Crack Propagation Research Articles

Long-term investigation of uniaxial compressive and self-sensing performances of ultra-high performance seawater sea-sand concrete incorporating superfine stainless wires: Insights into underlying mechanisms and theoretical models

Developing intrinsic self-sensing concrete is expected to provide a digital intelligence solution to address the infrastructure’s challenges in terms of safety, lifespan, resilience, and carbon emission reduction. Using seawater and sea-sand as well as corrosion-resistant conductive fillers to fabricate self-sensing ultra-high seawater sea-sand performance concrete (UHPSSC) with outstanding mechanical and durability performances is the prerequisite for realizing localized resource utilization and in-situ monitoring of marine infrastructure. However, the potential effect of ions from seawater and sea-sand on the long-term stability of mechanical and electrical performances cannot be ignored. This paper presents a long-term investigation of uniaxial compressive, electrical, and self-sensing performances of superfine stainless wires (SWs) reinforced UHPSSC under natural curing. The results show that incorporating 1.5% SWs improves the compressive strength, elastic modulus, peak strain, and toughness of UHPSSC by 25.0%, 11.7%, 33.8%, and 119.2%, respectively. The established statistical damage constitutive models based on Weibull strength theory highlight the roles of SWs in delaying crack initiation. The dense microstructure of SWs-reinforced UHPSSC inhibits the growth of expansive hydration products formed by corrosive ions, thus guaranteeing its long-term strength development. Additionally, due to incorporating 1.5% SWs, the electrical resistivity of UHPSSC is reduced by seven orders of magnitude, with a strain sensitivity of 119.1 under monotonic compressive loading and a monitoring stress range of 0-148.4 MPa, indicating that SWs-reinforced UHPSSC can sense its stress and strain while providing early warning of crack initiation and propagation. Benefiting from the rust-free property, micron diameter, and high aspect ratio of SWs, the primary conductive path of composites comprises the overlapping networks formed by low-content SWs, which are not influenced by the ion conduction of conductive ions. Hence, SWs-reinforced UHPSSC exhibits stable electrical resistivity and sensitivity during long-term curing, suggesting its considerable potential for long-term in-situ monitoring of marine infrastructure.

Creep-thermal fatigue behavior of thin-walled structures with holes and a creep-thermal fatigue-oxidation phase field model

An experiment was conducted to evaluate the creep-thermal fatigue (CTF) behavior of thin-walled structures with holes. To achieve this, a high-temperature hold phase was added in the testing. The crack propagation of CTF is driven by the combined effects of creep, thermal fatigue, and oxidation. Therefore, a creep-thermal fatigue-oxidation phase field model was developed to simulate CTF behavior. The model accounts for the interaction between creep damage and fatigue damage, as well as the oxidation effect. A creep degradation function was formulated based on classical damage theory, and two creep damage models were compared. Two physically meaningful strategies were proposed to describe oxidation-induced fatigue damage. Finally, the applicability of model to creep-fatigue was validated.

High-temperature behaviour of geopolymer composites containing carbon fibre and nano-silica: Mechanical, microstructure, and air-void characteristics

The fly ash–ground granulated blast-furnace slag-based geopolymer (FSG) represents a groundbreaking material, offering a sustainable and environmentally friendly alternative to conventional construction materials. This paper presents a systematic experimental study on FSG reinforced with carbon fibre (CF, 0%–1.0%) and nano-silica (NS, 1%–3%) at high temperatures (20, 200, 400, 600, 800, and 900 °C). Various tests concerning appearance, residual strength, uniaxial tension, XRD, FTIR, TGA, SEM, and air-void characteristics were conducted to analyse thermal effects. The findings show that as temperature increases, the colour of the geopolymer transitions from dark grey to yellow grey. Enhanced geopolymerisation during thermal solidification from 20 to 400 °C significantly increases the compressive, flexural, and tensile strengths of the geopolymer. The addition of CF and NS improves the residual strength of geopolymers at high temperatures, with optimal concentrations found to be 0.6% for CF and 2% for NS. CF maintains its bridging effect at high temperatures, enhancing fibre–matrix bonding, while NS promotes the formation of additional hydration products on CF surfaces, thereby strengthening the material further. After exposure to 800 °C, despite a weakening of the fibre–matrix interface, the modified geopolymers (FSGC0.6 and FSGC0.6N2) exhibit finer air bubble chord lengths and superior air-void characteristics than the control group. This supports the theory that the synergistic action of CF and NS not only enhances geopolymerisation but also fills pores, thus limiting crack propagation and densifying the pore structure.

