Crack Initiation Characteristics Research Articles

Crack propagation process in double-flawed granite under compression using digital image correlation method and numerical simulation

The existence of flaws seriously weakens the rock strength, directly affects the crack expansion morphology, and indirectly affects the slope stability. In this study, uniaxial compression tests were carried out on double-flawed granite to investigate the effects of flaw angle on its compressive strength and crack coalescence based on DIC technology and PFC2D numerical simulation. The result indicates that the UCS values of flawed specimens at angles of 15°, 30°, 45°, and 60° are approximately 25.5–51.7% lower than that of the intact specimen. The failure strength of the double-flawed sample increases with the increase of flaw inclination angle, and the fracture morphology shifts from no expansion to split expansion. When the flaw tip strain of each flaw sample exceeded 0.6%, the stress concentration was generated at the flaw tip, and the sample began to appear macroscopic large cracks. DIC technique can well observe the crack initiation and propagation process, recording inclined shear strain bands at flaw tips, tensile strain bands in the middle of flaws, and tensile shear O-shaped strain ring in rock bridge areas. Numerical simulations using PFC2D software were carried out and showed a good agreement with the physical results. In addition, the effect of structural inclination on specimen failure strength is clearly explained by the theory of compressive damage based on the projected size of flaws. The deformation of rock mass is not a simple material deformation but is composed of material deformation and structural deformation. These experimental and numerical results enhance our understanding of crack initiation and coalescence characteristics, aiding in the analysis of rock structure stability in scenarios such as excavated underground openings, slopes, and tunneling construction, where pre-existing cracks or step-path fractures are pivotal to structural integrity.

Study on thermal crack characteristics of granite in Shandong Province, China under different temperatures and heating/cooling treatments

The qualitative and quantitative analysis of crack initiation characteristics, extended procedure, and crack network morphology of rock under different temperatures and heating/cooling paths was conducted in order to delve deeper into the mechanism of thermal cracking, and the analytical perspectives mainly include microstructure, thermodynamics, and mineral crystal properties. Research results indicate that 200–300 °C and 600 °C are two obvious mutation points of water-cooled samples, while air-cooled samples show mutation points around 600 °C. The initiation positions of thermal cracking under all temperature paths are relatively similar, with intergranular crack located between particles at the edge of the sample or intercrystalline cracks in the middle of feldspar or quartz aggregates. Slow heating and rapid cooling contribute to the complexity of the thermal crack network, and rapid heating can form larger sized thermal cracks. The process of thermal cracking can be divided into three stages: initiation of small cracks, joint development of main cracks and small cracks, and interconnection of cracks to form a network structure. The thermal cracking mechanism is attributed to differences in expansion and thermal conductivity among various crystals.

Frost heaving and crack initiation characteristics of tunnel rock mass in cold regions under low-temperature degradation

Frost heaving and crack initiation characteristics of tunnel rock mass in cold regions under low-temperature degradation

Numerical investigation on the crack resistance curve of thermally treated quartz‐diorite rock under mixed mode I + II loading

AbstractTo investigate the effects of thermal treatment and loading conditions on the crack initiation and propagation characteristics of quartz‐diorite, mixed mode I + II fracture tests were conducted by using three‐point bending (TPB) specimens. The numerical model, established on ABAQUS, couples the initial fracture toughness criterion and cohesive zone model to predict the crack initiation and propagation. The effects of thermal treatment and loading conditions on the crack propagation process, crack resistance curve, fracture path, and some fracture parameters at peak load have been analyzed and discussed. The results demonstrate that the thermal treatment has an obvious influence on the crack resistance curve of the specimen. The crack of the specimen will be dominated by the mode I fracture component with crack extension. The numerical results also show that the temperature and loading condition affect the crack length and fracture toughness at peak load.

