Failure Mode Transition Research Articles

DEM study of structuralized cemented slopes under excavation conditions

Structuralized cementation has emerged as a promising method for reinforcing excavated slopes. The Discrete Element analysis is conducted on the structuralized cemented slopes under excavation with its validity verified through centrifuge model tests. The results indicate that structuralized cemented slopes exhibit progressive failure from the bottom to the top under excavation conditions. As the slope elevation decreases, the failure mode transitions from tension failure to shear failure, due to the presence of tensile stress only in the upper part of the slope. Microscopically, structuralized cementation prevents contact breakage, reducing fabric anisotropy and the variation of contact orientation from the vertical direction. Macroscopically, it increases the safety limit of slopes. The significant coupling between fabric evolution localization and local failure explains the failure mechanism of structuralized cemented slopes under excavation conditions. Increasing the size of the solidification zone reduces the localization extent of fabric evolution, thereby reinforcing the slope.

Effect mechanism of low-velocity impact loading rate on the failure modes transition of steel-UHPC-steel composite plates

This study delved into the transition mechanism of loading rate on the failure mode transition in steel-ultra-high performance concrete-steel (SUHPCS) composite plates under concentrated low-velocity impact loads. A series of drop-weight tests elucidated that, as impact velocity escalated, the composite plate shifts from local punching failure, characterized by circumferential cracks, to overall flexural failure, marked by radial cracks resembling yield lines. Given the constraints in capturing strain data in drop-weight test, a 3D nonlinear structural analysis using Abaqus software was conducted. This numerical simulation method was validated by drop-weight test results in terms of crack patterns and impact force time histories. Moreover, structural dynamic punching and flexural loads at a specific loading rate were obtained with the assistance of the dynamic increase factor (DIF). Failure mode could be judged through comparison between the two critical loads. The discrepancy in strain sensitivity exhibited by steel plates and UHPC accounted for the transition in failure modes of the SUHPCS composite plates. DIFs for UHPC compressive strength and steel plate strength were lower than DIF for UHPC tensile strength, as these factors were around 1.5, 1.7 and 2.6, respectively. Parameter analysis showed that the critical transition impact velocity for a composite plate with a core thickness of 80 mm was 11 m/s, surpassing the 13 m/s threshold for a composite plate with a 100 mm-thick core. Moreover, changes in DIFs of UHPC and steel strengths were less than 2 % with various impact masses and the composite plate maintained the same failure mode.

Triaxial compressive behaviour of ultra-high performance geopolymer concrete (UHPGC) and its applications in contact explosion and projectile impact analysis

Ultra-high performance geopolymer concrete (UHPGC) is increasingly recognised as a promising construction material in protective engineering owing to its exceptional material strength and eco-friendly characteristics. However, there is currently limited research data on the behaviour of UHPGC under complex stress states, and the applicability of some common concrete dynamic constitutive models to UHPGC requires calibration and evaluation. This paper presents an experimental study on the triaxial compression (TXC) behaviour of UHPGC under varying confining pressures with a range of 0–40 MPa. The stress-strain curves under various confinement levels and failure patterns of UHPGC are discussed. The results indicate that the peak axial stress and toughness of UHPGC increase with higher confining pressure levels, but it still exhibits noticeable softening behaviour. Under uniaxial compression, UHPGC typically fails with inclined shear and axial tensile cracks, while under TXC, particularly at confining pressures above 20 MPa, the failure mode transitions to shear failure with prominent oblique shear cracks. Additionally, existing failure criteria are calibrated to match the strength envelop of UHPGC, with the Power-law failure criterion providing the closest fitting results. Subsequently, utilising both current and pre-existing TXC data, three commonly used concrete constitutive models, including HJC, K&C and CSC, are calibrated with respect to the dominant strength parameters for UHPGC in the finite element (FE) hydrocode ANSYS/LS-DYNA. Further, numerical simulations are conducted to evaluate the effectiveness of such three calibrated concrete models in replicating the response of UHPGC panels exposed to the contact explosion or projectile impact. The simulation results demonstrate that the K&C and CSC models effectively replicate the localised damage of UHPGC panels under contact explosions, while the HJC model more accurately simulates the depth of penetration (DOP) for thick UHPGC panels under projectile impact.

