Crack Initiation Research Articles

Crack evolution mechanism of stratified rock mass under different strength ratios and soft layer thickness: Insights from DEM modeling

Research on stratified rock masses, which are common geological formations, has primarily focused on their mechanical properties, while studies on crack evolution and microscopic damage mechanisms remain limited. This study addresses this gap by investigating the combined effects of strength ratios and soft layer thicknesses on the microcrack evolution mechanism of stratified rocks using the discrete element method (DEM). Through FISH language programming in the particle flow code (PFC), this study reveals the acoustic emission (AE) characteristics, crack initiation and propagation, damage degree, and final failure characteristics. The key findings are: (1) Higher strength ratios between the hard and soft components of stratified rocks make specimens more sensitive to increases in soft layer thickness. (2) Three types of AE events were identified: continuous active, intermittent active, and silent. (3) Cracks initiate at the interface between components and propagate along the interface into the rock matrix. The strength ratios determine the crack propagation path and the damage extent of the components. (4) The failure of stratified rocks is primarily controlled by the soft component. Crack connections typically form vertical and sub-vertical tensile failure planes in the hard component, and a shear failure surface with a “V”-shaped intersection in the soft component.

Twinning mediated cryogenic toughness in an additively manufactured high Mn steel by manipulating stacking fault energy

The insufficient cryogenic toughness of wire-arc additively manufacturing (WAAM) component hinders their application in cryogenic fields. We report that tungsten is effective in enhancing the impact toughness of WAAM components, increasing it from 42 to 106J at 77K. The outstanding cryogenic toughness depends on the interaction between dislocations and twins, where the thickness of twins plays a pivotal role in determining whether they hinder or facilitate dislocation movement. Twins below 6.1nm hinder dislocations, impeding deformation, while those exceeding 21.7nm coordinate with the matrix orientation, aiding dislocation movement and forming microbands. The W element reduces the critical twinning stress from 112.3 to 83.5MPa by manipulating the stacking fault energy and grain size, thereby altering the twinning structure. However, excessive W content forms large-sized precipitates, serving as crack initiation sites and reducing crack initiation energy.

Detection of matrix cracks in composite laminates using embedded fiber-optic distributed strain sensing

We used an optical frequency-domain reflectometry fiber Bragg grating (FBG) distributed strain measurement system to detect matrix cracks in carbon fiber-reinforced plastics. Load/unload tests were performed on cross-ply laminates under ambient-, high-, and low-temperature conditions using embedded 100mm gauge FBG sensors. We measured the strain distributed along the gauge and examined the crack initiation using optical microscopy and soft X-ray inspection. A peak appeared in the distributed strain corresponding to the occurrence of cracks. The strain distribution during crack initiation was calculated in each temperature range via finite element analysis and compared with the measurement results. We determined that the number and location of cracks can be determined by extracting the peaks from the distributed strain. However, the peaks in the distributed strain do not correspond to the occurrence of cracks that are close to each other, warranting a measurement method with a higher strain resolution.

Deformation behavior study of SAC305 solder joints under shear and tensile loading by crystal plasticity finite element method.

Micro solder joints experience various loads during operation, and the failure of a single micro solder joint can impact the overall reliability of the entire microsystem. This study utilized the Crystal Plasticity Finite Element Method (CPFEM) to create a polycrystalline model that accurately represents the shape of solder joints. By calibrating the crystal plasticity characteristics of SAC305 material solder joints using macroscopic stress-strain curves, the model successfully simulates the deformation mechanisms of micro-solder joints under both tensile and shear loads. This paper highlights that equivalent plastic strain serves as a key indicator of solder joint fracture behavior, revealing that strain components have varying effects on cracking: shear strain was found to cause crack initiation, while normal strain was found to promote crack extension. Moreover, Grain boundary features and orientation lead to uneven plastic strain by affecting the slip system and the Schmidt factor, especially small Schmidt factor grains at triple grain boundary intersections are more susceptible to crack initiation. Additionally, the study found that solder joints exhibit more pronounced mechanical responses under tensile loading compared to shear loading. These insights offer valuable guidance for the design and manufacturing of micro-solder joints.

