Micromechanical Failure Research Articles

Micromechanics of fracture and failure in concrete under monotonic and fatigue loadings

In this research, the fracture processes in concrete subjected to monotonic and fatigue loadings are characterized and the differences in failure mechanisms are studied using acoustic emission (AE) and digital image correlation (DIC) techniques. Experiments are performed on notched plain concrete beams under three-point bending. The micro and macro structural activities are classified based on acoustic energy levels to differentiate between the formation of a fracture process zone (FPZ) under monotonic and fatigue loadings. It is observed that a FPZ within its conventional definition is formed under monotonic loading. On the contrary, the AE results under fatigue loading indicate isolated and dispersed micro cracks up to 95% of fatigue life. A damage index based on AE energy is proposed for concrete subjected to fatigue loading which could be used in health monitoring of structures.

Effect of tensile pre-strain on low-cycle fatigue behaviour of 7050-T6 aluminium alloy

The present study addresses the effect of tensile pre-strain histories on cyclic plastic deformation behaviour of 7050-T6 aluminium alloy. Low-cycle fatigue tests are conducted under fully-reversed conditions, with 8% tensile pre-strain, 4% tensile pre-strain, and 0% tensile pre-strain, at different strain amplitudes. After fatigue testing, fracture surfaces are examined by scanning electron microscopy and transmission electron microscopy to identify the main failure micro-mechanisms associated with the levels of pre-strain and strain amplitude. Microstructure is analysed by X-ray diffraction, transmission electron microscopy, and optical microscopy. The main findings suggest that tensile pre-strain histories are conducive to a reduction in fatigue life. This reduction increases with higher strain amplitudes.

Hydrogen embrittlement of the coarse grain heat affected zone of a quenched and tempered 42CrMo4 steel

The coarse grain heat affected zone (CG-HAZ) of welds produced in a quenched and tempered 42CrMo4 steel was simulated by means of a laboratory heat treatment consisting in austenitizing at 1200 °C for 20 min, oil quenching and finally applying a post weld heat treatment at 700 °C for 2 h (similar to the tempering treatment previously applied to the base steel). A tempered martensite microstructure with a prior austenite grain size of 150 μm and a hardness of 230 HV, similar to the aforementioned CG-HAZ weld region, was produced. The effect of the prior austenite grain size on the hydrogen embrittlement (HE) behaviour of the steel was studied comparing this coarse-grained microstructure with that of the fine-grained base steel, with a prior austenite grain size of 20 μm.The specimens used in this study were charged with hydrogen gas in a reactor at 19.5 MPa and 450 °C for 21 h. Cylindrical specimens were used to determine hydrogen uptake and hydrogen desorption behaviour. Smooth and notched tensile specimens tested under different displacement rates were also used to evaluate HE.Embrittlement indexes, EI, were generally quite low in the case of hydrogen pre-charged tensile tests performed on smooth tensile specimens. However, very significant embrittlement indexes were obtained with notched tensile specimens. It was observed that these indexes always increase as the applied displacement rate decreases. Moreover, hydrogen embrittlement indexes also increase with increasing prior austenite grain size. In fact, the embrittlement index related to the reduction in area, EI(RA), reached values of over 20% and 50% for the fine and coarse grain size steels, respectively, when tested under the lowest displacement rates (0.002 mm/min).A comprehensive fractographic analysis was performed and the main operative failure micromechanisms due to the presence of internal hydrogen were determined at different test displacement rates. While microvoids coalescence (MVC) was found to be the typical ductile failure micromechanism in the absence of hydrogen in the two steels, brittle decohesion mechanisms (carbide-matrix interface decohesion, CMD, and martensitic lath interface decohesion, MLD) were observed under internal hydrogen. Intergranular fracture (IG) was also found to be operative in the case of the coarse-grained steel tested under the lowest displacement rate, in which hydrogen accumulation in the process zone ahead of the notch tip is maximal.

Micromechanisms of Cortical Bone Failure Under Different Loading Conditions.