Fatigue response of wire-arc additive manufactured nickel-aluminum bronze (NAB) in the post-annealed condition

Nickel-aluminum bronze (NAB) is chosen for critical applications like marine propellers, pump components, and offshore structures due to its exceptional mechanical properties and corrosion resistance. Considering the utmost importance of the fatigue performance of NAB in such applications, this study investigates the fatigue performance of wire arc additive manufactured (WAAM) NAB in the annealed (675 °C for 6 h then furnace cooling) condition. To this end, static mechanical properties were evaluated using uniaxial tensile testing, while fatigue properties were assessed through fully reversed tests conducted at ambient temperature. Electron backscatter diffraction (EBSD) was utilized for the examination of the texture, and grain size distribution in annealed WAAM NAB. Additionally, scanning electron microscopy was employed to inspect fatigue-fractured specimens, while the analysis of EBSD was conducted to examine the deformation behavior beneath the crack initiation zone. The results provide insights into the fatigue life, crack initiation, and propagation characteristics of WAAM NAB relative to the cast counter material. The findings of this research provide valuable insights into the mechanical properties, microstructural characteristics, and fatigue response of WAAM NAB, informing the design of fatigue-resistant components and supporting the reliable utilization of WAAM NAB in demanding applications such as the marine and naval industries.

A Study on Crack Initiation and Propagation of Welded Joints under Explosive Load

Welded joints in naval ship hull structures are weak areas under explosive load, but there are relatively few studies investigating the failure characteristics of welded joints through dynamic fracture and explosion tests. In order to explore and predict the failure characteristics of welded joints under explosive load, instrumented Charpy impact tests, explosion tests, and numerical simulations were carried out. The dynamic fracture toughness of ultra-high strength ship hull structural steel welded joints was obtained, and the dynamic stress intensity factors, together with the correlation between stress wave and crack propagation at different positions, were acquired. The results showed that the stress state at the crack tip of a Charpy impact specimen was consistent with that of a welded joint under explosive loads, and the crack initiated when the dynamic stress intensity factor exceeded the dynamic fracture toughness. The results indicated that the dynamic fracture toughness obtained by instrumented Charpy impact tests could be used to predict the crack initiation characteristics of welded structures under explosive load, and the stress wave at the crack tip was basically perpendicular to the crack propagation surface and promoted the rapid propagation of cracks.

The Initiation Characteristics of Corrosion Fatigue Crack in 18Ni (250) Steel

Maraging steel is the material of deep sea pressure equipment. The characteristics of corrosion fatigue crack initiation in 18Ni (250) were investigated by performing a tensile fatigue test under different loading conditions. The mechanism of corrosion fatigue crack initiation was analyzed considering the influence of welding. The results showed that the crack initiation life decreases with increasing load and increases with the stress ratio. In a corrosive seawater environment, crack initiation is affected by both mechanical damage and electrochemical corrosion. The proportion of the crack initiation life of the total sample life is approximately 71.45–99.55%, where the larger this proportion is, the smaller the fatigue zone is. Crack initiation and microcrack propagation in maraging steel correspond to the microfracture mechanism of dimple fractures. The microfracture mechanism is strongly affected by the presence of a weld but weakly affected by the weld position. The results of this study provide a theoretical basis for predicting the corrosion fatigue life of deep-sea submersible pressure hulls.

Discontinuous fracture behaviors and constitutive model of sandstone specimens containing non-parallel prefabricated fissures under uniaxial compression

The deformation and damage laws of non-homogeneous irregular structural planes in rocks are the basis for studying the stability of rock engineering. To investigate the damage characteristics of rock containing non-parallel fissures, uniaxial compression tests and numerical simulations were conducted on sandstone specimens containing three non-parallel fissures inclined at 0°, 45° and 90° in this study. The characteristics of crack initiation and crack evolution of fissures with different inclinations were analyzed. A constitutive model for the discontinuous fractures of fissured sandstone was proposed. The results show that the fracture behaviors of fissured sandstone specimens are discontinuous. The stress–strain curves are non-smooth and can be divided into nonlinear crack closure stage, linear elastic stage, plastic stage and brittle failure stage, of which the plastic stage contains discontinuous stress drops. During the uniaxial compression test, the middle or ends of 0° fissures were the first to crack compared to 45° and 90° fissures. The end with small distance between 0° and 45° fissures cracked first, and the end with large distance cracked later. After the final failure, 0° fissures in all specimens were fractured, while 45° and 90° fissures were not necessarily fractured. Numerical simulation results show that the concentration of compressive stress at the tips of 0°, 45° and 90° fissures, as well as the concentration of tensile stress on both sides, decreased with the increase of the inclination angle. A constitutive model for the discontinuous fractures of fissured sandstone specimens was derived by combining the logistic model and damage mechanic theory. This model can well describe the discontinuous drops of stress and agrees well with the whole processes of the stress–strain curves of the fissured sandstone specimens.

