Crack Origin Research Articles

Fracture behavior, mechanical properties, and microstructural analysis of vacuum automated GMAW welded dissimilar A353 and A516 steels for pressure vessels

In this study, the performance of the automatic gas metal arc welding (GMAW) process in vacuum for similar and dissimilar joints of A353 and A516 steel plates with different thicknesses was intensively evaluated. Fracture of all vacuum-welded samples occurred in the heat-affected zone (HAZ) of A516 steel, highlighting the significant influence of HAZ microstructure and width on the cryogenic impact energy and strength of these joints. According to microscopic observations and impact tests, the fractures were fully ductile for notches in the weld and A353 HAZ, where dimples were observed on the fracture surfaces and the impact energy exceeded 50 J. This ductile behavior is particularly significant for notches in the weld due to the presence of a vacuum during welding prevents the formation of gas voids and oxide and hydride compounds that are normally the origin of cracks. However, for notches in the A516 HAZ, where the cleavage faces were observed on the fracture surfaces and the impact energy was less than 30 J, the fractures were quite brittle. For dissimilar vacuum-welded samples, the impact energy and fracture type for notches in the weld and A353 HAZ were nearly the same as those in similar A353 joints. However, for notches in the A516 HAZ, the impact energy and fracture type were close to those observed in the A516 joints. Yield strength and ultimate tensile strength (UTS) of A353-A353 non-vacuum-welded samples compared to vacuum-welded samples decreased by more than 78% and 40%, respectively. As well as, yield strengths and UTSs of A516-A516 non-vacuum samples compared to vacuum samples decreased by more than 34% and 4%, respectively. On the other side, the impact samples with a notch position in the weld suffered brittle fracture instead of ductile fracture because their impact energy decreased by about 40%.

Fatigue-induced fracture failure of acrylic-polycarbonate laminated aircraft canopy

The fatigue-induced fracture failure of the aircraft canopy occurred in the poly(methyl methacrylate) (PMMA) layer laminated on polycarbonate (PC) during flight. For more than 24 years, the aircraft had been operated at high altitudes and supersonic flight. To identify the root cause and the mechanism for the formation of the fracture, the fracture surfaces were investigated. The fracture morphologies were characterized using optical microscope (OM) and scanning electron microscope (SEM).In macroscopic observations, the main crack showed a total length of approximately 1.6 m from the front to the right of the crack stop groove when viewed from the front of the canopy. The main crack ran about 0.9 m including partly curved line from the front part to the upper middle one and then reached about 0.7 m in a straight line perpendicular to the right of the crack stop groove. There were two crack ends in the main crack: one was at the lower part of the front, the other was at the right end of the crack stop groove. Numerous macro-cracks visible to the naked eye were distributed only on the front surface of the canopy.In microscopic examination, the voids on the front surface of the outer PMMA layer were formed by the friction heat with air during the supersonic flight. The voids served as the origins, the actual starting point of the crack. The voids slowly grew to macro-cracks vertically or horizontally by the thermal stress during flying at high altitudes. Cracks proceeded in the direction of 90 degrees while being bisected in V-shapes downward from the surface of the PMMA layer with the action of thermal tension. The crack growth represents the typical characteristics of the fatigue crack: multi-origins, ratchet marks, and beach marks. The main crack grew further, forming a slight curved line by connecting adjacent macro-cracks arranged in an almost vertical direction. When crack growth reached a critical point, the catastrophic fracture progressed rapidly from the primary origin of the fatigue crack to both ends due to the action of lateral force. Fast crack zones on both sides showed the same dimple and river patterns.This study explains that the combined and synergistic interaction of the fatigue crack and environmental stresses iteratively occurred on the front surface in the outer PMMA layer of the aircraft canopy due to the continual exposure to high altitudes and supersonic flights, consequently resulting in the fatigue-induced fracture failure.

