Microscopic Cracks Research Articles

Viscoelastic-Thermodynamic Model and Crack Propagation in Asphalt Concrete Pavement

Asphalt concrete pavement has a wide range of applications in the construction of high-grade highways. Asphalt concrete pavement has elastic properties, and its viscoelasticity and viscoplasticity are obviously affected by temperature. To determine whether a structure can continue to be used safely, it is most important to determine whether microscopic or macroscopic cracks present in the structure continue to propagate and cause structural failure. The purpose of this study was to study the effects of different ambient temperatures and mobile vehicles on the stress around the crack tip and the propagation path of different types of asphalt pavements. Based on the theory of fracture mechanics and viscoelasticity, this work studies the problem of crack propagation from a macroscopic perspective. It proposes to use the finite element method to analyze the crack problem in the pavement structure. With the help of ABAQUS software, a curved beam model with cracks was established for calculation, and moving loads and ambient temperature fields were added for analysis. The experimental results in this study show that with the decrease in the ambient temperature, the stress at the crack tip increases continuously. As the analysis time increases, the stress also increases. When the crack is smaller, the stress value at the tip is larger. When a = 15 mm, the stress value at the crack tip is the largest. However, when a = 20 mm and a = 25 mm, the stress at the crack tip is significantly reduced. It shows that the crack is easier to crack when the crack value is smaller. After the crack has grown to a certain length, the growth rate will slow down.

Investigation on fracture characteristics and failure pattern of inclined layered cemented tailings backfill

The fracture characteristics of the layered cemented tailings backfill (LCTB) as an artificial false roof in underhand cut-and-fill mining would directly affect the personal safety of workers. For this reason, the non-layered cemented tailings backfill (NS-CTB), 2-layered (2-S), and 3-layered (3-S) LCTB with 0°, 5°, and 10° layered angles (LA) were prepared respectively for three-point bending experiments, and the microscopic crack evolution law and failure mode were simulated and analyzed by PFC 3D. The results revealed that the flexural strength (FS) and peak deflection (PD) of the LCTB decreased with the increase of the number of layers (NL), and the rate of decline increased gradually. The bending modulus (BM) and post-peak toughness (PPT) of the LCTB gradually rise with the increase of the NL. The FS and PD of the LCTB were inversely correlated with the LA, the BM and PPT tended to decline first and then increase, and reached the lowest when the LA was 5°. When the LA is greater than 5°, the bearing structure of the LCTB changes greatly, and the FS and PD of the LCTB have indeed fallen substantially. The numerical simulation results reveal that at the initial loading stage, the LCTB merely breaks and produces a small amount of cracks at the loading point in contact with the wall and layered contact surface. When the LCTB approaches the FS, a large number of cracks are generated and penetrate the LCTB, and then the total number of cracks curve enters the stable stage. All groups of CTB are destroyed by the lower part in tension initially, and then the cracks expand upward through the CTB, culminating in macroscopic failure to the CTB. The cracks of the NS-CTB expand in a smooth curve, while the cracks of the LCTB expand to the intersection of the layered contact surface and the loading direction with an obvious inflection point, and expand downward along the layered contact surface. Finally, this research explores the macroscopic fracture characteristics and microscopic failure patterns of non-horizontally LCTB, and the results of this study can provide a reference for the design and application of LCTB in the field of underhand cut-and-fill mining.

Investigation on steel fiber strengthening of waste brick aggregate cementitious composites

The waste red brick aggregate substituted with natural coarse aggregate can effectively alleviate environmental pollution in engineering concrete constructions. However, its mechanical performance lags far behind natural coarse aggregate due to micro-pore voids and micro-cracks. Three types of steel fiber contents (0, 1%, and 2%) are selected to improve the mechanical behavior of recycled concrete in this study. Cubic compression test, splitting tensile test, prism compression test, and static compressive elastic modulus test are carried out to study the influence of steel fiber content and replacement rate of recycled coarse aggregate. In addition, the stress-strain behavior and failure mechanism of steel fiber recycled brick aggregate concrete are discussed, and microscopic crack development is also analyzed. Test results show that the specimens without steel fiber are prone to aggregate destruction under loading. The cubic and prismatic compressive strength decreased by 48% and 34%, while the splitting tensile strength reduced by 40%. The mechanical properties substantially improve when the steel fiber content is 1%. The finite element model of steel fiber recycled brick aggregate concrete under uniaxial compression is established. The constitutive model for predicting uniaxial compression of steel fiber recycled brick aggregate concrete is also proposed.

