Radial Crack Length Research Articles

A Bilayer Method for Measuring Toughness and Strength of Dental Ceramics

The ever-increasing usage of ceramic materials in restorative dentistry necessitates a simple and effective method to evaluate flexural strength σF and fracture toughness KC. We propose a novel method to determine these quantities using a bilayer specimen composed of a brittle plate adhesively bonded onto a transparent polycarbonate substrate. When this bilayer structure is placed under spherical indentation, tunneling radial cracks initiate and propagate in the lower surface of the brittle layer. The failure analysis is based on previous theoretical relationships, which correlate σF with the indentation force P and layer thickness d, and KC with P and mean length of radial cracks. This work examines the accuracy and limitations of this approach using a wide range of contemporary dental ceramic materials. The effect of layer thickness, indenter radius, load level, and length and number of radial cracks are carefully examined. The accuracy of the predicted σF and KC is similar to those obtained with other concurrent test methods, such as biaxial flexure and 3-point bending (σF), and bending specimens with crack-initiation flaws (KC). The benefits of the present approach include treatment for small and thin plates, elimination of the need to introduce a precrack, and avoidance of dealing with local material nonlinearity effects for the KC measurements. Finally, the bilayer configuration resembles occlusal loading of a ceramic restoration (brittle layer) bonded to a posterior tooth (compliant substrate).

Spark Plasma Sintering of Fine-Grained WC-Co Composites.

Cemented carbides WC-Co are one of the basic tool materials. They constitute over half of the currently used tools intended for machining. The main advantages of WC-Co cemented carbides are high hardness and abrasion resistance. The properties of WC-Co sinters depend mainly on the content of the binding phase, the sintering method and the grain size of the powder from which the sinters were made. The aim of this study was to produce fine-grained WC-Co composites using SPS (spark plasma sintering) technology, as well as examine the effect of the applied technology on the basic properties of WC-Co sinters: microstructure, hardness, phase composition, compaction degree and tribological properties. In the processes carried out, no additives affecting the reduction in grain growth were used. Sintering was conducted at a temperature of 1200 °C with a holding time of 10 min. The process occurred under a load of 100 MPa. Finally, the samples were cooled in a vacuum of 10-6 mbar. We measured the hardness using a Vickers hardness tester. We took hardness measurements along the diameter of the sintered samples. In order to ascertain the fracture toughness (KIC), we measured the radial crack length around the Vickers indentation and applied Shetty's formula. The tribological tests were carried out with a tribotester using the T-01 ball-on-disc method. The obtained data enabled the characterization of the wear process of the tested materials.

Experimental investigation on de-icing by an array of impact rod-type plasma synthetic jets

Since flight accidents due to aircraft icing occur from time to time, this paper proposes an array of impact rod-type plasma synthetic jet de-icing methods for aircraft icing problems. The impact rod-type plasma synthetic jet actuator (PSJA) is based on the traditional PSJA with an additional impact rod structure for better de-icing in the flight environment. In this work, we first optimize the ice-breaking performance of a single-impact rod-type PSJA, and then conduct an array of impact rod-type plasma synthetic jet ice-breaking experiments to investigate the relationship between crack expansion and discharge energy, ice thickness and group spacing. The results show that the impact force and impulse of a single-impact rod-type PSJA are proportional to the discharge energy, and there exists a threshold energy Q min for a single actuator to break the ice, which is proportional to the ice thickness. Only when the discharge energy reaches above Q min can the ice layer produce cracks, and at the same time, the maximum radial crack length produced during the ice-breaking process is proportional to the discharge energy. When the ice is broken by an array of impact rod PSJAs, the discharge energy and group spacing together determine whether the crack can be extended to the middle region of the actuator. When the group spacing is certain, increasing the energy can increase the intersection of cracks in the middle region, and the ice-fragmentation degree is increased and the ice-breaking effect is better. At the same time, the energy estimation method of ice breaking by an array of impact rod-type PSJAs is proposed according to the law when a single actuator is breaking ice.