Bone being a hierarchical composite material has a structure varying from macro- to nanoscale. The arrangement of the components of bone material and the bonding between fibers and matrix gives rise to its unique material properties. In this study, the micromechanisms of cortical bone failure were examined under different loading conditions using scanning electron microscopy. The experimental tests were conducted in longitudinal and transverse directions of bone diaphysis under tensile as well as compressive loading. The results show that bone material has maximum stiffness under longitudinal tensile loading, while the strength is higher under transverse compressive loading. A reverse trend of compressive mechanical properties of bone is observed for longitudinal and transverse loading as compared to trends reported in the previous studies. Therefore, micromechanisms of cortical bone failure were analyzed for different loading conditions to reveal such type of behavior of cortical bone and to correlate bone microstructure with mechanical response of bone.

Analysis of interfacial adhesion properties of nano-silica modified asphalt mixtures using molecular dynamics simulation

The presence of nano-silica can reduce the sensitivity to moisture damage of asphalt mixtures and improve the anti-oxidation aging ability of asphalt binder. In the present work, the interface adhesion of nano-silica modified asphalt to aggregate surfaces at the different interfacial situations (aggregate surface irregularity and seawater erosion) were investigated using molecular dynamics (MD) simulations. The sodium chloride solution is used to simulate seawater erosion. The asphalt 12-component molecular model is used for the analysis of thermodynamic properties such as density, the ratio of free volume, and cohesive energy density. Molecular structure properties of asphalt on the aggregate surfaces were analyzed by radial distribution function (RDF), relative concentration (RC), and mean square displacement (MSD) of asphalt molecule. The interface adhesion properties were investigated by calculating the work of adhesion at the different interfacial situations. The results show that oxidation aging promotes the asphaltene aggregate on the surface of the silica aggregate, while the effect of nano-silica is reversed. Increasing the surface roughness of the aggregate enhanced its adhesion strength. The asphalt mixtures are less resistant to sodium chloride solution erosion than water damage. Compared with the original asphalt binder, oxidized asphalt is more susceptible to damage by water or sodium chloride solution. The presence of nano-silica reduces the susceptibility of asphalt mixture to water damage or sodium chloride solution erosion. The research findings provide insights to more fully understand the micromechanisms of interface adhesion failure between asphalt binder and aggregate.

Strength improvement of adhesively bonded single lap joints with date palm fibers: Effect of type, size, treatment method and density of fibers

In this paper, natural fibers and particles provided from different parts of date palm trees are considered to improve the static strength of adhesively bonded joints. Some influencing factors such as type, size, alkaline treatment and the weight ratio of the natural fibers/particles are the parameters that have been experimentally studied. The date palm fibers are provided from four parts of the tree including Mesh, Rachis, Petiole and Bunch. In order to investigate the effect of treatment of the fiber surface, the raw fibers and treated fibers with two different concentrations of aqueous NaOH solution were considered. To study the effect of fiber size, fibers with different sizes including short fibers (0.5–2 mm) and long fibers (30 mm) were separately added to the adhesive. To find the best weight ratio of the fibers corresponding to maximum joint strength, single lap joints with three different weight ratios of date palm fibers (2, 5 and 10%) were experimentally tested. The results show that date palm fibers significantly improve the strength of adhesively bonded joints. For example, the strength of adhesive single-lap joints reinforced by 2 wt% of Rachis fiber treated with 6 wt% NaOH solution, increased by 140%. To investigate the micromechanisms of failure, the fractured surfaces were analyzed. The results show that the date palm fibers can improve the strength of adhesive joints by slowing down the crack propagation, absorbing energy and load endurance.

