Final Fracture Research Articles

Study on the evolution rules of coal deformation and failure characteristics caused by multiple mining disturbances

During coal mining operations, the coal will be deformed and damaged due to multiple mining disturbances (MMD), often resulting in disasters, like rock burst. To understand the evolution rules of coal deformation under MMD and its final fracture characteristics after impact dynamic load loading, reduce the adverse effects of mining disturbances, and improve disaster prevention and control capabilities, quasi-static uniaxial cyclic loading-unloading (L-U) and dynamic axial compression tests were conducted on large-sized coal-like samples. During the tests, three-dimensional (3D) laser scanning and acoustic emission (AE) monitoring technology were utilized to accurately capture the full-field deformation and AE response data, facilitating a systematic analysis of deformation and fracture characteristics. The results show that: (1) Under the cyclic L-U effect induced by MMD, each loading cycle causes compression deformation with partial recovery during unloading, presenting an overall “wavy” variation trend. (2) The maximum load is the most critical factor affecting the damaged coal deformation, with smaller load resulting in less overall sample deformation. (3) After the impact dynamic loading, the damaged samples suffered large-scale impact splitting failure, with the compressive-shear layer failure mainly occurred inside the holes. (4) Lower loading during cyclic L-U process correlate with reduced damage degree, and smaller debris particles with a higher fractal dimension when impact failure occurs, indicating a more severe impact failure. (5) With multiple cycles of L-U, the cracks inside the sample gradually extend and expand from around the hole to the outside. The greater the load and the number of cycles, the more serious the crack damage will be. (6) In the practical mining process, it is crucial to reinforce roadway interiors while minimizing low-loading cyclic disturbances induced by MMD. The study has obtained the deformation evolution rules and failure characteristics of coal under MMD, providing a theoretical basis for the prevention and control of corresponding engineering disasters.

On the Competition between Pores and Hidden Entrainment Damage during In Situ Tensile Testing of Cast Aluminum Alloy Components

The competition between pores and hidden entrainment defects during tensile testing of specimens from Al-Si-Cu alloy high-pressure die castings has been characterized. In all tests, multiple strain concentrations have been identified by using the digital image correlation technique and the final fracture has been preceded by a competition between pores and hidden damage, later identified as oxide bifilms. The results have confirmed previous findings that overall damage to the metal during its liquid state is much more extensive than what can be assessed via X-ray inspection, which looks only for pores. It is concluded that current quality assurance techniques need to be updated.

Effect of torsional angle on the fretting wear behavior and fracture failure mechanism of steel wires with spiral contact structure

The fretting wear of steel wire will reduce the service life of wire rope and severely threaten the safety of mine hoisting. To investigate the influence of torsion on the fretting wear behavior of steel wire, the fretting wear tests of steel wires with spiral contact structure under the action of tensile-torsional coupling forces were conducted, and the fatigue fracture failure mechanism was revealed. The result indicates that the CoF increases with increasing torsional angle. As the cycles increase, the sliding distance between the wires decreases, while the adhesion increases. The fretting area gradually transitions from gross sliding status to partial sliding status. The sliding distance and energy loss between the wires increase with increasing torsional angle. The wear depth and coefficient increase with increasing torsional angle, and the wear level of peak contact structure is more serious than that of valley contact structure. The primary wear mechanisms of steel wire are abrasive wear, adhesive wear and fatigue wear. The fatigue fractograph of steel wire is obviously divided into fatigue source zone, crack growth zone and final fracture zone. Abundant secondary cracks and dimples exist in the final fracture zone, and the fatigue fracture failure mechanism is mainly ductile fracture.

