Mechanical Failure Modes Research Articles

Numerical study of the influence of loading rate on fracture mechanism in elastoplastic rock-like materials with a modified phase-field model

The failure mode in rock engineering structures is closely related to the loading mode and loading rate, because of the inertial effects under a wide range of geological conditions. In this work, we propose a modified framework of the phase-field model coupled with plasticity, to investigate the effects of loading rates on the fracture mechanism in elastoplastic rock-like materials. By means of this framework, the damage-plasticity coupling effects on the mechanical responses and failure mode are also studied with an optimized pressure-sensitive plasticity model. The capability and accuracy of the proposed phase-field approach in capturing the nonlocal damage evolution is validated based on the elastic/inelastic homogeneous case with analytical solutions. Then the mixed-mode cracking behavior of the representative numerical examples are demonstrated under quasi-static and dynamic loading conditions from low to high loading rates. The failure modes, dynamic damage profiles and the crack propagation velocities in the examples with various loading rates were captured. Numerical results are discussed with comparison to previous experimental observations and available numerical works.

Experimental and numerical simulation study of fiber-reinforced high strength concrete at high strain rates

With the widespread use of fiber-reinforced high strength concrete (FRHSC) in military and civil fields, it is necessary to have an accurate understanding of the mechanical properties and failure modes of FRHSC under impact. The dynamic compression experiments were performed on FRHSC using the large-size split Hopkinson pressure bar (SHPB) apparatus. The experimental results were analyzed to determine the reasonable strain rate range of FRHSC under large-size SHPB loading conditions. The strain rate effect, critical strain, and failure mode of FRHSC under dynamic impact were studied. The strain rate and strength parameters in the Holmquist-Johnson-Cook (HJC) model were determined based on the experimental results, and the numerical simulation of SHPB was completed by LS-DYNA. The results revealed that the reasonable strain rate of FRHSC under large-size SHPB loading did not exceed 100 s−1. The material presented an obvious strain rate effect, and the critical strain was almost unchanged with the increased strain rate. Steel fiber improves the toughness of FRHSC. Under quasi-static loading, the tensile-compression ratio of the material was 0.13, and there was no apparent damage under impact. The rationality of the strain rate and strength parameters in the HJC model was verified by comparing the stress wave curve of the pressure bar, the stress-strain curve, and the failure mode of the specimen obtained by experiment and simulation.

Investigation of mechanical characterization of hybrid sandwich composites with syntactic foam core for structural applications

In this work, mechanical performance on sandwich composites consisting of syntactic foam filled with hollow glass microsphere in an epoxy resin matrix as the core material and hybrid kenaf/glass fibers face-sheet based composites panel is studied. The main purpose of this work is to design a suitable lightweight composite panel material for structural application. Four different sequences of face-sheet reinforcement (e.g., kenaf-kenaf; glass-glass; glass-kenaf, and kenaf-glass) were adopted. In this study, the mechanical properties and failure modes are carefully investigated. The sandwich composites were characterized by compressive, tensile, and flexural properties. The results showed that compressive strength was highest at the “kenaf-glass” face-sheets, tensile strength was highest at “glass-glass” face-sheets, and flexural at “glass-glass” face-sheets with 52.1%, 67.6%, and 74.4% increase to “kenaf-kenaf” face-sheets, respectively. The tensile strength and modulus values with the specific strengths and modulus values showed that the mechanical properties of the sandwich composites can be influenced by hybridizing the face-sheet laminates of the composite. This shows that kenaf fiber has limited strength when used as reinforcement alone but gave an excellent performance when combined with glass fiber.