Assessment of foundation damage during earthquakes using acoustic emission monitoring: An experimental study on the shaking table test

Assessing structural foundation damage following an earthquake is critical for safety evaluation. However, assessing the damage to pile foundations with traditional visual inspections and non-destructive testing methods is challenging. This study evaluated the use of acoustic emission (AE) monitoring for damage detection and location in foundation structures during earthquake simulations by conducting shaking table experiments. Scaled models of foundation structures with and without surrounding soil were used in the experiments, and the performance of the AE monitoring system was evaluated by comparing the AE parameters via visual inspection. The experimental results showed that the AE monitoring system could effectively predict the initiation of cracks in foundation structures that experienced an earthquake. In addition, an appropriate filtering criterion for the shaking table experiments was established based on the AE characteristics of the foundation structures during the earthquake simulation, thereby improving the performance of the AE monitoring system for damage location. Consequently, this study contributed to a better understanding of the applicability of AE monitoring systems to foundation structures during earthquakes.

Heat treatment effects on the shear performance and microstructure evolution of W-core SiC fibers

SiC fiber reinforced titanium alloy matrix composites employed in elevated temperature aeroengine components are subjected to complex loading conditions. Investigating the shear performance of SiC fibers after heat treatment can help ensure the load-carrying capability of composites at service temperatures. This study presents a novel double shear testing method for W-core SiC fibers. The shear strength and stiffness of the SiC fibers in as-received and heat-treated conditions were measured. The morphologies of SiC fibers’ shear fracture surfaces and C coatings were thoroughly analyzed. The evolution of the internal microstructure and the composition of reaction layers after heat treatments were investigated. The shear strength of W-core SiC fibers at 95% survival probability reached 588.64MPa after 200 h of heat treatment at 600°Celsius, representing a 55% improvement over both the as-received and 1100°Celsius heat-treated conditions. After heat treatment at 1100°Celsius, the reaction layer between the W core and the SiC sheath thickened exponentially due to extensive elemental diffusion, resulting in the formation of numerous Kirkendall voids. The voids facilitated crack initiation and propagation, leading to a deterioration in fiber shear performance. The insights gained in this study regarding the shear properties of SiC fibers can provide support for predicting the mechanical performance of composites under complex loading conditions.

Crystal plasticity based investigation of the effects of additive manufactured voids on the strain localization behaviour of Ti-6Al-4V

The presence of defects produced by additive manufactured (AM) processes in structural Ti alloys such as Ti-6Al-4V is known to have serious implications on the deformation and fatigue behaviour of engineering components. However, there is little understanding about the localised plastic deformation patterns that develop around AM defects, and the associated local conditions that could lead to the nucleation of micro-cracks under creep loading conditions. In this work, the effects of the morphology and volume fraction of AM defects and temperature on the strain localization behaviour around such defects in Ti-6Al-4V will be addressed. To that purpose, a novel rate-dependent crystal plasticity formulation is proposed to describe the mechanical behaviour of the alloy’s predominant α′(HCP)-phase. Representative volume elements (RVEs) of the AM produced microstructures are digitally reconstructed from EBSD orientation maps obtained on planes perpendicular and transversal to the microstructure’s AM growth direction. Calibration of the single crystal model for the α′-phase is carried out from macroscopic uniaxial tensile data from polycrystalline AM specimens at different strain rates and temperatures and published creep data.Furthermore, RVEs containing AM defects of different morphologies and volume fractions are relied upon to investigate the strain localization behaviour around the defects under uniaxial loading at ambient and high temperatures. It is found that the extent of the localised accumulated plastic strain around defects depends greatly on whether the voids surface are smooth or have sharp corners, with the latter being associated with more severe localisation patterns. Moreover, a numerical investigation into the crack initiation behaviour of AM Ti-6Al-4V under uniaxial creep loading at 450 ° C revealed that the development of the local conditions suitable for the nucleation of creep damage/micro-cracks is accelerated in the presence of typical AM defects, and the extent of that acceleration depends strongly on their morphology. An AM defect shape parameter is introduced to quantify the way their morphology affect the time for creep crack initiation/damage.