Internal damage evolution of C/SiC composites in air at 1650 °C studied by in-situ synchrotron X-ray imaging

Carbon fibre reinforced silicon carbide (C/SiC) ceramic matrix composites have attracted considerable attention due to their exceptional properties and extensive potential applications as high-temperature structural materials. However, due to their complex structure and manufacturing defects, C/SiC composites exhibit intricate mechanical behaviour under thermal-mechanical-oxidative coupling environments. To date, systematic studies on the internal damage evolution and failure mechanisms of C/SiC composites under high-temperature oxidative environments are lacking. In this study, a combination of synchrotron X-ray micro-computed tomography (SR-μCT) and in-situ experiments under thermal-mechanical-oxidative coupling environments at room temperature and 1650 °C in air was used to characterize the internal microstructures and damage evolution processes of C/SiC composites at different loading levels. Additionally, the 3D strain fields during in-situ loading were quantitatively analysed using the Digital Volume Correlation (DVC) method. The findings underscore the substantial impact of oxidative damage on the mechanical response of C/SiC composites, particularly concerning tensile properties and fracture modes. At room temperature, severe delamination, fibre bundle pull-out and interfacial debonding occurred internally. Whereas, under high-temperature atmospheric conditions, severe fibre oxidation reactions occurred at the specimen edges, resulting in rapid porosity escalation. Crack initiation from surface defects followed by rapid inward propagation is observed. Moreover, while the strain distribution remains relatively uniform until fracture, a pronounced concentration of strain is evident near the fracture zones at room temperature, with an even greater concentration observed at 1650 °C. Notably, the region of concentrated strain within the 3D deformation field corresponds closely to the final fracture location, as revealed by quantitative DVC analysis.

Record statistics of fracture in the random spring network model.

We study the role of record statistics of damage avalanches in predicting the fracture of a heterogeneous material under tensile loading. The material is modeled using a two-dimensional random spring network where disorder is introduced through randomness in the breakage threshold strains of the springs. It is shown that the waiting strain interval between successive records of avalanches has a maximum for moderate disorder, thus showing an acceleration in occurrence of records when approaching final fracture. Such a signature is absent for low disorder when the fracture is nucleation-dominated, as well as for high disorder when the fracture is percolation type. We examine the correlation between the record with the maximum waiting strain interval and the crossover record at which the avalanche statistics change from off-critical to critical. Compared to the avalanche exponent crossover based prediction for failure, we show that the record statistics have the advantage of both being real-time as well as being a precursor significantly prior to final fracture. We also find that in the avalanche-dominated regime, the failure strain is at best weakly correlated with the strain at the maximum waiting strain interval. A stronger correlation is observed between the index of the largest record and the index of the record at the maximum waiting strain interval.

Crack Growth Patterns of Aluminum Tubular Specimens Subjected to Cyclic Tensile Loads

This study presents a detailed analysis of a fatigue test campaign in order to identify different crack patterns. It was conducted on 6061-T6 aluminum tubular specimens featuring an internal diameter of 10 mm and different thicknesses (2, 3 and 4 mm). These specimens were subjected to cyclic tensile loads with a load ratio of R = 0.1, utilizing a sinusoidal load function at a frequency of 3 Hz. The investigation examines the crack growth rates, the stress intensity factor, and the final and intermediate fracture zones by applying overloads in some cases. The differences with two-dimensional specimens and the importance of this study for the interpretation of results with biaxial loading states are highlighted. The different states of crack growth detected are analyzed using artificial vision techniques. The differences between the exterior and interior faces of the specimen are revealed, and a series of states prior to the formation of the radial crack front expected in these specimens are identified.

Lattice Defects Present beneath Crack Initiation, Propagation, and Final Fracture Regions on the Same Hydrogen Embrittlement Fracture Surface of Martensitic Steel

Lattice Defects Present beneath Crack Initiation, Propagation, and Final Fracture Regions on the Same Hydrogen Embrittlement Fracture Surface of Martensitic Steel

Percutaneous functional spinal unit cementoplasty versus percutaneous kyphoplasty for severe osteoporotic vertebral compression fracture complicated with endplate-disc complex injury: A retrospective case-control study.

