Microscopic Failure Modes Research Articles

Dynamic tensile mechanical properties of red sandstone under different pulse widths and amplitudes: Brazilian disk experiment and macroscopic and microscopic analysis

To investigate the dynamic tensile mechanical properties of red sandstone under varying impact pulse widths and amplitudes, we employed the response surface methodology to examine the dynamic response characteristics of sandstone under different bullet lengths and impact velocities. Subsequently, a dynamic damage constitutive model for sandstone was developed. Macroscopic and microscopic features of sandstone were analyzed through digital image correlation (DIC) and scanning electron microscopy (SEM) experiments. Additionally, ANSYS software was utilized to analyze stress wave characteristics under controlled shock waves and the impact of geological factors on rock fracturing effects. The findings revealed the following. First, the length and impact velocity of bullets exhibit an interactive effect on the tensile response characteristics of sandstone. Peak load, energy consumption rate, and energy density display a positive correlation with bullet velocity. Second, according to the DIC results, the fractal dimension of the crack is negatively correlated with the length of the bullet during equal energy impacts. Third, microscopic failure modes included fractures along particle cementation and through particles, with section roughness decreasing as bullet length increased under equivalent energy impacts. Fourth, a dynamic damage constitutive model (R2 ≥ 0.84) was established based on the Zhu-Wang-Tang (Z-W-T) model and Drucker-Prager (D-P) criterion, clarifying model parameters and influence rules. Fifth, under controlled shock waves, the pulse width facilitates crack propagation, while the pulse amplitude initiates crack formation. Optimal rock breaking efficiency is achieved when stress waves exhibit a large pulse width and low radiation values, meeting specific threshold conditions.

Study on the Preparation and Process Parameter-Mechanical Property Relationships of Carbon Fiber Fabric Reinforced Poly(Ether Ether Ketone) Thermoplastic Composites.

Carbon fiber fabric-reinforced poly(ether ether ketone) (CFF-PEEK) composites exhibit exceptional mechanical properties, and their flexibility and conformability make them a promising alternative to traditional prepregs. However, the formation of the CFF-PEEK composite is trapped in the high viscosity of PEEK, the smooth surface, and tightly interwoven bundles of CFF. It is more difficult for the resin to flow through the fibers of complex textile structures. Here, a simple film stacking method using the hot-pressing process of plain-woven CFF-PEEK thermoplastic composites is discussed. The uniform distribution of PEEK resin between each layer of CFF reduces the flow distance during the molding process, preventing defects in the composite material effectively. Four process parameters, including molding temperature (370, 385, 400, and 415 °C), molding pressure (1, 2, 4, 8, and 10 MPa), molding time (10, 20, 30, 40, 60, and 90 min), and pre-compaction process, are considered. Interlaminar shear strength (ILSS), tensile strength, and flexural strength of CFF/PEEK composites are evaluated to optimize the process parameters. Moreover, ultrasonic scanning microscopy and scanning electron microscopy are employed to observe the formation quality and microscopic failure modes of CFF/PEEK composites, respectively. The ultimate process parameters are a molding temperature of 410 °C, molding pressure of 10 MPa, molding time of 60 min, and the need for the pre-compaction process. Under the best process parameters, the ILSS is 62.5 MPa, the flexural strength is 754.4 MPa, and the tensile strength is 796.1 MPa. This work provides valuable insight for studying the process parameters of fiber fabric-reinforced thermoplastic polymer composites and revealing their impact on mechanical properties.

