Microscopic Failure Mechanisms Research Articles

Experimental study on constitutive relation of coal gangue coarse aggregate concrete under uniaxial compression

As an efficient method for utilizing coal gangue (CG), concrete incorporating coal gangue as coarse aggregate has significantly reduced the reliance on natural aggregates, offering substantial environmental and economic benefits. In this study, coal gangue concrete was prepared with coal gangue replacement rates of 0, 20, 40, 60, 80, and 100%, and mechanical tests under unconfined compression were conducted to evaluate the stress-strain behavior and failure mechanism of coal gangue coarse aggregate concrete (CGC). Utilizing scanning electron microscope (SEM) microscopic characterization, the microscopic failure mechanism of CGC was further elucidated. With increased coal gangue replacement, the CGC's uniaxial compression failure mode shifts from shear to longitudinal splitting failure. The slope, peak stress and elastic modulus of the stress–strain curve's rising section are negatively correlated with the coal gangue content, while the falling section's slope, peak strain and ultimate strain are positively correlated. Next, building upon the classical constitutive model, we adjust the constitutive parameters utilizing the uniaxial compressive strength and coal gangue content. Finally, we introduce a predictive model for the CGC's constitutive compressive behavior across various content levels. There is a notably high agreement between the model and experimental data.

Experimental and numerical study on CuO/ZnO-modified aramid fabric for ballistic and UV radiation protection

Aramid fabrics are widely used in bulletproof armor because of their excellent mechanical properties. Previous studies have shown that ultraviolet radiation has a negative effect on the mechanical properties of aramid yarn, so improving the mechanical properties and impact resistance of aramid fabrics under ultraviolet radiation has become a research focus. In this work, aramid fabric was modified with CuO and ZnO particles to improve its ballistic performance under ultraviolet radiation. The ballistic impact resistance response and microscopic failure mechanisms of aramid fabrics under ultraviolet radiation were analyzed in detail. Under ultraviolet radiation, the ballistic limit velocity (vbl) of the CuO/ZnO-modified aramid fabric was 185.1 % greater than that of a neat fabric with a similar areal density. The vbl of the single-layer modified fabric was 45.6 % greater than that of the two-layer neat fabrics. The decrease in the ballistic performance of the aramid fabric under ultraviolet radiation was attributed to surface damage caused by the fracture of the chemical structure of the fibers, which weakened the mechanical properties of the fabric. The numerical simulation results were highly consistent with the ballistic impact test results, and the error between the numerical simulation and experimental results was within 10 %. The effects of changes in the mechanical parameters of the fabrics on the protection mechanism and energy absorption structure during ballistic impact were investigated. The energy dissipation of the modified fabric was at least 147.7 % greater than that of the neat fabric, further explaining the significant improvement in the ballistic performance of CuO/ZnO-modified fabrics under ultraviolet radiation.

Enhancement of mechanical properties in coral concrete via seawater and sea-sand: Experimental insights into the use of dual plastic fibers

The South China Sea contains many coral reefs, and there is a lot of discussion about the best way to dispose of them. Finding ways to use these materials effectively in marine engineering could help address the shortage of natural aggregates in coastal engineering. One potential solution is using coral concrete, which offers cost-effective and enhanced properties. A type of coral fiber concrete using sea sand is created by combining coral rock, sea sand, seawater, PVA fiber, and PP fiber. The findings indicate that by utilizing the optimal ratio of the two fibers (3 kg PVA + 1 kg PP and 2 kg PVA + 2 kg PP), the cubic and axial compressive strength of the new concrete can be increased by 10 % and 20 %, respectively compared to ordinary coral concrete. Furthermore, incorporating these fibers can also reduce axial displacement, resulting in an elastic modulus that is 30–50 % higher than non-fiber variants while enhancing axial toughness. Finally, this study examines stress-strain curves at three different stages through analysis of macroscopic and microscopic failure mechanisms. It was found that there exists a correlation exceeding 0.9 between two mathematical models and measured stress-strain curves from this study. The effective use of composite plastic fiber significantly enhances concrete material performance while providing valuable data support for marine economic facilities construction.