Severe osteoporotic vertebral compression fracture (SOVCF) is frequently complicated by endplatedisc complex (EDC) injury. While percutaneous kyphoplasty (PKP) can offer rapid analgesia and facilitate early activity, it does not restore vertebral height and may result in intervertebral leakage and untreated EDC injury. This study aimed to evaluate the clinical outcomes of percutaneous functional spinal unit cementoplasty (PFSUP) for SOVCF complicated by EDC injury and compare its clinical and imaging outcomes with PKP. This was a retrospective case-control study. Patients with SOVCF complicated with EDC injury between January 1, 2018, and December 31, 2019, were recruited and assigned to the PKP group and PFSUP group based on their treatment procedures Back pain was evaluated using the visual analog scale (VAS) and daily life activities were assessed using the Oswestry disability index (ODI). X-rays were employed to observe the presence and location of cement leakage, as well as to measure the sagittal vertical axis (SVA) and local kyphosis angle (LKA). Loss of correction was calculated by subtracting the LKA after surgery from that at the final follow-up visit Subsequent vertebral fracture (SVF) was confirmed using the Genant semi-quantitative method and/or MRI. A total of 64 patients were included in this study. Among them 41 cases were assigned to the PKP group (28 females, 74.8 years on average), while the remaining 23 cases were assigned to the PFSUP group (15 females, 76.3 years on average). All surgical interventions were successfully completed without major complications. Compared to the PKP group, the PFSUP group had longer operation time (70.28 ± 11.44 vs 44.5 ± 10.12, P< 0.001) higher frequencies of radiation exposure (97.6 ± 19.85 vs 38.6 ± 9.53, P< 0.001), and a lower cement leakage rate (26.1% vs. 41.5%, P< 0.001). One day after surgery and at the final follow-up the PFSUP group had lower VAS and ODI scores, as well as lower LKA and Sva values compared with the PKP group (all P< .001). At the final follow-up visit, the PFSUP group demonstrated a lower loss of correction (4.38 ± 2.71 vs. 10.19 ± 3.41 P< 0.001) and a lower SVF rate compared to the PKP group (21.7% vs. 31.7%, P< 0.001). PFSUP outperformed PKP in alleviating pain restoring and maintaining sagittal balance, and lowering the incidence of cement leakage and SVF for SOVCF with EDC injury However, PFSUP was associated with longer operation time and high radiation exposure frequencies.

Corrosion fatigue cracking behaviors of Q690qE high‐strength bridge steel as a weld joint in simulated marine environment

AbstractCorrosion fatigue behavior and mechanism of the weld joint of Q690qE high‐strength bridge steel was investigated in a simulated marine environment. It reveals the sub‐critical heat‐affected zone (SCHAZ) and coarse‐grained heat‐affected zone (CGHAZ) are the most vulnerable sites to corrosion fatigue cracking. The CGHAZ of the Q690qE steel weld joint was prone to corrosion fatigue initiation mainly because of micro‐galvanic corrosion between CGHAZ and weld metal (WM). The corrosion fatigue failure in SCHAZ mainly resulted from local stress/strain concentration due to weld softening. The final fracture location was determined by the competition between these two effects.

Effect of cold expansion on the fatigue life of multi-hole 7075-T6 aluminum alloy structures under the pre-corrosion condition

The multi-hole structures with cold expansion are extensively utilized in the various joint parts of aircraft. However, the synergistic effect of stress concentration and atmospheric corrosion will cause severe deterioration of structural loading capabilities. Thus, it is crucial to investigate the fatigue life degradation mechanism of these structures under pre-corrosion conditions. In this paper, three-hole specimens with cold expansion (CE) and non-cold expansion (NCE) were used to investigate the effect of cold expansion on the fatigue performance of multi-hole structures of 7075-T6 aluminum alloy under pre-corrosion conditions. The life degradation law and damage mechanism of these two types of specimens under pre-corrosion conditions were analyzed. Furthermore, the cold expansion strengthening factor was proposed through the curve-fitting of the fatigue life attenuation ratio of these two types of specimens. Finally, a competition mechanism for the contribution ratio to the final fracture of the specimens in corrosive environments was proposed based on the SEM morphology analysis of the fatigue fracture of the specimens.