The effect of abrading treatment method on the bonding surface characteristics and fracture behavior of glass fiber-epoxy composite adhesive joints

The effect of the mechanical abrading method on the bonding surface characteristics and fracture response of glass fiber-reinforced polymer (GFRP) composite adhesively bonded joints was investigated. Double cantilever beam specimens were manufactured by bonding GFRP substrates with an epoxy adhesive to evaluate the mode-I adhesive joint fracture response. The bonding surfaces were treated with degreasing and abrading methods utilizing sandpapers with various grit sizes ranging from P60 to P800. The results showed that the sandpaper grit size can considerably change the bonding surface properties comprising the surface energy, roughness, morphology, and water drop contact angle and consequently, it can affect the joint fracture behavior. Reducing the sandpaper grit size continuously increased the surface roughness but there were optimum values for the surface energy and water drop contact angle. The fracture resistance of adhesive joints followed the trend of bonding surface energy as the higher the bonding surface energy, the higher the joint fracture resistance. The fracture energy and load-bearing capacity of adhesive joints were improved by 62 and 34%, respectively, when the joint bonding surfaces were abraded with P240. However, further reducing the sandpaper grit size to P60 caused a decrease in the fracture energy and load-bearing capacity of adhesive joints by 18 and 44%, respectively, compared to the joint with degreased and non-abraded bonding surfaces. Moreover, the abrading method considerably changed the microscopic and macroscopic failure modes of adhesive joints.

The performance transition of C/SiC composites induced by redistribution of residual stress

The residual stress generated from temperature changes and interference fit of rivet plays a significant role in the performance of C/SiC composites. To gain a thorough understanding of the residual stress evolution mechanisms and their influences on C/SiC composites, this work carries out comprehensive experimental research combined with simulation analyzes. The results of quasi-static tension tests present that as the environmental temperature rises from room temperature to 800 °C, the stiffness of structure increases a lot, while the strength of riveted joints significantly decreases. Furthermore, it is found that the microscopic failure modes are also changed. The cyclic unloading-reloading tests for calculating the residual stress quantitatively are conducted to explain the failure evolutions of structures. The obtained results reveal an important connection between the residual stress states and the microscopic failure modes of C/SiC composites. The environmental temperature and interference riveted joint both can affect stress state around the open hole, which are significant factors determining the damage modes and structural performance. In the process of C/SiC composites design, it is necessary to pay attention to the processing parameters of rivet interference fit to meet the performance optimization under service conditions.

Comparative study of different bonding interactions on the interfacial adhesion and mechanical properties of MXene-decorated carbon fiber/epoxy resin composites

Interfacial adhesion determines the mechanical performance of the carbon fibers-reinforced polymers composites (CFRPs), and different bonding interactions could lead to different degrees of interfacial adhesion and mechanical properties of CFRPs. In order to survey specific influence of different bonding interactions on the interface bonding, further boost the mechanical properties of CFRPs, MXene is decorated onto carbon fiber (CF) surface via Van der Waals force or hydrogen bonds (CF-v-MXene), ionic bonds (CF-i-MXene), and covalent bonds (CF-c-MXene), respectively. Besides, detailed interaction mechanisms of various bonding interactions are also comprehensively investigated. The results indicate that the introduction of MXene is obviously efficient for improving the interface adhesion and mechanical properties of CF/epoxy (CF/EP) composites, and the composites reinforced by CF-c-MXene exhibit the optimal properties. Tremendous improvements of 90.1 % and 110.3 % for the impact strength and interfacial shear strength (IFSS) are achieved compared with the composites reinforced by the unsized CF. The microscopic interfacial structure and fracture failure mode are further observed and analyzed to explore the enhancement mechanisms. This work provides effective guidance and reference for the application of MXene and other similar 2D layered materials, such as Transition Metal Dichalcogenides (TMDCs), to design and manufacture high-quality interface for CFRPs with excellent interfacial and mechanical properties.

