Microscopic Failure Mechanisms Research Articles

Effect of thermal and hydrothermal aging on the crashworthiness of carbon fiber reinforced plastic composite tubes

Thin-walled structures constructed of fiber reinforced composites are increasingly being employed in engineering practice. However, there is limited investigation into their residual properties after high temperature and hygrothermal aging. The aim of this experimental investigation is to study how moisture absorption and high temperatures affect the mechanical characteristics of fiber reinforced plastic (FRP) composite tubes. A set of accelerated thermal and hygrothermal aging experiments are designed to imitate the service conditions of carbon fiber reinforced plastic (CFRP) structure in harsh environment. Based on the glass transition temperature obtained by dynamic mechanical analysis (DMA), the specimens are divided into two groups for various temperatures (25°C, 70°C, 100°C, 160°C) and hydrothermal aging (25°C, 70°C). The specimens of hydrothermal aging group are immersed for about 700 h to monitor the water absorption of the specimens in hygrothermal environments with different aging time. Aged composite tubes subjected to quasi-static axial compression are tested to identify compressive strength, failure modes and structural degradation process. The scanning electron microscope (SEM) is used to explore the microscopic failure mechanism of CFRP tubes. As temperature and moisture absorption rate increase, the crashworthiness characteristics decrease drastically; and the failure modes differ substantially with three different failure types identified. The crashworthiness of CFRP has a nonlinear relationship with temperature, but it is related to the glass transition temperature of the matrix. Furthermore, moisture absorption can be divided into two stages and temperature affects the moisture absorption of CFRP. Microscopically, the morphology and bonding conditions between fiber and resin is found to change significantly on the microscale as a result of various temperatures and hydrothermal aging.

Effects of Accumulated Damage on the Dynamic Properties of Coal Measures Sandstone

The coupling effect of accumulated damage and impact load substantially affects the integrity of the surrounding rock structure in deep coal mining engineering, which has inhibited safe and effective coal mining. Therefore, dynamic compression tests were performed on coal measures sandstone specimens with accumulated damage using the SHPB device. The effects of a high strain rate and accumulated damage on the sandstone’s mechanical behavior and damage evolution were investigated. The results reveal that accumulated damage has a considerable impact on specimen stress–strain curves and lowers dynamic compressive strength and deformation modulus substantially. The sandstone failure mode looks to be shear failure from a macroscopic perspective, while it appears to be intergranular fracture between the mineral particles from a microscopic perspective. The macroscopic and microscopic failure mechanisms of the sandstone specimens likewise conformed to the energy absorption law. The accumulated damage factor and the accumulated damage correction coefficient were presented in order to construct a statistical damage constitutive model of rocks based on the Weibull distribution. This model provides a decent description of the effects of accumulated damage and the strain rate on sandstone’s mechanical behavior, with parameters that are all of evident physical significance.

Research on the mechanical property and failure mechanism of the dieless clinched joints of various aluminum alloys

This paper dealt with the mechanical property and failure mechanism of the dieless clinched joints of AL5052, AL5182 and AL6061 aluminum alloys. The cross-tensile test and lap-shear test were executed to assess the mechanical properties of various aluminum joints. Detailed fracture surface analyses were also performed to reveal the failure mechanisms of various aluminum alloy joints in different failure tests from both macro and micro perspectives. The forming curves, key geometrical parameters, static loads, absorbed energy, and fracture morphology of various aluminum alloy joints were obtained and analyzed. From the results, the AL6061 joint exhibits the most superior mechanical properties, i.e., the optimum static load and energy absorption, whereas the AL5182 joint exhibits the worst mechanical properties. Besides, although the macroscopic failure mode of various aluminum joints is the same, i.e., neck fracture mode, the joint microscopic failure mechanism varies with the types of sheet material and failure test.