Fatigue crack propagation behaviour of carboxyl-terminated polybutadiene solid rocket propellants

The fatigue crack growth behavior of composite solid carboxyl-terminated polybutadiene (CTPB) base propellants from a two stage-rocket has been analyzed. Both motors presented similar compositional percentage of the different constituents but while the booster motor presented aluminum as fuel and fine oxidizer particles, the sustainer motor presented nitroguanidine as fuel and coarse rigid inorganic particles. The fracture characterization revealed that the critical energy release rate values obtained from the grains of the booster motor were higher than those computed from the sustainer motor. The fatigue crack growth behavior of the propellant grains under study was comparable to that shown in rubber and the fatigue crack growth curves obtained from the booster motor were below those from the sustainer motor. The micromechanism of failure in both motors was microvoid nucleation and growth till the formation of a macro-crack capable of subcritical advancement. In the grains from the booster motor, the nucleation and progression of damage occurred through the matrix with fracture surfaces plain and with no trace of oxidizer particles. Instead, in the propellant grains from the sustainer motor, the damage was generated in the particle–binder interface and the progression occurred along these interfaces leading to an abrupt fracture surface with discernible oxidizer particles. The mechanism of failure in the booster motor led to a better fatigue crack growth behavior and the irregular crack advancement in the sustainer motor implied a lower exponent of the crack growth rate to the energy release rate power law.

Effect of fiber orientation angles on the material removal behavior of CFRP during cutting process by multi-scale characterization

CFRP composites have been widely used in many fields due to their excellent comprehensive performance. Precision machining of CFRP is the premise to ensure the efficient assembly of CFRP components. Therefore, it is important to clarify the material removal behavior for optimizing the process scheme and improving the machined surface quality. In this presented paper, the 3D finite element models including macro and micro models for CFRP cutting are found and verified by experiment. Based on four typical fiber orientation angles (FOAs), the micro-failure mechanism of CFRP and the formation mechanism of macro-chips under cutting load are studied. Finally, the machined surface quality of CFRP is studied using 3D nano system and scanning electron microscope (SEM). Results show that the damage of fiber and matrix on micro scale and the chip formation on macro scale are greatly affected by FOAs. Under different FOAs, the failure modes of fibers and matrix and the removal behavior of material are different, so that the machined surface quality is various. In this study, the relatively good surface quality is obtained when FOA is 45° or 90°, but a poor surface quality when FOA is 0°or 135°.

Numerical Simulation of Rock Failure Process with a 3D Grain‐Based Rock Model

A grain‐based rock model was developed and applied to study mechanical characteristics and failure micromechanics in thick‐walled cylinder and wellbore stability tests. The rock is represented as an assembly of tetrahedral blocks with bonded contacts. Material heterogeneity is modeled by varying the tensile strength at the block contacts. This grain‐based rock model differs from previous disk/sphere particle‐based rock models in its ability to represent a zero (or very low) initial porosity condition, as well as highly interlocked irregular block shapes that provide resistance to movement even after contact breakage. As a result, this model can reach higher uniaxial compressive strength to tensile strength ratios and larger friction coefficients than the disk/sphere particle‐based rock model. The model captured the rock fragmentation process near the wellbore due to buckling and spalling. Thin fragments of rock similar to onion skins were produced, as observed in laboratory breakout experiments. The results suggest that this approach may be well suited to study the rock disaggregation process and other geomechanical problems in the rock excavation.

Experimental and Mathematical Modeling of Monotonic Behavior of Calcareous Sand

The prominent performance of wave elimination and energy absorption makes calcareous sand important and useful in infrastructure construction and protection engineering. Due to the high compressibility induced by remarkable intragranular void and irregular shape, calcareous sand presents different mechanical behaviors from common terrestrial sands. Considerable efforts have been made to explore the static and dynamic mechanical properties of calcareous sand. In this paper, a series of monotonous experiments have been performed on calcareous sand utilizing the electrohydraulic servo‐controlled test apparatus designed by the Global Digital Systems Ltd (GDS). The effects of confining pressure and relative density on the mechanical properties of dry, drained, and undrained saturated sand were studied, and the underlying micromechanism of deformation and failure was discussed. It can be found that the residual stress of dry calcareous sand is independent of the relative density, while the peak stress and residual stress of drainage saturated sand have a positive correlation with the relative density. The increase of confining pressure makes the strain softening more remarkable and heightens the peak stress and residual stress. The stress‐strain curve of calcareous sand can be divided into two portions: prepeak portion and postpeak softening portion. For the dry sand and drainage saturated sand, the softened part can be partitioned into three phases, i.e., accelerated phase, steady phase, and degradation phase, while the undrained saturated sand tends to hyperbolic softening. A mathematical model composed of a hyperbolic function and an inverted S‐shaped function was formulated to describe the multiphase characteristic, in which the setting of parameter p expands its applicability. The experimental result validated the model, showing that the model can better describe the monotonic stress‐strain relationship of calcareous sand. Besides, the physical meanings of model parameters were discussed.