Effect of pavement layer temperature on fatigue performance of rib-to-deck weld details in orthotropic steel bridge decks

To investigate the effect of asphalt pavement layer temperature on the fatigue performance of welded details in orthotropic steel bridge deck (OSD), this study focuses on the critical fatigue crack regions of the rib-to-deck weld details. A systematic study was conducted based on the theory of linear elastic fracture mechanics (LEFM) and the finite element model (FEM). In this research, the elastic modulus of the pavement layer at different temperatures, and the initial crack shape, were considered as parameters. The maximum normal stress, stress intensity factor (SIF), and fatigue crack growth (FCG) shape and life of the weld details under the influence of these parameters were analyzed. The analysis results indicate that using a constant temperature of 20°C for the pavement layer will ignore the adverse effects of high summer temperatures. Under the influence of the pavement layer temperature effects, the weld toe of the top deck is the most critical welded detail. The FCG life and final crack shape at the weld toe of top deck are closely related to the effects of pavement layer temperature and the initial crack shape, higher pavement layer temperatures result in shorter fatigue life and larger final crack propagation sizes. Therefore, it is recommended that pavement layer and temperature effects be considered in the FCG analysis of welded details in OSDs.

Effects of supercritical CO2, water, and supercritical CO2-based water on the mechanical properties and fracturability of shale

The study of the effects of supercritical CO2 (ScCO2) under high temperature and high pressure on the mechanical properties and fracturing potential of shale holds significant implications for advancing our understanding of enhanced shale gas extraction and reservoir exploration and development. This study examines the influence of three fluids, i.e. ScCO2, deionized water (DW), and ScCO2+DW, on the mechanical properties and fracturability of shale at immersion pressures of 15 MPa and 45 MPa, with a constant temperature of 100 °C. The key findings are as follows: (1) Uniaxial compressive strength (UCS) of shale decreased by 10.72%, 11.95%, and 23.67% at 15 MPa, and by 42.40%, 46.84%, and 51.65% at 45 MPa after immersion in ScCO2, DW, and ScCO2+DW, respectively, with the most pronounced effect observed in ScCO2+DW; (2) Microstructural analysis revealed that while ScCO2 and DW do not significantly alter the microstructure, immersion in ScCO2+DW results in a more complex surface morphology; (3) Acoustic emission (AE) analysis indicates a reduction in stress for crack damage, with a decreased fractal dimension of AE signals in different fluids. AE energy is primarily generated during the unstable crack propagation stage; (4) A quantitative method employing a multi-factor approach combined with the brittleness index (BI) effectively characterizes shale fracturability. Evaluation results show that ScCO2+DW has a more significant effect on shale fracturability, with fracturability indices of 0.833% and 1.180% following soaking at 15 MPa and 45 MPa, respectively. Higher immersion pressure correlates positively with increased shale fracturability.

Shear tests and meso-scale simulation of cold joint structures in rock-filled concrete: Considering the mutual contact effects between component materials

To realistically reflect the shear performance of the rock-filled concrete (RFC) between layers, this study relies on an RFC dam project in Guizhou Province. Test blocks, measuring 5 m × 4 m × 2 m with cold joints, were cast on site and subjected to shear tests. Additionally, a three-dimensional four-phase meso-model was developed, considering the mutual contact effects between component materials. The mechanical response and damage mechanism of the interlayer were analyzed using the cohesive zone model. The results confirm that the cohesive zone model effectively simulates interlayer cold joint structures. The damage modes of the RFC interlayer include cold joint debonding damage (Mode I) and plastic extrusion damage of self-compacting concrete (Mode II). As the exposed height of the rocks increases, the damage mode shifts from Mode I to Mode II, and higher normal stresses intensify Mode II damage. Meanwhile, the paths of crack extension depend on the distribution of shear-resistant rocks, with weak interfacial transition zones serving as “bridges’’ for crack propagation. The interlayer shear strength is positively correlated with the average exposed height of rocks and normal stresses, with the former having a more significant impact. Finally, the accuracy of the proposed shear strength prediction model for the RFC interlayer was validated by comparing it with results from other studies, providing useful references for meso-numerical modeling of RFC.