A feasibility study of municipal solid waste leachate utilisation in concrete

Water was replaced with raw municipal solid waste leachate in a concrete mix. Compared with the control specimen (prepared with freshwater), the hardened concrete specimen made with 100% leachate and cured in freshwater for 28 days showed reductions of 20% in ultrasonic pulse velocity and 45% in unconfined compressive strength. Scanning electron microscopy revealed that an increase in leachate content increased the microscopic pores and cracks within concrete owing to organic matter in the leachate. Heavy metals are contaminants in leachate and can leak from leachate-produced concrete. The toxicity characteristic leaching procedure revealed that the heavy metal leakage of the 100% leachate specimen was lower than the maximum permissible range. Furthermore, the use of leachate in fresh cement paste induced a 56 min delay in initial setting compared with the control specimen. Slump measurements performed 0, 30 and 60 min after concrete fabrication indicated that leachate addition could double the concrete slump as compared with the control specimen. This research provides basic information for the use of raw leachate in concrete mixes. Fresh and hardened concrete properties decreased with an increase in the percentage of leachate in the mix design. Replacement of tap water with 10% leachate satisfied the US standard for ready-mixed concrete. Further investigations are recommended to approve the use of leachate as a part of concrete mixing water.

Discrete-element modelling of concrete behaviour under uniaxial compressive test

This paper presents a three-dimensional (3D) simulation of the concrete behaviour in a uniaxial compressive test using discrete-element modelling (DEM). The aim of this paper is to validate the numerical model developed and to study the cracking initiation and failure process in order to better understand the fracture behaviour of concrete. The particles were distributed using an algorithm that is based on sieve test analysis. The parameters were set up in order to validate the numerical model with the experimental result. It was observed that the 3D model is in line with the laboratory test in term of stress-strain response and macroscopic cracks development. Once the bond between the spheres was broken, it led to the formation of microscopic cracks, which were not visible in laboratory tests. From the observation recorded during the testing, it is clear that DEM is capable of capturing concrete behaviour both quantitatively and qualitatively. At the yielding point in the concrete the strain energy is released in the form of kinetic energy, frictional slip energy, energy of dashpot and local damping. Hence, it can be concluded that DEM can be used to study the random nature of the cracking and fracturing of concrete structures.

Macro- and meso- failure mechanism analysis for shale-like brittle materials under uniaxial compression

Understanding the failure mechanisms of the widespread shale-like brittle materials is of great significance to engineering safety. In this research, the discrete element model which can reflect the real rock mechanical properties and failure mode of shale specimens with different bedding angles has been reconstructed. First, microcrack has been introduced to characterize the meso-damage. The relationship between the microscopic crack numbers in the material and the macroscopic stress has been revealed. In addition, the relative movement between different deformed blocks has been analyzed by introducing the residual velocity field. The control factor of material failure varies with the bedding angle (β) in the macro scale. It manifests from the bedding plane and the matrix synergistically control the failure mode of rocks to mainly the bedding plane and then back to the bedding plane and the matrix synergistically control the rock failure mode. The competitiveness of the shear and tensile damages in the failure mechanism varies with the bedding angle in the meso scale. When 30°<β<75°, Shear damage dominates the failure mechanism. The established numerical model is capable of predicting the mechanical behavior of brittle rock, which may have guiding merits to the application of shale-like brittle materials.