Modeling of elastic plastic deformation of near-surface layers of materials

According to the results of strain measurements, a kinematically hardening body model is used to describe the process of elastic-plastic deformation of 1X18H9T steel. Special attention is paid to the processes occurring in the surface layers. The model takes into account the increasing tightness of shear deformations deep into the material. The increase in the stress of the plastic flow in depth is described by a second-order polynomial. The main parameters of the surface effect were determined experimentally and by calculations using the method of reduction coefficients in the process of successive approximations: depth, coefficient of hardening of the material, stresses of plastic flow on the surface and inside the material. It is shown that in order to study the near-surface effect by the strain gauge method, it is necessary to give preference to bending tests of samples rather than stretching. The presence of the surface effect explains the following facts: the destruction of the sample during the tensile test does not begin from the surface, but from the inside, the origin of fatigue cracks occurs under the surface, the surface effect practically does not affect the deformed state of the elastic body, but very strongly affects the stress state at the surface.

The Effect of Shot Blasting Abrasive Particles on the Microstructure of Thermal Barrier Coatings Containing Ni-Based Superalloy

Grit particles remaining on the substrate surface after grit blasting are generally considered to impair the thermal performance of thermal barrier coatings (TBCs). However, the specific mechanisms by which these particles degrade the multilayer structure of TBCs during thermal cycling have not yet been fully elucidated. In this study, the superalloy substrate was grit-blasted using various processing parameters, followed by the deposition of thermal barrier coatings (TBCs) consisting of a metallic bond coat (BC) and a ceramic top coat (TC). After thermal shock tests, local thinning or discontinuities in the thermally grown oxide (TGO) layer were observed in TBCs where large grit particles were embedded at the BC/substrate interface. Moreover, cracks originated at the concave positions of the TGO layer and propagated vertically towards BC; these cracks may be associated with additional stress imposed by the foreign grit particles during thermal cycling. At the BC/substrate interface, crack origins were observed in the vicinity of large grit particles (~50 μm).

Fe-based metallic glass coatings with suppressed cracks and enhanced wear resistance prepared by extreme high-speed laser cladding

Fe-based metallic glass (MG) coatings draw great attentions due to their excellent wear resistance. The recently developed extreme high-speed laser cladding (EHLC) provides a promising method for their fabrication but limited by the strong tendency to crack. Besides that, the deep insight into the structure evolution and the failure mechanism of the coating is still lacked. Thus, the understanding of the crack origin and the prevention of crack are of great importance. In the present work, Fe-Mo-Cr-Y-C-B MG coatings with adjustable glassy phase content were successfully fabricated by EHLC. The microstructure characterization of the coatings reveals that the cracks is main caused by the hard and brittle nanocrystalline phases precipitations of monoclinic Mo12Fe22C10 and (Fe, Cr)23C6 carbides in the heat affected zone. Therefore, the cracking of the coatings can be effectively reduced by increasing the glassy phase content. Subsequently, the hardness distribution, wear performance and wear mechanism of the coatings were investigated. The results showed that increasing the glassy phase content can effectively achieve low wear rate. The prepared Fe-Mo-Cr-Y-C-B MG coatings exhibit abrasion loss as low as 3.54·10−6 mm3 (N m)−1, which is an order of magnitude lower than that of the 45# steel substrate. The wear of the coatings is primarily attributed to the fatigue wear accompanied with slighting oxidative wear. By suppressing the brittle precipitated crystalline phases in the coating, the fatigue wear can be reduced and the wear resistance of the coatings can be improved. The present work provides insights into the crack prevention and were performance improvement of the Fe-based MG coatings prepared by EHLC.

Failure characteristics and pressure relief effectiveness of non-persistent jointed rock mass with holes

In tunnel engineering, the rock mass contains a significant number of irregularly distributed joints, and typically exhibits high energy accumulation, thereby posing a risk of rockburst occurrence. Therefore, it is of paramount importance to investigate the fracture propagation behavior in jointed rock masses and assess the impact of borehole pressure relief on mitigating rockburst occurrences for effective prevention and control measures. This paper focuses on failure characteristics and pressure relief effectiveness of non-persistent jointed rock mass with holes through laboratory testing and numerical simulation. In laboratory experiments, rock samples are prepared to include a range of crack dip angles and circular holes. Then, the crack propagation law of crack inclination and circular hole is studied by AE and DIC technology. The experimental results show that with the increase of fracture dip angle, the peak strength and energy change of the sample decrease first and then increase. Due to the existence of holes, the crack propagation direction of the original crack is changed. After drilling, the strain energy of the sample is obviously reduced, which shows that the drilling pressure relief effect is obvious, which can effectively reduce the energy accumulated inside the rock mass and reduce the risk of rockburst. Finally, the PFC numerical simulation software is used to analyze the micro-failure process and energy change law of the sample from three aspects: the relative position of cracks and holes, the diameter of boreholes and the spacing of boreholes. Further understanding of the energy dissipation law and mechanical behavior characteristics of jointed rock mass provides a reference for exploring the pressure relief effect of rock mass and preventing rockburst.