Dynamic properties and fragmentation fractal characteristics of water-saturated reef limestone concrete under impact loading

Reef limestone was used as a coarse–fine aggregate to prepare concrete with a strength class of C30. The mechanism of influences of water-saturated state on dynamic compressive strength, energy dissipation, and macroscopic and microscopic failure modes of reef limestone concrete was studied, and the relationships between fractal dimension and loading rate, fragmentation characteristics, and energy dissipation were discussed. The results indicated that the dynamic compressive strength of reef limestone concrete presents power function growth with increasing loading rate, the free water reduces the degree of cementation among particles and weakens the quasi-static state and dynamic mechanical properties of reef limestone concrete. Meanwhile, under the same loading rate, water-saturated reef limestone concrete demonstrates a stronger energy absorption effect and a greater energy dissipation density, and the energy dissipation density is linearly correlated with the dynamic compressive strength. In addition, the irregular shape and porous structure of reef limestone aggregate particles effectively enhances the friction strengths between the aggregates and cement slurry, causing most of the dynamic cracks in reef limestone concrete to occur in the cement-sand slurry and aggregates themselves. Finally, the results also show that the fractal dimension of the fragments of reef limestone concrete increases linearly with increases in the loading rate and fragmentation energy dissipation density. At approximately the same loading rate, the fractal dimension of dry specimens was larger, and when the fractal dimensions in the two states were similar, the energy consumed per unit mass water-saturated specimen exceeds that of dry specimens.

Study on Microscopic Mechanism and Failure Mode of Water–Rock Coupling in Fine Sandstone Under Uniaxial Compression

Study on Microscopic Mechanism and Failure Mode of Water–Rock Coupling in Fine Sandstone Under Uniaxial Compression

The Effect of the Geometrical Parameter and Stacking Sequence on Damage Behavior of Unidirectional Carbon / Epoxy Laminate

In this work, an experimental test series was carried out in order to evaluate the influence of the geometry of the specimen and stacking sequence on macroscopic behavior and the failure modes. A CFRP prepreg unidirectional was used to perform the tested specimens according to ASTM D790 standard. Five main lay-up configurations have been analyzed: [0]12, [±30]3s, [±45]3s, [±80]3s, and [0/90]3s subjected to flexural loading. The macroscopic behavior was followed by an MTS machine equipped with a bending fixture. A digital microscope is used to follow the microscopic failure modes during loading. These laminates exhibit a more complex behavior due to coupling effects and the combination of different failure modes. However, the most predominant damage observed is delamination accompanied with matrix cracking. Furthermore, the present work has shown a linear behavior of [0]12, [±80]3s, [0/90]3s, and a distinctive behavior of [±45]3s and [±30]3s laminate under flexural loading due to its pseudo ductile behavior.

Experimental study on Mode Ⅰ fracture characteristics of laminated flattened-bamboo lumber combined with VIC-3D system

According to ASTM E399, Mode I fracture tests were conducted on compact tension (CT) specimens of laminated flattened-bamboo lumber (LFBL) loaded parallel to grain, in combination with the optical measurement system VIC-3D (video image correlate, 3D) to observe the evolution of strains and crack propagation. In the test results, the average fracture toughness of the specimen gradually decreased as the ratio of initial crack length to specimen width increased, and after the ratio reached 0.45, the fracture toughness value tended to be stable; the fracture toughness tests could only be considered valid if the thickness of the specimen was greater than or equal to 25 mm. Compared to large-thickness specimens, small-thickness specimens had a lower degree of brittleness; microscopic failure modes were classified as ground-tissue cracking and fiber damage. During the test, the tension-strain field was always concentrated at the tip of the extending crack. When the cracks extended to equal length, the specimens were in the same state, indicating that crack extension followed a self-consistent pattern. Furthermore, the compliance calibration method was used to develop the equation for the function of crack length-compliance.