Research on Mechanical Properties and Failure Mode of Conglomerate Based on Discrete Element Method

Conglomerate reservoir is an important part of unconventional oil and gas resources, which has great developmental potential. However, its sedimentary environment and structural background are complex, and its cementation types, gravel volume fraction and shape are quite different, which leads to its strong heterogeneity. When developing a conglomerate reservoir, it is extremely difficult to drill because of its strong heterogeneity. It is difficult to obtain the mechanical properties and laws of the conglomerate through physical experiments, which further restricts the development process of conglomerate reservoirs. In order to study its failure law, a three-dimensional numerical model of a conglomerate is built based on the discrete element method, and the effects of cementation strength and gravel characteristics on the mechanical properties of the conglomerate are emphatically studied. The results show that the elastic modulus and uniaxial compressive strength of the conglomerate decrease obviously with the decrease in cementation strength. With the increase in cementation strength, the normal contact force of the conglomerate model increases significantly, the distribution of normal contact force changes from cylinder to sphere, and the heterogeneity of the conglomerate decreases. There is a threshold value for the influence of cementation strength on mechanical properties of the conglomerate, and when the threshold is exceeded, the mechanical properties of the conglomerate no longer change obviously. With the increase in gravel content, the uniaxial compressive strength of the conglomerate decreases at first and then increases, the phenomenon of penetrating gravels and bypassing gravels increases, and the single diagonal crack changes into diagonal cross cracks; the cementation strength and gravel content of gravel jointly affect the mechanical properties and fracture morphology of the conglomerate, and its stress–strain relationship is the external macroscopic expression of normal contact force of internal particles.

Steel wire prestress analysis of large section jacking prestressed concrete cylinder pipe (JPCCP): Site experiment and numerical investigation

As one of the key factors of Jacking Prestressed Concrete Cylinder Pipe (JPCCP), stress condition of the steel wire has a significant influence on the safety and durability of the JPCCP. In this paper, the stress of the steel wire was analyzed in the process based on the in-situ test monitoring results and numerical investigation in the water conveyance project in Shanghai. According to test results, the stress losses during the entire process were ruled out and turned out to be quantized. The reduction rate of steel wire was quite high in the early stage of prestress, and gradually slowed down with the time. The relaxation of steel wire was the main influencing factor of prestress loss. The numerical model of JPCCP was established and verified by field test data. Mechanical properties and failure modes of JPCCP under high inner water pressure and under different prestress losses were analyzed. After the initial prestress of steel wire was reduced from 1048 MPa to 888 MPa, the prestress of concrete in pipe core was reduced by 15%, while the compressive stress of the outer concrete was reduced by 23%. The water pressure carrying capacity of the structure was reduced by 10%. The larger the prestress loss of the steel wire was, the earlier the concrete in the pipe core entered the plastic phase. The results indicate that the prestress loss of steel wire has a direct and prominent influence on the bearing capacity of the water pressure on JPCCP. To help tunnel designers incorporate the prestress requirement into their basis of design and structural analysis, an assessment method was also proposed.

Mechanical behavior and failure modes of bamboo scrimber under quasi-static and dynamic compressive loads

Bamboo scrimber (BS), a renewable fiber-reinforced composite, is considered an ideal alternative to traditional construction materials. To investigate the compressive mechanical response of BS at different strain rates, experiments were conducted in three directions (directions X, Y, and Z) under quasi-static and dynamic loads. Results indicate that the mechanical properties in direction X are better than the other two. The failure modes in direction X include bulking, shearing and debonding, while shearing and crushing modes occur in directions Y and Z, respectively. Four compression phenomenological constitutive models are summarized via the stress–strain relationship. The strain rate effect represented by a DIF model and the micro-mechanism can be interpreted as micro-inertia and local densification. Compressive strength and Young’s modulus also can be influenced by the size and shape effect.

Experimental Investigation of Uniaxial Compressive Strength of Distilled Water Ice at Different Growth Temperatures

The existence of ice in nature will threaten the safety of navigation and water operations in cold regions. In order to improve the knowledge system of ice strength, the uniaxial compressive strength of distilled water ice grown at different temperatures is studied in this paper. Distilled water ice samples grown at −5 °C, −10 °C, −15 °C, −20 °C, −25 °C, −30 °C and −35 °C are prepared in the cryogenic laboratory. The density and grain size are measured. The uniaxial compressive strength tests are carried out at −10 °C. The stress-strain curves and the mechanical properties and failure modes of ice are obtained by loading along the vertical direction in the strain rate range of 10−6 s−1 to 10−2 s−1. It is found that the uniaxial compressive strength of ice is a power function of strain rate and a linear relationship with the −1/2 power of grain size. Combined with the relationship between strength and grain size and the relationship between grain size and temperature, it is deduced that the peak compressive strength has a logarithmic relationship with the growth temperature. In addition, it shows that the growth temperature affects the strength of ice by controlling the grain size.

