Failure Mechanisms Of Materials Research Articles

PRELIMINARY INVESTIGATION OF POWDER-MIXED MICROELECTRICAL DISCHARGE DRILLING OF CFRP COMPOSITE

This work investigates the fabrication of microholes in carbon-fiber-reinforced polymer (CFRP) composite by powder-mixed microelectrical discharge drilling (PM[Formula: see text]EDD) method. Initially, pilot experiments were carried out to understand the effect of powder concentration on the machining time and material removal mechanism. Then the full-factorial experimental analysis was performed to investigate the micromachining performance of CFRP composite. The input factors considered for the full-factorial analysis were powder concentration (2, 4, and 6 g/L), capacitance (100, 1000, and 10,000 [Formula: see text]F), and tool rotation (300, 400, and 500 rpm). The regression analysis was carried out to assess the significance of the chosen input factors. The surface morphology of the fabricated microhole was extensively studied to establish the material removal mechanism and failure mechanisms of the composite constituents.

Environmental Failure Behavior Analysis of 7085 High Strength Aluminum Alloy under High Temperature and High Humidity

High-strength aluminum alloys are exposed to more and more environmentally-induced cracking failure behaviors during service. However, due to the hard to detect nature of hydrogen, and the special working conditions, failure research has obvious hysteresis and complexity, and it is impossible to truly reflect the material failure phenomenon and mechanism. In this paper, 7085 high-strength aluminum alloy is selected as the research material to simulate and reproduce the environmental failure phenomenon of aircraft under extreme working conditions (temperature 70 °C, humidity 85%). The results proved that high-strength aluminum alloy has environmental cracking failure behavior under extreme working conditions. The failure mode that was determined was due to environment-induced hydrogen and hydrogen-induced cracking, which is the result of the combined action of hydrogen and stress. Meanwhile, we demonstrate that high-strength aluminum alloy’s environmental failure behavior in an environment of high temperature and high humidity is different from traditional stress corrosion cracking behavior.

Macro- and meso- failure mechanism analysis for shale-like brittle materials under uniaxial compression

Understanding the failure mechanisms of the widespread shale-like brittle materials is of great significance to engineering safety. In this research, the discrete element model which can reflect the real rock mechanical properties and failure mode of shale specimens with different bedding angles has been reconstructed. First, microcrack has been introduced to characterize the meso-damage. The relationship between the microscopic crack numbers in the material and the macroscopic stress has been revealed. In addition, the relative movement between different deformed blocks has been analyzed by introducing the residual velocity field. The control factor of material failure varies with the bedding angle (β) in the macro scale. It manifests from the bedding plane and the matrix synergistically control the failure mode of rocks to mainly the bedding plane and then back to the bedding plane and the matrix synergistically control the rock failure mode. The competitiveness of the shear and tensile damages in the failure mechanism varies with the bedding angle in the meso scale. When 30°<β<75°, Shear damage dominates the failure mechanism. The established numerical model is capable of predicting the mechanical behavior of brittle rock, which may have guiding merits to the application of shale-like brittle materials.

Elastic-plastic progressive damage model and low-velocity impact failure mechanism of composite materials

An elastic-plastic progressive damage model (PDM-3DEP) was proposed to study the mechanical response and damage development of composite laminates under low-velocity impact. The 3D Hashin failure criterion and an improved three-dimensional damage evolution model that considers strain component of ε33 and shear stain are implemented to describe the initiation and evolution of damage modes. The zero-thickness cohesive elements which follow bilinear traction-separation law are applied to simulate delamination. The numerical and experimental mechanical curves and damage distribution of composite laminate with a stacking sequence of 03o/45o/−45oS were compared to verify the proposed PDM-3DEP. The consistent results of the numerical and experimental data indicates that the proposed method can accurately describe damage mechanical behavior under low-velocity impact. Moreover, another type of composite laminate with a stacking sequence of 02o/902o/02o/902o/02oS is adopted for further verification. It is concluded that the proposed model can accurately analyze the mechanical responses of different composite laminates.

