Behavior Of Brittle Materials Research Articles

Influence of Carbon Nanotubes‐Based Coatings on the High‐Temperature Compression Strength of Al2O3‐C Foam Filter Structures

Materials for steel‐melt filtration have to fulfill different thermomechanical requirements and especially withstand the thermal shock attack in the beginning of casting. In particular, carbon‐bonded alumina materials provide an excellent thermomechanical performance and have been used in functional components such as submerged entry nozzles, stoppers, and sliding gates for several years. The present study investigates the mechanical high‐temperature behavior of Al2O3‐C foam filter structures with coatings based on carbon nanotubes+Al2O3‐nanosheets (CNT‐ANS) and carbon nanotubes+Al2O3‐nanospheres (CNT‐ANB) at 1100 and 1450 °C. The foam filters are tested in an electromechanical testing machine under argon atmosphere in quasi‐static compression. The tests show a brittle material behavior at 1100 °C with higher compressive strength of the coated filter samples. At 1450 °C, a loss of strength occurs, accompanied by an increase in plastic deformation. Rupture of struts and behavior of the coatings are microstructurally analyzed by scanning electron microscopy. As the mechanical load during the tests is much longer than in real immersion tests, the high‐temperature performance is considered trustworthy.

Experimental and numerical analysis of size effects on stress intensity in anisotropic porous materials

A prominent size effect has previously been reported for the fracture behaviour of brittle porous materials, with smaller specimens behaving quite differently to their larger counterparts. In such materials, the size of the K-dominant zone has been numerically found to be greatly affected by the presence of voids in the near-tip area, thus putting the assumption of a single fracture parameter under question. In order to address this, in this study mode I tests are conducted on porous double cantilever beam specimens, while the stress distribution in the near-tip area is being observed by means of photoelasticity. Results validate the predicted size effect and suggest that the voids can indeed alter the size and shape of the stress pattern in the specimens. A parametric study is then conducted to investigate the influence of void shape variations that can be caused by manufacturing inaccuracies on the stress concentration at the crack tip. It is found that although the stress intensity at the crack tip can be greatly affected by such factors, the size of the K-dominant zone remains unaffected.

Manufacturing of Titanium Components with 3DPMD

Within this work, the 3D plasma metal deposition (3DPMD) process is introduced as an additive manufacturing process for titanium components. For this purpose, demonstrators were designed, manufactured and subsequently analyzed. Process-structure-property relationships are discussed. By analyzing the microstructure, the chemical composition and the mechanical-technological properties, it is shown that the production of titanium parts with 3DPMD is possible. The micro tensile tests showed that a load parallel to the build direction is the most critical case for the component. Furthermore, a brittle material behavior could be determined due to enhanced oxygen content in the component. By subsequent heat treatment, the brittle failure behavior could be increased to a fracture elongation of 4.6%. In sum, the 3DPMD process has the potential to manufacture near-net-shape titanium parts out of metal powders. Critical issues are the protection of the weldment and the lack of ductility of the titanium component.

Thermal shock fracture behavior of wave-transparent brittle materials in hypersonic vehicles under high thermal flux by digital image correlation.

During diving, high orbit maneuver, or target detection and positioning, a hypersonic vehicle will experience high thermal flux. Aerodynamic heating will cause intense thermal shock to the antenna window and radar dome, which are usually made of brittle materials that are transparent to electromagnetic waves and are critical to the mission of target positioning and hitting. Therefore, determination of the time to fracture of the brittle materials under high thermal flux is of great significance. In this study, thermal shock tests were performed on two brittle materials, i.e., SiO2 and Al2O3, using a quartz lamp radiator test system with a maximum thermal flux of 1.5MW/m2. The difference between the pre-set and actual thermal fluxes for the tests was smaller than 1.0%. The time to fracture was determined for the two brittle materials by employing the digital image correlation method to capture and analyze changes in speckle images of the specimen's surface. The speckle image analysis also revealed variations in the surface strain values (εx and εy) prior to fracture. The test results provide important input for the safety and reliability design of radar domes and other electromagnetic wave-transparent signal detection and positioning components of hypersonic vehicles when subjected to high thermal fluxes.