Intralaminar crack propagation of glass fiber reinforced composite laminate

The intralaminar crack propagation of unidirectional glass fiber reinforced polymer composite (GFRP) laminate has been investigated by quasi-static tensile/compression experiment and complemented by finite element analysis (FEA). The FEA was conducted using a progressive damage model (PDM) -which was executed by the implicit user material (UMAT) and explicit user material (VUMAT) subroutines. Additionally, based on the UMAT implicit algorithm, the intralaminar crack propagation was predicted by the extended finite element method (XFEM). Finally, XFEM was used to simulate the microscopic crack propagation of representative volume element (RVE). The experimental results showed that the intralaminar crack propagation was accompanied by fiber bridging, which was further observed by scanning electron microscopy (SEM). Besides, multi-crack propagation occurred on the specimens without pre-existing crack. The calculated value of intralaminar fracture toughness in stable state was approximately 4.5 N/mm. The PDM revealed the damage status around crack path, stress field at crack tip and the progressive damage mechanical behavior. However, the predicted crack path did not fully agree with those obtained by experiments due to the element delete method. Although the crack propagation process obtained by XFEM was consistent with the experimental result, the damage status could not be observed. The XFEM predictions of the crack state in the RVE was in good agreement with the SEM results; this suggests that crack propagation underpins matrix cracking, interfacial debonding, fiber pulling out, and fiber fracture. In conclusion, using a blended approach by combining the PDM and RVE with XFEM could be a step forward to simulating the intralaminar crack propagation phenomenon in order to gain deeper insights into the micro–macro failure mechanism.

Investigation on lithium-ion battery degradation induced by combined effect of current rate and operating temperature during fast charging

Operating temperature and current rate are the main parameters that induce lithium-ion battery (LIB) degradation during the fast-charging process. In this study, fast-charging degradation was investigated using a commercial 18650 Nickel-Manganese-Cobalt battery at different charging current rates (C-rates) and operating temperatures. The degradation process was evaluated using electrochemical impedance spectroscopy and incremental capacity analysis. The electrode morphology change was investigated via a multiscale post-mortem analysis. The battery degradation mechanism at different C-rates and temperatures was characterized, and the coupled effects of the C-rate and operating temperature on the battery capacity fade were determined. With increasing operating temperatures, the most significant factor influencing the battery capacity fade changed gradually from lithium plating to the growth of the solid-electrolyte interface (SEI). An experimental analysis revealed that the battery capacity fade at the higher C-rates can be alleviated to some extent by employing higher operating temperatures. In comparison, the capacity fade at lower C-rates accelerated with increasing operating temperature. The changing and position shifting of the peaks in the incremental capacity curves were analyzed. It was found in the post-mortem analysis that the fast charging degradation was embodied in the macroscopic detachment of electrode material, the microscopic cracking of electrode particle, the SEI growth, the lithium plating, and the structural change of layered material crystal. The results of this study provide insight into the development of fast-charging strategies and the design of battery management systems.

Study of the surface integrity and mechanical properties of turbine blade fir trees manufactured in Inconel 939 using laser powder bed fusion

In this study, Inconel 939 turbine blade fir tree samples were fabricated by the additive manufacturing method using a direct metal laser sintering (DMLS) machine. This critical gas turbine connection part requires high geometric accuracy and excellent mechanical properties. The fir tree profiles of turbine blades were machined using creep feed grinding (CFG) and wire electrical discharge machining (EDM) methods. The effects of CFG and wire EDM on the recast formation, heat affected zone (HAZ), geometrical accuracy, microhardness, and tensile strength were compared. The experimental results revealed that the CFG process was superior in recast formation, HAZ, and microhardness, whereas better geometrical accuracy and ultimate tensile strength values were measured for the wire EDM method. The ultimate tensile strengths of turbine blades shaped using wire EDM were higher than the CFG method and measured 769 MPa and 748 MPa, respectively. Profile deviation and recast formation were the most influential parameters on the ultimate tensile strength of the fir tree geometry. The specimens were fractured from the pressure faces of the fir tree root during the tensile tests. Microscopic voids, cracks, and chloride impurities that would facilitate the propagation of fatigue cracks were observed on the fractured surface.

