Crack Propagation Mechanism Research Articles

In‐situ reaction sintered ReB2–B4C composite by reactive spark plasma sintering

AbstractThe high cost of ReB2 limits its application as a member of novel high‐hardness materials, which can be alleviated by forming composites with cheap compound(s). In the present study, a dense ReB2–B4C composite was fabricated via in situ reactive sintering by spark plasma sintering with Re and B13C2 (Re:B = 1:20) as raw materials. The phase composition, microstructure, Vickers hardness, and fracture toughness of the obtained composite were characterized, and the reaction mechanism, crack propagation, and toughening mechanism were clarified. XRD, SEM, Raman, and XPS results revealed that the obtained composite is mainly composed of ReB2 and B4C phases, with a small amount of free B and graphite coexisted. Vickers hardness of 33.5 ± 4.4 GPa under 0.49 N load and 24.6 ± 1.5 GPa under 4.9 N load and fracture toughness of 5.02 ± 0.37 MPa·m1/2 were observed for the obtained composite. The toughening mechanism of the obtained ReB2–B4C composite including crack deflection crack and cracking bridging was analyzed in detail.

Experimental investigation of crack propagation mechanism in refill friction stir spot joints of AA6082-T6

Since many aluminum alloys preferred in structural engineering can be welded conventionally only with great effort and energy input interest in alternative joining techniques is growing, such as solid state joining processes. In this work, the effect of refill friction stir spot welding (refill FSSW) on the crack propagation behavior in AA6082-T6 is studied. To be able to identify the individual fracture mechanism, refill FSSW was performed as a blind weld, i.e. only in one sheet designed as C(T)100 specimens. The vertical distance between notch and spot weld was varied and tested in two phases. First, a cyclic pre-crack was induced and then the specimen was caused to fail in quasi-static conditions, resulting in two different fracture modes. The results showed that the cyclic crack is dominated by residual stresses but the microstructure mainly influences the quasi-static crack propagation. It was also found that a stress concentration occurs in the transition area even without a hook. Furthermore, it was found that the crack propagation is not exclusively driven by the local strength but also by the angle at which the crack hits the spot weld.

Analysis of Micro-Evolution Mechanism of 3D Crack Initiation in Brittle Materials with Hole under Uniaxial Compression.

This article investigates the microscopic mechanism of crack initiation and propagation in three-dimensional embedded cracks in brittle materials containing circular holes. First, a method for the development of transparent, brittle materials is proposed. Second, UCS tests were conducted on transparent, brittle materials containing circular holes and internally embedded three-dimensional cracks. Finally, a numerical model was established in PFC3D to analyze the crack initiation and propagation mechanism. The results show that when α = 0° (α refers to the pre-existing crack inclination), the upper tip of the pre-existing crack appears as a tensile wing crack, and the lower tip of the pre-existing crack appears as a tensile-shear mixed crack. When α = 30°, no wing crack appears, and the tensile crack on the fracture surface only appears after the hole cracks. When α = 60 and 90°, a tensile wing crack and an anti-wing tensile-shear mixed crack appear at the upper tip of the pre-existing crack. A tensile wing crack appears at the lower tip of the pre-existing crack and appears "self-limiting". During the propagation of wing cracks to the surface of the specimen, the transition sequence of the crack propagation mechanism is tensile through failure-tension-shear mixed failure-tensile failure. It can be seen that the interaction between the crack and hole has an important influence on the evolution mechanism of the crack and the failure mode of the specimen.

Crack generation, propagation mechanism and thermal property of Zn-coated hot stamping steel

Crack generation, propagation mechanism and thermal property of Zn-coated hot stamping steel

Crack Propagation Phenomenon in Gangue Concrete Using the Digital Image Correlation (DIC) Method