Strength, crack resistance and optical losses of heat-treated silica fibers coated with non-ferrous metal

Metallic coatings currently provide the highest performance for silica fibers. The purpose of the present works predict of crack resistance, in determining the hardness and strength of uncoated and copper-coated optical fibers, as well as in the study of changes in strength parameters and optical losses after thermal effects on the studied optical fibers. The hardness and crack resistance values of silica fibers are measured by indentation. Crack resistance K1c is calculated using a semi-empirical Niihara relationship that relates the indentation size, radial crack length, and crack resistance. It was found that copper coating of silica fibers increases crack resistance 4 times, probably due to the affect of compressive stresses. The tensile strength of the fibers is determined by axial tension and two-point bending methods at a constant loading rate. Using a scanning electron microscopy, the initial crack length is calculated and the size of the characteristic defect is determined.

Study on Mechanical Cleavage Mechanism of GaAs via Anisotropic Stress Field and Experiments

To satisfy the increasing requirements for high-quality cavity mirrors for semiconductor lasers, the influence of the anisotropic stress field generated by scratching GaAs on the quality of cleavage planes was investigated. First, a novel method for calculating the interplanar spacing was proposed to estimate the favourable direction for the scratching operation. Some mechanical properties in the scratching direction were calculated. Consequently, an anisotropic calculation model of the stress field generated by scratching was established. The calculation model was used to analyse the y-component stress field distribution along different directions beneath the indenter. The results show that the maximum stress occurs at the contact area between the material and indenter tip, and the stress along the <100> orientation is lower than that along the <110> orientation. According to our experiments, a significant decrease in the average radial crack length and a slight decrease in the average kerf width were observed along the [011] direction compared to the corresponding results for the [001] direction. In addition, the (011) plane had a surface roughness of 0.43 nm and could become a preferential cleavage plane under a low scratching load. The results of this study contribute to advance our understanding of the mechanisms of mechanical cleavage processes.

Experimental Study on Crack Propagation of Rock by Blasting under Bidirectional Equal Confining Pressure Load

Rock blasting during tunneling has shown that the rock failure in high in situ stress environments is different from that in low in situ stress conditions or with a shallow rock mass. In particular, the propagation direction of the main crack induced by blasting is greatly affected by the in situ stresses. In order to study the law of crack propagation in rock during blasting under the conditions of an initial in situ stress, a transparent material that conformed to the mechanical properties of hard rock was used to carry out a similar model rock blasting test, under a unidirectional load. The results show that initial stress has a great influence on the propagation number, length, and direction of the main radial cracks. The specific performances were as follows: under the action of an equal confining pressure load, the longest main radial crack in the model specimen propagated along the diagonal direction, and the number and length of the main radial cracks propagated decreased with the gradual increase of confining pressure stress; in addition, the diameter of the circumferential cracks also decreased with the increase of stress, and there was a negative correlation between them. In view of the phenomenon where the longest main radial crack propagated along the diagonal direction in the model test, a mechanical model was established in this study to explain this process. This is of practical significance for understanding the mechanism of rock fracture when blasting with high in situ stresses.

Strength and fracture resistance of quartz fibers with polyimide coatings

Polyimide coatings currently provide the highest performance properties of quartz fibers. The purpose of this research is to determine the strength, hardness, dynamic fatigue, performance period and crack resistance of optical fibers with polyimide coatings. The strength limit of fibers determined by the method of axial stretching over the distance between capstans of 500 mm was 4.8–6.0 GPa at a loading speed of 10–500 mm/min. W. Weibull distribution curves were plotted in coordinates that relate the probability of failure to the strength, fiber length, and parameter describing the ultimate strength. The dynamic fatigue parameter n was found, which in physical sense corresponds to the slope tgα equal to 1/(1+n) in double logarithmic coordinates. Hardness and crack resistance values of quartz fibers were measured by indentation. Crack resistance K1c was calculated using the A. Niihara semi-empirical dependence, which connects the indentation size, radial crack length, and crack resistance. The initial crack length was calculated and the size of the characteristic defect was determined using scanning electron microscopy. Thermogravimetric analysis demonstrated that polyimide coated fibers maintain thermal stability up to 450 °С. The service life of optical fibers was determined based on the dynamic fatigue data, and it amounted to at least 25 years at a load of 0.2 GPa. The greater the difference between the lower strength level and the upper one in the stretch tests of fiber segments, the higher the distribution parameter m describing the ultimate strength of optical fibers. The values of this parameter are determined by the fiber quality: m = 50÷100 for coated fibers and m = 1÷5 for uncoated ones.