Enhanced an eutectic carbide reinforced niobium alloy by optimizing Mo and W alloying elements

Niobium alloys play an indispensable role in aerospace technology. However, traditional niobium alloys has unsatisfactory or high-temperature strength or limited room-temperature. This work introduces Nb2MoxWyC0.25 alloys with strength-plasticity balance by optimizing refractory alloy elements and eutectic carbides drawing on the design concepts of eutectic high-entropy alloys. The influence of Mo and W contents on the microstructure and mechanical properties of the alloys was studied. The Nb2MoxWyC0.25 alloy contain body-centered cubic (BCC) primary phase and eutectic structures composed of BCC and carbide phases with semi-coherent interfaces. Appropriate additions of Mo and W refine the grain size of the primary BCC phase and cause the carbide phase to evolve from Nb2C to NbC. The Nb2Mo0.5W0.5C0.25 hypoeutectic niobium alloy composed of BCC and Nb2C phases with a relatively small lattice mismatch has a room-temperature yield strength of 1.27 ± 0.04 GPa, compressive strength of 2.03 ± 0.06 GPa, and a fracture strain of 17.8 ± 2.2 %. Solid solution strengthening in the BCC phase and second-phase strengthening of the carbide phase simultaneously enhance the alloy. The fine grain strengthening, reduced crack origination at low mismatch interface, and the crack tip by soft BCC at the phase interface improve the plasticity simultaneously. This paper provides a method to improve the room-temperature plasticity and strength of refractory niobium alloys, laying the foundation for the industrial application of refractory niobium alloys.

An Underwater Crack Detection System Combining New Underwater Image-Processing Technology and an Improved YOLOv9 Network.

Underwater cracks are difficult to detect and observe, posing a major challenge to crack detection. Currently, deep learning-based underwater crack detection methods rely heavily on a large number of crack images that are difficult to collect due to their complex and hazardous underwater environments. This study proposes a new underwater image-processing method that combines a novel white balance method and bilateral filtering denoising method to transform underwater crack images into high-quality above-water images with original crack features. Crack detection is then performed based on an improved YOLOv9-OREPA model. Through experiments, it is found that the new image-processing method proposed in this study significantly improves the evaluation indicators of new images, compared with other methods. The improved YOLOv9-OREPA also exhibits a significantly improved performance. The experimental results demonstrate that the method proposed in this study is a new approach suitable for detecting underwater cracks in dams and achieves the goal of transforming underwater images into above-water images.

Flexural behavior and numerical simulation of reinforced concrete beams with a UHPC stay-in-place formwork

To overcome the shortcomings of traditional concrete structures such as long construction cycle, high construction cost and poor durability, seven composite beams with a UHPC stay-in-place formwork (UCB) and one reinforced concrete (RC) beam were designed. An experimental study was conducted to investigate the flexural behavior of composite beams under the static load. The research parameters include formwork thickness, reinforcement rate, and formwork surface treatment. The results revealed that roughening the surface of UHPC formwork has a positive impact on enhancing the integrity of composite beams. The utilization of UHPC stay-in-place formwork can effectively improve the stiffness, cracking load, and peak load of members. Compared with that of the RC beam, the cracking load and peak load of composite beams increased by 63% ~ 103% and 6% ~ 15%, respectively, and the yielding stiffness increased by 23% ~ 41%. Different from the RC beam, new cracks occurred on one side of the initial crack's end and continued to extend upwards rather than along the original crack. The existence of a significant quantity of tiny cracks in composite beams effectively slowed down the increase in the width of pre-existing cracks in the early stage of loading. Due to the multi-crack pattern exhibited by the UHPC formwork, the strain concentration of the steel bars was effectively mitigated. When subjected to the same load level, the strain of the longitudinal steel bars in composite beams was smaller than that of the RC beam. The formula for calculating the flexural bearing capacity of composite beams is established through theoretical analysis. In addition, the numerical model of the composite beam is established by the finite element method (FEM). The influence of the contact method of the UHPC-NC interface on the flexural performance of the composite beam is explored. The supplementary analysis of the parametric study of the reinforcement rate and the UHPC formwork thickness is carried out. When using ABAQUS for the numerical analysis of the composite beam, it is appropriate to adopt the cohesion model or the Tie model for the UHPC-NC interface, and it is not appropriate to use the Coulomb friction model.