Evaluation of the fractures of asphalt concrete added with rubber particles based on the fine aggregate mixtures

In railway construction, the solidified track bed has gained significant attention from researchers in recent years. Asphalt concrete added with rubber particles is a substitute for the polyurethane cured track bed. In this study, two fine aggregate mixtures (FAM) for different asphalt concrete mixed with rubber particles as the material for the solidified trackbed have been evaluated. Semicircle bending (SCB) test and CT scanning were used to evaluate the mechanical properties of four FAM. Acoustic emission (AE) was used to collect the AE signals during the SCB test. The energy analysis results of AE reveal that the temperature sensitivity of epoxy-FAM is better than that of rubber-FAM. Further, the AE signals can be classified according to the cracking type by clustering analysis. The results demonstrate that the epoxy-FAM with 11% rubber content has the best fracture resistance. AE technology provides an effective evaluation of the microscopic cracking process of FAM.

Initiation Behavior of Microscopic Short Cracks in Weld Toe of Structural Steel Q345B

Aiming at the base metal (BM) zone, heat affected zone (HAZ) and fusion zone (FZ) of the weld, the tip plastic zone and initiation behavior of microscopic short cracks (MSCs) in the toe of cross-welded joints are studied. Based on the theory of crack tip plastic zone and the fatigue index parameter (FIP), the shape and boundary size of the plastic zone at the tip of MSCs as well as the initiation life were calculated. The results show that the plastic zone was bone-like under plane stress and butterfly-like under plane stress; when the crack length increased from 2 μm to 30 μm, the boundary size of the plastic zone at the MSC tip increased by 15 μm and 2 μm under the action of plane stress and plane strain in the X-axis direction, respectively. As the grain gradient decreased from 0.5 to 0.1, the boundary size of the plastic zone at the tip of the MSC increased about 50%. MSC initiation occurs first in the BM zone, and life of initiation was about several hundred thousand cycles, which was consistent with the experimental result. The MSC expansion rate in BM zone was the fastest, with the average expansion rate about 2.5 × 10−3 μm/cycle; the MSC expansion rate in the FZ and HAZ was about 3.1 × 10−4 μm/cycle.

Micro and macro evaluation of tensile characteristics of anisotropic rock mass after high temperatures treatment-a case study of Lingshi Gneiss

The study of macroscopic and microscopic fracture behaviour and tensile strength evolution of anisotropic rock mass at high temperatures are of great significance to evaluation of stability of enhanced geothermal system (EGS). In this paper, Brazilian split test was adopted to study the action mechanism of different temperatures and bedding angles on the tensile strength of gneiss; the digital image correlation (DIC) was used for tracking the principal strain evolution throughout the whole split. Furthermore, the micro X-ray computed tomography and thin-section petrographic analysis were combined in studying the microscopic crack propagation trend and mineral component physicochemical changes of gneiss at different temperature stages. Results show that: (1) under the same bedding angle, the evolution of tensile strength of gneiss within 25–700 ℃ can be divided into three stages: at low temperature stage (25–200 ℃), the tensile strength of gneiss slowly increased, at medium-temperature stage (200–400 ℃), the tensile strength of gneiss slowly decreased, and at high temperature stage (400–700 ℃), the tensile strength of gneiss drops sharply. Under the same temperature, with increased bedding angle, the tensile strength of gneiss first decreased and then increased, with a minimum value when the bedding angle is 30° and a maximum value when it is 90°; (2) under the same temperature, with the increase of bedding angle, gneiss shows a single bedding plane tensile failure (0°), shear-type composite failure (15–30°), tensile-shear failure (45–75°), and base particle-bedding tensile failure (90°); (3) when the temperature is under 400℃, the initiation location is distributed at both ends of disc, whereas when the temperature is equal or above 400 ℃, it is distributed at the centre of disc. The results obtained are of great significance to safety evaluation of EGS and other high-temperature rock engineering.