Analysis of the crack initiation and short crack propagation of laser cladding IN718 enduring fretting fatigue at 650 ℃

In order to reveal the behaviors of crack initiation and short fatigue crack propagation in laser cladding IN718 under high temperature conditions, very high cycle fretting fatigue (VHCFF) experiments were conducted at 650 ℃ with a transverse clamping force of P=400N and different axial stress loading conditions to obtain S_N data. The surface wear morphology and fracture morphology characteristics were analyzed by scanning electron microscope (SEM) and white light interferometer. Additionally, the microstructure characteristics of grains in the short fatigue crack region were examined through electron back scatter diffraction (EBSD). Parameters such as equivalent grain size, grain elastic modulus, Schmid factor, geometric compatibility factor, twist angle, and tilt angle were considered in the analysis of the characteristics of fatigue crack initiation and propagation. The results indicate that fretting fatigue cracks tend to initiate on grains with higher Schmid factors, larger equivalent grain size, and lower grain elastic modulus on the fretting contact surface. Moreover, the short fatigue cracks tend to propagate along the grains with the microstructural features of the high Schmid factor, the geometric compatibility factor of m′>0.4, the twist angle of α<50°, and the tilt angle of β<50°.

Experimental study of precursory features of CO2 blasting-induced coal rock fracture based on grayscale and texture analysis.

CO2 blasting has been identified as a potent method for enhancing the permeability of coal seams and improving gas drainage efficiency. This study is focused on elucidating the deformation and fracture mechanisms of coal and rock during CO2 blasting and on identifying the precursor characteristics of these processes. To this end, a CO2 blasting-induced coal rock fracture pressure model and a gas pressure distribution model were developed. The research utilized a self-developed CO2 blasting test platform along with a non-contact full-strain field measurement analysis system. Briquette samples were subjected to CO2 blasting tests under controlled experimental conditions, which included an axial pressure of 1.0 MPa and variable gas pressures of 0.5, 1.0, and 1.5 MPa. This methodology enabled the capture of the principal strain field on the surface of the samples. The Gray Level Co-occurrence Matrix (GLCM) was employed to extract and analyze the grayscale and texture features of the strain cloud maps, facilitating a quantitative assessment of their evolution. The aim was to pinpoint the precursor characteristics associated with coal rock cracking and crack propagation. The results revealed that: (1) During the cracking and subsequent propagation of samples, the strain field's grayscale histogram underwent a transformation from a "broad and low" to a "narrow and high" configuration, with a consistent increase in peak frequency. Specifically, at 3 ms, a primary crack was observed in the sample, evidenced by a grayscale peak frequency of 0.0846. By 9 ms, as the crack propagated, the grayscale peak frequency escalated to 0.1626. (2) The texture feature parameters experienced their initial abrupt change at 3ms. Correlation with the gas pressure distribution model indicated that this was the crack initiation moment in the sample. (3) A secondary abrupt shift in the texture feature parameters occurred at 9ms, in conjunction with experimental phenomena, was identified as the crack propagation phase. Monitoring the grayscale and texture features of the principal strain field on the coal rock surface proved effective in recognizing the precursor characteristics of crack initiation and propagation. This research has the potential to reduce blasting costs in coal mines, optimize blasting effects, and provided theoretical guidance for enhancing gas extraction efficiency from deep and low permeability coal seams.