Coupling effect of roughness and normal pressure on mechanical characteristics and failure mode of deep-sea sediment interface

Revealing the failure mechanism at the soil–structure interface is an important prerequisite for foundations bearing capacity analysis. In deep-sea structures, due to the special sedimentary conditions and extremely low permeability coefficient of marine soil, obviously, the undrained behavior at the interface is becoming more complex and needs due attention. The objective of this study is to investigate the coupling effects of roughness and normal pressure on the interface mechanical characteristics and failure mode from the perspective of the pore pressure index through improved interface triaxial apparatus. The obtained results revealed that increased surface roughness makes it difficult for the soil particles in the grooves to climb along the structure surface and consequently the failure surface shifts from the interface to the soil. The confining pressure strongly influences the failure mode and critical roughness; Based on the response of two factors to the interface, a new piecewise Mohr-Coulomb failure criterion is established to reveal the real interface mechanical parameters, accordingly two new concepts are proposed and analyzed: transition roughness and critical normal stress; Furthermore, the relationship between the critical roughness and confining pressure is determined by the interface shear resistance equation considering the coupling effect; Finally, the interface failure mechanism and failure modes related to the confining pressure and surface roughness are revealed and distinguished. The dominant effect of effective interface friction parameters on the failure modes is evaluated for various surface roughness ranges.

Mechanical behavior and failure modes of jointed rock mass based on rough discrete fracture network models

Mechanical behavior and failure modes of jointed rock mass based on rough discrete fracture network models

Experimental study of CLT panels under combined out-of-plane bending and compression

Combined out-of-plane bending and compression is an inevitable loading condition for cross-laminated timber (CLT) walls due to the out of plumbness, initial panel curvature and load eccentricity. Because of the weaker cross layers, the mechanical performance and failure mode of the CLT panels under such loading significantly differ from those of the other wood members, such as sawn timber and glulam. This experimental study focuses on investigating the behavior of CLT panels under the stated loading scenario. The experimental program consisted of 36 specimens, including 12 five-layer CLT panels provided by two Canadian manufacturers, as well as 12 three-layer and 12 five-layer CLT panels provided by a Chinese manufacturer. All the specimens were tested in Canada and China, respectively, using the same test setup. It was found that in the majority of the specimens, the damage was caused by the local buckling near the mid-height of the outer layer on the concave compression side at about 45%–60% of the ultimate failure load and 25%–35% of the ultimate deflection. The dominant failure mode was characterized as the crushing yield of the outer layer on the concave side (fibers in compression) near the mid-height after the local buckling progressed. Signs of shear failure of cross layers were evident almost simultaneously with the outer layer failure. Hence, although the dominant failure mode was likely triggered by the fracture in the cross layers due to rolling shear, the load-carrying capacity of the CLT panels under the combined out-of-plane bending and compression is governed by the compression failure of the longitudinal outer layers. It was found that a cross-layered panel can maintain its integrity before failure. It was also noted that the interaction equations specified in the Canadian design standard are appropriate for the CLT panels under small load eccentricity, while for panels that have large load eccentricity, the appropriateness of the interaction equation is questionable. Based on the test results obtained, the validity of the interaction equation for the prediction of the load-carrying capacity of CLT columns, specified in the CSA O86-19, is confirmed for cases in which Pf/Pr>0.5 and Mf/Mr<0.3. For the cases of Pf/Pr<0.5 and Mf/Mr>0.3, modification of the equation is needed.