Cryogenic damage behavior of carbon fiber reinforced polymer composite laminates via fiber-optic acoustic emission

Cryogenic temperatures cause significant changes in the mechanical response and microscopic failure mechanisms of carbon fiber reinforced polymer (CFRP) composites. However, the mechanisms by which cryogenic temperatures affect composites are not yet fully understood due to a lack of adequate in-situ characterization techniques. Herein, the cryogenic damage evolution process of CFRP composites was investigated by constructed fiber-optic acoustic emission (AE) detection system. Tensile damage behavior of woven CFRP composites at 300 K, 153 K and 77 K was evaluated by AE characteristic response and cluster analysis combined with scanning electron microscopy. According to the results, increased damage activity within the composites at cryogenic condition promoted the release of mechanical energy, as well as an increase in the contribution of fiber damage to overall damage, which are the dominant micro-strengthening mechanisms of composite laminates under cryogenic conditions. This study provides a novel explanation for understanding the cryogenic damage behavior of composites.

Dynamic constitutive relation and macro/micro failure mechanism of fine-grained WC-Co composite

As the core of an armour-piercing projectile, it is crucial to provide a microscopic perspective to elucidate the macroscopic mechanical responses of fine-grained WC-Co composite under dynamic loadings. This study investigated the dynamic constitutive relation and macro/micro failure mechanisms of fine-grained WC-Co composite. Results showed that the compressive and tensile strengths increase with the strain rate influenced by the inertia effect of crack propagation and the presence of the Co phase. The strain rate dependency mechanism of the composite was obtained by comparing it with typical ceramic materials. Macro-crack propagation and distribution were discussed under different stress states in dynamic compression and Brazilian splitting tests. Microscopic failure mechanisms were obtained by SEM technique, revealing that the Co phase creates numerous localized dimple fractures and improves the plastic deformation of the WC-Co composite.

Experimental study on the failure behavior of rockburst induced by a dynamic disturbance in the circular chamber for deep underground engineering

Rockbursts induced by disturbance loads have become a significant engineering issue. Block specimens featuring a circular chamber structure were designed to investigate the effects of the disturbance frequency on rockbursts. The experiment simulated a series of rockburst failure processes through a disturbance load with four different frequencies by maintaining a constant preset confining pressure. The experimental results showed that an increased disturbance frequency intensifies the shear failure response and enhances the rockburst intensity. Furthermore, the particle image velocity technique was used to calculate the ejection velocity. The results suggested that the initial ejection velocity of fragments during rockburst surged by 253 % as the disturbance frequency was increased from 0.005 Hz to 0.625 Hz. The assessment level of rockburst failure transitions ranged from slight to moderate. These results indicated that an increase in disturbance frequency exacerbated rockburst failures. Two mechanisms were proposed to explain this effect. (i) The increase in disturbance frequency elevates the internal friction angle of rock units, enhancing their ultimate energy storage capacity. (ii) Increased energy input causes the energy storage in rock units to surpass the minimum energy required for rock failure, endowing the fragmented rock mass with greater velocity and leading to more intense rockburst failures. Ultimately, recommendations for early warning of rockburst failure are also provided by discussing the microscopic failure mechanisms of rockburst experiments and the rockburst in situ.

Experimental study on the engineering properties and failure mechanism of moraine in Southeast Tibet under freeze–thaw cycles conditions