Microscopic cracking behaviors of rocks under uniaxial compression with microscopic multiphase heterogeneity by deep learning

Cracking behaviors of rocks significantly affect the safety and stability of the explorations of underground space and deep resources. To understand deeply the microscopic cracking process and mechanical property of rocks, X-ray micro-computed tomography (X-μCT) is applied to capture the rock microstructures. The digital color difference UNet (DCD-UNet)-based deep learning algorithm with 3D reconstruction is proposed to reconstruct the multiphase heterogeneity microstructure models of rocks. The microscopic cracking and mechanical properties are studied based on the proposed microstructure-based peridynamic model. Results show that the DCD-UNet algorithm is more effective to recognize and to represent the microscopic multiphase heterogeneity of rocks. As damage characteristic index of multiphase rocks increases, transgranular cracks in the same grain phase, transgranular and intergranular cracks of pore-grain phase, intergranular and secondary transgranular cracks and transgranular crack between different grains propagate. The ultimate microscopic failure modes of rocks are mainly controlled by the transgranular cracks-based T1-shear, T3-shear, T1-tension, T2-tension and T3-tension failures, and the intergranular cracks-based T1-tension, T1-shear and T3-shear failures under uniaxial compression.

Fatigue of an Aluminum Foam Sandwich Formed by Powder Metallurgy.

In this paper, an aluminum foam sandwich (AFS) was prepared by the rolling composite-powder metallurgy method, and its fatigue properties were studied. It was compared with an AFS made by the adhesive method to study its fatigue properties more directly. In this experiment, the fatigue performance was investigated by studying the microscopic interface, fatigue life, deflection curve, and failure mode. The results show that the fatigue life of an AFS with the rolling composite-powder metallurgy method is much longer than that with the adhesive method. The failure mode of an AFS made by the rolling composite-powder metallurgy method is shear failure, and that of an AFS made by the adhesive method is shear failure and interface debonding. An AFS with the rolling composite-powder metallurgy method has better fatigue properties. This paper also explored the fatigue damage model using the fatigue modulus method, and the polynomial fitting method has a higher fitting degree.

Microscopic deformation and failure modes of high‐functionality epoxy resins from bond breaking molecular dynamics simulations and experimental investigation

AbstractThe material behavior of the API‐60 epoxy resin was investigated using atomistic molecular dynamics simulations. In this study, a hybrid, reactive force field (M‐GAFF2), which combined Morse and harmonic bond potentials from GAFF2, was developed to understand the elastic/plastic response of the cured epoxy polymer. We found that elastic deformation was caused by epoxy network rearrangement at low strain while plastic deformation was observed by stretching and breaking covalent bonds after the yield point. Ab initio calculations at the CASPT2(2,2)/6‐311+G** level of theory were performed to compute parameters for M‐GAFF2. These are necessary for breaking covalent bonds in the epoxy network backbone when they are deformed. The M‐GAFF2 was effective in revealing the ductile‐like deformation behavior of the crosslinked epoxy polymer at the microscopic level, including elastic/plastic deformations and progressive failure. The effect of testing temperature at 300 and 400 K and degree of cure (DoC) on tensile properties such as Young's modulus, ultimate strength, and failure strain was evaluated to verify the feasibility of the M‐GAFF2 in different thermodynamic constraints. This new approach is easy to use for nonequilibrium MD simulations and computationally efficient for studying the microscopic deformation and failure behavior of thermosetting polymers at the atomistic scale.

Process parameter–mechanical property relationships and influence mechanism of advanced CFF/PEEK thermoplastic composites

AbstractProcess parameters, as the core factor to control the forming quality of composites, are crucial for the processing of composites. In this paper, plain‐woven carbon fiber fabric reinforced polyether ether ketone (CFF/PEEK) thermoplastic composites are fabricated by hot‐pressing process. Influence of the critical process parameters, including molding temperature, molding pressure and holding time, on mechanical properties of CFF/PEEK composites are investigated. The tensile, flexural and short‐beam shear properties of CFF/PEEK composites fabricated by different process parameters were tested as evaluation indicators to achieve the optimization of process parameters. Most importantly, to deeply and systematically reveal the influence mechanism of process parameters, the high‐temperature contact angle of CFF‐PEEK and the viscosity‐temperature characteristics of PEEK were observed and analyzed. In addition, the forming quality and microscopic failure modes of CFF/PEEK composites were observed by optical, metallographic and scanning electron microscopy, respectively. This work provides a research idea and technical route for studying the process parameters of fiber reinforced polymer composites and revealing their influence mechanism on mechanical properties.

