Crack Propagation Mechanism Research Articles

Study on optimization of surface processing technology of silicon nitride bearing ring

Based on the difficult machining characteristics of silicon nitride materials, this manuscript focuses on optimizing the precision machining process of silicon nitride bearing components and improving the machining efficiency and quality of silicon nitride bearing components. Firstly, the mechanism of crack formation and propagation in hard and brittle materials under the action of abrasive particles is discussed in this paper, and based on this, a kinematic model of single abrasive grain cutting on hard and brittle materials is established. The effect of workpiece linear velocity, grinding wheel linear velocity, grinding wheel oscillation velocity and feed velocity on inner surface roughness of Si3N4 ring was discussed through grinding test. The experimental results show that the surface roughness can reach about Ra0.20 ∼Ra0.33 μm through a large number of grinding experiments by adjusting the combination of process parameters μm. On this basis, use the optimized process parameters calculated by the surface quality prediction model constructed in this manuscript to conduct grinding test again, and the surface roughness value reaches about Ra0.19 ∼Ra0.23 μm. The purpose of optimizing the process parameters is realized. Finally, the surface roughness can be further reduced and maintained at Ra0.05 ∼Ra0.06 μm by further superfinishing the surface after the process optimization with an oilstone. The research work of this manuscript realized the optimization of precision machining process of silicon nitride bearing ring and the rapid optimization of machining process through the established prediction model, which improved the precision machining efficiency of ceramic bearing components. Through the study of this manuscript, a process route of controllable processing of inner surface quality of Si3N4 ring is formed, from preliminary selection of process parameters by optimization model to precision grinding and then to ultra-finishing of whetstone, which provides theoretical reference for efficient and precise manufacturing of practical bearing components.

Effect of induced plastic strain on the porosity of PA12 printed through selective laser sintering studied by X-ray computed micro-tomography

3D printing is a widespread technology in different fields, such as medicine, construction, ergonomics, and the transportation industry. Its diffusion is related to the ability of this technique to produce complex parts without needing for assembly of different components or post-processing. However, the quality of the parts produced by additive manufacturing could be affected by the fabrication process, thus leading to the development of different kinds of defects such as porosity or inclusions. Understanding the role played by these defects and promoting strategies that could help reduce their occurrence represents a key point to allow using 3D printing for structural applications. In this work, 3D printed parts have been subjected to porosity characterization by using experimental tests on dogbone samples subjected to plastic deformation. In particular, multiple loading-unloading steps have been carried out and, at the end of each step, the X-ray computed micro-tomography (μ-CT) has been employed for the identification of fabrication defects and for analyzing the crack growth mechanism that occurs after quasi-static loading tests. Sample analysis reveals the presence of a high porosity that could be attributed to the fabrication process. After the sample’s plastic deformation, it was found an increase in both porosity percentage and pore dimensions. Moreover, the crack propagation mechanism is affected by porosity.

The anisotropy dependence of deformation mechanism of cleavage planes in β-Ga2O3 single crystal

β-Ga2O3 is a promising ultra-wide bandgap semiconductor. The deformation of β-Ga2O3 was few investigations, especially on the cleavage planes which hindered the high-efficiency and low-damage manufacturing of β-Ga2O3 wafers. Here, the anisotropy dependence of mechanical response and deformation mode of the (100) and (001) cleavage planes in β-Ga2O3 single crystal were studied by the nanoindentation method. The results showed that a broad deformation zone was formed on the subsurface of the β-Ga2O3 wafer after unloading, which was dominated by slippage, dislocations, stacking faults, and cracks. Cross stacking faults were found on the subsurface of the (100) plane. Under the same load condition, the damaged zone depth of the (001) plane was twice that of the (100) plane. At the same time, two kinds of cracks with different angles were found on the subsurface. The direction of the crack is related to stress distribution. The crack propagation mechanism transitioned to brittle cleavage fracture within the loading process. The crack did not extend to the indentation surface of the (100) plane. These results revealed the mechanism of deformation and damage in the manufacturing of low-symmetry β-Ga2O3 wafers.