Parameter estimation of lifetime distribution for the meta-action unit with uncertainty failure modes under type-I censored data

This paper presents a parameter estimation method for the lifetime distribution of the Meta-Action Unit (MAU) with uncertainty failure modes under type-I censored data. The MAU is regarded as the basic functional unit to accomplish the function of mechanical equipment, and its failure modes are classified according to the abnormal kinematic parameters in Meta-Action (MA), which are more succinct than the traditional mechanical failure modes on parts. However, there is some uncertain information about the failure data and censored data of MAU because of the technology limitations and the space accessibility constraints for monitoring the kinematic parameters of MA, which uncertainty information can impact the parameter estimates of MAU lifetime distribution. In order to avoid the impacts on the estimating accuracy of distribution parameters, the evidential likelihood function based on the belief function theory is constructed in view of the credibility level of the failure data and censored data. In addition, the Evidential Expectation Maximization (E2M) algorithm is proposed to estimate the parameters of the mixed exponential distribution of MAU lifetime under type-I censored data. Finally, an application of an Automatic Pallet Changer (APC) is used to illustrate the validity of the MAU failure modes classification. The simulations of the E2M algorithm are conducted to show that the proposed parameters estimation method can integrate uncertain information in the failure data and the censored data, and obtain more stable results than those based on the conventional Expectation-Maximization (EM).

Effect of bedding plane on mechanical properties, failure mode, and crack evolution characteristic of bedded rock-like specimen

To study the influence of bedding planes on mechanical properties and failure characteristics of specimens, an experimental test was conducted first on bedded rock-like specimens by uniaxial compression test with acoustic emission and digital image correlation. Experimental results show that both uniaxial compression strength (UCS) and failure strain (εf) present a tendency to first decrease and then increase with the increase in bedding angle (α). Moreover, the existence of bedding planes changes failure mode and fracture process of bedded specimen. Based on experimental results, three-dimensional numerical models containing bedding planes were used to determine the effect of orientation angle (α), spacing (s), and strength (λ) of bedding planes. Numerical results are as follows: (1) For α = 0° and 30°, the increase in s leads to the decrease in translaminar tensile crack and increase in translaminar shear crack, and the trend is reversed for α = 45° and 60°. Besides, (2) the variation of s leads to the variation of the percentage of the crack in bedding and rock matrix. However, (3) with the increase in s, the UCS and εf of bedded specimen at the same α remain almost unchanged. Unlike the influence of s, (4) the UCS and εf of bedded rock for α = 30°–60° are positively correlated with λ. (5) Moreover, with the increase in λ, the BP-crack percentage decreases dramatically, leading to the variation of the failure mode of bedded rock.

Mechanical behaviour of high strength steel T-stubs at elevated temperatures: Experimental study

At elevated temperatures, an experimental study on 28 high strength steel (HSS) T-stubs specimens was carried out by tensile tests, to reveal the mechanical performance of HSS T-stubs. The mechanical property and failure mode of each HSS T-stub specimen is obtained from the tensile tests. Further, the applicability of Eurocode 3 Part 1–8 both at ambient and elevated temperatures is verified with experimental results of HSS T-stubs. The experimental study includes 28 HSS T-stub specimens varying in flange strength, flange thickness, bolt strength, bolt diameter, bolt location, and temperature, to explore the effects of various influencing factors on the mechanical properties of HSS T-stubs. Compared with the deformation of the ambient-temperature experiments, the plasticity of the flange is not fully developed and the flexural deformation of the flange is small at elevated temperatures. When the bearing capacity of the flange is slightly lower than that of other components, the experimental results indicate that the predictions of Eurocode 3 Part 1–8 on failure modes are applicable for HSS T-stubs at elevated temperatures, while its theoretical predictions on the HSS T-stubs with large d (the distance between the bolt centre and the fillet weld) are inaccurate at ambient temperature. When e/d (e is the distance from the flange edge to the bolt centre) is less than 1, the initial stiffness and tensile load-bearing capacity will be significantly reduced by increasing d, because the increasing d will significantly improve flexural deformation and reduce flexural stiffness of the flange. Moreover, the initial stiffness of HSS T-stubs is overestimated by Eurocode 3 Part 1–8 both at ambient and elevated temperatures, especially the T-stubs with thick flanges, while the tensile load-bearing capacity of HSS T-stubs is conservatively predicted.