A comparative study of several life prediction models based on the critical plane approach for fretting fatigue crack initiation

AbstractTo reasonably forecast the crack initiation characteristics of fretting fatigue, this study evaluates the accuracy of several damage models. The existing experimental and simulation investigations are compared, noting the similarities and differences in the crack initiation location, crack orientation angle, and crack initiation life predicted by various damage models. It is crucial to employ a reasonable and reasonable damage prediction model of fretting crack initiation life, as fretting fatigue, a phenomenon of mechanically fastened connecting parts, can considerably reduce the fretting fatigue life of the components. This paper compares the fatigue damage prediction models of fretting fatigue specimens under partial slip conditions, and it will help researchers understand the benefits and drawbacks of various damage models and give them the knowledge and foundation to select the more reasonable fretting fatigue damage model.

Fracture analysis of chopped carbon fiber sheet molding compound composite under tensile loading via in-situ μXCT

This paper investigated the crack initiation in a chopped carbon fiber sheet molding compound (SMC) composite under different tensile loads by integrating synchrotron micro-X-ray computed tomography (μXCT) and micromechanics analysis. It was found that the varied distribution of fiber chips in the SMC composite led to distinct fracture propagation behaviors. With the loading increasing, the crack primarily occurred within the fiber chips, and the delamination exited when the crack propagated along the interface between adjacent layers. To understand the location and sequence of crack initiation in SMC composites, the sample were divided into identical cubes. After tracking the microstructure of each cube, the corresponding interfacial debonding strength as well as Young's modulus were predicted. Predictive modeling of crack initiation characteristics was conducted assuming homogeneous material deformation under small strain. The predicted outcomes closely matched with the experimental findings.

Effect of Heat Treatment on Fatigue Properties of TA15 Titanium Alloy Repaired by Laser Deposition

Laser‐deposition repair technology is employed to repair damaged TA15 titanium alloy forgings. Heat treatment is used to control the microstructure, strength, and plasticity of these repaired forgings. The fatigue properties and crack initiation and propagation characteristics of the repaired parts are studied. Optical microscopy and electron microscopy are used to analyze the microstructure, fatigue fracture, and longitudinal section near initiation. In the results, it is indicated that the tensile and fatigue properties of repaired parts after 900 °C heat treatment are superior to those treated at other temperatures. This is attributed to the strong tissue sensitivity of the repaired specimens’ performance. With increasing heat‐treatment temperature, the secondary α phase in the dimorphic structure tissue thickens. Additionally, the restoration zone develops in layered α phases and becomes truncated, leading to shortening and thickening. The heat‐affected zone undergoes a transformation from a transitional structure to a basket organization. After heat treatment at 1000 °C, the microstructure becomes relatively slender, cracks propagate readily, and the resistance is weakened.

Experimental and finite element analysis for evaluating influence of contact pressure on fretting fatigue mechanisms of A319‐T6 cast aluminum alloy

AbstractThis paper evaluates the influence of contact pressure on the surface contact state, fretting damage degradation mechanism, crack initiation and propagation characteristics, and fretting fatigue life of A319‐T6 cast aluminum alloy in fretting fatigue test using bridge‐type fretting pad. For a certain range of contact pressure, the fretting fatigue life exhibits a trend of increasing and subsequently decreasing with increasing contact pressure. With finite element analysis, the fretting contact region can be divided into two characteristic zones according to the dynamic evolution of the contact status within one fatigue cycle. The contact pressure affects both the distribution of the two characteristic zones as well as the relative slip range, which results in distinct fretting damage mechanisms. The nucleation characteristics and propagation paths of fatigue cracks under three typical contact pressures were analyzed considering the effects of fretting damage and heterogeneous microstructures of the alloy, accounting for the contact pressure dependency of the fretting fatigue life.

Experimental Studies on the Crack Initiation and Propagation of Hydraulic Fracturing with Different Coal Structure Combinations.