A rate‐dependent constitutive model for brittle granular materials based on breakage mechanics

AbstractModeling the rate‐dependent mechanical behavior of brittle granular materials is of interest to defense applications, civil and mining engineering, geology, and geophysics. In particular, granulated ceramics in armor systems play a significant role in the overall dynamic material response of ceramics, particularly in their penetration resistance. This paper presents a rate‐dependent constitutive model for brittle granular materials based on a recent reformulation of breakage mechanics theory. The rate‐dependency is introduced via the overstress theory of viscoplasticity. The proposed formulation incorporates the effects of relative density and particle grading on strength and porous compaction/dilation, and is capable of tracking their evolution. The model is devised with internal variables linked to underlying dissipative micromechanisms including configurational reorganization, particle breakage and frictional dissipation. A strategy for calibrating model parameters and required experiments are described. The impact of loading rate on shear strength and grading evolution are explored through a sensitivity analysis. The presented model is capable of capturing several key features of the experimentally observed behavior of brittle granular materials including stress‐, rate‐ and density‐dependent stress‐strain and volume change responses, the competition between dilation and breakage‐induced compaction, the evolving particle grading due to particle breakage, and the evolution toward a critical (steady) state under shearing. A possible application of this micromechanics‐inspired modeling framework involves integrating it into rate‐dependent models for ceramics to assist in improving the impact performance of next‐generation ceramics.

Tensile Piezoelectric Actuator: Fabrication, Characterization and Application

A multilayer piezoelectric actuator, with dimension of 10 mm×10 mm×60 mm, which can output a tensile displacement directly by exploring the d31 effect of piezoelectrics, was fabricated using soft Lead Zirconate Titanate (PZT) ceramics. The performance of the actuator was characterized by a special home-made experimental setup under electric fields varying from 200 to 1 000 V/mm with the preload varying from -150 to 100 N at room temperature. Results showed that it can output a maximum tensile displacement of 12 μm under unipolar electric field of 1 000 V/mm and preload was independent with generative displacement in the range of testing preload. Additionally, using the actuator, we built a micro loading apparatus, which can be used to test the fracture behavior of brittle materials.

Finite element analysis of the dynamic interaction between a single abrasive grain and a glass surface

The article presents the results of 2D and 3D formulation finite element analysis of glass surface scratching process using single abrasive grain. Johnson-Holmquist model JH-2 was chosen to describe the brittle behavior of the glass material. The cracked layer depth for a given depth penetration of grain and scratching speed were obtained.

The model of dynamic mechanical behaviour of brittle solids based on kinetic theory of strength

The paper presents the kinetic theory-based model of dynamic mechanical behavior of brittle solids. The model adopts the ideas of the kinetic theory of strength which postulate the finite time of nucleation of discontinuities and relaxation of local stresses in the material. The model is a generalization of the formalism of classical “quasi-static” Nikolaevsky’s plasticity model (non-associated plastic flow rule with the plasticity criterion in the form of Mises-Schleicher) and the Drucker-Prager model of failure to describe the material behavior at strain rates corresponding to dynamic loading. Three new dynamic (time) parameters are used to describe the dynamic mechanical behavior of brittle material. The dynamic model developed in the paper can be implemented within the framework of various Lagrangian numerical methods using an explicit integration scheme (including finite and discrete element methods).

Study on dynamic fracture behavior of mode I crack under blasting loads

Dynamic fracture behavior under impacting loads has been well studied, but for that under blasting loads, less attention has been paid. In order to investigate mode I crack propagation behavior of brittle materials under blasting, a new configuration specimen, i.e. single internal crack circular disc (SICCD) specimen was proposed in this paper, and it was applied in the blasting experiments. Crack propagation gauges (CPGs) were stuck along crack propagation paths to measure crack initiation and propagation time and crack propagation speeds. Green sandstone and PMMA were selected to make the SICCD specimens. Finite difference models were established by using AUTODYN code according to the SICCD specimen dimension and the loading curve measured near the borehole. Generally, the simulation results of crack propagation paths agree with the test results. Finite element code ABAQUS was applied in the calculation of dynamic stress intensity factors (SIFs), and the curves of dynamic SIFs versus time were obtained. By using these curves and the breaking time of the CPG wires, the mode I critical dynamic SIFs in initiation and in propagation were obtained. The results show that the measuring method of the critical dynamic SIFs of brittle materials under blasting presented in this paper is feasible and applicable. During crack propagations, the crack speed is not a constant, and the critical dynamic SIFs in propagation decreases with the increase of crack propagation speeds.