Nanoindentation based fracture studies of ITO coating

Nano-scale load (P) dependency of fracture toughness (K1C) of indium tin oxide (ITO) coating on silicon (Si) substrate is evaluated by nanoindentation based direct and energy methods. A Berkovich triangular diamond nanoindenter is used with nine different P values e.g., 10, 15, 20, 30, 40, 50, 100, 150 and 200 mN. Extensive utilization of both optical microscopy (OM) and field emission scanning electron microscopy (FESEM) techniques reveal that up to P ≤ 40 mN only the sharp radial cracks form from three corners of the nanoindent. However, for P > 40 mN, multiple cracks occur over and above the radial cracks. Finally, at P: 150–200 mN coating gets chipped off from the substrate. The K1C values of the ITO coating are calculated on the basis of both the conventional radial crack length measurement based method and the strain energy release based method. The K1C values are strongly sensitive to variations in P. Further, the strain energy method results in higher magnitudes of the K1C values.

Fracture Behavior of Cracked Ring Specimen at Different Crack Positions

Abstract The previous research review of piping systems revealed that the plastic pipe companies suffered from many problems in natural gas pipeline systems. One of the most significant problems that appeared in the piping systems are external cracks due to manufacturing processes, welding technique, and installation processes. The principal goal of the present experimental study is to predict the crack growth behavior and energy release rate of cracked ring specimens made from high-density polyethylene (HDPE) under different crack position angles and various crosshead speeds. The effect of loading rate on the external radial crack at different crack position angles plays an important role in the prediction of fracture behavior of plastic pipe materials. For this reason, it is necessary to conduct a study for the fracture analysis of pipe ring specimens under tension loading with double external cracks at constant radial crack length to width ratio equal a/W = 0.5. A precracking machine is designed especially in the present experimental study to simulate the actual radial cracks at outer surface of pipe ring specimens. The effects of crosshead speed and crack position angle revealed a significant effect on the energy release rate and maximum applied load under tensile load.

Interfacial energy driven distinctive pattern formation during the drying of blood droplets

HypothesisDried blood droplet morphology may potentially serve as an alternative biomarker for several patho-physiological conditions. The deviant properties of the red blood cells and the abnormal composition of diseased samples are hypothesized to manifest through unique cell-cell and cell-substrate interactions leading to different morphological patterns. Identifying distinctive morphological trait from a large sample size and proposing confirmatory explanations are necessary to establish the signatory pattern as a potential biomarker to differentiate healthy and diseased samples. ExperimentsComprehensive experimental investigation was undertaken to identify the signatory dried blood droplet patterns. The corresponding image based analysis was in turn used to differentiate the blood samples with a specific haematological disorder “Thalassaemia” from healthy ones. Relevant theoretical analysis explored the role of cell-surface and cell-cell interactions pertinent to the formation of the distinct dried patterns. FindingsThe differences observed in the dried blood patterns, specifically the radial crack lengths, were found to eventuate from the differences in the overall interaction energies of the system. A first-generation theoretical analysis, with the mean field approximation, also confirmed similar outcome and justified the role of the different physico-chemical properties of red blood cells in diseased samples resulting in shorter radial cracks.

Research on crack propagation and rock fragmentation efficiency under spherical tooth dynamic indentation

Rock dynamic indentation is the most basic form of rock dynamic fragmentation, understanding the rock failure mechanism and influence factors of breaking efficiency plays important roles in improving impact drilling efficiency. According to the research achievements of Marshall, the theoretical model of radial crack and lateral crack length considering confining pressure and hydraulic pressure was established, analyzing the influence of confining pressure, hydraulic pressure on crack propagation, and the influence of loading rate on rock fragmentation effect. In addition, the discrete element numerical simulation method was used to study the characteristics of crack formation and propagation and the influence of loading rate on breaking effect under spherical tooth dynamic indentation. The results indicate that theoretical analysis and numerical simulation results are basically identical, with the increase of dynamic load, the damaged area and lateral crack growth, and radial crack initiates from the damaged area and propagates towards the rock free surface, forming rock fragments eventually; under the same dynamic load amplitude and duration, the increase of confining pressure has an inhibitory effect on damaged area, lateral crack, and radial crack, the increase of hydraulic pressure has a promoting effect on damaged area and lateral crack but has an inhibitory effect on radial crack; under the same dynamic load amplitude, rock breaking volume first increases and then decreases with the duration shortening, and when the dynamic load amplitude is 8.5 kN, duration is 1.25 ms, the crushing volume is the largest. The research achievements of this paper can provide reference for understanding dynamic fragmentation mechanism and improving impact drilling efficiency.

