Microscopic Failure Research Articles

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.

Successful treatment with intense immunosuppressive therapy in an initially 100% crescentic lesion of anti-GBM nephritis

Anti-glomerular basement membrane (anti-GBM) nephritis is uncommon glomerular disease caused by autoantibodies targeting the capillary beds of the kidney. The clinical presentation of the disease is a variable nephritic syndrome, rapidly progressing to glomerulonephritis. Treatment outcomes are dependent on predictors at first diagnosis. We presented a case of 58-year-old man who did not have underlying disease presented with marked abdominal distension and acute kidney injury. He had no evidence of chronic renal disease before admission however, laboratory test showed microscopic haematuria (RBC 30-50 per high-powered field), proteinuria (2.9 g/d), and renal failure (serum creatinine 610 µmol/L) compatible with rapidly progressive glomerulonephritis; hence, a renal biopsy was conducted. The pathology showed 100% crescentic glomerulonephritis with IgG deposits in a linear pattern at the GBM. The initial serum anti-GBM titre was 105.59 RU/mL. This patient had poor renal prognosis factors for treatment response. After a discussion regarding treatment option with the patient, we decided to give intensive immunosuppressive therapy and plasmapheresis due to his good baseline functional status. The patient achieved partial remission and is not dialysis dependent. In conclusion, despite a poor renal prognosis with 100% crescents and serum creatinine ≥ 600 µmol/L, the treated patient had a good survival status and did not become dialysis-dependent. However, immunosuppressive treatment should be performed along with careful monitoring for infection to avoid infection-related morbidity and mortality.

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.

Ductile Fracture in Plane Stress

Abstract A micromechanics-based ductile fracture initiation theory is developed and applied for high-throughput assessment of ductile failure in plane stress. A key concept is that of inhomogeneous yielding such that microscopic failure occurs in bands with the driving force being a combination of band-resolved normal and shear tractions. The new criterion is similar to the phenomenological Mohr–Coulomb model, but the sensitivity of fracture initiation to the third stress invariant constitutes an emergent outcome of the formulation. Salient features of a fracture locus in plane stress are parametrically analyzed. In particular, it is shown that a finite shear ductility cannot be rationalized based on an isotropic theory that proceeds from first principles. Thus, the isotropic formulation is supplemented with an anisotropic model accounting for void rotation and shape change to complete the prediction of a fracture locus and compare with experiments. A wide body of experimental data from the literature is explored, and a simple procedure for calibrating the theory is outlined. Comparisons with experiments are discussed in some detail.

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.

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.

Effect of Matrix Resin Characteristics on the Fracture Behavior for Plain-Woven Carbon Fiber Reinforced Plastics fabricated by Vacuum Assisted Resin Transfer Molding

Acoustic emission (AE) measurement has been used to investigate microscopic failures and damage progress in carbon fiber reinforced plastics (CFRP). In this work, AE measurements were conducted on plain-woven CFRPs fabricated by vacuum-assisted resin transfer molding process and neat epoxy resins during tensile tests to evaluate the failure modes. Four types of epoxy resins were used as matrix resins to investigate the effects of matrix resin characteristics on the microscopic damage behavior and the mechanical properties of CFRP. As a result, the mechanical properties of on-axis CFRP were comparable regardless of matrix resins. On the other hand, in the case of off-axis CFRP, such as 45° direction, the tensile strength was remarkably lower with a brittle matrix resin. From the AE measurement results, the number of cumulative hits for CFRP was much larger than that for neat resins due to the occurrence of fiber/matrix interfacial cracks. Also, AE waves for CFRP had a longer duration than that for neat resin, and the relationship between the degree of duration and fracture mode was clarified.

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.

Progressive deformation and pore network attributes of sandstone at in-situ stress states using computed tomography

Understanding the mechanism of microscopic failure of sandstone is exceptionally crucial for various engineering projects. The current study investigates the real-time high-resolution micro-computed tomography (m-CT) based microscopic failure attributes of sandstone under unconfined axial compression. Specimens of sandstone were analysed under varying loading conditions (zero load, 25%, 50%, 75% of the peak loads, and at peak failure loads) for analysing their effects on various petrophysical properties such as total pores, connected pores, non-connected pores, development of microfractures, and different pore network attributes such as pore radius, pore-volume, pore coordination number (CN), throat radius and throat channel length. Such an investigation enables us to explore the evolution of failure within sandstone under in-situ compression. The results showed the appearance of macrofractures and an increasing pattern of various pore-network attributes in the sandstone after 50% of the peak load. However, the non-connected pores gradually decreased with the rising loading stages. Finally, a negative correlation was obtained between tortuosity of sandsone with effective porosity and coordination number.

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.

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.