Strain-rate sensitivity of brittle deformation and removal mechanisms of monocrystalline 3C–SiC induced by nano cutting process

In this work, the dynamic crack propagation mechanism of 3C–SiC at the micro-scale and the influence mechanism of the strain rate effect are studied, and the deformation mechanism under different strain rate is deeply analyzed. These simulations show many new phenomena, such as crack nucleation, deflection and arrest, defect accumulation and evolution near crack, strain rate effect on material deformation and removal. When the cutting velocity is equal to 5 m/s, the deformation mode of 3C–SiC is mainly sliding deformation, and the chip is separated from the matrix in the form of massive slump. When the cutting velocity reaches 500 m/s, the plastic deformation mode of the material in the machining region is the same, but the dislocation density is significantly reduced, and the material removal mode is transformed into a pure fracture mode. The brittle removal mode of monocrystalline SiC is closely related to the strain rate. The results will not only enhance the understanding the strain rate sensitivity of the damage evolution and removal behaviors of monocrystalline 3C–SiC involved at nano- and micro-scales, but also provide a theoretical guidance for optimizing process parameters during the SPDT process of monocrystalline 3C–SiC.

Atomic investigation on optimal interfacial bonding for enhanced fracture properties in polymer nanocomposites

Functionalization of nanofillers is known to have stiffening and strengthening effects on nanocomposites through improved interfacial bonding. However, the optimality in the interfacial bonding is generally not reported in the literature. This issue is addressed here through a series of reactive molecular dynamics simulations on carbon nanotube reinforced polymer nanocomposite. For various degrees of functionalization, uniaxial tension conditions are simulated to study the stress–strain behavior, crack propagation, and fracture toughness. The J-integral is used to quantify the fracture toughness. Through these simulations we demonstrate the existence of an optimal degree of functionalization for maximum enhancement in elastic property, tensile strength, ductility, and fracture toughness. The underlying mechanics behind this optimality is identified through careful studies on crack propagation mechanisms, including crack arresting and formation of new crack surfaces. This optimality, which might also be expected in other material systems, will help in designing efficient nanocomposites.

Peridynamic investigation on crack propagation mechanism of rock mass during excavation of tunnel group in cold regions

An extended ordinary state-based peridynamic model was developed to investigate the crack propagation of surrounding rock due to tunnel excavation. The frost heave effect on the crack surface was described by an equivalent pressure density. Two numerical examples were conducted to verify the effectiveness and robustness of the proposed model. Moreover, the influence of frost heave pressure, buried depth and rock elastic modulus on damage characteristics of the surrounding rock was investigated. The results demonstrate that massive microcrack damage will be formed at the invert when the tunnel is excavated in a deeply buried rock mass or soft rock stratum. HIGHLIGHTS A peridynamic model of rock mass under excavation and frost heave was constructed. Damage characteristics of surrounding rock could be simulated efficiently. Frost heave load was exerted on the crack surface by an equivalent pressure density. The effects of frost heave load, buried depth and rock elastic modulus were studied.

Molecular simulation of the plastic deformation and crack formation in single grit grinding of 4H-SiC single crystal

Silicon carbide (SiC) is a promising semiconductor material for high-performance power electronics devices, but difficult to machine. The development of cost-effective machining technology for SiC wafers requires a complete understanding of the interaction between diamond grit and workpiece material. This work systematically investigated the deformation and crack formation of monocrystalline 4H-SiC involved in single grit grinding using molecular dynamics (MD) simulation. The mechanism for crack initiation and propagation in monocrystalline 4H-SiC was revealed for the first time. Our simulation results showed that the subsurface damage at the earlier stage of plastic deformation was caused by amorphization and dislocation initiation. With the progress of penetration, slip bands were formed. The extension of slip bands in the later stage of plastic deformation was the cause of crack initiation. Stress analysis demonstrated that the tensile stresses parallel to the scratch direction and perpendicular to the scratch plane were the driving force for the crack initiation ahead of a diamond abrasive grit, while the tensile stress perpendicular to the scratch plane was responsible for the crack initiation underneath the grit. The propagation of the two cracks was driven by the maximum principal stress at the crack tip, and the propagation direction depends on the direction of the maximum principal stress.

Effects of spherical Al2O3 grades on crack initiation of thermal conductive silicone pads under high-temperature aging

ABSTRACT In this paper, the rule and mechanisms of crack initiation and propagation in the aging process of thermal conductive silicone pads (TCSP) were investigated by designing Al2O3 particle grading schemes with different particle sizes and mass ratios. Scanning electron microscope and energy dispersive spectrometer analysis showed that TCSP prepared with Al2O3 particles of various sizes according to the gradation ratio had the best anti-aging effect at high temperatures. At this time, the aging was mainly due to the aging degradation of the polymer matrix; When the proportion of small-size Al2O3 particles increases, aggregates and pore defects appear in TCSP. At this time, the main reason for its aging is that the high-temperature expansion of gas in pore defects accelerates the aging degradation of the polymer matrix; When the proportion of small-sized Al2O3 particles decreases, a large number of holes appear in TCSP and the coverage rate of the polymer matrix on Al2O3 particles decreases. The main reason for the high-temperature aging crack is that the gas between the holes expands at high temperature, resulting in the rapid extension of the crack and the polymer matrixes falling.