Coal structure is one of the key geological factors that affects the effect of coal reservoir stimulation. Based on the geological spatial combination characteristics, thickness, and proportion of different coal structures, the coal reservoir is divided into different coal structure combination types. The hydraulic fracturing device is used to carry out indoor fracturing experiments and dissect the crack initiation and expansion characteristics with different coal structure combinations. The results show that the coal structure combination is of the binary type (undeformed coal + granulated coal or cataclastic coal + granulated coal), and the undeformed coal (cataclastic coal) can overcome the tensile strength and minimum principal stress when it is driven by the high-pressure fluid. The undeformed coal (cataclastic coal) ruptures and forms longitudinal cracks. The increasing proportion of granulated coal inhibits crack expansion and promotes the transverse deformation of coal. The interface contact point between the undeformed coal (cataclastic coal) and granulated coal easily fractures along the cross section of the specimen. When the coal structure combination is the triplex type (undeformed coal + granulated coal + cataclastic coal or cataclastic coal + granulated coal + cataclastic coal), the undeformed coal or cataclastic coal is transformed. The forming fractures in the undeformed coal (cataclastic coal) can cut through the soft coal when the thickness of the undeformed coal (cataclastic coal) is large and the thickness of granulated coal is thin. On the contrary, it is not easy to cut through. When the coal structure combination is granulated coal + cataclastic coal + granulated coal, the cataclastic coal fails under shear stress and forms the crack along the cross section of the coal sample. The granulated coal inhibits the crack expansion at both ends. The research results have an important indicative significance for further understanding the fracture initiation and propagation mechanism of hydraulic fracturing with complex coal structures in coal reservoirs.

Effect of Slow Strain Rates on the Hydrogen Migration and Different Crack Propagation Modes in Pipeline Steel

The slow‐strain‐rate test (SSRT) is the most commonly used method for evaluating pipeline steel in service environments. However, to accurately assess the sensitivity of steel to hydrogen, it is necessary to investigate the effect of different strain rates, taking into account microstructure‐influenced hydrogen migration. Herein, a hot‐rolled X70 pipeline steel sheet is investigated by a SSRT at different strain rates with and without synchronous hydrogen charging. The influence of the pearlite content and different strain rates on the hydrogen‐assisted crack propagation and hydrogen diffusion in pipeline steels is discussed. Using scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and hydrogen microprinting, the hydrogen atoms are observed to be easily segregated at the ferrite/pearlite (F/P) interface without external stress. Under loading, a higher strain rate results in lower hydrogen permeation content in steel, penetrating into pearlite and interacting with its internal vacancies, leading to transgranular fracture initiating from pearlite. At a low strain rate, the F/P interface is more vulnerable to hydrogen degradation, leading to intergranular crack mode along the interface and increased tendency to form secondary cracks. Therefore, strain rate–induced crack initiation and propagation characteristics should be considered during the SSRT.

Strength and cracking resistance of concrete containing different percentages and sizes of recycled tire rubber granules

In order to use recycled materials in the composition of greener and cleaner concrete products, it is vital to ensure that the produced concrete have the appropriate level of strength. This paper investigates the effect of gradation (size) and percentage of recycled rubber granules on the physical parameters (slump and density), strength parameters (tensile and compressive strength values), and in particular the crack initiation and propagation characteristics (under the opening, shearing, and tearing modes). The tensile strength, compressive strength, fracture toughness (KIc, KIIc, and KIIIc), and fracture energy values at the initial crack initiation and ultimate failure propagation stages were determined by testing specimens containing 0, 1, 2, 4, and 12% of fine and coarse recycled rubber granules. Rubber particles were graded in two distinct ways so that they could serve as both filler and granules. In contrast to fine rubber granules, the addition of coarse granules had a positive effect on mode-II and mode-III fracture toughness. But, the addition of fine and coarse rubber granules reduces mode I fracture toughness. The analysis of fracture values indicates that coarse rubber granules have a positive effect on the post-peak failure energy absorption (i.e., the crack propagation stage) of all fracture modes. However, this increase in fracture energy for the fine rubber combination was only observed for mode-II fracture stress. The fracture toughness and fracture energy values of the similar concrete were empirically determined based on their corresponding tensile strength. The link between fracture toughness and fracture energy values was also discovered. According to the experimental results, the concrete containing 4% coarse rubber granules provides good mechanical and cracking resistance properties.