High-temperature low-cycle fatigue characteristics of the 12Cr10Co3MoWVNbNB turbine rotor steel

Low-cycle fatigue (LCF) experiments of the 12Cr10Co3MoWVNbNB steel at 598 °C and a strain amplitude ranging from 0.27 % to 0.55 % were executed to reveal the fatigue characteristics, fatigue fracture mechanism, and further to predict the high-temperature fatigue life of the new-style turbine rotor steel. The results indicate that the steel exhibits a high-temperature fatigue softening characteristic. During the high-temperature LCF, the cracks are generated at surface or subsurface defects of the specimens, and the fatigue striations and dimples are apparent on the fracture surfaces of the crack propagation zone and the final fracture zone, respectively. The Smith-Watson-Topper (SWT) and Manson-Coffin models are suitable to predict the high temperature fatigue life of the steel at high/low strain amplitudes, respectively. In terms of finite element analysis of the turbine set rotor, it is determined that the steady state strain of the rotor steel is approximately 0.2 %. And thus, the fatigue life of the steel rotor is estimated as 172,500 cycles at the strain amplitude, based on the Eq. (6) in this work. After 100,000 cycles at the strain amplitude, the yield and tensile strengths of the steel specimens are only reduced by 1 % and 3 %, respectively, indicating a long residual life, which is consistent with the above predicting result.

The progressive failure features of shield tunnel lining with large section

Investigating the failure process of segmental lining is of great significance to the design and maintenance of shield tunnel. Indoor scaled model tests are carried out to study the inner force, structural displacement, acoustic emission signals and damage pattern of lining. Meanwhile, a new numerical approach is put forward based on particle element method to study the influence of geo-stress field and the position of void or key block to the failure features of lining. The results showed that the failure of lining is progressive and can be divided into four stages, that is, the initial elastic stage, meso-damage stage, macroscopic failure stage and overall instability stage. For the key block is located at the crown, there are two concentrated intervals for the generation of micro-cracks. For the key block is located on the spandrel, micro-cracks are uniformly produced throughout the loading process after the initial elastic stage. For the key block is located on the haunch, micro-cracks distributed in a large concentrated range. The influence of the position of void on the damage of lining can be divided into three kinds, namely, no effect; affecting the emerging order of cracks while not final fracture pattern; affecting the emerging order of cracks and the final fracture pattern.

Bonding mechanism and fracture behavior of cold-sprayed Fe-based amorphous alloy on 6061 Al alloy

Using cold spray technology to prepare amorphous alloy coatings can effectively prevent crystallization, thereby preserving the excellent properties of amorphous alloys. However, interfacial bonding significantly affects performance, such as weak bonding impacting the durability and stability of the coating. Therefore, in-depth study of the interfacial bonding mechanism is crucial for a comprehensive understanding of coating properties. This paper uses cold spray technology to prepare Fe-based amorphous alloy coating on 6061 aluminum alloy substrates. The bonding mechanisms at the particle/substrate, particle/particle, and coating/substrate interfaces are studied, and the fracture behavior in the tensile bond strength test is analyzed. The results show that the heat accumulation and the increase in strain rate generated by the particles impacting the substrate cause the viscosity of the amorphous alloy to suddenly decrease from 1.2×103 g·cm−1 s−1 to 8.4×10−1 g·cm−1 s−1. Increased fluidity and micro-friction accelerate the mixing of elements and form a metallurgical bond at the particle/substrate interface. Meanwhile, the oxides at the particle/particle interface are extruded, and the exposed fresh metal is subjected to a huge shear rate to promote atomic fusion. Tamping of the subsequent particles leads to further deformation of the deposited particles, forming a stronger metallurgical bond and mechanical anchoring at the particle/particle interface. In addition, the continuous impact of particles triggers dynamic recrystallization of the substrate, and the average grain size decreases from 14.28 μm to 1.39 μm. This fine-grained structure strengthens the mechanical anchoring of the coating/substrate interface. Furthermore, the bonding strength measured by the tensile test is determined to be 18.51 MPa. The main reasons for the final fracture occurring in the coating are the coating pores, cracks formed by shear bands accumulated in free volume, and the low adhesion oxide layer at the particle interface.

Fracture in control rods of helicopters: Same failures but different reasons

This paper presents two incidences of failures of collective control rods which resulted in accidents to a particular type of helicopter. The control rods were made of 2024 Al-alloy tubes fitted with fork at one end and a spherical bearing at the other end (eye-end). In terms of physical appearance, the two failures in the control rods were identical to each other wherein there was fracture in the Al-alloy tube at the eye-end. Fractography study confirmed that in both the cases, multiple fatigue cracks had initiated over a sector at the internal thread roots on the Al-alloy tubes corresponding to 1st/2nd thread on the eye-end stud. After initiation, the cracks had propagated progressively into the tube-wall and then along the circumferential direction. As the crack propagation progressed, the load bearing capacity of the tubes diminished resulting in final overload fracture under the operational load at the time of accidents. Although failures in both the control rods looked alike in terms of failure location, fracture pattern and fracture mechanism, the primary reasons of failures were found to be different. Investigation revealed that premature fatigue crack initiation was due to non-uniform stress distribution on the internal threads of the control tube with increased level of stress over a sector. In one case, the non-uniform stress distribution resulted from thread manufacturing deficiency, and in the other case, it was due to incorrect assembly at the eye-end.