Imperfection-Enabled Strengthening of Ultra-Lightweight Lattice Materials.

Lattice materials are an emerging family of advanced engineering materials with unique advantages for lightweight applications. However, the mechanical behaviors of lattice materials at ultra-low relative densities are still not well understood, and this severely limits their lightweighting potential. Here, a high-precision micro-laser powder bed fusion technique is dveloped that enables the fabrication of metallic lattices with a relative density range much wider than existing studies. This technique allows to confirm that cubic lattices in compression undergo a yielding-to-buckling failure mode transition at low relative densities, and this transition fundamentally changes the usual strength ranking from plate > shell > truss at high relative densities to shell > plate > truss or shell > truss > plate at low relative densities. More importantly, it is shown that increasing bending energy ratio in the lattice through imperfections such as slightly-corrugated geometries can significantly enhance the stability and strength of lattice materials at ultra-low relative densities. This counterintuitive result suggests a new way for designing ultra-lightweight lattice materials at ultra-low relative densities.

The impact of lenses on the seepage failure of tailings dam.

The presence of lenses such as tailings slurry, frozen soil, and saturated zones disrupts the continuity of tailings dams and their normal seepage patterns, elevating the seepage line of the dam body and significantly impacting local stability. This study, to investigate how lenses affect the stability and failure mechanisms of tailings dams, employs numerical simulation and physical models and constructs a model of the tailings dam, incorporating tailings clay lens and void lens, to investigate variations in hydraulic gradients, seepage velocities, seepage flow, pore water pressure, and the patterns of seepage failure. This research reveals that the tailings clay lens within the dam body increases the hydraulic gradient in its vicinity due to its low permeability and raises the phreatic line. As the tailings clay lens approaches the dam body, the phreatic line tends to escape along the upper part of the lens towards the dam surface. In addition, the void lens could lead to a more pronounced seepage gradient along its path on the dam surface, with a liquefaction beneath it. As the void lens nears the toe of the slope, the dam failure mode transitions from a step-like progressive failure to an arch-shaped settlement failure along the void lens.

Tuning interfacial mechanical properties of Ti/CFRP laminates using customized intercalation of woven metallic fabrics

Improving the interlaminar fracture toughness of fiber metal laminates comprising titanium alloy (Ti) and carbon fiber-reinforced polymer (CFRP) holds considerable significance for their diverse applications. Here, an innovative and scalable strategy was developed to enhance the interfacial adhesion of Ti/CFRP hybrid laminates, which was achieved by introducing customized woven metallic fabrics to construct a heterogeneous interface through a simple and cost-effective spot welding method. Interestingly, it was found that the interlaminar mechanical performance and failure mode transition from snap-down instability to progressive softening behavior could be precisely tuned by adjusting the distribution configurations of the welded spots. The results demonstrated substantial improvements in interlaminar fracture energies with the implementation of customized intercalation. Specifically, mode I initiation and propagation were enhanced by approximately 276% and 144%, respectively, while mode II initiation improved by approximately 242%. These improvements were attributed to the activation of new dissipative mechanisms, including interfacial topological texture interlocking, adhesive failure, multiphase bridging (involving woven metallic fabric, welded spots and carbon fiber), as well as cross-layer migration of the interlaminar cracks. The use of customized intercalation thus proves to be a promising strategy for constructing heterogeneous interface and offers valuable insights into the interlaminar design for hybrid structures.

Experimental and numerical analysis of Brazilian splitting mechanical properties and failure mechanism for composite discs

The properties of structural planes and the strength of rock matrix significantly influence the splitting characteristics of composite rock masses. To investigate the effects of structural plane and differences in matrix strength on the mechanical behavior and failure characteristics of composite jointed rock bodies, composite jointed disc specimens with varying structural plane roughness and matrix combinations were prepared. These specimens were then subjected to Brazilian split tests under different loading angles ranging from 0° to 90°. Results indicate that the failure mode of the specimens is determined by the angle of the structural plane. As the structural plane angle increases, the failure mode transitions from structural plane failure to combination failure and finally back to structural plane failure. Increased roughness of the structural plane reduces the extent of structural plane failure, while matrix strength has no significant impact on the failure mode. The failure load of the specimens increases with increasing structural plane angle, roughness, and matrix strength. The influence of structural plane roughness and matrix strength on the mechanical properties of the specimens grows with increasing structural plane angle. Increases in structural plane roughness and matrix strength significantly enhance the strain energy and dissipated energy of the disc model. Finally, an analysis of the stress state on the structural planes is combined with an exploration of the failure mechanisms of the specimens.