Electromagnetical and ultrasonic characterizations of concretes subjected to internal swelling reactions

Among pathologies of reinforced concrete structures, internal swelling reactions (ISR), including alkali-aggregate reaction and delayed ettringite formation, are at the origin of cracks and major disorders due to rebar corrosion. Visual evaluation of crack density combined to non-destructive testing techniques can be used to characterize the global swelling and then give some structural diagnosis. For the last ones, an intermediate step (assimilated to a calibration step) can be performed at laboratory to evaluate the sensitivity of electromagnetic, electrical and ultrasonic properties of concretes subject to ISR.This present study focuses on such characterizations of concrete samples presenting different levels of ISR and for several water content. Numerous samples have been extracted from mock-ups representative of two massive concrete structures, affected one by alkali-aggregate reaction and the other by delayed ettringite formation, and conditioned in homogeneous and controlled conditions. After describing the experimental campaign, results are shown and commented.

Mechanism of methanol-to-olefin (MTO) reaction

The full name of MTO is methanol to olefin, which is a new chemical reaction process to gain an important chemistry industry raw material. In this paper, we mainly talk about several general reaction process as well as the mechanism of the reaction. The use of molecular sieve catalytic process is a key point where the new MTO process different form the original simple petroleum cracking in order to again olefin. In this work we mainly discuss how the MTO process could happen with the assist of the molecular sieve catalyst. There are direct mechanism Carbene mechanism and Methane-formaldehyde mechanism. The effect of the direct mechanism is to gain some simple and short Carbon-Carbon product. And then the indirect mechanism will take on to control the rate of the reacting process and the ration of different products. By studying on the mechanism of the MTO reaction, we can figure out how a powerful catalyst, molecular sieve catalyst, works in the reaction. Also, the MTO process is a meaningful way to take place of the origin cracking process to gain olefin, as the storage of petroleum is decreasing fast recently.

Molecular dynamics simulation of crack growth in nanocrystalline nickel considering the effect of accumulated plastic deformation

Molecular dynamics simulations are conducted to investigate atomic stress and plastic strain distribution in pre-cracked crystalline nickel, along with microstructure evolution. The plastic strain calculation method is improved by refining neighboring atom selection. A new parameter for measuring plastic strain concentration allows for comparison across different crystallographic directions. The effects of temperature, strain rate, crystallographic direction, loading mode, grain boundary and grain size are evaluated. At elevated temperature, crack tip blunting becomes obvious, and crack propagation decelerates. The crack propagates through void nucleation, growth, and eventual linkage with the original crack. Higher temperature increases the plasticity limit of void nucleation. Strain rates show a positive correlation with plasticity limit but a negative correlation with crack growth rate and final length. Crystallographic direction influences crack propagation, with crystal 100 exhibiting significant crack propagation, while 110 and 111 demonstrating marked resistances to crack propagation. Crystal 100 has the highest plastic deformation concentration near the crack tip. Loading mode affects plastic deformation accumulation. In-plane shear results in less plastic deformation and less obvious crack propagation. In bi-crystals, the relative rotation angle between grains significantly influences crack propagation. X-rotation of the right grain most significantly impedes crack propagation, while y- and z-rotation tend to cause the crack to cross the grain boundary. In polycrystals, plastic deformation accumulation at grain boundaries is significant, with the crack tending to propagate along grain boundaries. Plastic strain and concentration parameter peaks are lower in grain boundaries than in single crystals, indicating weaker stability. The results demonstrate that the failure process of crystalline nickel is intricately linked to the stress and plastic strain distribution around the crack tip. The plastic strain and concentration parameter are more accurate predictors of crack propagation than atomic stress.