Comparative study on modes I and II fracture characteristics of bituminous coal using asymmetric semi-circular bend specimen

Modes I and II fractures of coal play an important role in coal mining and have a serious impact on coal production and the control of mining hazards. Therefore, asymmetric semi-circular bend (ASCB) specimens with different notch depths were used to carry out modes I and II loading tests. The differences in the modes I and II fracture characteristics of coal specimens were studied. The internal damage characteristics, surface displacement characteristics, failure mode and microscopic damage mechanism of bituminous coal specimens were analysed by acoustic emission (AE) and digital image correlation (DIC) methods. The results show that the fracture toughness of coal under mode II loading is 1.02 times greater than that under mode I loading. The crack initiates along the centreline of the specimen and propagates in a roughly linear form under mode I loading. Under mode II loading, the curved crack mainly initiates from the upper left of the ASCB specimens. The notch length has a significant impact on peak load and fracture toughness, but it has a slight influence on crack propagation path. Larger notch depths are more suitable for modes I and II ASCB fracture tests. In this study, a method based on AE parameters is proposed to determine the real-time microscopic failure mechanism of coal. Combined with AE and DIC methods, it is revealed that the coal specimens experience tensile fracture (mode I) initiation under both mode I and mode II loading, and no specimen can be initiated with mode II shear fracture under pure mode II loading. Mode II loading and mode II fracture are not equivalent.

Intermittent failure mechanism and stabilization of microscale electrical contact

The stability and lifetime of electrical contact pose a major challenge to the performance of microelectro-mechanical systems (MEMS), such as MEMS switches. The microscopic failure mechanism of electrical contact still remains largely unclear. Here conductive atomic force microscopy with hot switching mode was adopted to simulate the asperity-level contact condition in a MEMS switch. Strong variation and fluctuation of current and adhesion force were observed during 10,000 repetitive cycles, exhibiting an “intermittent failure” characteristic. This fluctuation of electrical contact properties was attributed to insulative carbonaceous contaminants repetitively formed and removed at the contact spot, corresponding to degradation and reestablishment of electrical contact. When contaminant film was formed, the contact interface became “metal/carbonaceous adsorbates/metal” instead of direct metal/metal contact, leading to degradation of the electrical contact state. Furthermore, a system of iridium/graphene on ruthenium (Ir/GrRu) was proposed to avoid direct metal/metal contact, which stabilized the current fluctuation and decreased interfacial adhesion significantly. The existence of graphene enabled less adsorption of carbonaceous contaminants in ambient air and enhanced mechanical protection against the repetitive hot switching actions. This work opens an avenue for design and fabrication of microscale electrical contact system, especially by utilizing two-dimensional materials.

Characterization of crack resistance mechanism of fiber modified emulsified asphalt cold recycling mixture based on acoustic emission parameters

In the present study, to solve the problem of poor early strength and crack resistance of emulsified asphalt cold recycled mixture, the enhancement and fracture mechanism of fiber on emulsified asphalt cold recycled mixture were studied from multi-scale. The effect of fiber on the macroscopic mechanical properties of emulsified asphalt cold recycled mixture was studied by early and molding strength, dry-wet splitting ratio test and fracture energy index of semi-circular bending specimen. Optimum fiber types and dosages for emulsified asphalt cold recycled mixes have been selected. The relationship between crack evolution and acoustic emission parameters activity of fiber emulsified asphalt cold recycled mixture is studied, which reflects the microscopic failure mechanism of the material. The microstructure and morphology of fibers in the mixture were observed by scanning electron microscopy (SEM). The results show that both fibers can improve the strength and crack resistance of emulsified asphalt cold recycled mixture. Basalt fiber has better adsorption and anchoring effect on asphalt due to its high modulus and rough surface texture. The optimum dosage is 0.3 % of the mixture quality. Fracture characteristics of fiber emulsified asphalt cold recycled mixture are reflected by acoustic emission parameters. At the optimum fiber content, the acoustic emission parameters showed that the time from micro-cracks to macro-cracks in the fiber-emulsified asphalt cold recycled mixture was 122.22% longer than that in the control group. Basalt fiber plays the role of reinforcing crack resistance and anchorage in emulsified asphalt mixture, delaying the expansion of micro cracks.