Anisotropic characteristics of crack initiation and crack damage thresholds for shale

The inherent anisotropy is the most important structural feature of shale. Most research on the mechanical behavior of shale has focused on its anisotropic strength, deformation and failure patterns. Few studies have examined the anisotropic micro-failure mechanism and failure process of shale. To investigate the anisotropic crack initiation mechanism and evolution of shale during the progressive rock failure process, two stress thresholds (i.e., crack initiation stress (σci) and crack damage stress (σcd)) and two strain thresholds (i.e., crack initiation strain and crack damage strain) of shale under various bedding layer orientations and confining pressures are identified on the basis of the crack volumetric strain. The anisotropic characteristics of the four thresholds are described. The crack initiation stress can be seen as independent of the bedding layer orientation, but the other thresholds are strongly dependent on the bedding layer inclined angle. The physical mechanism of this phenomenon is analyzed in detail. The anisotropic features of the mutual relationships among the peak stress (σc), σci and σcd are presented. The ratios σci/σc and σci/σcd at confining pressures greater than 30 MPa and the ratio σcd/σc increase first and then decrease with the increasing inclination angle. A high confining pressure constraints shale from entering the unstable crack propagation stage but has little effect on preventing crack propagation once it has entered this stage. The anisotropic characteristics of these thresholds can provide meaningful information for understanding the anisotropic deformation and failure mechanism of shale.

A study on microstructural effect and mechanical behaviour of Al6061–5%SiC–TiB2 particulates reinforced hybrid metal matrix composites

In this present work, an attempt is made to prepare Al6061–5%SiC–TiB2 hybrid composites using stir casting process by considering optimal process parameters such as stirring temperature, stirring speed, stirring time, preheating temperature, grain size and wt% of particulate reinforcement and examine their metallurgical effect and mechanical behaviour. The metal composite samples were prepared using stir casting process in which the amount of reinforcement of 5%SiC is kept constant, and second reinforcement of TiB2 is varied by its weight fraction of 2, 4, 6, 8 and 10%. The prepared dual particulates reinforced hybrid composites are characterised, and their microstructural, mechanical properties and fractograpy were evaluated as per the standards. Strong interfacial bonding between matrix and reinforcements which are the significance of improved wettability is due to Potassium Hexafluorotitanate salt addition, and preheating of particles of SiC at 800℃ and TiB2 at 250℃ is made before adding into the melt. The results revealed that the increase in addition of TiB2 increases the tensile strength, flexural strength, impact strength and hardness by decreasing their weight density, and also the influence of SiC increases the hardness of the material of newly developed composite. Also, fracture morphology of tensile failure has been studied to examine micro-mechanisms of failure, and the mode of failure of the composites was found to be ductile.

Hydrogen embrittlement analysis in a CrMoV steel by means of sent specimens

In this study, single-edge notched tensile (SENT) specimens machined from a quenched and tempered CrMoV steel plate were hydrogen pre-charged using two different methods: in a high pressure and high temperature hydrogen reactor (19.5 MPa and 450 °C); and using an electrochemical method (2 M H2SO4 + 0,25 g/l As2O3, under a current density of 2 mA/cm2). The thickness of the specimens was only 1 mm in order to facilitate hydrogen entrance in a short time and the fracture J resistance curves were obtained using digital image correlation (DIC) to measure crack growth. The effect of both hydrogen pre-charged methods on the fracture toughness results was analysed, as was the influence of the applied displacement rate. Embrittlement indexes increase as the displacement rate decreases, with maximum embrittlement indexes of 20% and 84% being respectively measured for gaseous and cathodic pre-charging in tests performed at a displacement rate of 0.01 mm/min. Significant differences in the operative failure micromechanisms in the samples pre-charged via both methods were also detected and hydrogen embrittlement theories (hydrogen-enhanced localized plasticity, HELP, and hydrogen-enhanced decohesion, HEDE) were evaluated by mapping the strain distribution in the crack tip process region by means of DIC.