Effect of scanning strategy on the thermo-structural coupling field and cracking behavior during laser powder bed fusion of Ti48Al2Cr2Nb alloys

Laser powder bed fusion (LPBF) is characterized by complicated non-equilibrium processing characteristics, involving extremely high temperature gradient and cooling rate within the mesoscopic dynamic molten pool, which brings great challenges to the forming of metal materials with high crack sensitivity (such as γ-TiAl alloy). In this paper, based on the LPBF forming process of Ti48Al2Cr2Nb alloy, a corresponding multi-track and multi-layer thermal-structure sequential coupled finite element model was constructed, and the influence of four different scanning strategies on the temperature and stress evolution behavior in the powder bed forming region was quantitatively studied, and the crack initiation criterion was also established according to the relationship between nodal flow stress and equivalent stress. It was found that island scanning strategy could induce the higher molten pool temperature and attendant larger molten pool size compared with the non-island linear scanning strategy. Besides, island scanning strategy was conducive to relieving the thermal stress inside the division region but also inducing the stress concentration in the sub-island overlap region. Specially, the strip island scanning strategy exhibited the lowest residual stress and most homogeneous stress distribution. By the experimental measurements, the strip island scanning strategy contributed to the lowest crack density of 0.33mm/mm2.

Vibration fatigue of film cooling hole structure of Ni-based single crystal turbine blade: Failure behavior and life prediction

The vibration fatigue failure behavior and fatigue life of the film cooling hole (FCH) structure of Ni-based single crystal superalloy were investigated at high temperature. The vibration fatigue test of the FCH specimens were carried out based on the paired staircase method. The cracks are mostly initiated in the stress concentration area at the edge of the FCH, and may also be initiated in non-stress concentrated areas with large defects. At high temperature, the crack initiation mechanism of the FCH specimens of Ni-based single crystal superalloy is oxidative cracking nucleation in stress concentration area, and the crack propagation mechanism is the competition mechanism of dislocation slip and dislocation climbing. Based on the Fatigue Indicator Parameter (FIP) of the critical plane method, a vibration fatigue life prediction model for FCH structures considering stress concentration and high temperature oxidation damage is proposed. The life prediction model proposed in this paper is applied to the vibration fatigue life prediction of FCH specimens. The deviation between the predicted results and the experimental results is within 2.5 times at 850℃, and the deviation is within 2 times at 980℃. Besides, the life prediction method proposed is compared with other two FIP life prediction methods, and the high accuracy and effectiveness of the proposed life prediction method are verified.

Exploring mechanical properties and cracking behavior of AC-13 and OGFC-16 aggregate-segregated asphalt mixtures

Aggregate segregation compromises the homogeneity of asphalt pavement, which diminishes the road performance of the asphalt mixture. The segregation behavior and damage mechanism of asphalt mixture are complex. To investigate the mechanical properties and cracking behavior of asphalt mixtures with varying degrees of aggregate segregation, the dense-graded asphalt mixture with the nominal maximum size of 13.2mm (AC-13) and Open Graded Friction Course with the nominal maximum size of 16.0mm (OGFC-16) with varying degrees of segregation were designed. Indirect tensile test and the digital image correlation (DIC) technology was employed to analyze the strain distribution and strain evolution at the crack initiation point within the asphalt mixtures. The parameters were also proposed to evaluate the resistance to cracking for the segregated asphalt mixtures at a mesoscopic scale. The results show that increased segregation of coarse aggregates in AC-13 mixtures leads to decreased indirect tensile strength, lower maximum tensile strain along the horizontal direction (E11), and reduced failure displacement. Conversely, segregation of fine aggregates in OGFC-16 mixtures results in increased tensile strength, maximum strain, and failure displacement. Higher amount of coarse aggregates in the asphalt mixture contributes to a more intricate strain distribution and a greater number of strain peaks. Dense-graded mixtures show larger strain concentration zones prior to cracking, while smaller stress concentration zones are observed in open-graded mixtures. The horizontal strain-time curve displays distinct stages of stable growth, rapid growth, and rapid decline. Aggregate segregation results in more intricate crack patterns with larger inclination angles, influenced by the random distribution and shapes of coarse aggregates. Novel indexes such as strain uniformity (P1,2) and strain density during crack development (DW) effectively evaluate mesoscopic cracking resistance of asphalt mixtures. The results offer valuable insights into cracking behavior and improve evaluation methods for the mechanical performance of aggregate-segregated asphalt mixtures.