A three-dimensional coupled thermo-elastic-plastic phase field model for the brittle-ductile failure mode transition of metals

Dynamic brittle fracture and shear banding are two typical failure modes of metals, and the transformation of the brittle-ductile failure mode has been observed in the Kalthoff test. This paper establishes a thermo-elastic-plastic coupled three-dimensional phase field model to simulate brittle-ductile failure mode transition of metals. The expression for the variation of the Taylor-Quinney coefficient with stress triaxiality is adopted, and the critical energy release rate is automatically adjusted using the Taylor-Quinney coefficient. Then, the Kalthoff test is simulated using the proposed model. The brittle-ductile failure mode transformation phenomenon is reproduced, which agrees well with the experimental results. It can be well proved that impact velocity is crucial in determining the transition to failure mode. At low-velocity impact, the energy is insufficient to drive the plastic accumulation of the shear band, resulting in brittle tensile fracture. At high-velocity impact, the energy is sufficient to drive the formation of adiabatic shear bands, resulting in tensile shear failure. In addition, three-dimensional simulations show that the tip of the shear band exhibits a crescent-shaped non-two-dimensional extension state under finite thickness. This numerical framework provides a predictive tool to understand the evolution of the dynamic failure of metals under impact loading.

Shear strengthening of self-compacting lightweight concrete beams with steel fibers and unbonded prestressing bars

This study aims to investigate the shear behavior of self-compacting lightweight concrete (SCLC) beams strengthened with steel fibers and unbonded prestressing bars. For this, the experimental program included the use of hooked-end steel fibers with volume fractions of 0.5 %−0.75 %, and high-strength prestressing bars with varying prestressing forces and transverse reinforcement ratios. The addition of steel fibers in SCLC significantly increased the normalized shear capacity of beams, reaching up to 2.50 times higher values, whereas the substitution of normal coarse aggregates with lightweight aggregate resulted in a decrease of up to 10.1 %. The shear strengthening of SCLC beams with unbonded prestressing bars provided a substantial increase in the ultimate shear capacity up to 2.06 times higher than that of beam without reinforcement. Increasing the prestressing force of bars effectively restrained the occurrence of critical shear cracking, consequently enhancing the shear capacity. Utilizing hybrid reinforcement with fibers and prestressing bars resulted in a transition of failure mode from shear to flexural, attributed to the improved shear capacity. For the SCLC beams reinforced with steel fibers, the shear strengths were predicted by previous prediction models. When modifier for lightweight concrete was considered, the Imam’s model most precisely predicted the shear strengths with an average vu/vn ratio of 1.00 and SD of 0.07. The fib model Code, based on the modified compression field theory, provided accurate predictions for the shear capacities of SCLC beams with unbonded prestressing bars, exhibiting an average Vu/Vn ratio of 1.00 and SD of 0.09. The fiber-resisting component was reasonably considered in the model with an average Vu/Vn ratio of 1.02 and SD of 0.07.

Experimental study on the mechanical characteristics of weakly cemented mudstone under different loading rates