On the slow crack growth process and associated structure–property relationships in polyamide 12 grades

AbstractThe influence of micro‐structural changes within polyamide 12 (PA12) grades on their resistance against crack initiation and subsequent Slow Crack Growth (SCG) was examined. Possible micro‐deformation mechanisms and processes guiding slow crack extensions in PA12 are discussed. Special focus was put on the formation of plastic zones as predecessors to microscopic cracks prior to crack initiation via crack freezing analysis. Simultaneously, variations in the pure SCG resistance after a recorded craze‐crack transition were determined via cyclic Cracked Round Bar (CRB) tests. In that context, a homologous series of PA12 grades of increasing molecular weight (MW), as well as a systematically developed series of PA12 grades incorporating a pigment and/or an impact modifier, were selected for this study. Results show a good correlation between the crack initiation resistance and the disentanglement resistance of PA12 chains as well as the size of the plastic zone. In that context, an increasing average MW, leads to increasing disentanglement resistances due to hydrogen bond effects. An impact modification promotes the development of notably larger plastic zones. Contrarily, colored grades exhibit a reduced plastic zone size, rendering a lower amount of dissipated energy before physical crack initiation. Pure SCG resistance follows a similar trend and is improved by high average MW as well as the use of impact modifier. Crack growth acceleration, however, is assumed to occur alongside the relatively weak interfaces between polymer matrix and pigments.

Effect of time-varying corrosion on the low-cycle fatigue mechanical properties of wire rope

In this study, we investigate the macroscopic mechanical behaviour of wire rope corroded by seawater by analyzing the monotonic tensile and hysteretic behaviour of 6 × 19 S + IWR round strand steel wire ropes with different corrosion degrees. First, the wire rope samples were subjected to a full immersion accelerated electrochemical corrosion experiment, which revealed that the corrosion distribution is uneven, and that corrosion progresses from the outer wires to the inner wires through crevices in the wire rope. Scanning electron microscopy revealed that an increase in corrosion increases the number, length, and depth of microscopic cracks on the surface of the wire rope. The degradation effect of corrosion on the mechanical properties of wire rope differs significantly. As corrosion increases, the bearing and deformation capacities decrease substantially, and the fracture mode changes from ductile to brittle fracture. Based on the mechanical characteristics and the influence factors of a steady stress–strain cycle of a corroded wire rope, we proposed a hysteretic mechanical model consisting of a loading and unloading curve. The proposed model was verified using the experimental data, and can accurately reflect the influence of the corrosion degree and loading amplitude on the hysteretic characteristics of a wire rope.

Evaluation of macro- and meso-mechanical properties of concrete under the aggressiveness of landfill leachate

Current researches on the mechanical properties of concrete are mainly concentrated on the aggressiveness of sulfate, chloride ion, acid and freeze–thaw cycles. However, the evolution of concrete mechanical properties aggressived by landfill leachate remains to be revealed. As the micro-nano indentation test is a new method to measure the fine/micro mechanical behavior of materials, it is used in this research to evaluate the micro-mechanics of the landfill leachate of the concrete sample surface and different aggressived depths. In addition, the evolution of the macro-mechanical properties of concrete aggressived by the landfill leachate was studied through uniaxial compression tests. The results show that the hydration products with high elastic modulus are consumed by the aggressiveness of landfill leachate, which results in a large number of microscopic cracks on the surface of the concrete. Moreover, the longer the aggressive time, the shallower the aggressive depth, and the more severe the deterioration of the micromechanical properties of the sample surface. It is also notable that the uniaxial compressive strength of the concrete samples aggressived by landfill leachate showed a linear increase and then gradually decrease. Comparing with the sodium chloride and sodium sulfate solutions, the landfill leachate has the most significant weakening effect on the elastic modulus of concrete.

