Fracture Mechanical Properties Research Articles

Fatigue fracture analysis of the 42CrMo4 crankshaft used for mining dump truck

A failure investigation was performed into the fractured 42CrMo4 crankshaft used for wide-body mining dump truck in this paper. Fracture morphology, chemical composition, microstructure and mechanical properties of the crankshaft material were studied to investigate the premature failure mechanism. All the crankshaft broke at the 4th or 6th necks and a high-cycle bending fatigue type in this case could be identified by the observation of beach marks and fatigue striations in the propagation area according to fractography results. The fatigue source is underneath the hardening layer of R fillet and the root cause could be identified as the designed inadequate thickness of quenching layer at the fillets obtained through medium frequency induction quenching (MFIQ), which could not provide adequate fatigue strength upon cyclic loadings for long term. Fatigue validation test indicated that the bending moment increased with the increasing thickness of the quenching layer, and the appropriate thickness was recommended about 3.5 mm based on overall consideration.

Prevention and Control of Coal and Gas Outburst by Directional Hydraulic Fracturing through Seams and Its Application.

The regionalized pressure relief, permeability enhancement, and outburst prevention of "three high and one low" coal seams with high gas, high ground stress, high outburst risk, and low permeability have become important problems to be solved urgently in realizing the sustainable development of coal mines. In this study, a combination of theoretical research, RFPA2D-Flow numerical simulation, and field test was used to study the initiation mechanism and propagation law of directional hydraulic fracturing fractures through the seam. The results show that fracture initiation depends on the axial and radial horizontal stress of the fracture hole, the physical and mechanical properties of coal and rock, and the inside of the weakest layer. Single-hole hydraulic fracturing can achieve a pressure relief radius of 7-8 m, but there is a stress concentration zone outside, which is not conducive to regional pressure relief and permeability enhancement. Directional hydraulic fracturing with multiple holes produces an approximate cylindrical compression and crushing ring and a penetrating fracture surface along the center line of the pressure crack hole and the directional hole, which better eliminates the phenomenon of stress concentration in nondirectional hydraulic fracturing. The technology was applied to the 2238 auxiliary roadway of Chengzhuang Mine of Jinmei Group, and the field implementation results showed that field implementation results showed that directional hydraulic fracturing through the seam reduced the gas content in the coal seam to a great extent, and the coal seam gas content was reduced by about 42.3%, indicating that the technology can effectively reduce the risk of coal and gas outbursts.

Recycled Fine and Coarse Aggregates' Contributions to the Fracture Energy and Mechanical Properties of Concrete.

This paper investigates the fracture mechanical properties of concrete, using crushed concrete aggregates (CCA) and granulated blast furnace slag (GGBS) for partial cement replacement. CCAs made from prefabricated concrete replace 100% of the fine and coarse fractions in concrete recipes with w/c ratios of 0.42 and 0.48. Two pre-treatment methods, mechanical pre-processing (MPCCA) and accelerated carbonation (CO2CCA), are investigated for quality improvements in CCA. The resulting aggregates show an increased density, contributing to an increase in the concrete's compressive strength. The novelty of this paper is the superposition of the effects of the composite parts of concrete, the aggregate and the cement mortar, and their contributions to concrete fracture. Investigations are directed toward the influence of fine aggregates on mortar samples and the influence of the combination of coarse and fine aggregates on concrete samples. The physical and mechanical properties of the aggregates are correlated with mortar and concrete fracture properties. The results show that CCA concrete achieves 70% of the fracture energy values of concrete containing natural aggregates, and this value increases to 80% for GGBS mixes. At lower w/c ratios, MPCCA and CO2CCA concretes show similar fracture energies. CO2CCA fine aggregates are the most effective at strengthening the mortar phase, showing ductile concrete behavior at a w/c ratio of 0.48. MPCCA aggregates contribute to higher compressive strengths for w/c ratios of 0.42 and 0.48. Thus, mechanical pre-processing can be improved to produce CCA, which contributes to more ductile concrete behavior.