The effect of freeze–thaw cycles (FTCs) is a prevalent hydrothermal phenomenon in seasonal permafrost regions, significantly affecting the micro and engineering properties of moraine. This study investigates the engineering properties and failure mechanisms of moraine induced by FTCs, using samples with varying dry densities subjected to 0–20 FTCs. The permeability characteristics, particle breakage characteristics, elastic modulus, stress–strain curve, excess pore water pressure, peak friction angle, and critical state line characteristics of the moraine were analyzed before and after the FTCs, and the microscopic failure mechanism of moraine induced by FTCs was revealed. The test results show a degradation in the engineering properties of moraine induced by FTCs, exhibiting the greater the dry density the more obvious the damage to the engineering properties of the moraine. In addition, the particle fragmentation and fine particle aggregation caused by the FTCs changed the distribution, shape, arrangement, and particle intercontact patterns, which revealed the failure mechanism of moraine induced by FTCs. The correlation analysis between the shear strength index and microstructural parameters demonstrated that the particle arrangement has the greatest effect on the peak friction angle and particle shape has the greatest effect on the critical state line of moraine under the FTCs conditions. The findings offer experimental evidence and theoretical support for preventing and resolving geological risks and engineering project problems resulting from the deterioration of moraine shear strength during FTCs in the alpine mountain region.

Multiaxial creep–fatigue failure mechanism and life prediction of a turbine blade based on a unified numerical solution approach

AbstractExposure of turbine blades to cyclic torsional loading at high temperature, stemming from pre‐torque installation and the aerodynamic forces during operation, has the potential to induce substantial creep–fatigue damage, thereby contributing to the likelihood of premature failure. Investigating the deformation mechanisms and proposing a reliable life prediction method aiming at torsional loading is critical to ensure the structural integrity of turbine blades. This study conducted strain‐controlled fatigue and creep–fatigue tests on Inconel 718 superalloy, employing a multiaxial servo‐hydraulic testing machine. Electron backscattering diffraction elucidated deformation and damage mechanisms, forming a basis for subsequent constitutive modeling and life prediction. The lack of creep–fatigue mechanical behavior and microscopic failure mechanism when stress triaxiality equal to 0 is filled, which provides the theoretical basis and data support for the life design and damage assessment of this material under extreme service conditions. The unified viscoplasticity constitutive model effectively characterized macroscopic deformation under torsional loading. Prediction of creep–fatigue life under torsional loading, utilizing the multiaxial ductility factor‐modified strain energy density exhaustion model, demonstrated excellent alignment with experimental findings. Finally, parametric analyses of stress distribution and damage assessment under different conditions were carried out for the example of a turbine blade with relatively rarely considered aerodynamic loading as a variable. It is expected to be popularized and applied in life design and damage assessment of high‐temperature structures under multiaxial loading in engineering.

The macroscopic and microscopic fatigue failure mechanisms of high-temperature thermally-damaged granite under cyclic impact loading

The rock surrounding wells for developing geothermal energy are in an extreme environment with high temperature and long-term dynamic disturbance. To study the influences of different temperatures and dynamic disturbances on the damage, mechanical properties, and fatigue failure mechanism of rocks, a series of microscopic thermal damage analysis and dynamic cyclic impact tests were conducted on granite specimens treated at 25 ∼ 900 °C. Results show that after being thermally treated to different extents, granite specimens are weakened in terms of the dynamic strength and deformation resistance under cyclic impact loading. As the temperature rises, the number of impacts bearable by specimens before failure decreases accordingly. Under the joint action of temperature and strain rate, 450 °C is found to be the threshold temperature for the significant reduction of dynamic strength of the rock. Under action of temperature and strain rate, the proportion of dissipated energy gradually reduces with rising temperature. Moreover, the dynamic failure mode of rock specimens at high temperatures gradually changes from splitting failure to crushing failure, the fractal dimension gradually increases, and the cumulative dissipated energy density decreases. Finally, the deterioration degree of elastic modulus was adopted to describe the progressive damage evolution of thermally-treated granite under cyclic impact loading. Furthermore, the fracture morphologies of rock specimens after failure were analyzed from the microscopic perspective to reveal the macroscopic and microscopic fatigue failure mechanisms of the rock under the coupling of temperature and strain rate.

Electrochemical Failure Mechanism of δ-MnO2 in Zinc Ion Batteries Induced by Irreversible Layered to Spinel Phase Transition.