With the gradual shift of coal mining to the western coal mining region of China, floor heave in weakly cemented mudstone roadways has become an issue affecting the safety and efficiency of coal mine production. Additionally, different mining rates can lead to fluctuating support stresses on the roof and floor of weakly cemented mudstone roadways. Therefore, obtaining a comprehensive understanding of the mechanical properties of weakly cemented mudstone at different loading rates is conducive to improving the issue of floor heave in such roadways and provides a theoretical basis for further study. In this context, a series of uniaxial mechanical tests with concurrent acoustic emission monitoring were conducted on specimens of weakly cemented mudstone under various loading rates (0.005, 0.01, 0.05, and 0.1 mm/s). The stress‒strain and acoustic emission response curves were obtained to effectively characterize the strength, deformation, damage, macroscale instability, and crack propagation characteristics of the mudstone under the influence of loading rate effects. The research results support the following findings: (1) With increasing loading rate, the peak strength and elastic modulus of weakly cemented mudstone significantly increase, while the peak axial strain and peak radial deformation significantly decrease. (2) With increasing loading rate, the stress required to trigger the expansion of weakly cemented mudstone gradually increases, and a significant power-law relationship arises between the strain of the mudstone at the start of expansion and the loading rate. (3) With increasing loading rate, the acoustic emission ringing count of weakly cemented mudstone increases: The failure of weakly cemented mudstone changes from small-range progressive failure to sudden failure, and the failure mode transitions from shear failure to tensile‒shear composite failure. (4) The studied mudstone damage variables increase with increasing loading rate, following an approximate exponential function. The conclusions obtained in this work can provide a theoretical basis for the evolution mechanism and control of floor heave in deep roadway mining.

High temperature tensile properties and deformation behavior of CoCrFeNiW0.2 high entropy alloy

High entropy alloys (HEAs) have been a new-type of structural materials, which show good performance in various extreme conditions. In the present study, we systematically investigated the high temperature deformation behavior of as-cast CoCrFeNiW0.2 HEA under different temperatures from 298 to 1173 K and various strain rate of 1 × 10−4 s−1 to 5 × 10−3 s−1. An obvious temperature dependence of yield strength can be found with a remarkable decrease from 362 MPa at 298 K to 119 MPa at 1173 K. The normal Portevin–Le Chatelier (PLC) effect was presented in the intermediate temperature range of 673 K to 1073 K, the serrated flow transformed in the sequence of A → A + B → A + C → B + C → C with the increasing temperature, which is caused by dynamic strain aging (DSA). W atoms played an important role in the interaction between solute atoms and dislocations. Fractography analysis suggests the failure mode transition from transgranular fracture to intergranular fracture. Electron backscatter diffraction (EBSD) demonstrates that the high temperature deformation mechanism of the HEA is dominated by dynamic recovery (DRV). Additionally, transmission electron microscopy (TEM) was performed to observe the dislocation configurations and the interaction between dislocations and μ phase precipitates at various temperatures, and these results further identify the high temperature deformation mechanism. To summarize, this study indicates the proposed HEA has excellent high temperature mechanical properties and a wide range of potential applications in many fields.

Study on Seismic Performance of RC Frame Structures Considering the Effect of Infilled Walls

This paper studies the impact of half-height infilled walls on the failure modes of frame columns through quasi-static tests of both frame models and half-height infilled wall frame models. Based on the experimental results, a seismic analysis model of reinforced concrete (RC) frame structures is established, and parametric studies are carried out to analyze the effects of masonry materials and masonry heights on the seismic performance of structures. The results show that the load-bearing capacity and stiffness of the structure are improved, while the ductility of the structure is reduced because of the existence of infilled walls. As the height of infilled walls increases, there is a notable decrease in the free height of frame columns. At a wall-to-column height ratio of 0.2, the masonry walls exert a negligible effect on the frame structure’s seismic performance. In contrast, at a ratio of 0.6, there is a transition in column failure modes from bending to shearing. When evaluated at consistent masonry heights, aerated concrete block-infilled walls demonstrate the least impact on the seismic performance of RC frame structures. Thus, in the absence of additional structural enhancements, the use of aerated concrete blocks is recommended to mitigate the negative implications of infilled walls on the seismic integrity of RC frames.