Dynamic mechanical response and deformation behavior of a novel hierarchical cellular structure material – The thousands of eyes Bodhi

Natural cellular structures encourage researchers to seek new design inspirations for impact protection engineering materials. The thousands of eyes Bodhi (TEB) is a novel hierarchical cellular structure material, which includes the first order filled-cells, the second order novel-closed-cells, and the third order open-cells. In this study, dynamic compressive tests at different high strain rates in the longitudinal and transverse directions were conducted by a split Hopkinson pressure bar (SHPB), respectively. A high-speed camera was employed to capture the macroscopic deformation mechanisms and a scanning electron microscope (SEM) was employed to study the microscopic failure mechanisms. The dynamic mechanical response and strain rate effects of TEB samples in the transverse and longitudinal directions were investigated. The anisotropic behavior of TEB samples is observed under dynamic compression tests. The tests results show that the dynamic mechanical properties of TEB samples and dynamic deformation behavior of each order cellular structure exist a significant strain rate dependence. The TEB material dissipates the impact energy through the dynamic compression mechanisms of each order cellular structure which improves the energy absorption and impact resistance. Thus, it provides a bio-inspired template for the design of energy absorption and crashworthiness engineering materials.

Strain-Rate-Dependent mechanical behavior of a non-Equimolar CoCrFeMnNi high entropy alloy with a segmented coarse grain structure

The mechanical response of a Co11.3Cr20.4Fe22.6Mn21.8Ni23.9 (wt%) high entropy alloy with a segmented coarse grain structure is investigated using strain-rate-dependent uniaxial compression experiments. The composition is verified using ab-initio simulations by examining thermal stability through the reduction of Gibbs free energy relative to similar equi-molar alloys. The segmented grain structure, combining high-angle coarse-grains with low-angle sub-grains, provides good hardness (∼3 GPa) and strain-hardening (∼2600 MPa/ε). Yield strength shows strain-rate-dependent behavior with high strain-rate sensitivity. Here, the macroscopic deformation is correlated with the microscopic plastic failure mechanisms, facilitated by advanced in situ visualization and analysis. Results reveal that the formation of deformation bands occur before yield, while the interactions between microscopic plastic deformation mechanisms and the low-angle sub-grain boundaries significantly affect the apparent surface deformation features and mechanical properties.

Application of a geometrically nonlinear elastoplastic gradient-enhanced damage model with incremental potential to composite microstructures

The prediction of inelastic processes like plastic deformations and cracks within the microstructure of modern man-made materials by realistic, yet simple and efficient continuum models remains a major task in material modelling. For this purpose, gradient-extended standard dissipative solids represent one of the most promising model classes, which is also formulated and applied in this work to investigate microscopic failure mechanisms in three exemplary three-dimensional composite microstructures. The model combines geometrically nonlinear isotropic elastoplasticity with an isotropic damage model with gradient-extension. For the numerical treatment, a variational constitutive update algorithm based on the exponential map is applied. The model is used to provide insight into the microscopic failure of a brittle woven composite material, a particle-reinforced plastic and a carbon fiber reinforced composite. The influence of different microstructural and material parameters on the overall failure behavior is characterized. Adaptive meshing is used to enable a refined numerical resolution of the cracked regions.

High-Temperature Compressive Response of SiCp/6092Al Composites under a Wide Range of Strain Rates.

The high-temperature dynamic compressive properties of a 30 vol.% SiCp/6092Al composite, fabricated using powder metallurgy, were experimentally investigated using the split Hopkinson pressure bar system with an electric furnace. Three different ambient temperatures, namely, room temperature, 200 °C, and 350 °C, were adopted, and the dynamic tests of the composite specimens were conducted at strain rates ranging from 1500 to 4500 s−1. The experimental results showed that the flow stress of the composite was generally insensitive to strain rates at room temperature. However, the composite started exhibiting different strain-rate-dependent behaviors as the temperature increased, and the flow stress nonlinearly varied with increasing temperature. In addition, the microscopic images of the specimens showed that the microscopic failure mechanisms of the composite were greatly influenced by the ambient temperature and strain rate. Specifically, the percentage of failed particles decreased with rising temperature and the dominating failure mode of particles changed significantly as the strain rate increased.