Promoted cutting and energy transition efficiency by an asymmetric cutter

Laboratory and numerical tests were performed to understand the rock breakage mechanism of a CCS (constant cross section) cutter and an asymmetric cutter. The laboratory and numerical tests consistently show that the asymmetric cutter frequently generates larger breakage areas and consumes less indentation energy than the CCS cutter, but there is a defect spacing of 70 mm. Thus, the asymmetric cutter frequently has a higher cutting efficiency than the CCS cutter. In addition, the numerical tests reveal that the CCS cutter first generates a plastic zone and subsequently forms cracks to connect with the defect tips. In this process, a large proportion of the energy is wasted in the particle friction in the plastic zone. However, the asymmetric cutter tends to form much smaller plastic zones, which makes it difficult to generate particle friction. Only a small amount of friction energy is consumed in the later crack propagation process. Thus, the asymmetric cutter has a higher energy transition efficiency than the CCS cutter.

Numerical investigation of patch geometry effect on the fatigue life of aluminum panels containing cracks repaired with CFRP composite patch using XFEM and CZM approach

This study investigated numerically the effect of using composite patches with various geometric shapes to improve the fatigue life of aluminum panels of AA7075-T6 and AA2024-T3 containing cracks and compared with the available experimental results. The Cohesive Zone Model (CZM) and Extended Finite Element Method (XFEM) were applied to static and fatigue analyses. Four types of rectangular, trapezoidal, left-oriented and right-oriented triangular composite patches were used in the numerical analysis. With the combination of XFEM and CZM methods, and using the Paris equation, the fatigue crack propagation and fatigue life of aluminum panels containing cracks can be reasonably predicted. A composite patch with a suitable shape and geometry, fatigue life of the repaired panel can be improved considerably. With a 204% improvement in fatigue life in comparison to the unrepaired panel, the repaired sample made of aluminum alloy AA2024-T3 alloy achieved the most significant improvement in fatigue life. In AA7075-T6 alloy samples repaired with composite patches, the samples repaired with rectangular and trapezoidal patches experienced the highest fatigue life increases, respectively with 342 and 290% compared to the unrepaired samples. The comparison of numerical simulation results with the available experiment results shows a good agreement.

Enhancing Shear Strength in Hybrid Metal-Composite Single-Lap Joints Using Z-pins Fabricated via Fused Filament Fabrication

This paper investigates the interfacial shear strength of hybrid metal-composite single-lap joints (SLJs) reinforced with stainless steel Z-pins fabricated by fused filament fabrication (FFF). The joints were created by 3D printing an orthogonal array of 2 mm diameter steel Z-pins onto a steel substrate using FFF. The Z-pins were then embedded into a basalt fibre (BF)-fabric/epoxy resin composite using the Ultrasonically Assisted Z-FibreTM (UAZ) method to form a high-strength and tough interface. The results demonstrate that steel Z-pins produced via FFF effectively enhance the shear strength of the hybrid metal-composite SLJs, significantly improving joint performance. The study also explores the influence of Z-pin volume fraction and embedding height on SLJ shear strength. It was found that higher volume fractions and greater embedding heights of the Z-pins contribute to the increased shear strength. Compared to unreinforced joints, the maximum shear strength of the steel Z-pin reinforced joints increased by 120.1%. This enhancement is attributed to the effective energy absorption mechanisms, primarily facilitated by the frictional pull-out, plastic deformation and shear fracture of Z-pins accompanied by the formation of ductile dimples. These mechanisms suppress crack propagation and improve joint integrity. This study presents an innovative approach for fabricating hybrid metal-composite joints with enhanced toughness and strength.