Phase-Field Simulation of Temperature-Dependent Thermal Shock Fracture of Al2O3/ZrO2 Multilayer Ceramics with Phase Transition Residual Stress.

Compared with single-phase ceramics, the thermal shock crack propagation mechanism of multiphase layered ceramics is more complex. There is no experimental method and theoretical framework that can fully reveal the thermal shock damage mechanism of ceramic materials. Therefore, a multiphase phase-field fracture model including the temperature dependence of material for thermal shock-induced fracture of multilayer ceramics is established. In this study, the effects of residual stress on the crack propagation of ATZ (Al2O3-5%tZrO2)/AMZ (Al2O3-30%mZrO2) layered ceramics with different layer thickness ratios, layers, and initial temperatures under bending and thermal shock were investigated. Simulation results of the fracture phase field under four-point bending are in good agreement with the experimental results, and the crack propagation shows a step shape, which verifies the effectiveness of the proposed method. With constant thickness, high-strength compressive stress positively changes with the layer thickness ratio, which contributes to crack deflection. The cracks of the ceramic material under thermal shock have hierarchy and regularity. When the layer thickness ratio is constant, the compressive residual stress decreases with the increase in the layer number, and the degree of thermal shock crack deflection decreases.

Crack propagation and arrests in gelatin hydrogels are linked to tip curvatures.

Gelatin hydrogels are attractive scaffold materials for tissue engineering applications as they provide motifs for cell attachment, undergo large deformations, and are tunable. Low toughness and brittle fractures however limit their use in load bearing applications. An investigation of crack tip processes and mechanisms of crack propagation is warranted to link fracture properties with material microstructure. We cross-linked gelatin using glutaraldehyde to obtain low cross-linked control hydrogels and used an additional cross-linker, methylglyoxal, to fabricate MGO hydrogels with higher cross-links. We quantified fractures in the gelatin hydrogels from both groups using pure shear notch tests and characterized strain fields near the crack tip using 2-D digital image correlation. We used a numerical method based on Taylor's series expansion to measure the crack tip curvatures in the hydrogels. This method captures tip curvatures better than the parabolic method routinely used in studies. Results from our study show that cracks in gelatin hydrogels underwent frequent arrests during propagation through the specimen width in both groups. MGO hydrogels had 85% enhanced fracture toughness and a significantly higher number of stalls compared to the control group. Crack initiations following stalls in the sample correlated with low tip curvatures in both hydrogel groups. We also show that mechanical stretching blunts the crack tip before crack propagation; the degree of blunting was independent of the cross-link density and elastic modulus of the gelatin hydrogels. These results show a link between crack growth and the tip curvature in cross-linked gelatin hydrogels, and offer potential insights for the development of tougher hydrogels.

Fabrication of optical glass lenses using laser slicing technology (Stress formation and crack propagation mechanism in internal laser processing)

Fabrication of optical glass lenses using laser slicing technology (Stress formation and crack propagation mechanism in internal laser processing)

Crack initiation mechanism of laser powder bed fusion additive manufactured Al-Zn-Mg-Cu alloy

Crack is a serious defect limiting the additive manufacturing of metal materials. The morphologies and crystallographic orientation of cracks for Al-Zn-Mg-Cu alloy prepared by laser powder bed fusion (LPBF) were clarified. The mechanisms of crack initiation and propagation were expounded. Results show that the cracks were crisscrossed horizontally and propagated through multilayers along the building direction in the as-printed specimens. The cracks were classified as solidification cracks, which occurred in the final stage during solidification when the liquid fraction was <20%. Al-Zn-Mg-Cu alloy has a wide temperature range between the liquidus and solidus, leading to high cracking susceptibility. Solidification cracks were prone to initiate at high-angle grain boundaries (HAGBs) and terminate at low-angle grain boundaries (LAGBs) or within grains. LPBF process could affect crack initiation and propagation by refining the grain size and inhibiting the growth orientation. Solidification and thermal shrinkage of adjacent grains during solidification provided subjective factors for cracking initiation. Directional preferential growth of dendrites along the 〈001〉 crystal orientation and the untimely filling of the liquid phase provided secondary factors. This study can provide theoretical guidance for crack suppression in additive manufacturing of 7075 aluminum alloy and other metals with high cracking susceptibility. • Crack initiation and propagation mechanisms of Al-Zn-Mg-Cu alloy in LPBF were investigated. • Internal cracks of as-printed specimens are solidification cracks. • Cracks occur in the final stage during solidification. • Oriented crystallization and texture affect the initiation and propagation of cracks. • Cracks initiate at straight HAGBs and terminate at LAGBs or within grains.