In order to study the mode I crack propagation mechanism of coal gangue concrete with different contents, the digital image correlation (DIC) method was used to carry out the three-point bending fracture tests on coal gangue concrete with different contents. The results show that the process of the mode I crack propagation of coal gangue concrete with different contents can be divided into three stages as follows: the elastic stage before crack initiation, extended viscoelastic stage, and extended fracture stage. The amount of coal gangue has a significant impact on the crack propagation path. The more the amount of coal gangue, the more the crack penetrates through the coal gangue coarse aggregate, the smaller the bending degree of the failure path, and the faster the crack propagation to the penetration speed. The crack initiation load, ultimate load, external force work, gravity work, and fracture energy all decrease with the increase of the coal gangue content. The data obtained by the DIC method and displacement extensometer are in good agreement, which proves that the DIC method is feasible. Based on the DIC method, before reaching the horizontal displacement on both sides of the crack tip, the horizontal displacement of the horizontal pixel is very small and there is a jump increase after the ultimate load. There are obvious inflection points on the left and right, and the horizontal displacement remains unchanged after the inflection point. After the horizontal displacement field of crack propagation reaches the limit load, there is an obvious limit; the limit gradually extends upward, and the corresponding crack tip strain field is also gradually enhanced. The more the coal gangue is added, the smaller the corresponding horizontal displacement and strain field is at the same limit load moment. The shape of the crack generation area of coal gangue concrete takes the crack tip as the axis of symmetry and is “gourd shaped.” The more the content of the crack, the larger the crack generation area.

Research on ultrasonic bone cutting mechanism based on extended finite element method.

The research on the crack propagation mechanism of bone has important research significance and clinical medical value for the selection of cutting parameters and the development of new surgical tools. In this paper, an extended finite element method (X-FEM) model of ultrasonic bone cutting considering microstructure was developed to further study the ultrasonic bone cutting mechanism and to quantitatively analyze the effects of cutting direction, ultrasonic parameters, and cutting parameters on the mechanism of ultrasonic bone cutting crack propagation. The results show that ultrasonic bone cutting is essentially a controlled crack propagation process, in which brittle crack and fatigue crack are the main crack propagation mechanisms. In order to improve the efficiency of ultrasonic bone cutting, large amplitude and high-frequency ultrasonic vibration are preferred. Compared with the other two cutting directions, the crack propagation deflection angle in the transverse cutting direction is the largest, resulting in the worst cutting surface. Therefore, in the path planning of orthopedic surgical robots, the transverse cutting direction should be avoided as much as possible. Frequency only has a significant effect on the crack propagation rate and has a positive correlation. There is a positive correlation between the deflection angle, propagation length, propagation rate, and amplitude, which provides the possibility to control the direction and length of crack propagation by controlling the amplitude of ultrasonic. The feed speed is much lower than the ultrasonic vibration speed, which makes the influence of ultrasonic vibration speed on the crack propagation characteristics dominant. The X-FEM model of ultrasonic bone cutting provides an effective method for selecting reasonable machining parameters of orthopedic robot and optimize the design of ultrasonic osteotome.

Numerical study on the characteristics of roadway failure and instability in coal seam with rock parting

In order to explore the mechanism of rockburst in coal seam with rock parting, a combination of on-site and numerical experiment is used to study the failure and instability process, crack propagation mechanism, and influencing factors. The following four points were addressed: (1) the instability is a process that roadway in coal seam with rock parting go through from stable locking in the initial stress unloading stage to slipping unlocking, and then to spatter ejection in slipping dynamic load disturbance stage. (2) The fracture development caused by unloading excavation of coal seam with rock parting will change from shear crack to tensile crack. In this process, coal-rock contact surface slip and coal-rock fracture are coupled with each other. (3) The greater the mining depth is, the greater the lateral pressure coefficient is, and the higher the rockburst risk is. On the contrary, the lower the risk of rockburst. (4) When choosing the support form of roadway in coal seam with rock parting, the two supporting forms of bolting (cable) and supplementary masonry support should be preferred. The results enrich the theory of the dynamics of surrounding rock fracture in coal mine, further clarify the potential dangers to mining-area roadways and working faces, and provide technical information to ensure the safe and efficient mining of bifurcated coal seam.

SEM-Guided Finite Element Simulation of Thermal Stresses in Multilayered Suspension Plasma-Sprayed TBCs

This study presents novel insights into thermal stress development and crack propagation mechanisms in single- and multilayered suspension plasma-sprayed (SPS) coatings of gadolinium zirconate (GZ) and yttria-stabilized zirconia (YSZ), thermally treated at 1150 °C. By combining image processing with finite element simulation, we pinpointed sites of high-stress concentration in the coatings, leading to specific cracking patterns. Our findings reveal a dynamic shift in the location of stress concentration from intercolumnar gaps to pores near the top coat/thermally grown oxide (TGO) interface with TGO thickening at elevated temperatures, promoting horizontal crack development across the ceramic layers. Significantly, the interface between the ceramic layer and TGO was found to be a critical area, experiencing the highest levels of both normal and shear stresses. These stresses influence failure modes: in double-layer SPS structures, relatively higher shear stresses can result in mode II failure, while in single-layer systems, the predominant normal stresses tend to cause mode I failure. Understanding stress behavior and failure mechanisms is essential for enhancing the durability of thermal barrier coatings (TBCs) in high-temperature applications. Therefore, by controlling the interfaces’ roughness along with improving interfacial toughness, the initiation and propagation of cracks can be delayed along these interfaces. Moreover, efforts to optimize the level of microstructural discontinuities, such as intercolumnar gaps and pores, within the creaming layer and close to the TGO interface should be undertaken to reduce crack formation in the TBC system.