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.

Experimental study of bonded, bolted, and hybrid braided CFRP joints with different stacking sequences and lapping patterns

Tensile tests of bonded, bolted, and hybrid braided CFRP joints were conducted using an MTS tester. Twenty-four groups of specimens were designed with four stacking sequences and two lapping patterns. The strain data around the bolt holes were recorded using a dynamic acquisition device. The macroscopic and microscopic failure modes obtained by SEM were analysed. The strength, stiffness, and bolt load distributions with different variables were investigated to determine the mechanical properties of the CFRP joints. The results show that the load–displacement curves for S1, S2, and S4 are approximately linear before fracture; the curve for S3 is nonlinear. Bolts 1 and 2 near the loading side have large strain values; bolts 3 and 4 have smaller strain values. The failure modes include adhesive debonding, fibre fracture, net-section failure, tearing failure, and bolt fracture. Hybrid joints exhibit a combination of failure modes with single joint failure. The SLJ exhibits better strength and stiffness than the SSBJ. The strength of bonded joints with high-performance adhesives can be greater than that of hybrid joints, and is related to the stacking sequence and lapping pattern. Bolt load distribution is a dynamic process. With addition of an adhesive layer, there is greater balance between the four bolts, which can effectively reduce fluctuation of the load distribution.

Topological study about failure behavior and energy absorption of honeycomb structures under various strain rates

High-speed impact threats and terrorist actions on the battlefield require the development of more effective protective materials and structures, and various protective structure is designed according their energy-absorbing characteristics. In this research, the deformation behavior, microscopic failure modes and energy absorption characteristics of re-entrant hexagonal structure, regular hexagonal structure and regular quadrilateral structure are studied under different strain rates impact. The re-entrant hexagonal structure forms a “X”-shaped deformation zone, the regular quadrilateral and regular hexagonal structure form an “I”-shaped deformation zone. The microscopic appearance of the section is a mixed fracture form. The effects of the topological shape, cell angle, and cell height on the impact behavior of the structure were evaluated. When the cell height is fixed and the cell angle is changed, the energy absorption of the structure increase and then decrease as the relative density increase. The mechanical properties of the structure are optimal when the relative density is about 18.6% and the cell angle is 22.5°. When the cell angle is fixed and the cell height is changed, as the relative density increases, the energy absorption of the structure gradually increases. The regular quadrilateral structure and the re-entrant hexagonal structure experienced clear strain rate effects under dynamic impact conditions; the regular hexagonal structure did not exhibit obvious strain rate effects. The results presented herein provide a basis for further rational design and selection of shock-resistant protective structures that perform well in high-speed impact environments.

Research on Macro-Micro Failure Mechanism of Interbedded Sandstone in the Deep of Lannigou Gold Mine

Abstract The surrounding rock of the Lannigou gold mine roadway is mainly sandstone, most of which contains thin interlayers of argillaceous and carbonaceous minerals. To explore the deformation and failure mechanism of interbedded sandstones under static load caused by surrounding rock stress, the macroscopic failure modes, macroscopic physical properties, and microscopic fracture surface characteristics of sandstones were studied from different inclination angles. Triaxial compression tests and scanning electron microscopy tests were conducted. The results show that the peak strength of the specimen changes in a “spoon”-shaped pattern with increasing inclination. With increasing confining pressure, the peak strength, peak strain, residual strength, and elastic modulus of specimens increase, which damages interlayers. The macroscopic failure mode is mainly affected by the interlayer without confining pressure, and both tensile-shear failure (0° and 90°) and composite failure (30° and 60°) occur. Under higher confining pressure, the effect of the interlayer is smaller, and the specimen shows shear-tensile failure (0°, 30°, 60°, and 90°). Furthermore, the propagation mode of microcracks and the microscopic failure mode of mineral crystals differ for different macroscopic failure modes. In this study, the microscopic mechanism of macroscopic failure of interbedded sandstone was identified, which is significant for practical engineering guidance.