Damage accumulation in carbon steel under low-cycle loading at the crack initiation stage and strain ageing temperature

Based on the strain criterion for fatigue fracture and experimental studies, the damage accumulation kinetics from recoverable plastic strain (plastic strain in the tensile half-cycle), from the unilateral accumulation and elastic strain with the increasing number of loading cycles was studied. The influence of stress ageing on the redistribution of the levels of these components of damage, depending on the number of cycles before destruction, was studied. Depending on the load level, the limit states (formation of a surface macro-crack or buckling failure of strain stability due to the formation of an internal crack in the quasi-static fracture range) were established. Strain ageing reduces the range of quasi-static fracture. It is shown that the critical values of strains and stresses correspond to the limit states. The parameters determining the capacity of structural materials under static and cyclic loading under conditions of stress ageing are confirmed.

Mechanical property and cracking mechanism of Cr-coated Zr alloy under tensile and compressive loading

Cr coatings are considered as the most promising candidate materials for accident-tolerant fuel cladding coatings. In combination with optical microscopy, focus ion beam, energy dispersive spectrum and universal testing machine, the effect of Cr coating on hoop tensile and compressive mechanical properties of Zr alloy was studied and the underlying cracking failure mechanism of Cr coating was revealed. The results indicate that Cr coating slightly increases tensile/compressive yield strength and tensile strength compared with those of uncoated Zr alloy. This is attributed to high hardness/elastic modulus of Cr coating. Interrupted hoop tensile and compressive tests show that Cr coating cracking concentrates in the range of 0-30°. Further, crack initiation of the coating occurs either at the surface or at the interface between Cr coating and the matrix. Compared with the former, the latter is dominant cracking mode, irrespective of tensile and compressive loading. In lights of the advanced microstructural analysis, one possible reason, i.e., the existence of nano-scaled Al-enriched loose layer between the interface, in which nano-cracks and voids exist, is proposed to explain the interfacial cracking. The result indicates that the optimization of the interfaces can improve the resistance of coating failure. In addition, crystallographic study demonstrates that the crack propagation in the Cr coating is along the high-angle grain boundaries with the misorientation of 50°.

Damage regularity and multifractal analysis of sol-gel reflection coating of KDP crystal under low UV irradiation flux.

This study employed multifractal analysis to investigate the changes in surface morphology of SiO2 anti-reflective coatings prepared on KDP substrates using the sol-gel method, under various conditions of ultraviolet (UV) irradiance. The coatings were successfully fabricated, and the chemical structure of the SiO2 sol was comprehensively characterized using Solid-State Nuclear Magnetic Resonance (SSNMR) technology. Under low UV irradiance (4 J/cm2), repeated experiments revealed a crack-induced mechanism of surface fatigue damage. Utilizing Scanning Electron Microscopy (SEM), the study discovered the induction effect of initial crack defects in UV-damaged coatings and established a damage model. Furthermore, Atomic Force Microscopy (AFM) was used to acquire images of the coatings' surface morphology at different damage levels, which were analyzed using the multifractal spectrum f(α). This analysis confirmed the multifractal nature of the coatings both before and after damage. This study identified significant effects of UV irradiation on the width of the multifractal spectrum and Δf, indicating that the SiO2 anti-reflective coatings exhibit multifractal characteristics under various damage states. The coatings displayed a pattern of decreasing and then increasing singularity spectrum width, height distribution unevenness, and surface roughness with increasing damage. This study demonstrates that multifractal analysis is an effective tool for describing the complexity of the surface morphology of sol-gel-derived anti-reflective coatings for the first time and for validating their multifractal properties across different stages of UV damage. HIGHLIGHTS: Damage dynamic process of KDP crystal sol-gel coating was described by SEM&AFM; The crack propagation mechanism of sol-gel coating under UV radiation is proposed; The damage evolution of sol-gel coating was described by multifractal analysis.