Phase transitions of Mn-based cathode materials associated with the charge and discharge process play a crucial role on the rate capability and cycle life of zinc ion batteries. Herein, a microscopic electrochemical failure mechanism of Zn-MnO2 batteries during the phase transitions from δ-MnO2 to λ-ZnMn2O4 is presented via systematic first-principle investigation. The initial insertion of Zn2+ intensifies the rearrangement of Mn. This is completed by the electrostatic repulsion and co-migration between guest and host ions, leading to the formation of λ-ZnMn2O4. The Mn relocation barrier for the λ-ZnMn2O4 formation path with 1.09eV is significantly lower than the δ-MnO2 re-formation path with 2.14eV, indicating the irreversibility of the layered-to-spinel transition. Together with the phase transition, the rearrangement of Mn elevates the Zn2+ migration barrier from 0.31 to 2.28eV, resulting in poor rate performance. With the increase of charge-discharge cycles, irreversible and inactive λ-ZnMn2O4 products accumulate on the electrode, causing continuous capacity decay of the Zn-MnO2 battery.

Delamination of Plasticized Devices in Dynamic Service Environments.

With the continuous development of advanced packaging technology in heterogeneous semiconductor integration, the delamination failure problem in a dynamic service environment has gradually become a key factor limiting the reliability of packaging devices. In this paper, the delamination failure mechanism of polymer-based packaging devices is clarified by summarizing the relevant literature and the latest research solutions are proposed. The results show that, at the microscopic scale, thermal stress and moisture damage are still the two main mechanisms of two-phase interface failure of encapsulation devices. Additionally, the application of emerging technologies such as RDL structure modification and self-healing polymers can significantly improve the thermal stress state of encapsulation devices and enhance their moisture resistance, which can improve the anti-delamination reliability of polymer-based encapsulation devices. In addition, this paper provides theoretical support for subsequent research and optimization of polymer-based packages by summarizing the microscopic failure mechanism of delamination at the two-phase interface and introducing the latest solutions.

LCF life prediction of turbine disc alloy using crystal plasticity and 3D representative volume element containing twin boundaries

The traditional LCF life prediction method based on macroscopic behavior is difficult to reveal the microscopic fatigue failure mechanism. Therefore, the crystal plasticity finite element method and the representative volume elements considering the real microstructure were used to simulate the cyclic deformation of a nickel-based superalloy FGH4098, and a fatigue failure criterion is established to predict the LCF life. The influence of twin boundary on the prediction accuracy is particularly discussed. Meanwhile, the hotspots which are potential sites for fatigue crack initiation were analyzed. The results show that the consideration of twin boundary can improve the accuracy of life prediction, with a scatter factor of ±1.3 only.

Studies on the life, damage evolution, and crack propagation behaviors of TC18 titanium alloy under repeated impact loading

TC18 titanium alloy has been widely used as the key structure which is often subjected to repeated impacts in service in aerospace engineering. In this study, the repeated impact responses of TC18 were investigated by conducting various repeated drop hammer impact tests. Abundant microscopic characterization technologies were used to reveal the correlation between damage evolution and the impact response. The results show that the average impact fatigue life (Nf¯) of the notched three-point-bend specimen decreases exponentially with the impact energy. Microscopic observation shows that cracks nucleate at the initial 40%∼50% of Nf¯, then propagate steadily during the next 50% to 90% of Nf¯. During the last 10% of Nf¯, the cracks propagate rapidly, resulting in the failure. During crack propagation, the repeated impacts induce complex stress states in the vicinity of cracks, leading to grain refinement, phase transition, and strengthened texture. The phase transition and strengthened texture mechanisms are mainly due to the generation of the platelet-like α subgrains inside the refined grains. The results in this paper are crucial for understanding the microscopic damage and failure mechanism of TC18 under repeated impacts, which helps to establish the damage accumulation-based life prediction model in the future.