Research on Crack Propagation Mechanism of Silicon Nitride Ceramic Ball Bearing Channel Surface Based on Rolling Friction Experiment

The application feedback on existing silicon nitride ceramic bearings and RCF experimental research all indicate that the primary failure mode of silicon nitride ceramic bearings is material spalling on the contact surface. Spalling failure occurs due to the initiation and propagation of cracks under rolling contact. However, silicon nitride ceramic bearings, owing to their unique manufacturing method, inevitably exhibit defects and cracks. Therefore, as silicon nitride ceramic bearings are increasingly prevalent, reducing the probability of spalling failure is crucial for extending their service life. This can only be achieved by gaining a clear understanding of the crack initiation and expansion mechanisms in silicon nitride ceramic bearings. This paper is based on silicon nitride rolling friction experiments. It involves the joint simulation of Franc3D-V8.4 and ABAQUS2020, wherein the crack front SIFs are calculated for each load contact position of the surface crack on the silicon nitride ceramic bearing ring during cyclic movement. The study also delves into the determination of the maximum effective stress intensity factors and explores the influence of the initial crack depth on the cycle life and direction of crack propagation. The research yields several valuable conclusions. The findings of this research offer theoretical guidance for formulating grinding technologies for silicon nitride rings and adjusting and controlling working parameters of silicon nitride ceramic ball bearings. These insights are crucial for enhancing the reliability and longevity of silicon nitride ceramic bearings in practical applications.

Experimental study on the effect of unloading rate on gneiss rockburst

Rockburst are often encountered in tunnel construction due to the complex geological conditions. To study the influence of unloading rate on rockburst, gneiss rockburst experiments were conducted under three groups of unloading rates. A high-speed photography system and acoustic emission (AE) system were used to monitor the entire process of rockburst process in real-time. The results show that the intensity of gneiss rockburst decreases with decrease of unloading rate, which is manifested as the reduction of AE energy and fragments ejection velocity. The mechanisms are proposed to explain this effect: (i) The reduction of unloading rate changes the crack propagation mechanism in the process of rockburst. This makes the rockbursts change from the tensile failure mechanism at high unloading rate to the tension-shear mixed failure mechanism at low unloading rate, and more energy released in the form of shear crack propagation. Then, less strain energy is converted into kinetic energy of fragments ejection. (ii) Less plate cracking degree of gneiss has taken shape due to decrease of unloading rate, resulting in the destruction of rockburst incubation process. The enlightenments of reducing the unloading rate for the project are also described quantitatively. The rockburst magnitude is reduced from the medium magnitude at the unloading rate of 0.1 MPa/s to the slight magnitude at the unloading rate of 0.025 MPa/s, which was judged by the ejection velocity.

Molecular dynamics simulation of crack propagation in very small grain size nanocopper with different grain size gradients.

In this paper, we use molecular dynamics to simulate the crack propagation behavior of gradient nano-grained (GNG) copper models with different grain size gradients, compare the crack propagation rates of different models, and analyze the microstructural changes and the mechanism of crack propagation. The simulation results show that the increase of the grain size gradient of the GNG copper model can improve the fracture resistance of the material, and the crack propagation mode undergoes a transition from brittle propagation along the grain boundaries to the formation of pores at the grain boundaries, and then to ductile fracture along the inclined plastic shear zone. The number of dislocations increases with the grain size gradient, while the crack passivation is more serious, indicating that a larger grain size gradient is more effective in inhibiting crack propagation. The introduction of gradient grain size promotes crack propagation and weakens the plasticity of the material relative to the nano-grained (NG) copper model.

Effect of specimen thickness on fatigue crack shape evolution for aluminum alloy

The investigation of fatigue crack propagation and fracture mechanisms in high-thickness metal plates holds pivotal importance in the advancement of a three-dimensional damage tolerance assessment methodology. This research also bears significant implications for ensuring the safe operation of large-scale mechanical equipment. In this study, we examine the influence of specimen thickness on the evolution of fatigue crack shapes in an aluminum alloy. Fatigue crack growth tests were conducted on single-edge notch tension specimens to explore the impact of specimen thickness on the shape of fatigue crack growth. Additionally, we propose a calculation model for crack growth shape based on an energy model in this paper. Both experimental and analytical findings indicate that specimen thickness does indeed exert an influence on fatigue crack shape. Specifically, as the specimen thickness increases, the crack shape transitions from resembling a “fingernail” to a “saddle” shape.