Multi-crack propagation in PMMA plates under dynamic out-of-plane impact

In this paper, the multi-crack propagation within polymethyl methacrylate (PMMA) plates under out-of-plane impact is experimentally investigated. First, the dynamic propagation behaviors of in-plane radial and circular cracks are directly recorded using a non-contact optical measurement platform in which a high-speed imaging system is combined with a low-speed impact loading device. Second, the dynamic energy release rates and crack velocities of multiple cracks are studied using caustics, and the effects of impact speed and plate thickness are studied. Finally, a mathematical model of dynamic energy release rate which is related to the radial crack length is established based on the principle of energy conservation. The fracture mechanism of multi-crack propagation in PMMA plates is quantitatively analyzed.

Initial-contact-induced bending failure of a semi-infinite ice sheet with a radial-before-circumferential-crack pattern

Ice loadings induced by the bending failure of a semi-infinite ice sheet have been investigated extensively for ice-capable vessels or sloping offshore structures in the arctic regions. In recent years, real-time simulators of ice-sloping structure interactions are rapidly gaining traction due to their potential applications. For such purposes, analytical or semi-analytical bending failure models are required to achieve reasonable levels of efficiency and accuracy. Motivated by this need, the current paper focuses on the bending failures of a semi-infinite ice sheet, with a breaking pattern of radial before circumferential cracking. Departing from existing work done in literature, the proposed framework does not assume a large initial radial crack length, nor associate the maximum ice load with the initiation of a circumferential crack. Instead, the proposed framework tracks the competition between component forces for the initiation and propagation of radial cracks, as well as that for the initiation of the circumferential crack, to determine the maximum ice load. For sea ice with large fracture energy, it is shown that the maximum ice load does not occur at the initiation of the circumferential crack, as commonly assumed in literature. The research methodology and conclusions presented herein thus facilitate a more accurate real-time description of ice-sloping structure interactions.

Analysis of fracturing characteristics of unconfined rock plate under edge-on impact loading

The objective of this study is to investigate the fracturing characteristics of granite under impact loading. Generation and propagation of cracks from the specimen edge subjected directly to impact loading were captured by the ultrahigh-speed photography. Testing results indicate the existence of a critical impact velocity. At an impact velocity lower than the critical impact velocity, a damage zone will be generated on the edge subjected directly to impact and the damage zone area is varied, depending on the magnitude of impact velocity. At an impact velocity greater than the critical impact velocity, radial cracks appear on the boundary of the damage zone and propagate deep into the rock plate with further increasing impact velocity. The increases in radial crack length and damage zone area are affected by impact velocity and impact angle. However, at some impact velocity, damage zone area decreases while radial crack length increases, demonstrating the existence of an incubation period of crack forming. Comparison between testing and numerical results shows that at a constant impact velocity the fractured zone area and the crater depth are varied, depending on impact angle. The results are helpful to understand the fracturing process in rock subjected to impact loading.

Strengthening of slab-column connection against punching shear failure with CFRP laminates

This paper presents the findings of an experimental investigation undertaken to evaluate the influence of externally bonded CFRP laminates on the punching shear capacity of a two-way spanning slab-column connection. Two control and five strengthened RC slabs were prepared and tested under concentrated load. The variables of the experiment were the anchorage type and width of the CFRP plates used. The structural response of the samples in terms of load, deflection, strain and cracking was monitored and analysed throughout the experimental procedure. It was found that strengthening increases the ultimate load by up to 25% and reduces the maximum deflection by up to 50%. The strengthening increases the cracking load and significantly reduces the cumulative length of radial cracks. The samples strengthened with transverse anchorages exhibited significantly high level of residual strength. The experimental findings for unstrengthen cases are compared with several design codes.