A novel fixture for mixed mode I/II/III fracture testing of brittle materials

AbstractA novel test‐loading device was suggested in order to study the fracture behavior of brittle materials under mixed mode I/II/III loading conditions. A version of the compact tension shear specimen was used as the test configuration. Using a three‐dimensional finite element analysis, the influence of mode mixity on the stress intensity factors, the T‐stress, and 3‐D plastic zone around the crack tip was investigated. In addition, an experimental study was performed on an epoxy polymer using the proposed setup. Finally, the fracture toughness of pure epoxy was measured under several loading conditions. The numerical and experimental results manifested that the proposed setup is able to determine a full range of mixed mode I/II/III fracture properties. At the end, the fracture envelope obtained using the practical study was compared with various three‐dimensional fracture criteria. A negligible discrepancy was concluded between the practical data and the theoretical data estimated by the maximum mean principle stress criterion.

Time-dependent behavior of subcritical crack growth for rock plate: Experimental and numerical study

Time-dependent behavior of subcritical crack growth is one of the main characteristics in rocks. The double-torsion test is commonly used to study the slow crack growth behavior of brittle and quasi-brittle materials. However, double-torsion specimen is difficult to processing, the process of the laboratory test is irreversible, and the current numerical simulation is difficult to consider the time-dependent behavior, and so on. In view of all above problems, an idealized particle model was built, and the crack was identified in this article, based on the theory of particle flow. The numerical model was built using Particle Flow Code in 3 Dimensions, and the macromechanical and micromechanical parameters of the model were calibrated. The process of the macroscopic crack propagation and its evolution were analyzed. The intrinsic relations with the load, the displacement, and the time were established. The results show that the Particle Flow Code in 3 Dimensions can reproduce the time-dependent behavior of subcritical crack growth in double-torsion test. And, the peak and the law of the curves are in good agreement with the laboratory test results. Therefore, the Particle Flow Code in 3 Dimensions numerical simulation can be used as a new effective method to reveal the slow crack growth behavior, to get the relevant parameters such as V, KI, and KIC, and to build the relationship between V and KI. The results of this article will have some reference value for the simulation and application of double-torsion test.

A unified experimental-theoretical approach to predict the critical stress characteristics of failure and yielding under quasi-static and dynamic loading

The results of experimental and theoretical studies on the dynamic response of brittle and ductile materials are presented. The work shows the possibility of predicting the critical stress of brittle materials or yield strength of ductile materials over a wide range of strain rates on the basis of a combination of a simple theoretical approach to dynamic strength and a set of experimental data obtained using a conventional technique. The studies were carried out using fine-grained concrete and ceramic brick as representatives of brittle materials, and copper and titanium as representatives of ductile materials. The traditional Kolsky method was used to determine the strength and ductility characteristics under dynamic loading conditions of the experimental samples. The sensitivity of the critical stress characteristics of failure and yielding of the studied materials to the strain rate is shown. The incubation time approach is used to interpret the effects of strain rate. The applicability of the incubation time criterion to prediction of the dynamic behaviour of both brittle and ductile materials is shown. The presented unified experimental-theoretical approach can be applied in engineering practice to model the dynamic response of structural materials.

Meshfree analysis on dynamic behavior of hard brittle material in abrasive flow machining

Erosion mechanism of hard brittle target subjected to abrasive particles has been investigated numerically in the present study. A coupled model based on smoothed particle hydrodynamics and finite elements was established. The large deformation and high strain rate of eroded material were concerned by using a strain rate-dependent constitutive model. Dynamic processes of material deformation and fracture growth were analyzed. The results indicated that the radial and lateral cracks were the main fracture modes of aluminum nitride target material. Moreover, the effects of different incident parameters such as impact angle and velocity on erosion mechanism were also investigated.