Strain Distribution and Its Relationship to Crack Initiation and Propagation Behavior During Cyclic Bending Test on Commercial SUS301 Sheet

Strain Distribution and Its Relationship to Crack Initiation and Propagation Behavior During Cyclic Bending Test on Commercial SUS301 Sheet

A FEM-based model for failure initiation in the microstructure of gray cast iron under uniaxial compression

PurposeThe purpose of this study was to validate the feasibility of the proposed microstructure-based model by comparing the simulation results with experimental data. The study also aimed to investigate the relationship between the orientation of graphite flakes and the failure behavior of the material under compressive loads as well as the effect of image size on the accuracy of stress–strain behavior predictions.Design/methodology/approachThis paper presents a microstructure-based model that utilizes the finite element method (FEM) combined with representative volume elements (RVE) to simulate the hardening and failure behavior of ferrite-pearlite matrix gray cast iron under uniaxial loading conditions. The material was first analyzed using optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) to identify the different phases and their characteristics. High-resolution SEM images of the undeformed material microstructure were then converted into finite element meshes using OOF2 software. The Johnson–Cook (J–C) model, along with a damage model, was employed in Abaqus FEA software to estimate the elastic and elastoplastic behavior under assumed plane stress conditions.FindingsThe findings indicate that crack initiation and propagation in gray cast iron begin at the interface between graphite particles and the pearlitic matrix, with microcrack networks extending into the metal matrix, eventually coalescing to cause material failure. The ferritic phase within the material contributes some ductility, thereby delaying crack initiation.Originality/valueThis study introduces a novel approach by integrating microstructural analysis with FEM and RVE techniques to accurately model the hardening and failure behavior of gray cast iron under uniaxial loading. The incorporation of high-resolution SEM images into finite element meshes, combined with the J–C model and damage assessment in Abaqus, provides a comprehensive method for predicting material performance. This approach enhances the understanding of the microstructural influences on crack initiation and propagation, offering valuable insights for improving the design and durability of gray cast iron components.

Dielectric Hybrid Optimization Model Based on Crack Damage in Semi-Rigid Base Course

To accurately predict the relative permittivity of cement-stabilized base materials, a study on the dielectric mixing model for cracked base materials was conducted. Based on the electromagnetic mixing theory of multiphase composites, a comprehensive dielectric mixing model of cement-stabilized base materials was derived. The volume ratios and relative permittivity values of the specimen constituents in different cracking states of the cement-stabilized base were determined using industrial CT and a Percometer relative permittivity meter, with comprehensive consideration given to the effects of different initial porosities and crack widths on the dielectric properties. Based on the volumetric and dielectric properties of the base material specimens in both intact and cracked states, as well as the error analysis between the predicted and measured values of the relative permittivity constant, the u-optimal solution of the dielectric mixing model for cement-stabilized base material was determined to be 1. Consequently, an optimization dielectric mixing model for semi-rigid base course materials in a cracked state was developed. The optimization model proposed is suitable for predicting the dielectric properties of cement-stabilized base material with crack widths generally greater than 3 mm during the service life of semi-rigid base course in engineering practice.

Numerical Study on the Determination of Crack Initiation Angle, Stress Field and Stress Triaxiality at Crack Tip in a Cracked Beam of Functionally Graded Material

Numerical Study on the Determination of Crack Initiation Angle, Stress Field and Stress Triaxiality at Crack Tip in a Cracked Beam of Functionally Graded Material

Insights into the Influence of Tensile and Compressive Strain on the Microstructure and Corrosion Performance of 304 L Stainless Steel