Influence of graphene oxide content on the morphology and properties of carbon fiber/epoxy composites

AbstractIn this article, the influence of graphene oxide (GO) content on the morphology and properties of carbon fiber/epoxy (CF/EP) composites was investigated. To obtain CF/EP composites with uniform dispersion of GO, electromagnetic stirring and ultrasonic dispersion techniques were employed in the wet transfer process. Combined with a new molding process, CF/EP composites with different GO content were prepared. The test results showed that the bending strength of CF/EP composites could be effectively improved by adding GO, among which 0.5 wt% GO–CF/EP composite exhibited the best bending strength and modulus increased by 47.20% and 21.24%, up to 1158.9 MPa and 115.3 GPa. Likewise, the mode I interlaminar fracture toughness (GIC) of the composite at the beginning of crack and the maximum during the propagation increased by 28.13% and 21.57%, reached 327.5 and 656.6 J/m2, respectively. The distribution of GO in the composites and the fracture morphology of the composites were observed. Finally, according to the test results and microscopic morphology analysis, the enhancement and failure mechanism of GO in CF/EP composites were clarified. It was found that GO could improve the properties of CF/EP composites by enhancing the interface properties of CF/matrix, enhancing the strength of the matrix and forming a net‐like structure inside the matrix.

Fatigue Performance Analysis of GRE Composite Pipes by Conducting Tension-Tension Tests on the Rings Cut from the Pipe

Abstract The main purpose of this study is to analyze the fatigue behavior of an industrial (±55°)9 filament-wound glass reinforced epoxy pipe. Fatigue tests are conducted in accordance with ASTM D2992, Standard Practice for Obtaining Hydrostatic or Pressure Design Basis for Fiberglass (Glass-Fiber-Reinforced Thermosetting-Resin) Pipe and Fittings, but instead of the entire pipe, rings cut from the pipe are used in the tests. This standard proposes a frequency of 0.42 Hz and stress ratio of R = 0.05. Tension-tension fatigue tests are performed on the rings at different load levels from 60 to 90 % of the ultimate hoop strength of the pipe. After the tests, the maximum pressure and maximum hoop stress curves versus the number of cycles to fatigue failure are presented. A comprehensive discussion about the importance of substituting the ring test method with ASTM D2992 standard test methods to determine the long-term performance of the composite pipes in the future is presented. The microscopic fatigue failure mechanisms of the rings are also characterized and interpreted using a scanning electron microscope.

Effects of foliation on deformation and failure mechanism of silty slates

The objective of this study was to investigate the deformation and failure mechanism of silty slates loaded with an angle 0 and 90° between foliation and direction of uniaxial force. For this purpose, mineral composition and microstructure test, uniaxial compression test, acoustic emission (AE) monitoring and scanning electron microscope (SEM) were carried out on Silurian silty slates. Based on the results of tests to analyze the deformation and failure mechanism as well as the macroscopic failure modes, microscopic rupture surface characteristics and acoustic emission characteristics of silty slates under different test conditions. Tests results showed that the silty slates are mainly composed of clay minerals (chlorite and illite) and brittle minerals (quartz and feldspar) and in a layered structure. Under the two angles, silty slates specimens exhibited large differences in the deformation and damage process, macroscopic failure modes, microscopic rupture surface characteristics, deformation and failure mechanism. Besides, there are large differences in the relative parameters of AE event. These results reveal that there is a certain relationship between mechanisms of microscopic fractures and macro failure of silty slates.

High-strain-rate mechanical response of HTPE propellant under SHPB impact loading

The high-strain-rate dynamic response of a hydroxyl-terminated polyether (HTPE) propellant during impact loading is essential for assessing the structural reliability and impact safety of HTPE propellant. In this study, a modified split Hopkinson pressure bar (SHPB) apparatus has been developed to research the stress–strain behavior of the HTPE propellant over strain rates ranging from 470 to 5910 s−1 at room temperature, and the validity of the SHPB test is analyzed in detail. Meanwhile, the evolution of deformation to failure of the HTPE propellant was recorded by a high-speed digital camera synchronized with the SHPB test, which revealed the correlation between mechanical response and failure mode. Scanning electron microscopy was applied to investigate the microscopic failure mechanism of the post-test HTPE propellant, which indicated two characteristic failure modes: cracking propagates along the (1) debonding surface and (2) transgranular damage path. Finally, based on the stress–strain plots derived from the SHPB tests, the ultimate stress, strain energy density, and adiabatic temperature-rise effect of the HTPE propellant were seen to show strong strain rate dependence by following an empirical power law function.