Near Real-Time Characterization of Spatio-Temporal Precursory Evolution of a Rockslide from Radar Data: Integrating Statistical and Machine Learning with Dynamics of Granular Failure

This study builds on fundamental knowledge of granular failure dynamics to develop a statistical and machine learning approach for characterization of a landslide. We demonstrate our approach for a rockslide using surface displacement data from a ground based radar monitoring system. The algorithm has three key components: (i) identification of a regime change point t 0 marking the departure from statistical invariance of the global velocity field, (ii) characterization of the clustering pattern formed by the velocity time series at t 0 , and (iii) classification of velocity patterns for t > t 0 to deliver a measure of risk of failure from t 0 and estimates of the time of emergent and imminent risk of failure. Unlike the prevailing approach of analysing time series data from one or a few chosen locations, we make full use of data from all monitored points on the slope (here 1803). We do not make a priori assumptions on the monitored domain and base our characterization of the complex spatial patterns and associated dynamics only from the data. Our approach is informed by recent developments in the physics and micromechanics of failure in granular media and is configured to accommodate additional data on landslide triggers and other determinants of landslide risk readily.

Modified micro-mechanics based multiscale model for progressive failure prediction of 2D twill woven composites

To consider fiber random distribution at the microscale for the multiscale model based on the micro-mechanics failure (MMF) theory, clustering method is used for the extraction of amplification factors. As the clustering method is a kind of unsupervised machine learning method, the elements with similar mechanical behavior under external loading can be included in a cluster automatically at the microscale. With this modification, the fiber random distribution model can be used for multiscale damage analysis in the framework of MMF theory. To validate the modified multiscale analysis method, progressive damage analysis of a kind of 2D twill woven composites is conducted based on different microscale models. The stress values for microscale models with fiber hexagonal and random distribution patterns are compared first. Much higher stress concentration is generated in the fiber random distribution model due to the smaller inter-fiber distance especially under longitudinal shear loading. The obtained cluster distribution results exhibit the characters of the stress distribution in the two microscale models. Thereafter, tensile and compressive responses of the 2D twill woven composite are predicted with the modified multiscale analysis method and accuracy of the method is verified through comparison with published experimental results. From the simulation results, it can be found that the matrix damage initiation from the model based on the fiber random distribution model is premature compared with that from the model based on the fiber hexagonal distribution model. Besides, under tensile loading, the damage all initiates from the fill tows and propagates to the wrap tows. However, under compressive loading, the matrix damage initiates from the wrap tows in the model based on the fiber random distribution model.

Fatigue failure micromechanisms in recycled aggregate mortar by μCT analysis

The recycling of the fine fractions from construction and demolition wastes is an environmental and technological challenge. In view of its incorporation as recycled aggregate in the manufacture of concretes, this work analyzes its influence on the mechanical response of these concretes to the action of variable loading, which can lead to failures due to fatigue. To do this, 3 different recycled mortar were manufactured using fine aggregate: a standardized siliceous aggregate and two others with recycled fine aggregate, coming from the crushing of out of use basalt ballast and concrete sleepers from the railway superstructure. Plain mortars have been subjected to fatigue compressive loading, following the Locati methodology, and have been characterized both by conventional methods and by micro-Computerized Tomography (μCT) during the fatigue testing. From the observation made on the specimens, it appears that the cracks are generated, mostly, in the paste-aggregate interface and at the aggregate corner vertex of the flakiest aggregates, propagating through the weakest planes, until the failure of the concrete occurs.

