by various heat treatment processes. The room-temperature tensile properties, deformation microstructure and fracture mechanism of the alloys with different microstructures were investigated. Furthermore, the mechanism by which the microstructure affects the mechanical properties of Ti-5111 alloys with three typical microstructures was confirmed. The Ti-5111 alloy with a bimodal microstructure has minimum grain size and a large number of αs/β phase boundaries, which are the primary reasons for its higher strength. Simultaneously, the excellent coordination in the deformation ability between the lamellar αs and β phases is what enables the alloy with a bimodal microstructure to have the most outstanding mechanical properties. Additionally, the presence of a grain boundary α phase and the parallel arrangement of a coarse αs phase are the main reasons for the inferior mechanical properties of the Ti-5111 alloy with a lamellar microstructure. The fracture mechanism of the alloy with an equiaxed microstructure is a mixed fracture mechanism including ductile fracture and destructive fracture. The fracture mechanisms of the Ti-5111 alloy with bimodal and lamellar microstructures are typical ductile fracture and cleavage fracture, respectively. These findings serve as a guide for the performance improvement and application of the Ti-5111 alloy.

Fracture behaviour assessment of the additively manufactured and HPT-processed Al–Si–Cu alloy

Ultrafine-grained Al–9%Si–3%Cu alloy was achieved by a combination of laser powder bed fusion (LPBF) additive manufacturing and high-pressure torsion (HPT) processing in this investigation. The alloy was initially deposited layer-by-layer using a bi-directional scan strategy in LPBF with a scan rate of 1000 mms−1, a layer thickness of 40 µm and a hatch spacing of 200 µm, leading to a melt pool morphology with an average width of 150 µm and differing lengths. This led to a grain size of 722 nm and a dislocation density of 1.1 × 1014 m−2. This as-deposited alloy was then processed using HPT at room temperature using an applied pressure of 6.0 GPa and at a speed of one revolution per minute for different numbers of turns: half, one, five and ten turns. The alloy after HPT processing showed ultrafine grains with a grain size of 66 nm, well-dispersed nanosized intermetallic particles with sizes of 50–90 nm, the disappearance of the pool morphology and a notable dislocation density of about 6.2 × 1014 m−2 for the ten turns HPT-processed alloy. The as-deposited and subsequently HPT-processed samples were tensile tested at 298 and 573 K at different strain rates between 10−4 and 10−1 s−1. The elongation-to-failure and tensile strength were recorded and the fracture surfaces were also inspected using scanning electron microscopy and then correlated with the manufacturing, processing and tensile testing conditions. The alloy performance in tensile testing has been evaluated at ambient and elevated temperatures in terms of structural evolution and fractography for the first time. Ultrafine α-aluminium grains and nanosized eutectic silicon particles obtained by room temperature HPT-processing of the alloy have significantly improved the mechanical properties and microstructural stability at ambient and elevated testing temperatures for the HPT-processed additively manufactured alloy compared to the as-deposited additively manufactured and counterpart conventional alloys. The HPT-processed tensile samples showed a significant tensile strength of 700 MPa at 298 K and elongation-to-failure of 220% at 573 K, which is higher than that seen in the as-deposited tensile samples where 400 MPa and 106% are observed under the same testing conditions. Fractographic observations demonstrated that mixed brittle and shear ductile fractures dominated in the as-deposited tensile samples at 298 K, and tension ductile fracture dominated at 573 K. However, the HPT-processed tensile samples exhibited tension ductile and shear ductile fractures at 298 K, and tension ductile fracture at 573 K. The ultrafine-grained microstructure produced by the HPT application in the LBPF-manufactured alloy controls effectively the fracture mechanisms, dimple morphology and thus strength and elongation in comparison with the as-deposited additively manufactured microstructure.

Microstructure Evolution and Mechanical Properties of AlCoCrFeNi2.1 Eutectic High-Entropy Alloys Processed by High-Pressure Torsion.

High-entropy alloys (HEAs) have garnered significant attention for their exceptional properties, with eutectic high-entropy alloys (EHEAs) emerging as particularly notable due to their incorporation of eutectic structures comprising soft and hard phases. This study investigated the influence of shear strain on the microstructural refinement and mechanical properties of AlCoCrFeNi2.1 EHEAs, which were subjected to high-pressure torsion (HPT) at room temperature under a pressure of 6 GPa across 0.5 to 3 turns, compared to the initial material. After HPT treatment, significant grain refinement occurred due to strong shear strain, evidenced by the absence of B2 phase peaks in X-ray diffraction (XRD) analysis. Microhardness increased substantially post-HPT, reaching a saturation point at approximately 575 HV after three turns, significantly higher than that of the original sample. Moreover, the ultimate tensile strength of HPT-treated specimens reached around 1900 MPa after three revolutions, compared to approximately 1100 MPa for the as-cast alloy, with a mixed fracture mode maintained. This investigation underscores the efficacy of HPT in enhancing the mechanical properties of AlCoCrFeNi2.1 EHEAs through microstructural refinement induced by shear deformation, offering insights into the design and optimization of advanced HEAs for various engineering applications.