Experimental study on the Mode l fracture toughness of frozen silty clay incorporating Digital image correlation

To investigate the effects of initial moisture content and temperature on the Mode I fracture toughness (KIc) of frozen silty clay, a series of three-point bending tests were conducted. Rectangular specimens with prefabricated cracks were tested, and digital image correlation (DIC) technology was employed to analyze the microscopic characteristics at the crack tip. The results indicate that the Mode I fracture toughness of frozen silty clay increases with decreasing temperature and increasing initial moisture content. Temperature governs the failure mode, with plastic failure predominantly occurring at high temperatures and brittle failure at low temperatures. Based on the load–displacement curves and DIC recordings, the macroscopic fracture process of the specimens can be categorized into three stages: elastic deformation, formation of the failure surface, and specimen failure. Additionally, the crack propagation process can be further divided into three stages: initiation and development of microcracks, transition from microcracks to macroscopic cracks, and rapid development of macroscopic cracks. These findings provide critical insights for slope stability research in cold regions and offer new perspectives for engineering design and construction in similar environments.

Nonlinear vibration characteristics and damage detection method of blade with breathing fatigue crack

In aero-engine, blades operate under harsh conditions for extended periods, which makes them highly susceptible to fatigue cracks. The localized and nonlinear structural changes caused by blade crack lead to complex vibration characteristics that are not well understood, posing challenges to the identification of these cracks. This paper proposes an approach for identifying breathing fatigue crack in compressor blade, leveraging the nonlinear features induced by the cracks. Crucial sensitive characteristics are extracted and a crack indicator for accurate crack detection is defined. Initially addressing the typical morphology of fatigue breathing cracks, a dynamic model of blade with cracks is developed to analyze their impact on the natural frequencies. Nonlinear contact forces are introduced to characterize the "breathing" effect of the fatigue crack, allowing for the determination of the harmonic distribution patterns in the blade's vibration response under various excitation conditions. A finite element model is then established considering the crack morphology, by adopting the contact element. Based on that, the nonlinear response characteristics associated with the blade's primary, sub-harmonic, and super-harmonic resonances are explored. A crack-sensitive parameter is defined and extracted from the nonlinear responses, under different crack propagation stages. At last, effectiveness and reliability of the defined parameter for identifying cracks is validated through a vibration fatigue test.

Cracking and deformation behaviors of overhanging rock: Laboratory tests and optical monitoring

The destabilization of overhanging rock is a dangerous geological problem. In this study, a generalized model of typical overhanging cliffs from the Three Gorges Reservoir area in China with different fracture angles, fracture lengths, and free surface depths is constructed to investigate the cracking and deformation behavior of overhanging rocks. Laboratory tests and deformation field monitoring using the digital image correlation (DIC) method are performed on these specimens to reproduce the destabilization and failure process of overhanging rock under external loading. The influence of peak load is found to be the most sensitive to the fracture length, followed by the free surface depth, and to be the least sensitive to the fracture angle. The DIC-based strain fields reveal that the fracture angle and free surface depth significantly alter the crack propagation paths, whereas the influence of the fracture length is weaker. These parameters also affect the crack initiation time. The relative displacement evolution characteristics indicate that fracture angle, fracture length, and free surface depth affect the shape and size of the rotating block, the rotation center, and the rotation pivot point and degree, respectively. The grayscale characteristic evolution trends are similar for all examined overhanging rock specimens. The evolution of the grayscale indices based on DIC can be divided into high-frequency oscillation, smooth decline (or smooth downward concavity), and stable development stages. Furthermore, the multistage properties of the indices can be used to identify the fracture state of overhanging rocks, providing a theoretical basis for graded early warning of rockfall disasters.