The microscopic failure mechanism and energy evolution law of foamed concrete under uniaxial compression

This paper uses the macroscopic stress–strain curve of foamed concrete under uniaxial compression as a baseline reference, combining the micro-failure mechanisms of the internal structure and the laws of energy evolution to explain the macroscopic mechanical behavior. This study proposes the mechanisms behind pore fracture and recombination, as well as the mechanical interactions within thin-walled structures. The fractured structure swiftly fills and restores the framework, while the reticulated thin-walled structure adjusts transmission pathways for forces, inducing dormant elastic strain energy and thereby prolonging the interval of residual strain. Coupled with macroscopic laws dictating energy evolution, this paper elaborates on the influence of microscopic structures on stress–strain characteristics. The capacity of foam concrete to proficiently absorb and dissipate external energy, thereby extending the duration of the strain process, and its potential as a buffering material are substantiated.

Study on microscopic failure mechanism and numerical simulation of sandstone under different saturated pressure

X-ray diffraction and SEM scanning are conducted to examine the alterations in sandstone under different saturation conditions to reveal the water-rock softening effect on sandstone from the Qilicun tunnels in China under varying saturation pressures. The mechanisms underpinning the strength softening of sandstone are analyzed using uniaxial compression tests. The experimental results demonstrated that after immersion in water, the internal cementing material within the sandstone dissolves, and the mineral particles fragment or disintegrate, increasing porosity. In the presence of water, the macroscopic compressive strength of sandstone exhibits a declining trend. Concurrently, as the saturation pressure escalates, the compressive strength diminishes by approximately 10%, the elastic modulus decreases by about 30%, and Poisson's ratio incrementally falls by about 25%. The sandstone's failure is characterized by both axial multiple splitting surface failure and shear failure surface. Finally, a strain-softening numerical model is employed to simulate the failure behaviors of sandstone under various saturation pressures. The findings indicated that the sandstone sample exhibits plastic failure characteristics under high saturation pressure.

Research on the joining of aluminum alloy and high-strength steel by dieless clinched-adhesive processes

Aluminum and high-strength steel are the best choices for modern cars due to their excellent ratio of strength to weight. As application support, the clinched-adhesive process for aluminum alloys and high-strength steels is also attracting increasingly attention. In this paper, a novel dieless clinched-adhesive process was proposed, which greatly improves the joint strength. It also has unique advantages such as low alignment requirements and low joint bulge due to the use of dieless clinched mould. In the background of automotive light weighting, this joining technology has great prospects for development and application due to its many advantages. Meanwhile, the geometric and mechanical characteristics of various material types of clinched-adhesive joints and clinched joints were analyzed. The key geometric parameters, static strength, and absorbed energy of the joints were obtained. Besides, the microscopic/macroscopic failure mechanisms of joints were investigated. The results showed that the shear strength and energy absorption of the clinched-adhesive joints were much higher than the clinched joints. The highest shear strength of 8159.7 N was obtained for the AL5052-AL5052 material combination, which was 298.3% enhanced compared with the clinched joints. The enhancement of the joint strength was mainly caused by the adhesive. Additionally, the macroscopic failure modes of clinched-adhesive joints and clinched joints with the same material combination are identical, but there are some differences in the microscopic failure mechanisms.

Investigation of strain rate dependent microscopic failure mechanisms in short fiber reinforced plastics using finite element simulations

For predicting the strain rate dependent failure of short fiber reinforced plastics (SFRP) a two-phase simulation model is developed using the finite element method and comparing the results to microscopic specimen tests for uniaxial tension under quasi-static (0.007/s) and dynamic loads (250/s). Experimentally the failure behavior of SFRP is observed to be strain rate dependent. The global strain at failure and the absorbed energy increase with strain rate. Moreover, locally an influence of the strain rate on the amount of material involved in the deformation can be observed. The suggested model can represent these effects accurately. Also, the present micro-mechanical effects and their influence on the strain distribution are investigated by unit cell simulations. Thereby the material model of the fibers, the matrix, and the boundary layer are varied respectively. These reveal the important role of strain rate dependent decohesion leading to a correct representation of the plastically deformed volume. Consequently, the distortion energy density is evaluated and is found to be constant at failure for all strain rates.