Kinetic approach to the development of computational dynamic models for brittle solids

The paper presents an approach to developing the mathematical formalism of the discrete element method to numerically study the inelastic behavior and fracture of brittle materials under dynamic loading. The approach adopts the basic principles of the kinetic theory of strength which postulate the finite time of nucleation of discontinuities and relaxation of local stresses in the material. A general methodology is proposed for constructing dynamic (kinetic) models of the mechanical behavior of a discrete element based on quasi-static models and using three dynamic material parameters (time parameters). The physical meaning of these parameters is discussed, and a method is proposed for estimating the magnitude of the parameters for a considered material using standard experimental data on its mechanical characteristics. The approach is verified by a dynamic formulation of two-parameter models of inelasticity and strength of brittle materials within the method of simply deformable discrete elements. The proposed way to the dynamic generalization of conventional quasi-static mechanical models is applicable to various Lagrangian numerical methods and makes it possible to numerically study the dynamic behavior features and to predict the mechanical characteristics of brittle materials at different strain rates (up to strain rates 103 s−1) and different types of stress state.

Quantifying Topological Uncertainty in Fractured Systems using Graph Theory and Machine Learning

Fractured systems are ubiquitous in natural and engineered applications as diverse as hydraulic fracturing, underground nuclear test detection, corrosive damage in materials and brittle failure of metals and ceramics. Microstructural information (fracture size, orientation, etc.) plays a key role in governing the dominant physics for these systems but can only be known statistically. Current models either ignore or idealize microscale information at these larger scales because we lack a framework that efficiently utilizes it in its entirety to predict macroscale behavior in brittle materials. We propose a method that integrates computational physics, machine learning and graph theory to make a paradigm shift from computationally intensive high-fidelity models to coarse-scale graphs without loss of critical structural information. We exploit the underlying discrete structure of fracture networks in systems considering flow through fractures and fracture propagation. We demonstrate that compact graph representations require significantly fewer degrees of freedom (dof) to capture micro-fracture information and further accelerate these models with Machine Learning. Our method has been shown to improve accuracy of predictions with up to four orders of magnitude speedup.

SEISMIC VULNERABILITY ASSESSMENT OF A 17th CENTURY ADOBE CHURCH IN THE PERUVIAN ANDES

ABSTRACT A large part of Peruvian heritage corresponds to Colonial Andean churches, which are economically and culturally essential for the development of several villages. Particularly, adobe churches are very vulnerable to earthquakes due to the brittle material behavior, the weak connections, and the absence of a rigid diaphragm. This article aims at performing the seismic vulnerability assessment of an adobe heritage church: “Virgen de la Asuncion” of Sacsamarca located in the province of Ayacucho in the Andean region of Peru; using a rapid screening method and a more complex numerical analysis. First, the damage survey was carried out through visual inspection and infrared thermography, while the material characterization was performed by applying flat-jack tests. Later, the seismic vulnerability assessment was carried out in two stages. The first was based on simplified criteria for masonry structures, and the other on a pushover analysis of global performance. From this evaluation, it was possible to conclude that the church is indeed vulnerable to Peruvian seismic demands.

Vinyl Sulfonate Esters: Efficient Chain Transfer Agents for the 3D Printing of Tough Photopolymers without Retardation.

The formation of networks through light-initiated radical polymerization allows little freedom for tailored network design. The resulting inhomogeneous network architectures and brittle material behavior of such glassy-type networks limit the commercial application of photopolymers in 3D printing, biomedicine, and microelectronics. An ester-activated vinyl sulfonate ester (EVS) is presented for the rapid formation of tailored methacrylate-based networks. The chain transfer step induced by EVS reduces the kinetic chain length of the photopolymer, thus shifting the gel point to higher conversion, which results in reduced shrinkage stress and higher overall conversion. The resulting, more homogeneous network is responsible for the high toughness of the material. The unique property of EVS to promote nearly retardation-free polymerization can be attributed to the fact that after the transfer step no polymerizable double bond is formed, as is usually seen in classical chain transfer agents. Laser flash photolysis, theoretical calculations, and photoreactor studies were used to elucidate the fast chain transfer reaction and exceptional regulating ability of EVS. Final photopolymer networks exhibit improved mechanical performance making EVS an outstanding candidate for the 3D printing of tough photopolymers.