Thermal shock and thermal fatigue resistance of Al2O3/(W, Ti)C/TiN/Mo/Ni multidimensional graded ceramics

AbstractAl2O3/(W, Ti)C/TiN/Mo/Ni multidimensional graded ceramics and homogeneous reference ceramic were prepared by two step hot press sintering. The thermal shock and thermal fatigue resistance of the multidimensional graded ceramics were tested using the water quenching method. Scanning electron microscopy (SEM) and optical microscope were used to investigate microscopic failure mechanism of ceramics. The results showed that the retained flexural strength of two‐dimensional and one‐dimensional graded ceramics was almost same, but higher than that of the homogeneous ceramic. The crack growth (∆c) of homogeneous ceramic increased rapidly, while that of two‐dimensional graded ceramics is the lowest. Hence, thermal fatigue resistance of the two‐dimensional graded ceramics was highest. The residual compressive stress in the first layer induced by the optimal graded structure played an important role. In addition, the increasing toughness on the crack propagation path by adding different amounts of metals was also a contributing factor.

Microscopic Failure Mechanism Analysis of Rock Under Dynamic Brazilian Test Based on Acoustic Emission and Moment Tensor Simulation

The dynamic tensile failure of rock is a main failure mode in deep underground engineering projects. The microscopic failure mechanism analysis of this failure mode plays a key role in dynamic disaster warning. Moment tensor inversion is a very well-known method used to analyze failure mechanisms. However, an acoustic emission (AE) event cannot be accurately distinguished in rock dynamic experiments at the laboratory scale, because there are hundreds of AE events generated within a few hundred microseconds in one dynamic test. Therefore, moment tensor analysis is rarely applied in rock dynamic tests with laboratory scale. In this paper, AE and moment tensor simulations with the discrete element method (DEM) are introduced to analyze the microscopic failure mechanism of rock under a dynamic Brazilian test. Comparing the simulation results of AE and moment tensor analysis with the simulation results of micro-crack with DEM, the moment tensor discriminant method can obtain the mechanical mechanism and energy level of micro-cracks. Furthermore, R, which is the ratio of isotropic and deviatoric components of the moment tensor, is used to analyze the AE source mechanism. The implosion, shear, and tensile of the AE source mechanism can better explain the evolution process of main axial crack and the shear failure zones of the Brazilian disc specimen under dynamic tensile simulation. These findings contribute to a better understanding of the microscopic failure mechanism of rock under a dynamic tensile test than the statistical types of micro-cracks based on break bonds with DEM.

Effects of porosity on tensile mechanical properties of porous FeAl intermetallics

Abstract Uniaxial tensile tests and scanning electron microscopy (SEM) experiments were carried out on the porous FeAl intermetallics (porosities of 41.1%, 44.2% and 49.3%, pore size of 15–30 μm) prepared by our research group to study the macroscopic mechanical properties and microscopic failure mechanism. The results show that the tensile σ–ɛ curves of the porous FeAl with different porosities can be divided into four stages: elasticity, yielding, strengthening and failure, without necking phenomenon. The elastic modulus, ultimate strength and elongation decrease with the increase of porosity and the elongation is much lower than 5%. A macroscopic brittle fracture appears, and the microscopic fracture mechanism is mainly intergranular fracture, depending on the Al content in the dense FeAl intermetallics. In addition, the stochastic porous model (SPM) with random pore structure size and distribution is established by designing a self-compiling generation program in FORTRAN language. Combined with the secondary development platform of finite element software ANSYS, the effective elastic moduli of the porous FeAl can be determined by elastic analysis of SPM and they are close to the experimental values, which can verify the validity of the established SPM for analyzing the elastic properties of the porous material.