Harmful Effects, Origin, and Control of Banded Structures in Electroplated Fasteners of High‐Carbon Steel

In this study, the harmful effects of a banded structure on the bending fracture of electroplated fasteners are investigated. The origins and ways to control these structures are also explored. The fracture surfaces of the fasteners exhibit stepwise cracks and numerous intergranular facets associated with ductile tearing, which is a characteristic of hydrogen embrittlement. The origin of these stepwise cracks can be traced to the internal banded structures. Therefore, the internal banded structures promote fracturing during the bending test. The internal banded structures in the fasteners can be traced back to the internal cracks in the bloom, and inappropriate soft‐reduction processes lead to internal cracks. Notably, the carbon segregation at the internal cracks is severe. This is attributed to the proximity of the internal cracks to the center of the bloom, causing the enriched liquid in the interdendritic region to be sucked into these internal cracks. The optimal reduction position and amount are determined by employing heat‐transfer and soft‐reduction models. Fluctuations in superheat and casting speed significantly influence this optimal position and reduction amount.

Effect of water quenching temperature on microstructure and tensile properties of IN718 coatings during heat treatment

To enhance the strength and toughness of laser-clad IN718 coating on a 27SiMn steel surface, the heat treatment process of "heating + water quenching + natural cooling+ tempering+ natural cooling" was used. Keeping other parameters unchanged, different water quenching temperatures were used to heat treat the forming IN718 coatings. The properties of IN718 coatings after heat treatment were analyzed. The results show that with the increase of water quenching temperature, the elemental distribution of the coating is more uniform, the elemental diffusion tends to be stable, and the organization grain size of the coating is gradually reduced. The mechanical strength of the coating increases and then decreases, and the elongation gradually increases. The coating is well combined with the substrate, and the crack origin areas of the tensile specimens are located in the substrate area. Under the process of "heating temperature of 900 ℃+ water quenching to 450 ℃ + natural cooling + tempering temperature of 480 ℃ + natural cooling", the comprehensive performance of IN718 coating is optimal. Its organization is a mixture of a large number of fine dendrites and cellular crystals interspersed with a small number of coarse columnar crystals and dendrites. Its yield strength is 1293±23 MPa, tensile strength is 1407±30 MPa and elongation is 14.4±0.8 %.

An adaptive phase field approach to 3D internal crack growth in rocks

Modeling the 3D internal crack under compression entails complex fracture mechanics (mode I-II-III fracture), resulting in substantial computational costs and challenges in characterizing fracture morphology characterization for Phase Field Method (PFM) simulations. In the present paper, we developed a 3D adaptive crack propagation model integrated into the PFM to study the internal cracks in rocks. The proposed locally refined solution was implemented within the isogeometric analysis (IGA) framework, utilizing PHT-splines and employing the phase field variable as an error indicator to accurately capture non-smooth crack surfaces. The simulation begins with a coarser mesh, and the spatial discretization is adaptively refined in real-time as the computation progresses and the phase variable threshold is reached. By comparing our results with previous numerical tests, we validated the effectiveness of our model in simulating crack growth. Notably, a smaller length scale parameter led to a higher peak force in notched square plates under tension, and our adaptive PFM successfully reduced errors caused by length scale parameters. We further validated our model by comparing it with uniaxial compression experiments on 3D-printed resin samples with internal penny-shaped cracks, achieving good agreement with the experimental data. Using the 3D adaptive PFM, we predicted changes in crack morphology and fracturing mechanisms by varying the inclination angles of internal penny-shaped cracks. Our numerical results showed that as the inclination angle increased, wing cracks initiated closer to the end tips of the original crack, and their length decreased due to the wings wrapping effect. The corresponding fracturing mechanisms transition from mode I-III fracture (0°) to mode I-II-III fracture (30°, 45°, 60°), and eventually to mode I fracture (90°) as inclination angles increase. Comparing 2D and 3D models, we found that the growth of wing cracks in 3D was significantly influenced by the intermediate principal stress. We conclude that the extended 3D adaptive PFM-based model accurately predicted complex mechanical behavior and crack growth patterns in rock.