Shear Failure Mechanism and Numerical Simulation Analysis of Rock-like Materials with an Embedded Flaw

In this study, the failure characteristics of self-made rock with internal flaws under shear were studied and a numerical simulation analysis was carried out. Firstly, based on basic physical and mechanical tests, the shear strength characteristics of rocks with built-in 3D defects were summarized. PFC3D simulation software was used to model the samples with flaws, and the microscopic parameters were calibrated according to the test results. From the simulation results, it was found that the generation mode of microcracks from the flaw tip was different. The microcracks of forward shear and reverse shear were mainly generated from the horizontal direction, while the microcracks of lateral shear gradually increased from the upper and lower ends of the flaw in the opposite direction. When the peak shear strength was reached, the total number of cracks was the largest in lateral shear and the smallest in forward shear. When studying the particle velocity vector field, it was found that when reaching the peak shear strength, the particles on both sides of the prefabricated flaw appeared to be in vortex motion. When α = 45° and σn = 2 MPa, the failure mode of forward shear and lateral shear was shear-tensile-shear (S-T-S), and that of reverse shear and the intact specimen was shear-shear-shear (S-S-S). The lateral shear tensile effect was the most obvious and was mainly concentrated in the middle part of the sample.

Interactions between multiple rigid lamellae in a ductile metal matrix: Shear band magnification and attenuation in localization patterns

A ductile matrix material containing an arbitrary distribution of parallel and stiff lamellar (’rigid-line’) inclusions is considered, subject to a prestress state provided by a simple shear aligned parallel to the inclusion lines. Because the lamellae have negligible thickness, the simple shear prestress state remains uniform and its amount can be high enough to drive the matrix material on the verge of ellipticity loss. Close to this critical stage, a uniform remote Mode I perturbation realizes shear band formation, growth, interaction, thickening or thinning. This two-dimensional problem is solved through the derivation of specific boundary integral equations, holding for a nonlinear elastic matrix material uniformly prestressed; the related numerical treatment is specifically tailored to capture the stress singularity present at the inclusion tips. Results show how complex localized deformation patterns form, so explaining features related to the failure mechanisms of ductile materials reinforced with stiff and thin inclusions. In particular, the influence of the inclusion distribution on the shear bands pattern is disclosed. Conditions for the magnification (the attenuation) of the localized deformations are revealed, fostering the progress (the setback) of the failure process.

Initiation and Fracture Characteristics of Different Width Cracks of Concretes under Compressional Loading

A stress concentration at a crack tip may cause fracture initiation even under low-stress conditions. The maximum axial stress theory meets the challenges of explaining the fracture propagation of a non-closed fracture of cracked concretes under compressional loading. Uniaxial loading tests of single-crack concrete specimens were carried out and a numerical simulation of fracture propagation under uniaxial compression was performed. The radial shear stress criterion for a mode-II fracture is proposed to examine the stress intensity factor (SIF) of the pre-crack specimens under compressional loading. When the maximum radial shear stress at the crack tip is larger than the maximum axial tensile stress, and the maximum dimensionless SIFs can satisfy frθmax/fθmax &gt; 1 and frθmax/fθmax &gt; KIIC/KIC (fθmax=KIe/σyπa and frθmax=KIIe/σyπa are maximum dimensionless mode-I and mode-II SIFs, respectively), the crack will extend along the direction of the maximum radial shear stress. The influence of the single-crack angle and width on the mechanical properties of the specimens was examined. The experimental and numerical results indicate that the existence of cracks can considerably weaken the strength of the specimen. The distribution and width of the cracks had a significant effect on the specimen strength. The strength of the concrete specimen initially decreased and then increased with increasing fracture angle. The failure mechanism and rupture angle of pre-crack brittle material while considering crack width will be discovered.

Fatigue prediction of composite materials, based on serial/parallel mixing theory

In this work, a numerical procedure to obtain the fatigue performance of composite laminates is proposed. The formulation used is based on the Serial/Parallel Mixing Theory (SP-RoM) and a fatigue damage model. The SP-RoM acts as a constitutive law manager, obtaining the mechanical performance of the composite by solving the behaviour of each constituent material. The fatigue damage model modifies the constitutive performance of the composite constituents in order to consider the degradation of their mechanical properties due to cyclic loading.The proposed formulation is validated for two composite systems, a carbon/epoxy system and a glass/ polyester system. In addition, different composite configurations are numerically evaluated and compared with experimental data, in order to prove the capabilities of the formulation. All results prove that the numerical procedure develop is an excellent tool for obtaining the fatigue life and failure mechanisms of composite materials.