Study on the influence of cement slurry viscosity on acoustic emission response and deformation characteristics of slurry-coal cemented body

In the process of grouting sealing, the cementation failure of slurry and coal interface exists in the gas extraction borehole, which leads to unstable and gas leakage in the sealing section and affects the efficient gas extraction in the borehole. In order to study the influence of slurry viscosity on the failure characteristics of slurry-coal cemented body, this paper explores the law of influence of slurry viscosity on the acoustic emission response characteristics and deformation characteristics of slurry-coal cemented body under uniaxial compression through laboratory tests, data analysis and quantitative research of image processing. The results show that: (1) Changing the slurry viscosity can achieve the strength control of the cemented body, which can be increased by up to 42.65 %. (2) In the process of uniaxial compression, the cementation structure of slurry-coal cemented body breaks down successively, and its proportion and strength lead to the distribution difference of AE signals and the fluctuation of b-value. Combined with the phase response characteristics of the two, the change of failure mode can be analyzed to provide a reliable evaluation of crack propagation in the cemented body. (3) The b-value of the sample of slurry-coal cemented body with high-viscosity slurry does not fluctuate significantly, and is relatively stable at a low level near 1.0. The results are helpful to promote the research and development of sealing materials and sealing technology.

Monte Carlo simulations of the fracture resistance of an asphalt pavement layer

The purpose of the proposed numerical model is to analyze the cracking of the wearing course in a pavement overlay, assuming a pre-existing crack that passes through the binding layer and base. The computations employed the author's simulation-based Monte Carlo material model, which describes the failure process of a Semi-Circular Bend (SCB) specimen during standard laboratory testing of asphalt concrete. A key feature of this model is the incorporation of the random nature of material parameters, allowing for the simulation of result dispersion when analyzing a sufficiently large population of samples. The proposed FEM model and obtained material data were directly applied to the numerical analysis of the pavement structure. The comprehensive computational algorithm allows for a random description of the load that induces crack propagation in the pavement wearing course, leading to the creation of a histogram that defines the range of failure load dispersion. Such supporting calculations can assist in optimizing asphalt mix design and, in the future, may allow for the estimation of pavement structure reliability.

Influence of cavitation erosion on crack propagation behavior of hydraulic concrete under tensile stress

Influence of cavitation erosion on crack propagation behavior of hydraulic concrete under tensile stress

Fracturing Process Visualization of Cracked Chevron-Notched Brazilian Discal and Notch Semicircular Bend Rock Specimens and Comparative Analysis by Combined AE and DIC

Abstract A series of homogeneous granite specimens were prepared to investigate the Mode I fracture toughness (KIC) using the cracked chevron-notched Brazilian disk (CCNBD) method and semicircular three-point bending (NSCB) method recommended by the International Society for Rock Mechanics (ISRM). A combination of acoustic emission (AE) and digital image correlation (DIC) was utilized to monitor the crack propagation and analyze the fracture modes at various test stages. The results indicated that the average value of Mode I fracture toughness (KIC-) of the CCNBD specimen was approximately 10% higher than that of the NSCB specimen. The coefficient of variation in the NSCB test was small, which could provide more stable result of Mode I fracture roughness. Compared to the CCNBD test, the failure mode of NSCB specimens was mainly tensile damage with instantaneous damage during the experimental process. Furthermore, the failure process of the NSCB test was mainly controlled by tensile fractures, which exhibited instantaneous failure. The CCNBD test was first controlled by tensile fractures and then by shear fractures when the peak was reached, exhibiting progressive failure. The failure mode and the process may be the main factors controlling the difference between the results of the NSCB and CCNBD tests. This work helps select appropriate methods to measure the Mode I rock fracture toughness.

Rock fragmentation of simulated transversely isotropic rocks under static expansive loadings

Rock fragmentation is a critical process for mineral extraction and for mitigating overstressed rock in geotechnical applications. In this study, 3D-printed concrete was used to simulate the stratified rock mass, and experimental and numerical methods were employed to investigate crack propagation under static expansive loadings in transversely isotropic rocks. Two types of cracks were observed in the experiments: P-type (a crack propagates primarily along the weak layer) and T-type (a crack propagates across the weak layers) cracks. The findings revealed that the orientation of layers significantly influenced the initiation and propagation of cracks, with P-type cracks commonly observed in simpler P-P mode fragmentations and more complex P-P-T modes emerging under higher expansive loadings. P-T-T modes were characterized by the simultaneous presence of the T-type crack after an initial P-type crack. The AE energy levels in the P-P-T and P-T-T modes were much higher than those in the P-P mode. 2D-DDA models were further built to understand the effects of the loading scales, layer angles, and locations of weak layers on the cracking sequences. The results provided detailed insights into stress evolutions and the impact of expansive loadings on crack initiation and propagation.