Crack Propagation and Coalescence Mechanism of a Rock Bridge between a Parallel Fissure Pair in a Direct Shear Test with Unloading Normal Stress

Crack Propagation and Coalescence Mechanism of a Rock Bridge between a Parallel Fissure Pair in a Direct Shear Test with Unloading Normal Stress

Propagation behavior of coal crack induced by liquid CO2 phase change blasting considering blasting pressure effects.

To investigate the crack propagation mechanisms in low-permeability coal seams induced by liquid CO2 phase change blasting under different blasting pressures, this research presents an experimental study conducted on a small liquid CO2 phase change blasting test system. The failure mode, crack morphology, and distribution characteristics of the coal rock model specimens under different liquid CO2 phase change blasting pressure were revealed, analyzing the crack shapes and expansion process. The results show that with increasing blasting pressure, both the number and complexity of cracks significantly increase under liquid CO2 phase change blasting, evolving from simple linear cracks to more complex multi-directional networks. Furthermore, the process of crack generation and expansion during liquid CO2 phase change blasting in coal and rock is controlled by the interaction of shock waves and quasi-static stress resulting from high-pressure CO2 phase transition in the borehole. Cracks form in distinct zones: the broken zone, where shock waves cause severe crushing near the borehole; the crack zone, where quasi-static tensile stress drives crack propagation. Higher confining and CO2 blasting pressures increase crack propagation. The research results offer valuable insights for optimizing blasting design in liquid CO2 phase change fracturing.

Molecular dynamics study of the trans-grain extension behavior of crack tips under synergistic effects.

In this paper, the trans-grain extension behavior of crack tips is investigated. The crack propagation mechanism of BCC-Fe model of single and combination orientation under type I loading was studied by molecular dynamics method. It is found that crystal orientation has an important effect on the activation and evolution of crack growth. The propagation mechanism of each orientation in the combination model works together to control the propagation behavior of the crack. The results show that the combination of brittle cleavage and grains with weak ductility exhibits better ductility expansion. For the combination of strong ductility and weak ductility, the ductility behavior decreases, which will seriously harm engineering safety. The limiting effect of combination orientation can enhance the toughness of crack propagation. Stress concentration and shear bands are easy to form along the single layer twin, and the formation of slip bands in the weak toughness orientation reduces the crack propagation toughness behavior of the combined model.

INFLUENCE OF PLY ORIENTATION ON INTERLAMINAR FRACTURE TOUGHNESS OF CARBON/EPOXY COMPOSITES

Delamination is a concern for the structural integrity of fiber-reinforced polymer composite structures. The decisive material parameter for delamination is fracture toughness (G<sub>Ic</sub>). The main objective of this study was to investigate the in-plane, quasi-static fracture toughness (both initiation and propagation) of two laminate composites. Both composites contain carbon fibers in an epoxy matrix. The tests were performed using double cantilever beam specimens with different ply orientations according to ASTM standards. The effect of ply orientation was evaluated using data reduction methods to obtain fracture toughness. The experiments revealed that the ply sequence changed the crack propagation mechanism, increasing the initiation and propagation fracture toughness values for 90/90 layup during mode I interlaminar tests, by 80% and 69%, respectively. In addition, the mode I fracture toughness increased by 40% after 50 mm crack extension.

Numerical investigation of thermal fatigue crack growth behavior in SAC305 BGA solder joints