Study on in-plane compression properties and numerical modeling of three dimensional five-directional braided composites

Delamination phenomenon often occurs on traditional laminated composites while 3D five-directional braided composites (3D5dBC) could avoid this defect in practical application. In this work, the mechanical properties and failure mechanisms of 3D5dBC with different braiding angles are investigated at room temperature under transversal and longitudinal compression via experiments and finite element analysis (FEA) simultaneously. Meanwhile, the compression properties at elevated temperature are also researched. It is found that mechanical properties of composites under longitudinal compression are superior to those under transversal compression. Composite with small braiding angle possesses higher strength and modulus comparing to those of large braiding composite. Moreover, at room temperature, 3D5dBC exhibits 45º shear crack failure mode under transversal compression originated from matrix cracking, fiber failure along Z-direction obtained from FEA, and interfacial debonding; Under longitudinal compression, the structure shows shear expansion failure feature due to thorough fracture of axial yarns along L-direction, braiding yarns fracture along L-, T- and Z-direction, and matrix cracks obtained from FEA. With temperature increasing, interfacial debonding of fiber/matrix gets obvious. The microscopic failure modes and process at room temperature are well explained by FEA method.

Mechanical characteristics, constitutive models and fracture behaviors of short basalt fiber reinforced thermoplastic composites under varying strain rates

Mechanical properties and fracture behaviors of polypropylene (PP) and short basalt fiber reinforced polypropylene (BFRPP) composites were studied at varying strain rates ranging from 0.001 s−1 to 400 s−1. Full-field strain analyses were performed via Digital Image Correlation (DIC) methods to evaluate the entire progressive failure process. The strain localization phenomenon caused by the strong toughness of PP matrix is found. Based on Halpin-Tsai model, the tensile strength prediction model of BFRPP composites was established, and the tensile strength under different fiber content and strain rate was predicted. Based on Cowper-Symonds model, a one-dimensional phenomenological constitutive model related to the strain rate of BFRPP composites was established by introducing strain rate related functions. It reveals the strain rate dependent behavior of the stress-strain curve of BFRPP composites. Results show that the tensile strength of PP and BFRPP has strongly positive strain rate sensitivity. Microscopic failure modes of BFRPP composites mainly include matrix cracking, interfacial debonding, fibers pulled out and fibers breakage. As the strain rate increases, the fracture behavior of PP matrix in BFRPP composites changes from ductile to brittle, however basalt fiber is always brittle fracture from relatively flat transverse fracture to oblique fracture with an angle.

Strain evolution of L-shaped precast columns spliced by laminated metal plates, part I

ABSTRACT Mechanical joints for both irregular steel-concrete composite precast frames and reinforced concrete precast frames were developed to replace conventional cast-in-place concretes in the previous study of the authors. Novel connections modified from conventional steel joints having fully restrained moment connections offer rapid and facile erections. This study aims to numerically identify microscopic strain evolution and failure modes of the laminated metal plates splicing L-shaped precast composite columns. Numerical parameters including material properties of structural components of mechanical joint were implemented to determine damages and strength degradations with associated fractures. Comprehensive numerical results agreed well with test data, which explored structural behavior and contribution of each structural element of mechanical joint to flexural capacity. Strain evolution showed that degradation of the mechanical joints was retarded by interior bolts to a level similar to monolithic columns, resulting in contribution of mechanical joint to flexural capacity of columns. Influence of axial loads on the behavior of columns with mechanical joints was also evaluated to provide design recommendations.