Experimental investigation on seismic performance of CHS T-joints in salt spray environment

This paper aims to investigate the seismic performances of circular hollow section (CHS) T-joints with different regions of corrosion damage. A total of seven CHS T-joint specimens with various corrosion degrees were applied to conduct the quasi-static tests of brace axial cyclic loading, and the effects of corrosion damage on the hysteresis behavior and failure mode were discussed. Experimental results indicated the brace corrosion only had a slight influence on the seismic performance of CHS joints, while the corrosion difference on two sides of the brace might cause an additional bending moment and damage accumulation to accelerate the plastic collapse of the connection zone. Compared with the corrosion on the brace, the resistance of CHS joints was more susceptible to the corrosion located in the chord wall, resulting in the axial cyclic carrying capacity and stiffness of CHS joints significantly decreasing with the increase of chord corrosion level. However, the increasing corrosion on the chord would affect the final fracture behavior and fracture position, so the total energy dissipation and deformation of CHS joints presented a complicated and nonlinear degradation trend. In addition, corrosion damage simultaneously had an impact on reducing the bending capacity of the chord, and this phenomenon not only caused the differences between the degradation rates of axial tensile and compressive strength of CHS joints but also affected the failure mechanism and local deformation of CHS joints under the brace axial cyclic loading.

Fracture propagation characteristics of pre-cracked red clay in roadbed filling under three-point bending

Red clay was a special roadbed filling soil that was easy to expand and shrink because of the characteristics of high liquid limit and high porosity, leading to the appearance of a large number of cracks. In order to understand the cracking mechanism and nonlinear fracture failure process of red clay, the fracture toughness and crack sensitivity of red clay with different saturation, compaction and gravel content were analyzed by three-point bending cracking experiment and Image processing technology, and then the cohesive crack model was established to describe the fracture toughness. The results showed that the cracking process of red clay could be divided into three stages: crack incubation period, crack initiation and propagation period and visible cracks appearance stage. There was a conic function relationship between fracture toughness and saturation and a positive correlation between fracture toughness and compactness. The influence of gravel content on the final fracture path of red clay was the most significant, while the effects of saturation and compaction could be ignored. In addition, the fracture toughness converted from the cohesive crack model was basically as the same as the measured. This work provided a basis for theoretical analysis and numerical calculation for the fracture toughness of red clay under three-point bending.

Fracture behaviour of reaction-bonded silicon carbide-boron carbide using digital image correlation

The current research uses the Digital Image Correlation (DIC) approach to study the fracture behaviour of Reaction-Bonded Silicon Carbide-Boron Carbide (SiC–B4C) ceramic (RBSBC). A Photron camera with 137,500 fps was used to capture the crack propagation at an interframe time frame of 2.6 ms before final fracture. The purpose was to capture the rapid brittle behaviour of the ceramic during the onset of crack propagation. The RBSBC failed in a transgranular manner like most monolithic SiC-based materials as Hexoloy-SA. The presence of randomly dispersed coarse B4C grains helps in deflecting the crack path which increases the overall crack length. The material’s resistance to crack propagation (R-curve) was measured using the crack tip opening displacement, the stress intensity at the crack tip KI, and the J-integral against crack extension. Two methods were used to determine the KI: experimentally based on the displacement values at the crack tip as obtained from the DIC images, and quasistatic based on the fracture load and the geometrical dimension of the crack. The experimentally-determined KI and J resulted in higher values compared to the quasistatic ones. However, they reflect the actual conditions at the crack tip, while the quasistatic conditions underestimate the resistance-plot. The RBSBC has higher R-curve compared to commercially available SiC-only materials as Hexoloy-SA and ABC-SiC. To improve the crack-resistance of RBSBC, the Si amount should decrease to a nm level, while the percentage of B4C grains should increase and be evenly dispersed across the material.