Investigation on microstructure and mechanical properties of GH5188 superalloy TLP joints using nickel based alloy interlayer

GH5188 was bonded successfully with a commercial BNi-2 interlayer through transient liquid phase (TLP) diffusion bonding. The characteristic bonded joint was formed by following three characteristic zones: diffusion affected zone (DAZ) mainly consisting of boride precipitates, athermal solidification zone (ASZ) composed of multiple silica-boride phases and isothermal solidification zone (ISZ) containing γ solid solution phases. As the holding time increases, ISZ gradually replaced ASZ and a defect-free joints was obtained then. However, excessive holding time can cause ASZ to reappear because of the extravagant dissolution of the base metal. The joints’ shear strength from 574 MPa (1190 °C/5 min) increased to 777 MPa (1190 °C/60 min), and the fracture location shifted from ASZ to DAZ with the fracture mode transferred from cleavage fracture to mixed fracture.

Blind prediction of curved fracture surfaces in gypsum samples under three-point bending using the Discontinuous Galerkin Cohesive Zone method

A computational framework based on a Discontinuous Galerkin (DG)/Cohesive Zone formulation is utilized to simulate the experiments of the Purdue Damage Mechanics Modeling Challenge. The inelastic response of the additively-manufactured gypsum material used in the experimental tests is modeled via a dilatational plasticity model. The constitutive and fracture model parameters are calibrated using the load–displacement curves corresponding to three-point bending tests initially provided by the Challenge organizers. The test samples contained initial notches especially designed to force specific types of mixed fracture modes. The calibrated computational modeling framework is used to blindly simulate the more complex configuration of the Challenge experiments. The numerical predictions of the load–displacement curve and the shape of the curved fracture surface are compared to the experimental data provided a posteriori. It is found that the computational method is able to quantitatively describe the fracture response of the material including crack propagation, plastic wake, and the curved geometry of the fracture surface that results from the evolving fracture mode mixity with significant fidelity.

Fracture mechanical and ablation behaviors of C/SiC–Cu3Si–Cu interpenetrating composites and their dependence on metal addition and interface thickness

The C/SiC–Cu3Si–Cu interpenetrating composites were prepared by the ceramization and metallization of carbon fiber reinforced carbon aerogel (C/CA) preforms. Fracture mechanical and ablation behaviors as well as their dependence on metal addition and PyC interface thickness were investigated. With the presence of metallic components, the C/SiC–Cu3Si–Cu composites show pseudo-plastic fracture, typical brittle-plastic mixed fracture, and then ladder-like fracture with the PyC interface thickness increasing due to the co-existence of ductile and brittle components, obviously different from the fracture of conventional ceramic matrix composites. The composite with 3.0 μm-thick interface has the highest flexural strength of 292.8 MPa due to the proper interfacial bond strength and residual compressive stress. The composite with 1.9 μm-thick interface has the highest work of fracture but relatively low fracture toughness due to the abundant modes of crack propagation. The anti-ablation property is deteriorated with increased PyC thickness due to the weakened transpiration cooling effect and reduced oxide scale quality. The composite with 0.9 μm-thick interface exhibits lower mass and linear ablation rates of 0.054 mg cm−2 s−1 and 0.018 μm s−1 after ablation at 2200 °C for 600 s, respectively. This work provides vital insights into the control of the overall performance of carbon fiber reinforced ceramic-metal matrix composites.

Revealing the failure mechanism of industrial Fe-25Cr-20Ni stainless steel plate: Fine-grained segregation band and brittle phase induced delamination cracking

The use of heat-resistant stainless steel thick plates in industrial production often presents challenges due to the presence of uneven microstructure and segregation, which can pose a persistent challenge for the prolonged safe operation of critical components made from such materials. This study aimed to investigate the fracture mechanism of an industrial Fe-25Cr-20Ni stainless steel plate at room temperature through uniaxial tensile experiments and fully reversed strain-controlled tension-compression fatigue experiments. Delamination cracks were observed on the fracture surfaces of both experiments. The specimens with fracture delamination were systematically studied by detailed microstructural evolution, and the failure mechanism and main reasons of fracture delamination were analyzed. The results show that the fracture delamination is related to the fine-grained segregation band (FGSB) consisted of fine grains and sigma phase located at the center of the thickness of the stainless-steel plate. The FGSB demonstrates low transgranular and intergranular fracture modes and exhibits bamboo knobs crack during plastic deformation, leading to a mixed fracture mode of ductile and brittle fracture. Furthermore, FGSB, as an anisotropic microstructure, which has a high stress concentration with nearby grains during plastic deformation. The sigma phase fracture and the weakening of the interface strength of the microstructure in FGSB provide a potential initiation site for fatigue crack initiation, thereby contributing to the possibility of aggravating fatigue damage and deteriorating fatigue performance. It is concluded that FGSB, as a metallurgical defect in stainless steel plates, is responsible for delamination failure.