Crack propagation law of rock with single fissure based on PFC2D

In the study of pre-fabricated single-fissured rock, the failure mode, mechanical properties and propagation law of rock are affected by the length and inclination angle of fissure. In this study, artificially prepared marble-like rocks with different fissure characteristics were used as rock representatives, using PFC2D software to establish a uniaxial compression model tests for rocks with fissures. The effects of different fissure lengths and inclination angles on the failure mode and crack initiation and propagation of rock-like structures were systematically studied, revealing the mechanism of macroscopic mechanical behavior of rocks containing single fissures in the process of crack initiation, propagation, and failure at the micro-level. The results show that: 1) Failure mode of the rock sample is mainly controlled by fissure inclination angle and fragmentation degree is mainly controlled by fissure length. 2) The initiation stress, damage stress, and peak stress of rock samples during loading deformation increase with the increase of fissure inclination angle, and decrease with the increase of fissure length. The crack initiation angle decreases with the increase of the fissure inclination angle. 3) Crack initiation characteristics: at a low fissure inclination angle (α&amp;lt; 45°), the crack initiation position has a certain offset to the center of the prefabricated fissure, or starts from the tip of the prefabricated fissure. At high fissure inclination angles (α≥45°), the crack initiates from the tip of the prefabricated fissure and forms a “dispersed” distribution. 4) The crack propagation law is mainly shear cracks, at a low fissure inclination angle (α&amp;lt; 45°), the crack propagation has obvious aggregation band formation. At high fissure inclination angles (α≥45°), the crack aggregation zone gradually weakened and expanded in the direction of dispersion.

Microstructures and Fatigue Properties of High-Strength Low-Alloy Steel Prepared through Submerged-Arc Additive Manufacturing

Submerged arc additive manufacturing (SAAM) is a viable technique for manufacturing large and complex specialized parts used in structural applications. At present, manufacturing high-strength low-alloy steel T-branch pipe through SAAM has not been reported. This paper uses this technology to manufacture low-alloy structural steel parts. The microstructures of the samples were characterized, which revealed that they were mainly composed of polygonal ferrites. The tensile properties in the horizontal and vertical directions of deposits were studied. Results show that the horizontal tensile strength of deposits was quite close to the vertical one, while the elongation rate in the vertical direction was obviously lower than that in the horizontal direction. Fatigue results indicate that the strain fatigue limit of high-strength low-alloy steel samples in vertical direction was 0.24%. The fatigue fractures of fatigue samples of deposits showed multi-source crack initiation characteristics and the crack propagation regions exhibited typical fatigue striations, so the final instantaneous fracture region showed a ductile fracture. Fatigue performance is very important for the safe service of structural parts, but there is a lack of relevant research on this additive manufacturing part. The results of this paper may support the popularization of the SAAM for high-strength low-alloy steel T-branch pipe.

Cumulative Damage in Very High/Low Cycle Combined Fatigue for TC21 Titanium Alloy

The effect of low cycle fatigue (LCF) predamage with no precracks on very high cycle fatigue (VHCF) properties, and crack initiation characteristics for TC21 titanium alloy, was investigated. The results showed that LCF predamage with less than 5% of fatigue life had little influence on fatigue limit, but reduced its fatigue life. Fatigue cracks were initiated on the surface of the specimen at high stress amplitude, whereas fatigue cracks were initiated on the subsurface of the specimens at low stress amplitude. Based on Lemaitre damage theory, a very high/low cycle combined fatigue damage model was established to analyze the fatigue damage process, which was consistent with the experimental data. It was indicated that 5% LCF predamage value was the equivalent damage value, which was close to the critical value of VHCF crack initiation. The fatigue crack initiation of the specimens with LCF predamage less than 5% took up the major components of fatigue life.