Poroelastic modeling of pore pressure development in granular pavement layers

ABSTRACT Moisture excess in granular pavement layers can decrease pavement performance leading to moisture-related distresses. The combination of trapped water and heavy traffic loads may substantially increase pore pressure and lead to shear failure if layer hydraulic conductivity is insufficient. Currently, most pavement analysis models are unable to predict pore pressure development, so the design and evaluation methods ignore this dynamic effect which may significantly underestimate the moisture damaging effect on pavements. This paper proposes the use of Biot’s poroelasticity theory for modelling fully saturated unbound materials. The proposed methodology – Poroelastodynamic Finite Integration Technique (PEFIT) – was used to analyze axisymmetric multilayered pavement structures consisting of elastic and poroelastic layers. The model was verified and validated using a well-known commercial finite element solver and field pore pressure data, respectively. A sensitivity analysis was conducted by varying critical pavement design parameters. The PEFIT analysis of a three-layer pavement system with a poroelastic base layer has shown that a decrease in base hydraulic conductivity significantly increases the maximum pore pressure, especially for high-speed loads. Results also indicate substantial pore pressure and water movement in the base layer, which may induce a potential shear failure mechanism for unbound materials.

Numerical analysis of oblique hypervelocity impact damage to space structural materials by ice particles in cryogenic environment

A phase-aware constitutive model is developed and integrated with the meshfree simulations to study the failure mechanisms of space structural materials under the oblique hypervelocity impact of ice. The proposed constitutive model covers material’s dynamic behavior in a wide range of temperature, pressure, and strain rates. The numerical analysis combines the Hot Optimal Transportation Meshfree method with the EigenErosion algorithm to explicitly account for large deformation, brittle and ductile failure, phase transition, and debris cloud formation in the target materials and ice projectile. Specifically, the surface erosion and crater formation in the target under different impact speeds and angles are characterized by the damage’s shapes, depth, and length. The predicted crater depths in the Al6061 targets are validated by the Cour-Palais empirical model. It is evident that the damage to the space structural material by ice particles results from the competition between fracture, plasticity, and phase transition in the materials. The impact angle and velocity determine the crater deformation and the potential thread of the ejected debris.

3D meso-scale investigation of ultra high performance fibre reinforced concrete (UHPFRC) using cohesive crack model and Weibull random field

Ultra high performance fibre reinforced concrete (UHPFRC) is composed of fibres, mortar, fibre-mortar interfaces, and pores randomly distributed in space, and thus has heterogeneous mechanical properties and complicated failure mechanisms at micro/meso-scales. This study aims to develop a highly effective 3D finite element modelling approach to simulate both the mortar heterogeneity and the complicated failure mechanisms in UHPFRC materials and structural members. In this approach, Weibull random fields are used to characterize random properties (such as moduli, strength and fracture energy) of the mortar caused by defects such as pores and microcracks; pre-inserted cohesive interface elements with softening constitutive laws are applied to model multi-cracking behaviour; specific probability density distributions are adopted to describe the random orientation of fibres in the mortar, and orientation-dependent bond-slip relations are developed to model fibre-mortar interactions. The fibres are rigidly embedded in the mortar so that non-conforming finite element meshes can be used conveniently. The modelling approach has been validated by extensive simulations of single fibre pull-out tests and direct tensile tests of UHPFRC specimens with thousands of fibres in agreement with experimental data. It is found that the modelling approach is highly efficient in capturing the fibre crack-bridging effect, simulating the damage and fracture initiation and evolution, and quantifying the load-carrying capacities with statistical analyses considering fibre orientation and mortar heterogeneity.

Research on the back analysis and failure mechanism of recycled concrete aggregate meso-parameters based on Box-Behnken Design response surface model

To obtain the mesophase material parameters of recycled concrete aggregate and analyze the material failure mechanism, based on the Box-Behnken Design, a non-linear material parameter back analysis model of recycled concrete is proposed, and the failure mechanism of recycled concrete materials is explored. Based on the indoor experiment, the numerical simulation experiment of 25 sets of inversion parameters was constructed with the help of Design-Expert. The response surface relationship between different response variables (inversion parameters) and independent variables (mortar elastic modulus, interface elastic modulus, mortar tensile strength, and interface tensile strength) was obtained, which realized the explicit expression of the implicit relationship between mesophase parameters and macroscopic mechanical properties. The mesophase parameters are obtained by inverse analysis, and the stress-strain curve is obtained by bringing it into the numerical model. The comparison with the test results shows that the peak stress error is only 2.8%, the peak strain error is close to 8%, and the fitting degree is good. Combined with the grey relational degree theory, the material failure mechanism is analyzed from the perspectives of the material area ratio of each phase and the mesophase material parameters: the effect of the mortar phase parameters on the stress-strain process is greater than that of the interface phase. The elastic modulus of the mortar has the greatest influence on the stress of each stage, and the tensile strength of the mortar has the greatest influence on the peak strain. This study provides a theoretical basis for the follow-up research on the mechanical properties of recycled concrete.