A unified hardening/softening elastoplastic model for rocks undergoing brittle-ductile transition with strength-mapping and fractional plastic flow rule

A novel unified hardening/softening model is presented, addressing challenges in the constitutive modelling of rocks’ strain-hardening and strain-softening behaviours with brittle-ductile transition. The model highlights the impact of confining pressure on failure mode transition when capturing variations in initial yield, peak, and residual strength. The yield criterion and hardening/softening law are developed by a strength-mapping method, where the peak strength is considered the upper bound. The strength-mapping method relies on a mapping index formulated by plastic shear strains. The mapping index is then incorporated into the fractional plastic flow rule, leading to the proposed constitutive model with 9 easy-to-calibrate parameters. The model predictions have been validated by 4 series of rock samples on triaxial tests, where the brittle-ductile transitions have been well captured. The results indicate that it is reliable to capture the rocks’ complicated mechanical responses, particularly the brittle-ductile transition, with our proposed strength-mapping method and fractional plastic flow rule.

Effect of Fe-Al intermetallics on fatigue properties of aluminum to steel dissimilar spot welds

Lightweight structures made of aluminum alloys and high strength steels are increasingly used for improving vehicle fuel efficiency. One barrier for joining aluminum and steel is the formation of brittle Fe-Al intermetallics that can have deleterious effects on joint ductility and strength under static loading conditions. However, it is unclear how the intermetallics affect the joint fatigue properties. In this study, the effect of intermetallics on fatigue properties of dissimilar metal joints between a 6xxx aluminum alloy and a high-strength steel is investigated on welds created by ultrasonic interlayered resistance spot welding (Ulti-RSW). Joint efficiency calculations are used to normalize the peak load so the fatigue properties of Ulti-RSW are compared with a wide range of joining processes in the literature. During fatigue testing the failure mode transitions from interfacial or button pull-out failure during low cycle fatigue loading (below 100,000 cycles) to through-thickness failure during high cycle (above 100,000 cycles). Based on the stress intensity factors calculated using a simple 2D fracture mechanics model, the transition in failure mode is attributed to the plastic deformation near the notch tip. Overall, this study shows that the intermetallics have little effect on the high-cycle fatigue properties of dissimilar spot joints.

Numerical Investigation of Failure Mode Transitions in Rock Specimens Containing Non-persistent Joints Under Compression-Shear Conditions

Numerical Investigation of Failure Mode Transitions in Rock Specimens Containing Non-persistent Joints Under Compression-Shear Conditions

In situ SEM side observation of asperity behavior during sliding contact

The performance and evolution characteristics of the friction interface are crucial for the design optimization of material friction and wear, as well as the revelation of the mechanism of seismic sliding motion. To understand the friction behavior of rough surfaces, it is essential to understand the physical process of interaction among asperities. However, visualizing the contact of asperities is challenging because materials are typically opaque, and the dimensions of asperities are usually in the micron range. This study developed an in-situ scanning electron microscope friction device and a micro speckle fabrication method to measure the strain field of regular asperities from the side, and synchronously measure the macroscopic friction coefficient. In-situ friction experiments were conducted on brass and silicon materials. The results indicate that directly correlating the friction coefficient with the deformation and failure images of asperities can effectively explain the evolution process and differences in friction coefficient for the two materials, validating the performance of the device. Different failure modes of asperities were observed, including severe plastic deformation in brass asperities and fracture-producing large particles in silicon asperities. The critical transition of asperity failure modes in the experiments was analyzed based on a theoretical model. The phase diagram of asperity failure modes and friction coefficient evolution was plotted, providing potential explanation for the evolution of friction coefficient in friction experiments on randomly rough surfaces. The developed device in this study can be used for non-transparent materials and helps reveal the microscopic mechanisms behind experimental phenomena.

The effects of temperature and hygrothermal aging on the properties of CF/PA6 composite and its compression failure mechanisms

AbstractCarbon fiber reinforced polyamide composites have outstanding properties, but their applications are significantly affected by the service environment. This work evaluates the effects of temperature (25, 80, 140, and 200°C) and hygrothermal conditions on the mechanical properties of CF/PA6 composite containing delamination damage, and studies its damage behaviors and failure mechanisms. After hygrothermal aging, recrystallization of PA6 improves the heat resistance of CF/PA6 composites, but water absorption reduces the interfacial strength of composites, resulting in the decrease of modulus and compressive strength (24% and 42%, respectively). With the increase in temperature, the compressive strength of CF/PA6 composites gradually decreases (from 132 MPa at 25°C to 24 MPa at 200°C), and the failure mode transitions from fiber fractures, interlayer shear fractures, delamination to kinking. In addition, the compressive failure process of CF/PA6 composites is divided into three stages: buckling, delamination bulging and kinking failure.Highlights Hygrothermal aging reduces the fiber/resin interface performance. Fiber fracture, interlayer shear and delamination convert to kinking failure. The failure process is divided into buckling, delamination and kinking.