PurposeThis study aims to investigate the reliability issues of microvoid cracks in solder joint packages exposed to thermal cycling fatigue.Design/methodology/approachThe specimens are subjected to JEDEC preconditioning level 1 (85 °C/85%RH/168 h) with five times reflow at 270°C. This is followed by thermal cycling from 0°C to 100°C, per IPC-7351B standards. The specimens' cross-sections are inspected for crack growth and propagation under backscattered scanning electronic microscopy. The decoupled thermomechanical simulation technique is applied to investigate the thermal fatigue behavior. The impacts of crack length on the stress and fatigue behavior of the package are investigated.FindingsCracks are initiated from the ball grid array corner of the solder joint, propagating through the transverse section of the solder ball. The crack growth increases continuously up to 0.25-mm crack length, then slows down afterward. The J-integral and stress intensity factor (SIF) values at the crack tip decrease with increased crack length. Before 0.15-mm crack length, J-integral and SIF reduce slightly with crack length and are comparatively higher, resulting in a rapid increase in crack mouth opening displacement (CMOD). Beyond 0.25-mm crack length, the values significantly decline, that there is not much possibility of crack growth, resulting in a negligible change in CMOD value. This explains the crack growth arrest obtained after 0.25-mm crack length.Practical implicationsThis work's contribution is expected to reduce the additional manufacturing cost and lead time incurred in investigating reliability issues in solder joints.Originality/valueThe work investigates crack propagation mechanisms of microvoid cracks in solder joints exposed to moisture and thermal fatigue, which is still limited in the literature. The parametric variation of the crack length on stress and fatigue characteristics of solder joints, which has never been conducted, is also studied.

The Explosive Crack Propagation Mechanism of Layered Rock Mass

Layered joints significantly affect the propagation and attenuation of engineering blasting stress waves and have a significant influence on rock breakage. In this study, the explosion crack propagation mechanism of a stratified rock mass with weak or no cementation is analyzed. Multilayer plexiglass plates are employed to simulate a layered rock mass, establishing an experimental model to investigate the vertical interaction between explosive charges and layered rock masses. The extent of ruptures and damage within the charging and sealing sections of the rock mass is analyzed. The explosion cracks at the bottom of the hole are relatively dense, whereas those at the hole opening are relatively sparse. The range of the crushing and fracture zones around the gun hole in the charging section is larger than that in the blocking section. Moreover, as the blocking section approaches the free plane, the crushing zone size decreases. A notable area of rapid damage escalation is identified between the blocking and charging sections, leading to a significant increase in damage intensity. The numerical analysis results indicate that the cumulative explosive energy within the charging section surpasses that of the blocking section, and the rate of energy accumulation is notably higher in the charging region.

Effect of dendritic structure and secondary phases on the fatigue behavior of ERNiCrMo-3 weld metal

The effect of dendritic structure and secondary phases on the fatigue crack propagation mechanism of ERNiCrMo-3 weld metal was investigated. Element segregation in the weld metal induced severe lattice distortion in the interdendritic region, creating substantial variations in mechanical properties between the dendrite core and interdendritic region. This disparity resulted in an unstable crack propagation rate. The limited plastic deformation capacity of the interdendritic region caused a contraction in the local plastic deformation zone at the crack tip, increasing stress concentration and accelerating crack propagation. The fatigue crack in the interdendritic regions exhibited a rate of approximately 2.6 μm/cycle, surpassing the 1.5 μm/cycle observed in the dendrite cores. Meanwhile, element segregation promoted the precipitation of numerous Laves phases in the interdendritic regions. With increasing stress, micro-voids formed by broken Laves phases served as rapid channels for crack propagation. Post-weld heat treatment weakened the non-uniformity of the microstructure. The fatigue crack propagated at a rate of about 1.2 μm/cycle. Furthermore, the dissolution of Laves phases and the precipitation of γ″ enhanced the fatigue performance of the weld metal. Under equivalent stress levels, the fatigue life of post-weld heat-treated weld metal increased by approximately 1.2 times compared to that of as-welded metal.

Mechanism of crack evolution and strength failure in chemo-mechanical induced fracture

Chemo-mechanical coupled fracture is ubiquitous among various application fields. Understanding the mechanism of crack propagation is critical to the prediction and control of fracture behavior. In this paper, mechanical damage and chemical erosion have been investigated by a combination of experiments and theoretical analysis. We developed a theoretical model to analyze the interface evolution and the mechanical state of the crack tip in chemically active environments based on the transition-state theory. This model enables us to predict mechanical failure and chemical corrosion of materials exposed to external acid attack. Theoretical predictions of the corrosion rate and fracture strength have been validated by fracture experiments performed in different concentrations of corrosive solutions. In particular, we discovered a non-monotonic and non-linear relationship between the degree of corrosion and fracture strength, which demonstrates that corrosion-induced crack tip blunting and mass loss of materials together affect the cracking critical state. We further conducted the thermodynamic analysis of a quasi-static cracked body to investigate the effect of corrosion on energy stored in the crack tip. Our theory-experiment-combined study reveals the mechanism of coupling chemical and mechanical damage and provides significant insight into corrosion-induced fracture behavior in aggressive environments.