The effect of strain rate on compressive behavior and failure mechanism of CMDB propellant

The compressive mechanical behavior of composite modified double base (CMDB) propellant was investigated across a wide scope of strain rates ranging from 10−3 s−1 to 4210 s−1 at room temperature, by applying a conventional universal testing machine and a split Hopkinson tension bar (SHPB), respectively. The derived stress-strain curves at different strain rates show a strong rate dependence, indicated that yield stress, ultimate stress and strain energy density of CMDB propellant all increase with strain rate by following a power law function, while the amplification of increase are different. The deformation and damage modes of CMDB propellant has changed from a typical ductile manner (cracking along the axial direction) to a brittle manner (maximum shear failure) with increasing of strain rate. Scanning electron microscopy (SEM) was employed to explore the microscopic failure characteristics of CMDB propellant. Under quasi-static loading, the nearly parallel micro-cracks propagating along the axial direction and the debonding of RDX particle without particle crushing can be observed. While under dynamic loading, the micro-crack is 45° angle to the axial direction, and multiple cracking modes of RDX particles appeared. Finally, the correlation between strain energy density and failure mechanisms of CMDB propellant was revealed by developing four characteristic failure modes. The findings of this study is very important to evaluate the structural integrity of CMDB propellant.

Experimental Study on the Effect of Brittleness on the Dynamic Mechanical Behaviors of the Coal Measures Sandstone

As one of the most crucial mechanical parameters of the rock materials, the effect of brittleness on the deformation and failure is of great practical significance for geotechnical construction and disaster prevention and mitigation. In this paper, the deformation and failure behaviors of the different brittle samples under dynamic loading were investigated using a split Hopkinson pressure bar (SHPB) experimental system. Besides, scanning electron microscopy (SEM) was also employed to study the relationship between the microscopic failures and rock brittleness and strain rate effects. The results revealed that the brittleness indexes BI3 and BI5 of the samples under uniaxial compression follow a linearly decreasing trend affected by the temperature changes, while the brittleness of the sample shows an increasing trend with the increase of strain rate under the dynamic loading. Also, the decline in the brittleness leads to an increase in the prepeak yield deformation phase of the sample under dynamic loading; after the peak point, the sample failure mode transitions from type I to type II with self-sustaining failure. Moreover, it was found that the dynamic strength increase factor presents a negative correlation with the sample brittleness. Finally, the macroscopic failure mode of the sample changes from split failure with multiple cracks to shear failure with few cracks due to the effect of decreasing brittleness. The failure surface of the sample gradually becomes smooth with the increase of brittleness, which manifests as a decrease in microcracks, and the gradual increase of the strain rate makes the failure surface rough, accompanied by an increase in microcracks.

The effect of secondary sawtooth height on shear properties of rock joints with rigid contact

A variety of joints of various shapes are widely developed in rock masses. An accurate account of the behavior of a joint rock mass requires a realistic model for the tribological behavior of joints, which requires further study to understand the influence of secondary asperities on shear strength and the shear failure mechanism of joints. In view of the current lack of research on the effects of secondary asperities, a test scheme including first-order and second-order sawteeth of different heights was developed. By means of a physical model test and numerical model test under different constant normal stress conditions, the macroscopic mechanical properties and the microscopic deformation failure process affected by secondary sawtooth of joint are revealed, respectively. The results of the direct shear tests under different normal stresses on these sawtooth profiles demonstrate that the height of the second-order asperity has a certain effect on the shear strength. When the secondary sawtooth height is less than half the height of the first-order sawtooth, the shear strength increases with secondary sawtooth height. Compared with the joint surface of the same sawtooth height, the presence of a secondary sawtooth causes local stress concentration of the joint under the direct shear test, resulting in different microcrack production, expansion sites, and numbers of microcracks. We believe that this study is useful for understanding the shear characteristics of saw-toothed joints.

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.

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.

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.

Effect of sand grain size and fibre size on macro–micro interfacial bond behaviour of steel fibres and UHPC mortars

In this paper, the macro–micro interfacial bond behaviour of steel fibres embedded in ultra-high-performance concrete (UHPC) matrix was investigated using a single-fibre pull-out test with the parameters of fibre size, maximum sand grain size and fibre type. Test results showed that the maximum sand grain size (1·18 and 2·36 mm) had a slight effect on the pull-out responses while increasing the maximum sand grain size tended to change the microscopic interfacial failure mode according to observations made using scanning electron microscopy (SEM). Increasing the fibre size led to an increase in the maximum pull-out load, average bond strength and pull-out work but a decrease in the equivalent bond strength. The percentage of end hook contribution to the total pull-out energy ranged from 54·0 to 60·3%, indicating a significant mechanical anchorage contribution. A new evaluation index on the fibre–matrix interfacial bond behaviour was firstly proposed using the concept of the contribution of unit volume from the perspective of structural demands. It was found that, according to the proposed index, hooked-end fibres with a smaller diameter would be a better choice for structural applications.