The Time-Dependent Interfacial Adhesion between Artificial Rock and Fresh Mortar Modified by Nanoclay.

The time-dependent interfacial adhesion between rock and fresh mortar is key for printing concrete linings in mountain tunnels. However, a scientific deficit exists in the time-dependent evolution of the interfacial adhesion, which can cause adhesion failure when printing tunnel lining. Nanoclay has the potential to increase the interfacial adhesion and eliminate the adhesion failure. Before the actual printing of tunnel linings, the time-dependent interfacial adhesion between artificial rock and fresh mortar modified by nanoclay should be understood. This paper studied the time-dependent interfacial adhesion based on fast tack tests, fast shear tests, and isothermal calorimetry tests. With the addition of nanoclay, the maximum tensile stress and the maximum shear stress increased. Compared with a reference series, the maximum interfacial tensile stress in a 0.3% nanoclay series increased by 106% (resting time 1 min) and increased by 209% (resting time 32 min). A two-stage evolution of the interfacial adhesion was found with the addition of nanoclay. In the first stage, the time-dependent interfacial adhesion increased rapidly. A 0.3% NC series showed an increase rate six times higher than that of the reference series. As the matrices aged, the increase rate slowed down and followed a linear pattern of increase, still higher than that of the reference series. The stiffening of fresh matrices resulted in the interface failure mode transition from a ductile failure to a brittle failure. The effect of nanoclay on flocculation and on accelerating the hydration contributed to the time-dependent interfacial adhesion between artificial rock and fresh mortar.

Microstructural Evolution and Failure in Fibrous Network Materials: Failure Mode Transition from the Competition between Bond and Fiber.

For the complex structure of fibrous network materials, it is a challenge to analyze the network strength and deformation mechanism. Here, we identify a failure mode transition within the network material comprising brittle fibers and bonds, which is related to the strength ratio of the bond to the fiber. A failure criterion for this type of fibrous network is proposed to quantitatively characterize this transition between bond damage and fiber damage. Additionally, tensile experiments on carbon and ceramic fibrous network materials were conducted, and the experimental results show that the failure modes of these network materials satisfy the theoretical prediction. The relationship between the failure mode, the relative density of network and strength of the components is established based on finite element analysis of the 3D network model. The failure mode transforms from bond damage to fiber damage as increasing of bond strength. According to the transition of the failure modes in the brittle fibrous network, it is possible to tailor the mechanical properties of fibrous network material by balancing the competition between bond and fiber properties, which is significant for optimizing material design and engineering applications.

Importance of cohesive failure mode in fracture toughness enhancement of polymer nanocomposites with covalent grafting: A multiscale study

The covalent grafting between the nanoparticles and polymer matrix is considered a useful design factor to control the mechanical properties (e.g., fracture toughness). Meanwhile, in the theoretical viewpoint, the toughening mechanisms of the epoxy nanocomposites with covalent grafting are still unclear. In this study, a new multiscale fracture model is proposed to quantify the toughness enhancement of epoxy nanocomposites induced by the covalent grafting. Especially, it was concentrated on the investigation of the influences of the cohesive failure mode on the fracture toughness and revealed that the failure mode transition (from the adhesive failure mode to cohesive failure mode) near the nanoparticles with increasing grafting ratio is critical factor for the toughening mechanisms of the epoxy nanocomposites with covalent grafting. The theoretically derived multiscale model demonstrated that the cohesive failure mode significantly enhanced the fracture toughness of polymer nanocomposites by dissipating more energy compared to the adhesive failure mode. This study underscores the critical importance of rationally designing nanocomposites with an optimal grafting ratio that properly reflects the cohesive failure mode. It is anticipated that the multiscale analysis approach outlined in this study will provide valuable design guidelines and insights for polymer nanocomposites incorporating covalent grafting.