Investigation on the Fracture Properties of Layered Carbonaceous Slate: Experiment and Numerical Simulation

In order to study the influence of bedding structure on the mechanical properties of rock mass structure, this paper takes the layered carbonaceous slate in the Muzhailing tunnel as the research object, and studies the mechanical properties of layered slate under uniaxial compression and Brazilian splitting. In the numerical simulation, the discrete element software MUSEN (bonded particle model, BPM) is used to simulate the failure behavior of layered rock mass in the process of quasi-static compression. The main research contents of this paper are as follows: (1) the relationship between the microscopic Young’s modulus and the macroscopic Young’s modulus. (2) Influence of tangential shear strength and normal tensile strength on particle model. (3) The influence of different particle distributions on the simulation results. (4) The particle model with different bedding inclinations was simulated numerically. Through the comparative analysis of simulation and experiment, the results show that the mechanical response and failure mode have good consistency, and the splitting evolution process of layered slate with different bedding inclination angles is reproduced.

Mechanical behavior and constitutive model of shale under real-time high temperature and high stress conditions

The high temperature and high in-situ stress geological environment can significantly affect the mechanical properties, failure modes, and deformation characteristics of deep shale reservoirs. In this study, real-time high temperature triaxial compressive tests simulating the deep shale formation environment (temperature: 25–150 °C, confining pressure: 0–100 MPa) are carried out. The GSI-strength degradation and constitutive models are derived based on the Hoek–Brown criterion. The results show that in low confining pressure conditions, the mechanical behavior of shale is greatly influenced by temperature. Compared with shale at 25 °C, the compressive strength of shale at 150 °C decreases by up to 13.7%, and the elastic modulus decreases by up to 36.9%. The peak strain was increased by a factor of up to 1.4, and the yield stress level was advanced by as much as 7.4%. However, in high confining pressure conditions, the shale plasticity characteristics are significantly enhanced and the failure mode is relatively single. The GSI-strength degradation model can well characterize the variation law of shale strength with confining pressure under high temperature conditions. The statistical damage constitutive model matches the actual stress–strain curve very well, and it can fully reflect the deformation and failure characteristics of deep shale. The findings of this study can help us better understand the variation of mechanical properties of deep shale.

Investigation on Mechanical Properties, AE Characteristics, and Failure Modes of Longmaxi Formation Shale in Changning, Sichuan Basin, China

Investigation on Mechanical Properties, AE Characteristics, and Failure Modes of Longmaxi Formation Shale in Changning, Sichuan Basin, China

Anisotropy of mechanical properties in directionally solidified Nb-Si alloys at room temperature compression

The room-temperature compressive strength and deformation mechanisms in directional solidified Nb-Si alloy with different orientations were studied. The Electron Backscattered Diffraction (EBSD) experiment further explained that the orientation relationship between the Nbss phase and the α-Nb5Si3 phase is [001] Nbss // [001] α-Nb5Si3, (110) Nbss // (110) α-Nb5Si3, and the corresponding relationship between the material crystal structure and macroscopic spatial coordinates was established. The anisotropy of mechanical properties and failure mode in the alloys are attributed to the phase structures which are the long rod-like morphology of the α-Nb5Si3 phase growing along [001] and the spatial asymmetry of a single (110) Nbss // (110) α-Nb5Si3 phase interface. The Tsai-Hill criterion was applied to explain the anisotropy of the compressive strength of alloy at an angle to the direction of the heat flow. In addition, phase boundary separation was considered to be an important reason for the higher plasticity exhibited by the [100] compression orientation.