Fracture failure analysis of a lightning rod on a substation frame considering residual thermal stress

The collapse of a typical lightning rod on a 500-kV high-voltage substation frame in central China is investigated to identify the mechanism of fracture. Comprehensive analyses are performed on the fracture morphology, metallurgical structure, chemical composition and mechanical properties of the materials at the fracture site. A finite element (FE) model of a typical lightning rod structure with an intersecting joint weld is developed and verified using experimental results. Based on the FE results and wind tunnel tests, the fatigue life is estimated for the lightning rod structure at the accident site considering residual stress. The results indicate that the fracture failure of the typical lightning rod structure is mainly caused by wind-induced fatigue damage in the weld area of the intersecting joint of the rod base and the frame beam. The welding residual stress has a severe impact on the fatigue life of the structure. Regardless of the welding residual stress, the actual fatigue life of the typical lightning rod structure under a long-term wind load is lower than the design service life. Therefore, complete analysis of the structure under complex wind-induced dynamic effects is needed in engineering design. Additionally, attention should be given to the quality control of the welding, and measures should be adopted to reduce residual stresses during manufacturing and installation.

Study on Mechanism of Cumulative Directional Blasting of Brittle Karst Limestone in the Guizhou Province

The theoretical analysis, mechanical test, numerical simulations, and model tests are carried out on the Taoziya tunnel in Guizhou to investigate the influence law of cumulative blasting technology on directional rock breaking and fracture propagation of brittle limestone. The penetration effect of the triangular-shaped charge cover angle on the metal jet, the expansion law of the main explosion crack, and the influence of the limestone directional damage effect are explored. The results show that high-energy metal jet is formed when the initial velocity of the shaped charge cover is lower than the sound velocity of the shaped charge cover material. The values of B1 and B2 of the original rock are 22.1 and 248.76, respectively, demonstrating the mechanical properties of high strength, high brittleness, and easy fracture. The evaluation index is brittle limestone, and the directional precracking effect of shaped charge blasting technology is remarkable. When the angle of the shaped charge cover is 90°, a penetrating crack is almost consistent with the direction of the precrack generated after the explosion. Moreover, the width of the slit is approximately 2 cm. The borehole imager scans show that the rock wall in the hole is smooth and stable, the test results are consistent with the simulation results, and the angle of the shaped charge cover is 90°. The results have certain guiding significance for rock blasting technology, surrounding rock control technology, and similar directional fracturing engineering in the karst area of Guizhou province.

The unique flexibility feature of bamboo: Cantilever-beam loading form the coupling bending-shear effects

Bamboo's remarkable attribute of flexibility has earned it widespread recognition, positioning it as an exemplary material for mimicking biological structures in advanced engineering construction. While the tension, compression, bending, shear, and torsion properties of bamboo have been extensively documented, its response under combined loads remains a topic that warrants comprehensive investigation. Filling this knowledge gap, the present study employs the cantilever-beam principle as a methodological framework to scrutinize the fracture mechanism and mechanical properties of bamboo under the coupling effect of bending-shear. The obtained results reveal a linear decrease in the bending moment from the fixed support to the free end of the strip, while the shear force maintains a constant distribution along the strip's length. Within a specific segment of the strip, the bending stress exhibits a linear increase with the distance from the neutral axis, whereas the shear stress manifests a parabolic variation above or below the neutral axis. During the plastic stage, simultaneous transverse and radial fractures occur, effectively dissipating crack energy through interfacial debonding, fiber sliding, and fiber bridging mechanisms. The presence of stepwise deflection between adjacent internodes allows for substantial overall angular deflection of the bamboo culm without incurring damage. Furthermore, the mechanical properties of the strips demonstrate a positive linear correlation with both the fiber volume fraction and the density of vascular bundles. A notable achievement of this study is the establishment of a numerical structure-activity relationship, elucidating the bending-shear coupling performance. This relationship serves to clarify the fracture toughness exhibited by bamboo and provides a valuable reference for performance testing and the formulation of standardized protocols in the realm of biomimetic material production.