Enhancing Machining Efficiency: Real-Time Monitoring of Tool Wear with Acoustic Emission and STFT Techniques

Tool wear in machining is inevitable, and determining the precise moment to change the tool is challenging, as the tool transitions from the steady wear phase to the rapid wear phase, where wear accelerates significantly. If the tool is not replaced correctly, it can result in poor machining performance. On the other hand, changing the tool too early can lead to unnecessary downtime and increased tooling costs. This makes it critical to closely monitor tool wear and utilize predictive maintenance strategies, such as tool condition monitoring systems, to optimize tool life and maintain machining efficiency. Acoustic emission (AE) is a widely used technique for indirect monitoring. This study investigated the use of Short-Time Fourier Transform (STFT) for real-time monitoring of tool wear in machining AISI 4340 steel using carbide tools. The research aimed to identify specific wear mechanisms, such as abrasive and adhesive ones, through AE signals, providing deeper insights into the temporal evolution of these phenomena. Machining tests were conducted at various cutting speeds, feed rates, and depths of cut, utilizing uncoated and AlCrN-coated carbide tools. AE signals were acquired and analyzed using STFT to isolate wear-related signals from those associated with material deformation. The results showed that STFT effectively identified key frequencies related to wear, such as abrasive between 200 and 1000 kHz and crack propagation between 350 and 550 kHz, enabling a precise characterization of wear mechanisms. Comparative analysis of uncoated and coated tools revealed that AlCrN coatings reduced tool wear extending tool life, demonstrating superior performance in severe cutting conditions. The findings highlight the potential of STFT as a robust tool for monitoring tool wear in machining operations, offering valuable information to optimize tool maintenance and enhance machining efficiency.

Inter-laminar Strength of NITE-SiC/SiC Composites with Various Fiber Reinforcing Architecture

The mechanical performance of SiC/SiC composites is significantly influenced by the architecture of fiber reinforcement. Among the various fabrication methods, the nano-powder infiltration transition/eutectic (NITE) process is a promising technique that is capable of achieving a dense and stoichiometric SiC matrix. The reinforcement architecture, such as cross-ply (CP) or woven prepreg (WP), is determined during the preform stage of the NITE process, which is crucial in determining the mechanical properties of SiC/SiC composites. In this study, the tensile test and double notch shear (DNS) test were conducted using NITE-SiC/SiC composites to investigate the effect of the fiber reinforcing architecture on the fracture mechanism of SiC/SiC composites. The tensile strength and maximum shear strength of both CP and WP specimens were nearly identical. However, other mechanical properties, particularly those of CP specimens, exhibited significant variability. A comparison of fracture surfaces and load-displacement curve analyses from the DNS tests revealed that the cross points of the longitudinal or transverse fibers act as obstacles to both deformation and crack propagation. These obstacles were found to be more densely distributed in WP specimens than in CP specimens. The variability observed in the mechanical properties of CP specimens is likely due to size effects caused by the sparser distribution of these obstacles compared to the WP specimens.

Security Length Associated with the Risk of Ammonia Tank Leak Using CTOA Criterion and ALOHA Software

Ammonia is a toxic gas and can cause tragic consequences for humans. The damage level depends on concentration and duration of exposure. The security length associated with the risk of a tank leak at the acute exposure level of 30 ppm (AEGL-1) has been computed. Two tools have been combined: the CTOA criterion and the ALOHA software. The CTOA, a measure of fracture resistance against ductile crack propagation, is implemented in Abaqus software to compute the size of a breach in a tank submitted to internal pressure. This breach is assumed to be initiated by a gouge-dent defect provoked by a shock. The ALOHA software introduces the tank's characteristics, contents, and breach size. This allows us to determine and visualize the security length. The security length depends on geographic and climatic conditions, and for an incident localized in Metz (France), a value of 436 m was found. The effects of internal temperature, wind speed, and breach position are studied. A comparison for the same reference state with hydrogen is also made.