Axial-compression performance and finite element analysis of special-shaped tubular three-dimensional woven composites with different thicknesses and shapes

In this research, the self-reinforced structures tube preforms, which are the special-shaped tubular three-dimensional woven preforms, had been manufactured on semi-automatic looms. To prepare special-shaped tubular three-dimensional woven composites with different wall thicknesses and shapes, the vacuum-assisted resin transfer method was used by performing the prepared preform and resin as the reinforcement and matrix. Then, the axial compression performance was tested on a universal material testing machine. The results revealed that both the wall thicknesses and the shape had remarkable effects on the composites’ axial-compression properties. The performance of special-shaped tubular three-dimensional woven composites and normal tubular three-dimensional woven composites were compared and special-shaped tubular three-dimensional woven composites were better. Finally, finite element analysis was adopted in this research. The initial damage, stress evolution, and final failure of special-shaped tubular three-dimensional woven composites had been shown by the analysis. The stress concentration and failure mechanism of the composite materials after axial compression was revealed and the finite element model was correct and can predict the mechanical properties of special-shaped tubular three-dimensional woven composites.

A Defect Localization Approach Based on Improved Areal Coordinates and Machine Learning

The defects are usually generated during the structural materials subjected to external loads. Elucidating the position distribution of defects using acoustic emission (AE) technique provides the basis for investigating the failure mechanism and prevention of materials and estimating the location of the potentially dangerous sources. However, the location accuracy is heavily affected by both limitation of localization area and reliance on the premeasured wave velocity. Here, we propose a novel AE source localization approach based on generalized areal coordinates and a machine learning algorithmic model. A total of 14641 AE source location simulation cases are carried out to validate the proposed method. The simulation results indicate that even under various measurement error conditions the AE sources could be effectively located. Moreover, the feasibility of the proposed approach is experimentally verified on the AE source localization system. The experiment results show that the mean localization error of 3.64 mm and the standard deviation of 2.61 mm are obtained, which are 67.55% and 75.46% higher than those of the traditional method.

Size-and-thickness-dependent fracture patterns of hollow core–shell electrodes during lithiation

Conventional materials for lithium-based rechargeable batteries are reaching their limit. Material failure is also a huge barrier to fully exploit the potential. Novel design of nanomaterials provides possible solutions for these problems. However, the mechanism of material failure during lithiation/delithiation cycling is still not well understood. Here we develop a concurrently coupled chemo-mechanical model based on peridynamics to study the fracture-pattern formation of hollow core–shell structures during lithiation. We show that the geometric parameters of the nanostructures may profoundly influence the behaviors of dynamic fracture, i.e., different coating thicknesses and sizes of the nanotubes lead the cracks to branch in different directions, making them grow into different patterns. We investigate the stress state to explore the reason for the differentiation. A phase diagram for fracture patterns is constructed to have a systematic view. The results in this work provide a basis of designing the nanostructures of the electrode materials in the future.

Adaptive wavelet-enhanced cohesive zone phase-field FE model for crack evolution in piezoelectric composites

A novel finite deformation FE model is developed in this paper for coupled electro-mechanical response with crack evolution in heterogeneous piezoelectric composite microstructures. Fracture in these composites is driven by complex mechanical and electrical mechanisms leading to a set of strongly coupled, nonlinear governing equations. Crack propagation and interfacial decohesion in the presence of electro-mechanical fields are modeled by a unified phase-field model integrated with cohesive traction–separation laws, derived from a cohesive potential function at material interfaces. To incorporate different electrical crack face conditions, degradation functions are employed in terms of the crack phase-field order parameter. An adaptive wavelet-enhanced hierarchical framework is developed to improve computational efficiency of the coupled electromechanical-phase-field FE solver based on a phase-field error indicator. Numerical examples representing different failure mechanisms in heterogeneous piezoelectric materials and piezo-composite microstructures are solved to demonstrate the efficacy of the model.

Detection of sub-micrometer thermomechanical and thermochemical failure mechanisms in titanium with a laser-based thermoreflectance technique

This work reports on a pump–probe laser-based heating and sensing metrology to study the failure mechanisms of materials during extreme heat fluxes localized near surfaces, the localization of which is controlled by the focus of the laser beam and sensed by the reflection of a secondary probe laser. We focus the demonstration of these power density at failure tests on the damage mechanisms of commercially pure titanium metal during and after high heat fluxes induced from the absorbed laser energy. Using this steady-state thermoreflectance pump–probe metrology, a localized region of the material was irradiated at a low modulated frequency, while the average change in the thermoreflectance signal was monitored. We observe surface and cross-sectional oxidation of the titanium, revealing correlations between microstructural evolution events and shifts in thermoreflectance trends as a function of absorbed power density. Furthermore, the damage morphology was shown to be heavily influenced by the size of the heater (dictated by the radius of the pump laser beam), which controlled the relative degree of thermomechanical, melting, and oxidative decohesion failure mechanisms in the samples. The analysis of the temperature distribution coupled with the observed microstructural damage gives rise to a high-throughput experimental technique to induce desired deformation modes through cyclic thermal testing.