Meso-Complexity Computer Simulation Investigation on Antiexplosion Performance of Double-Layer Foam Aluminum under Pore Grading

Foam aluminum is an energy-absorbing material with excellent performance. The interlayer composed of multiple layers of foam aluminum and steel plate has good antiexplosion ability. In order to explore the antiexplosion performance of double-layer foam aluminum under different porosity rankings and to reveal its microscopic deformation law and failure mechanism, three kinds of aluminum foams with a porosity of 80%, 85%, and 90% were selected to form six different structures. Based on the Voronoi algorithm, a three-dimensional foam aluminum generation algorithm with random pore size and random wall thickness was written by using the Python language and Fortran language. The three-dimensional mesoscopic model of double-layer closed-cell aluminum foam sandwich panel is established by using LS-DYNA and ABAQUS software. The explosion process was simulated, and the flow field movement of explosion shock wave of aluminum foam under different porosity rankings was analyzed. Two groups of aluminum foam were randomly selected for the explosion test and compared for the strain and compression. The test results are consistent with the simulation results, which verifies the correctness of the three-dimensional meso-model. The results show that when the porosity of the upper layer of aluminum foam is greater than that of the lower layer of aluminum foam, the sandwich structure of double-layer aluminum foam has a large compression and the bottom plate has a small displacement; it is not that the greater the compression amount of aluminum foam is, the better the antiexplosion and wave absorption ability is. When the aluminum foam reaches the ultimate load-bearing capacity, the aluminum foam transfers the load due to compaction, resulting in stress enhancement phenomena. Through the analysis of the compression amount, floor deformation, wave dissipation capacity, and energy ratio of aluminum foam, it is concluded that the antiexplosion wave absorption effect of the sandwich structure of aluminum foam with 80%/85% group is the best; the changes of porosity and cell wall are important factors affecting the energy absorption capacity of aluminum foam.

The Statistical Damage Constitutive Model of the Mechanical Properties of Alkali-Resistant Glass Fiber Reinforced Concrete

Alkali-resistant glass fiber reinforced concrete (AR-GFRC) has greatly improved in terms of tensile strength, toughness, durability, and reduction of cracking, which has been proven by testing. However, the constitutive relationship of fiber reinforced concrete under complicated stress represents a complex theoretical problem. In order to investigate the microscopic damage evolution and failure mechanism of AR-GFRC, the meso-statistical damage theory, microcontinuum theory, and composite material theory were considered, and uniaxial tensile tests of two types of AR-GFRC were conducted. A new damage variable expression of the AR-GFRC was proposed, and the stress-strain curve was redefined by considering the residual strength based on experimental fitting parameters and statistical parameters. A Weibull distribution was assumed and a statistical damage constitutive model was developed of the deformation process of the AR-GFRC while considering the residual strength effect; detailed calculation methods to determine the mechanical and statistical parameters of the concrete were developed. The validation results show that the theoretical stress-strain curve of the constitutive model is in good agreement with the experimental curve and the trend is consistent.

A multiscale analysis of instability-induced failure mechanisms in fiber-reinforced composite structures via alternative modeling approaches

Multiscale techniques have been widely shown to potentially overcome the limitation of homogenization schemes in representing the microscopic failure mechanisms in heterogeneous media as well as their influence on their structural response at the macroscopic level. Such techniques allow the use of fully detailed models to be avoided, thus resulting in a notable decrease in the overall computational cost at fixed numerical accuracy compared to the so-called direct numerical simulations. In the present work, two different multiscale modeling approaches are presented for the analysis of microstructural instability-induced failure in locally periodic fiber-reinforced composite materials subjected to general loading conditions involving large deformations. The first approach, which is of a semi-concurrent kind, consists in the “on-the-fly” derivation of the macroscopic constitutive response of the composite structure together with its microscopic stability properties through a two-way computational homogenization scheme. The latter one is a novel hybrid hierarchical/concurrent multiscale approach relying on a two-level domain decomposition scheme used in conjunction with a nonlinear homogenization scheme performed at the preprocessing stage. Both multiscale approaches have been suitably validated through comparisons with reference direct numerical simulations, by which the ability of the latter approach in capturing boundary layer effects has been demonstrated.