Multi-regime fatigue failure model based on damage theory

The prediction of a spatial fatigue crack development under arbitrary loading conditions is a complex problem. This paper introduces a new approach to predict crack initiation and propagation development based on damage theory. The fatigue degradation of the material within a physically small volume is described by the kinetic equation for damage function. The mechanical properties of the material (elastic moduli) depend on the damage function value. The uniform descriptions of the left and right branches of the full SN-curve allow the introduction of the general numerical procedure for fatigue ranges from HCF to VHCF. At the initial stage, the values of the damage function are calculated based on the elastic or elastic-plastic solution. The damage function is associated with two different mechanisms of fatigue damage accumulations. The first one is due to normal stress and the second one is due to shear. The contribution of each mechanism to damage function development is calculated simultaneously. As soon as the damage function gets the critical value the dominant mechanism is established. Therefore, the proposed multi regime model and numerical procedure are capable to predict crack initiation, spatial crack propagation and dominant crack opening mechanism at different stages of crack growth.

Усталостная прочность сварных соединений сталей 30ХГСА–40ХМФА, полученных ротационной сваркой трением

Rotary friction welding (RFW) is used in the production of drill pipes for solid mineral prospecting. The need for the creation of the lightened drill strings for high-speed diamond drilling of ultradeep wells dictates the necessity of a greater focus on the study of a weld zone and setting the RFW technological parameters. This paper presents the results of experimental studies of a welded joint of a drill pipe of the H standard size according to ISO 10097, made of the 30ХГСА (pipe body) and 40ХМФА (tool joint) steels under the cyclic loads. The authors evaluated the influence of the force applied to the workpieces in the process of friction of the contacting surfaces (force during heating), and postweld tempering at a temperature of 550 °С on the cyclic life of welded joints, under the conditions of alternate tension-compression at the cycle amplitude stress of ±420 MPa. The study determined that with an increase in the force during heating, the microstructure changes occur in the zone of thermomechanical influence, contributing to an increase in the fatigue strength of welded joints. The authors identified the negative effect of postweld tempering on the fatigue strength of welded joints, which is expressed in the decrease in the number of cycles before failure by 15–40 %, depending on the magnitude of the force during heating. The optimal RFW mode of the specified combination of steels is determined, which provides the largest number of cycles before failure: the force during heating (at friction) Fh=120 kN, forging force Ffor=160 kN, rotational frequency during heating n=800 Rpm, and upset during heating l=8 mm. A series of fatigue tests have been carried out at various values of the cycle amplitude stress of the welded joint produced at the optimal mode and the 30ХГСА steel base metal; limited endurance curves have been plotted. It is shown that the differences in the limited endurance curves of the pipe body material (30ХГСА steel) and the welded joint are insignificant. The obtained results are supplemented by the microhardness measurement data and fractographs of fractured samples, revealing the mechanism of crack propagation under the cyclic loads.

Designing a Computer Simulation Model to Study the Failure Mechanism in Rotating Shaft

The phenomenon of the failure of rotating mechanical structures is considered one of the dangerous phenomena that researchers are trying to study to find effective ways to avoid it. Many mathematical models describing the mechanism of crack formation in structures and the occurrence of failure in them have been adopted. In this paper, a computer simulation model of the failure mechanism in the intermediate conduction shaft of the ship's propeller (oil shuttle tanker) was designed based on the mathematical model that describes the stages of failure according to the number of revolutions that the shaft bears and the dimensions of fatigue cracks in it. In the beginning, a detailed mathematical model of the crack propagation mechanism was deduced as an initial stage, through which the differential equations that express the failure of the shaft were described. In the second stage, numerical simulations were implemented using the Euler and Rung-Kutta numerical methods used in solving ordinary differential equations. Also, for both methods using the Matlab language to reach the results and accurate graphs and analyze them. In this way it is possible to predict the number of cycles that the column will bear before the collapse, and this will reduce the occurrence of sudden collapses and save time, effort, and costs.