A Study on the Propagation of Chloride-Induced Stress Corrosion Cracking for Austenitic Stainless Steel at Corrosion Environment

Austenitic stainless steels have been widely used in the petrochemical industry, nuclear power plants and the ship industry due to their high toughness and corrosion resistance. However, these materials are susceptible to environmentally-assisted degradation, such as chloride-induced stress corrosion cracking (CISCC). In this respect, this study investigated CISCC crack propagation rate for austenitic stainless steels, such as type 304L and type 316L, in the corrosion environment. Considering the environment of the spent nuclear fuel canister used in the dry storage system, the test chamber is always maintained at 3.5% NaCl and related humidity (RH) 95% at 60℃. For initiation of CISCC, a test procedure, which has nine pre-cracking procedures for a CT specimen, is suggested. In a test procedure, crack transitioning, which induces the environmental corrosion crack, is observed after mechanical precracking. To determine the CISCC crack length corresponding to each test step, the direct current potential drop (DCPD) method is measured in real-time. In addition, the CISCC crack observed after final fracture is measured using optical microscopy in order to improve the accuracy of the measured crack length. In the test results, CISCC crack resistance of type 316L is better than that of type 304L. Finally, the test results show that the CISCC crack propagation rate is 2.784E-11 m/s for type 304L and is 9.93E-11 m/s for type 316L.

A remaining multiaxial ductility-based fracture toughness prediction model for metallic alloys

Predicting fracture toughness is an important part of developing an overall structural integrity methodology for components. This study therefore tries to establish a fracture toughness prediction model based on the concept of remaining multiaxial ductility exhaustion. Numerical simulation model which takes into account the level of constraint at the crack tip and the material’s inherent multiaxial ductility is established to research fracture initiation and propagation. The core of the model is based on increasing load to consume the remaining multiaxial ductility of metallic alloys until final fracture. The presence of grains (G) and grain boundaries (GB) in the simulation model is used to highlight a sensitivity analysis on the local interaction of microstructural at meso-scale with regards to crack initiation and growth. The model has been validated with fracture toughness tests by using additively manufactured Ti6Al4V alloy compact tension (CT) specimens. It is shown that the predicted plane strain KIC compares well with results from both wrought and heat-treated additively manufactured test specimens. Future tests on different other materials using low/high constraint cracked geometries should strengthen the model’s predictive capabilities for different fracture scenarios.

Structural Health Monitoring of Fiber Reinforced Composites Using Integrated a Linear Capacitance Based Sensor.

The demand for fiber-reinforced polymers (FRPs) has significantly increased in various industries due to their attributes, including low weight, high strength, corrosion resistance, and cost-efficiency. Nevertheless, FRPs, such as glass and Kevlar fiber composites, exhibit anisotropic properties and relatively low interlaminar strength, rendering them susceptible to undetected damage. The integration of real-time damage detection processes can effectively mitigate this issue. This paper introduces a novel method for fabricating embedded capacitive sensors within FRPs using a coating technique. The study encompasses two types of fibers, namely glass and Kevlar fiber/epoxy composites. The physical vapor deposition (PVD) technique is employed to coat bundle fibers with conductive material, thus creating embedded electrodes. The results demonstrate the uniform distribution of nanoparticles of gold (Au) along the fibers using PVD, resulting in a favorable resistance of approximately ≈100 Ω. Two sensor configurations are explored: axial and lateral embedding of the coated yarn (electrodes) to investigate the influence of load direction on the coating yarn. Axial-sensor configuration specimens undergo tensile testing, showcasing a linear response to axial loads with average sensitivities of 1 for glass and 1.5 for Kevlar fiber/epoxy composites. Additionally, onset damage is detected in both types of fiber composites, occurring before final fracture, with average stress at the turning point measuring 208 MPa for glass and 144 MPa for Kevlar. The lateral-sensor configuration for glass fiber-reinforced polymer (GFRP) exhibits good linearity towards strain until failure, with average gauge factors of 0.25 and -2.44 in the x and y axes, respectively.