The Tensile Deformation of Multiphase Al2O3: Insights from Molecular Dynamics Simulations

Multiphase Al2O3 samples are simulated using molecular dynamics method. These samples consist of γ‐Al2O3 and α‐Al2O3 crystals embedded in a disordered phase matrix. The stress–strain curves of these samples show elastic and plastic deformation. Structural analysis indicates that the AlO bond lengths are stretched and the geometries of the AlOx (x = 4, 5, and 6) units are distorted during tensile deformation. AlO bond breakage causes the transformation of AlOx units; AlO4 increases while AlO6 decreases with increasing strain. The γ‐Al2O3 and α‐Al2O3 crystals transfer into the disordered phase, while a small number of atoms recrystallize under tensile loads. In plastic deformation, large simplexes with radii above 1.958 Å increase rapidly with increasing strains above 0.116, with the large simplexes coalescing to form the clusters in the disordered phase. The number of large simplexes in the largest cluster increases with increasing strain values above 0.127. The growth and coalescence of large simplexes in the disordered phase cause microscopic crack formation at high strains.

Research on evolution behavior of 42CrMo microscopic fatigue short crack based on microstructure effect of material

To investigate the causes of damage dispersion in fatigue life of alloy materials, fatigue tests on 42CrMo are carried out under microscope with the aid of a micro fatigue testing device. The fatigue crack initiation and propagation evolution process of 42CrMo are studied on the micromechanical scale. The fatigue short crack and short crack growth rates are measured respectively under loads of 1400, 1550, and 1650 N. The experimental result shows that the initiation of fatigue short cracks is related to the randomness of microstructure defects. The initiation and propagation of fatigue short cracks on the surface of samples are random and local, while the propagation is characterized by first deceleration and then acceleration.

Experimental investigation of microscopic crack development and damage characteristics of sandstone based on acoustic emission characteristic parameters

Experimental investigation of microscopic crack development and damage characteristics of sandstone based on acoustic emission characteristic parameters

Fatigue crack growth under mixed mode I-II loading with a single tensile overload at negative load ratios

Fatigue crack growth (FCG) behavior under mixed mode loading has been received extensive attention, including overload condition. The mixed mode I-II fatigue crack growth behavior of commercial pure titanium (CP-Ti) with tensile overload at negative load ratios was investigated by experiment and finite element method (FEM). The effects of loading angle, load ratio and overload on mixed mode I-II fatigue crack growth were discussed with a modified compact-tensile-shear (CTS) specimen. The mechanism of mixed mode crack growth with overload was revealed. Fatigue crack growth path and fatigue crack growth rate (FCGR) with/without overload were compared experimentally, and overload retardation effect was investigated. The variation of plastic zone and the distribution of stress and strain were simulated through finite element method. The mechanism of overload retardation effect of mixed mode I-II crack growth under compressive load was illustrated by microscopic perspective, crack closure and residual compressive stress at crack tip, and the weakening effect of mode II shear component and compressive load effect on overload retardation effect was demonstrated.

Synergistic Effect between CaCO3 Whisker and Steel-PVA Fiber Cocktail in Cement-Based Material at Elevated Temperature

A new fiber cocktail reinforced cement–based materials (NFCRCM) with steel, polyvinyl alcohol (PVA) fiber, and calcium carbonate whisker (CCW) was developed to obtain powerful fire resistance in this paper. The flexural properties, synergistic effects, and microscopic mechanism of NFCRCM at elevated temperature were particularly studied. With the introduction of CCW, the degradation of flexural strength and toughness of steel-PVA fiber cocktail reinforced cement–based materials below 800°C was decreased. Below 800°C, the NFCRCM with 1.5% steel fiber +0.5% PVA fiber +3% CCW showed a significant positive hybrid effect in flexural strength and toughness resulting from microscopic crack inhibition and nucleation effect of CCW. When the temperature reached 800°C or above, a negative hybrid effect was generally observed due to thermal decomposition of CCW.