Cyclic contact fatigue behavior of baria-silicate glass-ceramics as a function of crystal aspect ratio

Ceramic materials are potentially useful for dental applications because of their esthetic potential and biocompatibility. However, evidence of contact fatigue damage in ceramics raises considerable concern regarding its effect on the survival probability predicted for dental prostheses. To simulate intraoral conditions, Hertzian indentation loading with steel indenters was applied in this study to characterize the fatigue failure mechanisms of ceramic materials. Baria silicate glasses and glass-ceramics with different aspect ratios of crystals were selected because the glass and crystal phases have similar density, elastic modulus, and thermal expansion coefficients. Therefore, this system is a model ceramic for studying the effect of crystal geometry on contact cyclic fatigue failure. The subsequent flexural strength results show that the failure of materials with a low fracture toughness such as baria-silicate glass (0.7 MPa m1/2) and glass-ceramic with an aspect ratio of 3.6/1 (1.3 MPa m1/2) initiated from cone cracks developed during cyclic loading for 103 to 105 cycles. The mean strengths of baria-silicate glass and glass-ceramics with an aspect ratio of 3.6/1 decreased significantly as a result of the presence of a cone crack. Failures of baria-silicate glass-ceramics with an aspect ratio of 8.1/1 (Kc = 2.1 MPa m1/2) were initiated from surface flaws caused by either grinding or cyclic loading. The gradual decrease of fracture stress was observed in specimens with an aspect ratio of 8.1/1 after loading in air for 103 to 105 cycles. A reduction of approximately 50 % in fracture stress levels was found for specimens with an aspect ratio of 8.1/1 after loading for 105 cycles in deionized water. Thus, even though this glass-ceramic with an 8.1/1 crystal aspect ratio material is tougher than that with a 3.6/1 crystal aspect ratio, the fatigue damage induced by a large number of cycles is comparable. The mechanisms for cyclic fatigue crack propagation in baria-silicate glass-ceramics are similar to those observed under quasi-static loading conditions. An intergranular fracture path was observed in glass-ceramics with an aspect ratio of 3.6/1. For an aspect ratio of 8.1/1, a transgranular fracture mode was dominant.

Mode III dynamic fracture performance of carbon-epoxy laminates

The aim of this work is the development of a new test methodology for the characterization of delamination of composites under mode III of fracture and dynamic stress (fatigue).&nbsp;The phenomenon of delamination is one of the main failure mechanisms in composite materials configured by stacking several layers.&nbsp;The mode of fracture that has been studied is mode III to fatigue, with the difficulty that its simulation implies.&nbsp;The test device employed, which allows subjecting to the laminates to this loading mode, has already been previously tested in static tests. Characterization tests were performed on two different materials, both with different matrices and carbon fibers.&nbsp;An analytical model developed for the case of static fracture has been used and a statistical analysis of the results obtained has been carried out.&nbsp;A fractography of the fracture surface was also performed in order to determine the characteristics of this type of fracture in a dynamic regime.

3D numerical investigation of the detonation wave propagation influence on the triangular plate deformation using finite rate chemistry model of LS-DYNA CESE method

In this article, an immersed boundary method which couples a Lagrangian structure solver with an Eulerian fluid solver with a chemistry model capable of detonation computation for H2O2 is developed to achieve the fluid-structure interaction, the deformation, and the damage of a thin plate subjected to a gaseous detonation loading. The Johnson-Cook material model was used for the plate which incorporates strain hardening, temperature softening, and strain rate effects. The detonation wave is modeled using LS-DYNA finite rate chemistry model incorporated in conservative element solution element solver. Other objectives of this numerical simulation such as estimation of deflection and stress state failure mechanism of material at high strain rate were derived based on a strain-rate dependent failure criterion. The simulated deformation pattern and the effect of pre-detonation pressure are compared with experimental results and a good agreement was acquired. Further additional simulations revealed that the pre-detonation pressure, ignition point location, and longitudinal capacity of the cylinder have great influences on the deformation and sensed pressure of the plate.