Mechanical Properties and Failure Modes of Additively Manufactured Ti6Al4V Lattice Structures Under Quasi-Static Compressive Loading

Additive manufacturing (AM) has many advantages over traditional manufacturing technologies as it allows the fabrication of lattice structures with complex designs and inherent features within a single part without any separation. Currently, lattice structures have wide application prospects due to their excellent mechanical performance and design freedom. This paper provides both experimental and numerical investigations for the failure behaviors of selective laser melting (SLM) Ti6Al4V lattice structures under uniaxial compressive loading. Lattice structures with different cell topologies and strut radii were chosen to conduct quasi-static compression simulations to examine their mechanical properties and failure modes. It is found that adding [Formula: see text]-direction struts in the loading direction could significantly improve the load-carrying capacity and the most superior mechanical properties were presented by FCCZ. The slopes of the double logarithmic relationship between the equivalent stiffness and the relative density of lattice structures can be distinguished as close to 1.0 and 3.0, implying bending-dominated or stretch-dominated behavior of lattice structures, respectively. For stretch-dominated lattice structures under uniaxial compression, FCCZ, BCCZ and FBCCZ, the failure modes would experience a transformation from strut buckling to fracture with the increase of strut radii, which is different from FCC and BCC demonstrated as the bending-dominated lattice structures.

Failure modes and excavation stability of large-scale columnar jointed rock masses containing interlayer shear weakness zones

The mechanical properties of large-scale columnar jointed rock masses (CJRMs) after excavation would be reduced due to the unloading relaxation, in addition to its influence on the stability of underground caverns . Moreover, the cutting of the interlayer shear weakness zones through the cavern complicates the overall stability of CJRM, and brings challenges to the stability control of project. In view of such problems encountered during the construction of underground caverns, we reconstructed the large-scale CJRMs containing interlayer shear weakness zones to conduct the 3D rock failure process analysis. The numerical models of surge tank caverns during excavation were established and the mechanical properties, failure modes and acoustic emission (AE) characteristics were analyzed. The simulation results indicate that the large-scale columnar joints and C2 interlayer shear weakness zones influence the mechanical properties and failure modes significantly. Three types of damage modes in numerical models containing crush-shear failure along columnar joints, tensile-slip failure along C2 interlayer shear weakness zone and crush failure inside and around caverns were summarized. The concentration zones of tensile stress were found around each cavern and gradually shrunk from sector shape to linear shape. The caverns were divided into two parts along C2 shear weakness zone in the process of X stress loading and two sector damage zones occurring on both horizontal sides of each cavern. In the process of Y stress loading, some crack networks consisting of large-scale columnar joints, intralayer joints and C2 shear weakness zone were formed which led to the complete failure of the numerical models. A new method based on BP neural network was proposed to evaluate the stability of rock mass with large-scale columnar joints and interlayer shear weakness zones. The training results show that the method can distinguish the stability of rock mass well, while its accuracy reaches 97.6%.

The Influence of Grain Size Distribution on Mechanical Compaction and Compaction Localization in Porous Rocks

AbstractThe modes of formation of clastic rocks result in a wide variety of microstructures, from poorly‐sorted heterogeneous rocks to well‐sorted and nominally homogeneous rocks. The mechanical behavior and failure mode of clastic rocks is known to vary with microstructural attributes such as porosity and grain size. However, the influence of the grain size distribution, in particular the degree of polydispersivity or modality of the distribution, is not yet fully understood, because it is difficult to study experimentally using natural rocks. To better understand the influence of grain size distribution on the mechanical behavior of porous rocks, we prepared suites of synthetic samples consisting of sintered glass beads with polydisperse grain size distributions. We performed hydrostatic compression experiments and found that, all else being equal, the onset of grain crushing occurs much more progressively and at lower pressure in polydisperse synthetic samples than in monodisperse samples. We conducted triaxial experiments in the regime of shear‐enhanced compaction and found that the stress required to reach inelastic compaction was lower in polydisperse samples compared to monodisperse samples. Further, our microstructural observations show that compaction bands developed in monomodal polydisperse samples while delocalized cataclasis developed in bimodal polydisperse samples, where small grains were systematically crushed while largest grains remained intact. In detail, as the polydispersivity increases, microstructural deformation features appear to transition from localized to delocalized through a hybrid stage where a compaction front with diffuse bands propagates from both ends of the sample toward its center with increasing bulk strain.