Modeling the effects of loading scenario and thermal expansion coefficient on potential failure of cryo-compressed hydrogen vessels

A multiscale thermomechanical model for a simplified Type-3 cryogenic, compressed-hydrogen (H2) storage vessel is described in this paper. The model accounts for the temperature-dependent elastic-plastic behavior of the vessel's carbon/epoxy composite overwrap and aluminum alloy liner. The homogenized thermo-elastic-plastic behavior for the individual laminae of the vessel layup is obtained using an incremental Eshelby-Mori-Tanaka approach associated with a micromechanical failure criterion to predict laminar failure while a standard elastic-plastic constitutive model is used to describe the behavior of the typical aluminum alloy assumed for the liner. The vessel's response to external loadings is modeled using a finite element method. Four loading scenarios, representing four thermomechanical cycles applied to the vessel, are analyzed to evaluate constituent and laminar stresses as well as the associated failure criterion during the cycle according to these scenarios. The model can provide helpful guidance to mitigate thermal stresses by selecting a suitable loading scenario, optimizing the layup, and tailoring the thermomechanical properties of the resin matrix.

Investigation of large strain extrusion machining (LSEM) of pure magnesium (Mg)

Large strain extrusion machining (LSEM) is used to produce pure magnesium strips directly from as-cast billets. LSEM reduces manufacturing steps and the special shear direction inclines the unfavorable strong basal texture of Mg. Finding suitable parameter ranges to control geometry and properties of the produced strip is a great challenge. In the present research, a finite element model is proposed and verified with experiments and previous findings. Next, effects of process parameters (temperature and pressure) on geometrical and microstructural properties of the produced strips are investigated. Studying the involved failure micro-mechanisms shows that the process temperature and hydrostatic pressure have adverse effects on the production of a flawless strip. To reduce the temperature and hydrostatic pressure throughout the LSEM process many different conditions are tested by the FEM model. Among all the examined conditions, 0.85 mmRev feed and 530 Revmin rotation speed yields the most promising results it terms of low temperature, low hydrostatic pressure, and stable machining force. The proposed conditions are applied in the experimental setup and a uniform and flawless strip is obtained.

Prediction of static tensile failure of double-bolt joint for composite laminate

Based on the micro-mechanics of failure (MMF) theory, the accelerated testing methodology (ATM) and the VUMAT subroutine in ABAQUS, this study put forward a novel microscopic analysis approach for the large deformation failure analysis of composite. The predicted strength accuracy of the MMF approach for the bolted structure was then compared with those calculated based on the Tsai–Wu and Hashin theories, and with the experimental results. Results showed that the MMF method enjoyed the highest strength accuracy with an error of 7.2%, and the component damage initial occurred in matrix of the 90-degree layer and the initial failure of fiber occurred in the 0-degree layer. The failure mode predicted by the MMF approach behaved as shearing mode, which was in agreement with the experimental results. The long-term static strength of the bolted structure was predicted based on the MMF/ATM approach, and a comparison with the test results was conducted.

Elastic-plastic fracture assessment of CNT-reinforced epoxy/nanocomposite specimens weakened by U-shaped notches under mixed mode loading

For the first time, mixed mode I/II fracture of U-notched semicircular bend (USCB) specimens fabricated from a ductile carbon nanotube (CNT)-reinforced epoxy/nanocomposite material is investigated experimentally and theoretically. The U-notched nanocomposite specimens are prepared with different notch rotation angles and various notch tip radii. Mixed mode I/II fracture tests are conducted on these specimens by applying the three-point bending loading in order to measure their load-carrying capacities (LCCs) and fracture initiation angles (FIAs). For theoretical fracture estimations, the well-known U-notched maximum tangential stress (UMTS) criterion is employed in conjunction with the reformulated Equivalent Material Concept (EMC). To compare the experimental and theoretical results, the effective notch stress intensity factor (ENSIF) and the notch mode mixity parameter are defined and computed for the notched nanocomposite specimens tested. It is found that the EMC-UMTS combined criterion can provide good predictions of the mixed mode notch fracture toughness, as well as the FIAs, without performing any nonlinear analyses. The micromechanical failure processes of the nanocomposite specimens are also investigated using the pictures taken by the scanning electron microscope (SEM) from the fracture surfaces. It is shown that the SEM pictures confirm well the fracture results obtained from the macro-mechanical analyses.