Rate‐Dependent Mechanical Behavior and Amorphization of Ultrafine‐Grained Boron Carbide

An investigation into mechanical properties and amorphization behavior of ultrafine‐grained (0.3 μm) boron carbide (B4C) is conducted and compared to a baseline coarse‐grained (10 μm) boron carbide. Static and dynamic uniaxial compressive strength, and static and dynamic Vickers indentation hardness were determined, and Raman spectroscopy was then conducted on indented regions to quantify and compare the intensity of amorphization. In relation to coarse‐grained B4C the ultrafine‐grained material exhibited, on average, a 33% higher static compressive strength, 20% higher dynamic compressive strength, 10% higher static Vickers hardness, and 23% higher dynamic Vickers hardness. In addition, there was an 18% reduction in indentation‐induced radial crack length in ultrafine‐grained B4C, which corresponded to an increase in estimated fracture toughness. Although traditional coarse‐grained B4C exhibits an 8.6% decrease in hardness from the static to dynamic regimes, ultrafine‐grained B4C showed only negligible change under similar conditions, suggesting a reduced propensity for amorphization. Raman spectroscopic analysis confirmed this result by revealing significantly lower amorphization intensity in ultrafine‐B4C compared to coarse‐grained B4C. These results may have significant positive implications in the implementation of ultrafine‐grained boron carbide as a material for improved performance in impact and other high‐pressure applications.

Blast-induced damage and its impact on structural stability of underground excavations

Analysis of rock wedge stability plays important role in tunneling and hard rock mining. Today there are many procedures for prediction of wedge formation and their stability. These procedures consider preexisting joints in rock mass, and there is no procedure that considers blast-induced cracks and their impact on structural stability of excavations. Using main principles of new rock blasting theory it is possible to assess length and density of radial cracks around blastholes. Herein, one simple excavation situation in hard rock is presented, where only two joint sets were present. This made it impossible for rock wedges to form. After blast-induced crack with limited persistence was added to analysis as result unstable rock wedge was formed. in addition to presented problem it is suggested how mechanical properties of blast-induced cracks could be estimated using Barton-Bandis failure criterion. As final result it was clear that without blast- induced cracks no wedge stability analysis is complete.

Effect of processing routes on the synthesis and properties of Zn4Sb3 thermoelectric alloy

In this work, sintering of Zn4Sb3 was performed by open die pressing at 558 K, investigating the role of different starting materials (powders obtained from slowly cooled master alloy ingots or rapidly solidified ribbons) on microstructure, phase stability, thermoelectric properties and mechanical properties.With respect to the master alloy, sintered samples show lower porosity, a slight decrease in Zn content and a partial decomposition of Zn4Sb3 into ZnSb and Zn. Sintering of ground rapidly solidified ribbons further reduces porosity and partially inhibits the decomposition of Zn4Sb3.As sintered samples show a temperature behaviour of the Seebeck coefficient that is intermediate between those of Zn4Sb3 and ZnSb. Thermal cycling between 300 K and 700 K favours the homogenization of the microstructure, due to the recombination of ZnSb and Zn into Zn4Sb3, leading to a Seebeck coefficient typical of Zn4Sb3 single phase. The phase evolution observed upon sintering and thermal cycling is described in terms of competing reactions ruled by thermodynamics and kinetics, and the different behaviour of the samples is explained by the different free energy level of the starting materials.All samples show a brittle behaviour upon stress-strain compressive test and microindentation measurements. A remarkable increase of the fracture strength is observed as the porosity decreases, whereas microhardness values are similar for all samples (220–240 HVN). For the sintered samples, values of the indentation fracture toughness (0.8–0.9 MPa m1/2) are estimated from the length of Palmqvist-type radial cracks.

Crack Evolution and Estimation of Fracture Toughness of HfO2/SiCN(O) Polymer Derived Ceramic Nanocomposites

In this study the crack evolution and fracture toughness of HfO2/SiCN(O) ceramic nanocomposites were studied. These ceramics crack radially under indentation in contrast to Hertzian crack in SiOC ceramics. This difference was attributed to the variance in Poisson's ratio and elastic modulus to hardness ratio. Fracture toughness by crack opening displacement and radial crack length measurement was 0.8–1.2 MPa.m1/2 and 2.5–3 MPa.m1/2, respectively. The open network forming structure resulted in the over estimation of fracture toughness value by RCL.

Fracture Toughness and Adhesion of Transparent Al:ZnO Films Deposited on Glass Substrates

Al-doped zinc oxide films (AZO) of different thicknesses were deposited by reactive magnetron sputtering on glass substrates. Fracture toughness and adhesion of transparent AZO films were measured by indentation in air and water. Fracture toughness of glass is about 0.63 MPa center dot m(1/2). Under the same normal load, radial crack length shortened with the increase of AZO film thickness. When indented under deionized water, cracks in both glass and AZO films got longer, signifying corresponding decrease in the fracture toughness.