Assessing Activation Quality through Evaporative Drying Patterns of Zr-MOF (UiO-66) Colloidal Droplets.

We demonstrate a simple droplet diagnostic approach to monitor the UiO-66 MOF (metal-organic framework) synthesis and its quality using the sessile droplet drying phenomenon. Drying a sessile droplet involves evaporation-driven hydrodynamic flow and particle-nature-dependent self-assembled deposition. In general, the MOF synthesis process involves different sizes and physicochemical nature of particles in every synthesis stage. Equivalent quantities of each of purified pore-activated UiO-66 MOF, yet-to-be-purified pore-inactivated UiO-66 MOF, and reaction precursors of UiO-66 MOF give different deposition patterns when a well-dispersed aqueous droplet of these materials undergoes drying over substrates of varying stiffness and wettability. Yet-to-be-purified, pore-inactivated UiO-66 MOF nanoparticles undergo transport toward the droplet periphery, leading to a thick ring-like deposition at the dried droplet edge. Under appropriate drying conditions, such a deposit leads to desiccation-type mud-like reticular cracking. We study the origin of such ring-like deposits and cracks to understand how the surface charge density of UiO-66 particles controls their stability. We demonstrate that ZrOCl2 salt trapped in a nonpurified pore-inactivated UiO-66 MOF moiety is the principal reason for ring-like deposit formation and subsequent cracking in its dried aqueous droplet edge. Qualitatively, we identified Lewis acid salts that are capable of acting as Bro̷nsted acid upon hydrolysis (like FeCl3, SnCl2, and ZrOCl2), influence surface charge density and colloidal stability of dispersed UiO-66 MOF particles. As a result, immediate particle coagulation is avoided, so those travel to the droplet edge, forming ring-like deposition and subsequent cracking upon drying. Further, we show that crack patterns on such deposits are highly dependent on the stiffness and temperature of depositing substrates via a competition between axial and lateral strains at the deposit-substrate interface.

Coherence analysis of the crack strain field in coal rock with borehole-crack composite defects

Cracks in the coal mass surrounding boreholes for gas extraction will significantly impact the efficiency of gas extraction. The primary objective of this research is to explore the effects of original cracks and borehole grouting on cracks propagation in the coal mass around the boreholes. Accordingly, this research employs coal mass with borehole, borehole-cracks composite coal mass, and borehole-cracks-grouting coal mass as experimental subjects. The digital image correlation (DIC) platform of the coal deformation and damage was used to carry out the damage experiment of the coal mass around the boreholes. The Matlab GUI crack detection system was used to extract the crack skeleton, and time domain characteristics of strain and strain rate coherence coefficients in the area near the cracks before and after grouting were obtained according to the Pearson product-moment coherence analysis method. The research findings obtained in this article are as follows: (1) The tips of the prefabricated fractures act as stress concentration zones, where the further extension of wing cracks severely degrades the continuity and uniformity of the coal mass. The injection of ultrafine cement significantly enhances the overall deformation resistance of the coal mass. However, the local stiffness under specific stress conditions shows limited improvement due to the presence of micro-damage around the prefabricated cracks. (2) The expansion of the cracks around the borehole and the wing cracks at the tips of the prefabricated cracks is primarily due to tensile forces. As the cracks extend to the top of the specimen, they transition to a mixed mode of tensile-shear, with the extension of Type II wing cracks being faster and more unstable compared to Type I. (3) The correlation coefficient between multiple crack fields and their highly significant characteristics indicate that crack propagation influences each other. This influence diminishes as the specimen progressively damage. (4) The mechanical properties of the specimen show a strong correlation with the average interference coefficient R̅ of the cracks. Better mechanical properties of the coal rock are associated with higher geometric similarity in the strain and strain rate curves across multiple strain fields, the cracks tend to propagate synchronously.