Investigation of notch position and fiber ratio on the fracture mode and energy of sfrc beam under the impact load

When the literature is examined, it is seen that a lot of study has been done on the crack propagation and fracture modes that occur under the influence of static loads in reinforced concrete (RC) structures. Studies in the literature show the effect of crack initiation, concrete compressive strength, and fiber volume fraction on the tension mode (Mode-I), shear mode (Mode-II), and mixed fracture mode that occurs in crack propagation under static load. However, the literature has not found a comprehensive study in which the fracture modes are examined under the effect of dynamic impact loading. For this reason, an experimental study has been planned in which hybrid steel fiber reinforced concrete (SFRC) notched beams with varying compressive strength will be tested under impact loading with a free-fall drop weight device designed by the authors. The experimental variables studied were determined as the change of the crack initiation point in the beams, the compressive strength of the concrete used to produce the beams, and the fiber volume fraction in the concrete mixture. The acceleration and impact load-time histories under the constant-energy-level impact loading applied to the beams were measured. Also, the crack shape, fracture mode, crack angle, and energy absorption capacities of the beams under impact load were interpreted. While the transition from Mode-I fracture form to Mode-II fracture form occurs under impact loading, the effects of concrete compressive strength and steel fiber ratio variables on energy absorption capacity, crack angle, and crack shape are studied. Increasing the distance of the notch from the beam midpoint in notched beams produced with concrete without fiber from 0 to 40, 80 and 120 mm caused the energy absorption capacity values to increase by an average of 39 %, 66 % and 86 %, respectively. Increasing the distance of the notch to the beam midpoint from 0 to 40, 80 and 120 mm in beams with 0.75 % and 1.5 % fiber ratio increased the energy absorption capacity values by an average of 40 %, 65 % and 86 %, respectively. In beams with 2.25 % fiber ratio, increasing the notch distance from 0 to 40, 80 and 120 mm increased the energy absorption capacity values by an average of 38 %, 62 % and 83 %, respectively.

Improving fatigue resistance of ultrafine bainitic steel by exploiting segregation-induced bands

An ultrafine bainitic steel was designed with segregation-induced martensitic/austenitic (MA) bands. The steel comprises almost 21 % retained austenite (RA); the rest is mainly bainitic ferrite and a small fraction of martensite. The steel exhibits room temperature (RT) yield strength of ∼914 MPa, ultimate tensile strength of ∼1969 MPa, and total elongation to failure of ∼11.5 %. Low cycle fatigue (LCF) behavior was investigated under a fully reversed strain-controlled waveform at RT, with a loading-axis parallel to the MA bands. LCF stress response reveals initial cyclic hardening followed by softening or saturation, depending on the applied strain amplitude. This is attributed to the interplay of strengthening mechanisms, which include RA transforming to martensite and dislocation multiplication/interactions and the softening effects of dislocation annihilation and crack formation/propagation. Damage studies reveal a mixed fracture mode, displaying brittle striations, secondary cracks, quasi-cleavage facets, and dimples. The secondary cracks mainly appear to originate from the main crack and propagate predominantly along the matrix-bands interphase regions or within MA bands. Additionally, surface-initiated cracks along their propagation path were deflected and branched around the interphase regions. This tortuosity reduces the crack propagation rate. Overall, the significant work hardening capability of the steel, along with its banded microstructure, contributes to its superior fatigue resistance compared to non-banded bainitic steels reported in the literature.

Influence of microstructure evolution on hot ductility behavior of a Cr and Mo alloyed Fe-C-Mn steel during hot deformation

Cr and Mo alloyed Fe-C-Mn steel was conducted by hot tensile experiments under different strain rates and deformation temperatures. The influence of microstructure evolution on fracture mechanism was studied. The principal failure mechanism was attributed to intergranular crack propagation and microvoid aggregation when deformed at 800–1100 ℃, changing to a mixed ductile-brittle intergranular fracture when deformed at 1200 ℃. After hot deformation, the microstructure transformed from a mixed phase of pearlite and ferrite to a lath martensite structure. Moreover, the lath martensite structure transitioned from disorder to order as the deformation temperature increases. The study revealed that in case of lower temperature when dynamic recovery (DRV) was dominated, the growth of voids was impeded by fine dynamic recrystallization (DRX) grains, resulting in an irregular void morphology. In case of higher temperature, the degree of DRX was high, voids were surrounded and impeded by newly formed DRX grains, which were gradually elongate with deformation. On the other hand, when there were DRX grains present between voids, void coalescence was less likely to occur, thereby inhibiting damage propagation.