The effects of tensile and compressive strain, originating from U-bent deformation, on the corrosion behavior of 304 L stainless steel were studied via analyses of the material’s microstructure and electrochemistry in a 3.5% NaCl solution. In contrast with the as-received 304 L steel with the largest grain size, the deformed 304 L material with a small grain size had the lowest number of Σ3 grain boundaries and an overall low fraction, with special low-Σ values (≤29). Moreover, the dislocation density increased to 1.13 × 1016/m2 and 1.4 × 1016/m2 for the tensile and compressive 304 L steel testing, respectively. The decrease in Epit and increase in ipit suggested that there was a decrease in anti-corrosion properties due to tensile and compressive deformation. This might be attributed to the higher plastic strain found in deformed 304 L steel, which can induce the rupture of passive film and have a harmful influence on corrosion resistance. In particular, the compressive 304 L steel with the highest content of deformed grains (42.12%) promoted the formation of microgalvanic cells, thereby facilitating the nucleation of pits. Then, these pits grew to a large size through grain shedding. Subsequently, massive chloride ions were generated during metal dissolution and diffused along grain boundaries, which promoted the initiation and propagation of intergranular corrosion cracks.

A numerical investigation of the structural behavior of reinforced concrete beams fully or partially encased with UHPC layers in flexure

Weakness in the structural facilities appears due to design faults, poor construction, usage change, maintenance lack, material defects, and environmental conditions. Therefore, strengthening structures becomes necessary for deteriorated concrete and enhancing aging concrete elements to achieve satisfactory performance perfectly. This study numerically investigated the structural behavior of conventional concrete beams reinforced with Ultra-High Performance Concrete (UHPC) layers on different sides. The arrangement of UHPC layers, bonding method between traditional concrete and UHPC, layer thickness, and reinforcement ratio were examined using Abaqus software. The interface between the UHPC layer and conventional concrete sustains compressive, tensile, and shear stresses, which cause weak resistance to the stresses at the contact surfaces. Therefore, the simulation of contact face in Abaqus needs a proper numerical model relying on practical methods. However, using the bonding models available in Abaqus is crucial for reaching logical results comparable to the practical ones. The practical bonding approaches are highly consequential, such as roughening surfaces by sandblasting to attain a firm connection between the contact surfaces, which permits using a surface-based tie constraint for simulation. Further, epoxy resin conducts as a thin coating between contact surfaces, making it possible to simulate via a cohesive surface. The results proved that increasing the area encased by the UHPC layer improves load capacity further and reduces the deflection at the maximum load. Surrounding the beam on whole sides with a UHPC raises load capacity by 108 % and reduces deflection by 46 %. Adding a UHPC layer to the beam delays crack initiation and reduces their number and spacing. Increasing the UHPC layer thickness or reinforcement ratio enhances the load capacity of the beam at cracking and the ultimate state.

A novel 'stop-hole' strategy to extend the fatigue life of orthodontic thermoformed retainers: an in vitro study.

This study aimed to investigate the potential to limit crack propagation in thermoformed retainers (TRs) and extend their fatigue life by placing a 'stop-hole'. Thirty-two TRs, fabricated from 1mm thick, round polyethylene terephthalate glycol (PETG) blanks of ICONIC®, underwent testing within a custom-built fatigue tester. After initial crack growth of 3mm, the TRs were divided into three groups: a control group (C) and two experimental groups based on the stop-hole placement method. In the first experimental group, stop-holes were placed using a 1mm round carbide bur on a slow-speed handpiece (ES), while the second group utilized a heated 1-mm-width ball-ended CPITN probe (EH). Following the placement of a stop-hole at the crack-tip, all 32 TRs resumed testing until the final crack length of 10mm was reached. Stop-hole placement significantly prolonged the delay in crack re-initiation in both experimental groups compared to the control group (P < .001 for both). The mean re-initiation times for the ES and EH groups were 104.50 and 129.50min, respectively, whereas the control group was 28.94min. Consequently, this discrepancy was reflected in the total testing times, with both experimental groups requiring significantly longer time to reach the final crack length (P < .001). There was no significant difference in the testing times between the two experimental groups. This is an in vitro study. The stop-hole methodology significantly extended the fatigue life of a TR in a controlled laboratory environment.