Role of Cr addition in Ni-based interlayer on strengthening titanium and steel dissimilar bimetallic structures enabled by directed energy deposition with laser beam

For searching alternative strategies to improve reliability of titanium and steel dissimilar bimetallic joints manufactured by directed energy deposition with laser beam (DED-LB), pure titanium was considered as cladding deposited on carbon steel substrate with Ni-based alloy interlayers in this work. Effect of different interlayer modification methods on the microstructure evolution and mechanical properties of joints was analyzed systematically. The distribution of intermetallic compounds (IMCs) such as β-Ti, Ti2Ni, TiNiFe0.2, Ti2Ni3Si and TiB2 in joints was revealed. The results showed that original deposition cracks caused by residual stress during processing could be alleviated by substrate preheating treatment while suppressed by the modified interlayer with Cr completely. Notably, additional Cr could reduce reaction activity between Ti and Ni atoms by raising laser molten pool liquidus, leading to fewer IMCs in joints. As a result, both bonding strength and toughness of joints were remarkably improved. The findings emphasize more significance of optimizing Ni-based interlayer composition with Cr than preheating method to improve the mechanical performance of DED-LB joints.

Study on the mechanism of energy evolution and bearing degradation in pre-cracked sandstone under non uniform cyclic loading

The engineering rock mass is affected by the original cracks, which increases the uncertainty of instability and failure mechanisms. Under the action of excavation unloading or other disturbance loads, its bearing properties are more complex and the deformation characteristics is more special, which has always been a difficult issue in the analysis of rock mechanics. In order to explore the propagation and failure mechanism of spatial micro cracks in fractured rock masses, acoustic emission (AE) experiment of sandstone was carried out under pre-static loading and low-frequency stepwise cyclic loading–unloading. The research results indicate that the peak strain and crack initiation stress of rocks with different pre-cracked positions have the highest differences of 12.62% and 18.65%, respectively. The difference between peak stress and crack initiation strain is within 4%. The pre-cracked position mainly affects the peak strain and crack initiation stress of pre-cracked rocks, and its effect on peak stress and crack initiation strain is limited. The crack initiation angle of prefabricated cracks is mainly distributed within the range of 45° − 90°, and it decreases with the increase of quantities of prefabricated cracks. The crack initiation angle of rocks with prefabricated three cracks is 30% lower than that of single crack. The research results have deepened the analysis of energy evolution and deformation damage of fractured rock masses.

Biaxial flexural strength of hydrothermally aged resin-based materials

PurposeThe strength of temporary restorations plays a vital role in full-mouth reconstruction, and it can be impacted by the aging process. The aim of this in vitro study was to evaluate the biaxial flexural strength and fractographic features of different resin-based materials submitted to thermal aging. Material and methodsOne hundred and ninety-two resin disc-shaped specimens (6.5 mm in diameter and 0.5 mm in thickness) were fabricated and divided into six experimental groups according to the resin-based materials (Filtek Bulk-Fill flowable resin; J-Temp temporary resin; and Fuji Lining glass ionomer cement) and aging process (before and after thermal cycling). Biaxial flexural strength test was performed using a universal testing machine at a crosshead speed of 0.5 mm/min before and after thermal cycling (5 °C and 55 °C, 5760 cycles, 30 s). The mechanical properties were assessed using Weibull parameters (characteristic strength and Weibull modulus) (n = 30). Fractured specimens were examined under a polarized light stereomicroscope to identify crack origin and propagation direction. The surface microstructure of the resin-based materials was assessed by scanning electron microscopy (n = 2). The Weibull modulus (m), characteristic strength, and reliability properties were calculated, and a contour plot was used to detect differences among groups (95% confidence interval). ResultsThe Weibull modulus (m), characteristic strength, and reliability of the resin-based compounds were influenced by material type and thermal aging (p < 0.05). Weibull modulus (m) revealed no differences when comparing the materials and aging process (p > 0.05), except for the preceding aging period where Filtek Bulk-Fill exhibited higher values compared to J-Temp (p < 0.05). Filtek Bulk-Fill demonstrated superior characteristic strength and reliability compared to J-Temp and Fuji Lining before and after thermal cycling (p < 0.05). Fractography of the resin-based materials showed fractures originating from surface defects exposed to tensile side and their propagation toward the compressive side. Generally, no differences in surface microstructure were observed on micrographs before and after thermal aging for Filtek Bulk-Fill and Fuji Lining. However, the aging process developed flaws in J-Temp. ConclusionResin-based material composition resulted in different flexural strength performance, impacting the Weibull modulus (m), characteristic strength, and reliability of the resin-based restorations.