Experimental and numerical investigation on mechanical behavior of Q355 steel and connections in fire conditions

Understanding the temperature-dependent mechanical behavior of steel materials is the basis for evaluating collapse resistance of a steel structure and assessing its after-fire safety. Tensile experiments are conducted on Q355 steel to study the influence of heating histories and stress triaxialities on mechanical behavior in the whole process of fire including heating, heating-cooling and after-fire stages. The true stress-strain curves are measured, and microscopic failure mechanism is investigated. A SMCS model is used to simulate fracture behavior of steel and the model parameters are calibrated based on experimental and numerical results. A parametric study is conducted to study mechanical performance of steel connections under and after fire. The experimental results show that Q355 steel exhibits ductile fracture behavior under and after fire. The higher the tensile and peak temperatures, the greater the fracture strain and the better the ductility. The stress triaxiality may affect the sensitivity of fracture behavior of steel to the temperature condition. The SMCS model is applicable to evaluating fracture performance of Q355 steel in fire, with errors less than 10%. The fracture model of steel materials has a great influence on the mechanical performance of connections in fire. The ductility coefficient of steel connections in fire can be increased to 1.8 times the ambient-temperature value.

Interplay Between Friction and Cohesion: A Spectrum of Retrogressive Slope Failure

AbstractRetrogressive failures occur in slopes consisting of sensitive materials such as snow or quick clay. They can be triggered by a small disturbance at the slope toe, but can cause propagated failure spreading miles away. Understanding the physical mechanism and predicting the retrogressive failure process are particularly important. Previous studies have discussed the failure criteria, the soil properties or the method of numerical modeling of retrogressive slope failure. However, little attention has been paid to the microscopic failure mechanism, especially relating to various possible failure patterns. In this study, multiscale modeling is incorporated to study the physical mechanism of different retrogressive failure patterns, including earth flow, flowslide and spread failure, within a unified framework. Utilizing multiscale analysis, we found that earth flow failure is related to the shear failure of granular materials. In contrast, the development of macroscopic shear bands is accompanied by tensile failure. As shear and tension failures are typical failure mechanisms of frictional and cohesive materials, it is deduced that friction and cohesion effects play key roles in different retrogressive failure patterns. Therefore, the distributions of attractive and repulsive contact forces are explored and a novel parameter η is proposed to quantify the interplay between friction and cohesion. Further analysis proves that η can capture the effect of friction and cohesion and distinguish different retrogressive failure patterns. Finally, a spectrum of retrogressive failures for a granular slope is established, in which the failure mechanism is explained by the changeable dominant effect, that is, frictional or cohesive in soil.

Research on Splitting-Tensile Properties and Failure Mechanism of Steel-Fiber-Reinforced Concrete Based on DIC and AE Techniques.

Concrete presents different internal micro-structure and damage characteristics because of the different content of steel fibers and the randomness of its distribution. Therefore, the failure process of steel-fiber-reinforced concrete (SFRC) should be divided into different stages and the damage types should be classified to further clarify the strengthening mechanism of steel fibers. The role of volume fractions of steel fibers in the splitting-tensile strength of concrete was investigated by split tensile tests for concrete with four different volume fractions of steel fibers (0.0%, 1.0%, 1.5%, 2.0%). The acoustic emission energy and horizontal displacement of concrete in the splitting-tensile process were monitored by combing digital image correlation (DIC) and acoustic emission (AE) techniques, and the microscopic failure mechanism of SFRC was analyzed emphatically. The results showed that the addition of steel fibers improved the splitting-tensile strength of concrete. With the increase of the volume fraction of steel fibers, the splitting-tensile strength of concrete increased first and then decreased, and reached the maximum value of 5.294 MPa when the content was 1.5%. It was observed that the overall failure mechanism could be divided into four stages: slow accumulation of elastic energy (I); rapid accumulation of elastic energy (II); rapid accumulation of dissipated energy (III); a slow decrease of elastic energy and a slow increase of dissipated energy (IV). Tensile failure dominated the failure process of concrete splitting-tensile resistance, while there was a part of shear failure.