Flexural fracture mechanisms and fatigue behaviors of Bi4Ti3O12-based high-temperature piezoceramics sintered at different temperatures

In this paper, a series of Aurivillius phase Bi4Ti2.95W0.05O12.05 + 0.2 wt% Cr2O3 (ab. BTWC) ceramics were prepared by a solid-reaction process and sintered at different temperatures (1050 ℃~1150 ℃), their microstructures, fracture mechanisms and fatigue behaviors were investigated under three-point-bending mode. The results show that the grain size of BTWC ceramics increases with increasing the sintering temperature. The typical transgranular mode dominates the fracture behavior of the samples sintered at lower temperatures, while the intergranular fracture can be also observed in the samples sintered at higher temperatures. Besides, the storage modulus (E’) and mechanical loss (tanmδ) of BTWC ceramics have a subtle variation with the increase of sintering temperature. In addition, the high-temperature environment could not only decrease the fracture toughness and bending strength of ceramics but also change their fracture mode. On the other hand, the bending strength also decreases with the decrease of loading rates, which could be attributed to the slow crack growth referring to the dynamic fatigue behavior of brittle materials. The slow crack growth parameter (n) of BTWC ceramics shows a downtrend with increasing their sintering temperature, indicating that those high temperature sintering samples possess a higher susceptibility when subjected to the long-term stress corrosion. Furthermore, the sample sintered at 1125 ℃ exhibits an excellent fatigue resistance when subjected to the cyclic stress. The corresponding fatigue mechanism is not only related to the ferroelectric domain configuration but also involved with the pores or impurities.

Structure Behavior of Beams Combining Ultra High Strength Concrete and Normal Strength Concrete

The concept of "Composite structures or partial elements" was mainly carried out from an economical point view. The basic idea for using the composite elements is combining the Normal strength concrete and Ultra High Strength concrete (UHSC) or any recent advanced cementitious material in composite structures in order to exploit the advantages of the two materials in an optimal way. Ultra-high Strength concrete (UHSC) which was used in this research have exceptional material properties, however, their material costs are significantly higher than those of normal strength concretes. It is defined as a concrete which is meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituent materials and normal mixing, placing, and curing practices,(ACI Committee 116). UHSC is characterized by extraordinary mechanical properties and durability properties. The UHSC-Matrix is very brittle material behavior. In this research an experimental program is being carried out to study the behavior of UHSC beams and composite beams using UHSC (141MPa) and Normal strength concrete with studying the effect of different parameters tested under static loads. This program mainly aims to reach to the optimum thickness of ultra-high strength concrete layer in the composite beams, it also aims to make a validation for using the Egyptian code equation for the maximum reinforcement ratio of longitudinal steel in UHSC and composite beams. This paper presents the results of this experimental program, which consists of ten reinforced concrete beams. The main parameters of this program were: longitudinal reinforcement ratio, the type of used concrete and the thickness of UHSC layer. Particular attention is paid to the effect of each variable on the strength enhancement, stiffness degradation, toughness and ductility of the tested beams. Valuable conclusions were obtained from the research results which stated that; in case of high reinforcement steel ratios the optimum thickness of UHSC in the concrete beams cross section should be not less than the third of the cross section depth only in order to achieve the economic point of view.

A simplified model of fracking based on the topological derivative concept

This paper deals with a simplified hydraulic fracture model based on the concept of topological derivatives. It means that we consider a two dimensional idealization in which the rock is assumed to be impermeable, while the fracturing process is activated by a given pressure acting within the existing geological faults. The basic idea consists in adapting the Francfort–Marigo damage model to the context of hydraulic fracture. The Francfort–Marigo damage model is a variational approach to describe the behavior of brittle materials under the quasi-static loading assumption, focusing on the evolution of damaged regions under an irreversibility constraint. In our model problem, the loading comes out from a pressurized damaged region embedded into the rock, which is used to trigger the hydraulic fracturing process. In particular, a shape functional given by the sum of the total potential energy of the system with a Griffith-type dissipation energy term is minimized with respect to a set of ball-shaped pressurized inclusions by using the topological derivative concept. Thus, the topological asymptotic expansion of the shape functional with respect to the nucleation of a circular inclusion endowed with non-homogeneous transmission condition on its boundary is obtained. The associated topological derivative, which corroborates with the famous Eshelby theorem, is used to devise a simple topology optimization algorithm specifically designed to simulate the whole nucleation and propagation process of hydraulic fracturing. To assess our model, some numerical examples are presented, showing typical features of hydraulic fracture phenomenon, including the characterization of the fault-activation pressure and specific crack path growth, allowing for kinking and bifurcations.