High-cycle fatigue behavior of IN 738LC superalloy at high temperatures

The presented study is focused on experimental results of high-cycle fatigue life of a cast polycrystalline nickel-based superalloy IN 738LC at temperatures of 800, 900, and 950°C. The absence of hot isostatic pressing (HIP) led to the occurrence of casting defects in the microstructure. The fatigue tests were performed in symmetrical loading in laboratory air. The fracture surfaces of specimens were studied by scanning electron microscopy to describe the influence of temperature on the fatigue damage mechanism and fatigue crack initiation. Change in the primary crack propagation mechanism from crystalline (stage I regime) to non-crystalline (stage II regime) mechanism was observed with increasing temperature. However, the effect of changing mechanism caused by temperature dependence was unfavorably influenced by casting defects.

Experimental Study on the Effects of Geometric Parameters of Filled Fractures on the Mechanical Properties and Crack Propagation Mechanisms of Rock Masses

Experimental Study on the Effects of Geometric Parameters of Filled Fractures on the Mechanical Properties and Crack Propagation Mechanisms of Rock Masses

Influence of V addition on the mechanical properties of FeCo alloys: a molecular dynamics study

Interatomic potentials for the Fe-Co and Fe-V binary systems have been developed based on an Fe interatomic potential with enhanced mechanical capabilities and merged to describe the Fe-Co-V ternary system based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. Emphasis has been placed on the correct prediction of FeCo elastic properties and antiphase boundary (APB) energies to ensure proper description of the mechanical behaviors. The potentials are then used to analyze the crack nucleation and propagation mechanisms in polycrystalline FeCo and FeCo-2V alloys under different ordering degrees at grain boundaries (GB) and within the grains. The results show that disordering inside grains has a larger impact on the ductility of the binary FeCo. The mobility of the antiphase domains (APD) inside the grains delays the ultimate tensile strength (UTS) and fracture point. The observed fracture procedure shows that triple junctions of GBs serve as the focal points of stresses concentration. The poor mobility at the triple GBs promotes crack nucleation and crack propagation, leading to an intergranular fracture in ordered and disordered binary alloys. The effects of adding 2 at. % V to the FeCo alloy have also been investigated via molecular dynamics (MD) and hybrid grand canonical Monte Carlo-MD (GCMC-MD) simulations. It was shown that V prefers to migrate to (a) the GBs in an ordered state, and (b) GBs and APBs in a disordered state. The ordered FeCo-2V displays a decreased UTS and loss of ductility at room temperature. Introducing V exclusively to the APBs greatly increases the mobility of APD and migration of atoms to GBs boosting the ductility of the alloy.

Microscopic mechanisms of coral reef limestone crack propagation

Coral reef limestone (CRL) is a marine sedimentary rock dominated by biogenesis, and its composition as well as internal structure characteristics are different from traditional terrigenous rocks. This article used CT scanning and petrographic thin section observations to visually and quantitatively analyze crack morphological characteristics after triaxial shear failure at a microscopic scale. The analysis revealed the mechanism of crack initiation and propagation from the perspectives of geological genesis and fracture mechanics. Results show that the crack propagation mechanism in the bio-cemented framework includes propagation along crystal boundaries and across crystals. Cracks can be classified as intergranular cracks, transgranular cracks, and bifurcation cracks based on crack morphology. The destruction mode of the specimen is controlled by the pore structure and cemented framework. Based on the pore distribution, the destruction mode can be divided into local tension failure and monoclinic shear failure. Understanding CRL crack propagation mechanisms has important scientific significance and practical engineering value for enriching rock damage theory and long-term stability analysis of island reef engineering foundations.

Understanding the fracture toughness of gadolinium‐ and lead‐containing plexiglass

AbstractToughened gadolinium‐ and lead‐containing plexiglasses (GLCPs) were prepared via copolymerization using ethoxylated bisphenol A dimethacrylate (BPAxEODMA, x = 2, 4, 10) as comonomer and network regulator. As‐obtained modified GLCPs possessed highly improved toughness, with satisfactory shielding and optical performances. The fracture mechanism was studied then in terms of fracture surface analyses. The results disclosed that both the impact strength and the fracture toughness (KIC) increased with the increase of fatty chain lengths or concentrations of BPAxEODMA, and the surface morphology strongly depended on the fracture processes. A morphological evolution, from mirror region to haze one, and finally to hackle region, was visible on all GLCP samples. However, the morphological characteristics of each region, such as size and roughness as well as the appearance of ring‐banded structure and firework‐like pattern, were completely different. The crack propagation mechanism was utilized to correlate the fracture surface morphology with fracture process of the samples. The relationship between KIC of the samples and their crosslinking networks was well‐built in this way. This work proposes valuable information around fabricating the anti‐radiation GLCPs with high and tailorable mechanical performance.