Mechanical Characteristics of Precast Cracked Concrete under Freeze-Thaw-Load Action

Four types of precast double-cracked concrete specimens and non-cracked control group were prepared, and the specimens were subjected to freezing and thawing cycles and uniaxial compression tests, with a total of five freezing and thawing cycles, namely 0, 25, 50, 75 and 100, in order to analyse the fracture phenomena and mechanical properties of the specimens after freezing and thawing in uniaxial compression, and to determine the damage variables of the precast cracked concrete under the action of freezing and thawing loads on the basis of strain energy and fracture mechanics theories. The test results show that: (1) the crack aggregation mode generated by the compression damage of precast cracked specimens is mainly influenced by the crack inclination angle, when the crack inclination angle is 30°, the specimen undergoes wing tension aggregation, and when the crack inclination angle is 60°/90°, the specimen undergoes a mixed tension-shear aggregation mode; (2) After the freeze-thaw cycle test, the degree of deterioration of the prefabricated cracked specimens is significantly higher than that of the non-cracked specimens, and the loss of compressive strength and Young’s modulus of the specimens with crack inclination angle of 60° and 90° is the most severe, and the decrease of the stress-strain curve is the most obvious; (3) Based on fracture mechanics and damage-strain energy to derive the damage variables of the prefabricated cracked loaded specimens, and based on the Weibull function and the principle of equivalent effect of Lemaitre effect, the damage variables under freeze-thaw loading are also derived based on the Weibull function and Lemaitre equivalent strain principle.

Mechanical properties and fracture behavior of Al(111)/MgAlB4(0001) interface in Al matrix composites: a first-principle calculation study

Hexagonal crystal MgAlB4 is a strengthening phase in Al matrix composites, which can significantly improve ultimate tensile strength. In this paper, the surface perform, interfacial bonding characteristic, fracture mechanism, and electronic properties of the Al(111)/MgAlB4(0001) interface were thoroughly investigated by the first principles method. The results reveal that the top-site and bridge-site configurations were more unstable than the hollow-site. Besides, from the calculated results of interfacial energy and work of adhesion, the hollow-stacked Al(111)/B(Al)-terminated/MgAlB4(0001) interface expresses stronger stability than other interfacial models, which is attributable to the higher work of adhesion and lower interfacial energy of the hollow-stacked Al(111)/B(Al)-terminated/MgAlB4(0001) interface. Analysis of electronic structure reveals that the Al-termination and Mg-termination Al(111)/MgAlB4(0001) interface presents Al-Al and Al-Mg metallic bonds at the interface, respectively, but the B(Al)-termination Al(111)/MgAlB4(0001) interface expresses strong Al-B covalent bonds characteristic, which leads to the highest interface stability. The results of tensile fracture revealed that the HCP stacked B(Al)-termination interface transferred the external stress to Al bulk, due to the Al-B covalent bond formed near the interface. Therefore, ceramic phase MgAlB4 can effectively promote the particle reinforcement of Al matrix composites.

Ductile fracture simulation on 10.9 HP bolts under composite loading at elevated temperatures

AbstractThe mechanical properties of the bolts deteriorate significantly at elevated temperatures; accurate prediction of the performance and fracture behaviour of steel bolted connections is essential for the fire safety assessment of steel structures and is a prerequisite for performance‐based fire safety design. Existing studies on the fracture mechanical properties of bolts at elevated temperatures are limited due to the difficulty and economics of testing. This paper presents the numerical analysis of combined loads on grade 10.9 high‐performance (HP) bolts. Based on the proven fracture analysis method, the fracture behaviour of HP bolts at high temperatures was investigated by fitting material parameters. Significant changes in the fracture shape of the bolts at high temperatures were found. The ultimate resistance, fracture deformation behaviour and strength reduction factors were analyzed. This study demonstrates the potential of using numerical analysis to reasonably predict the fracture behaviour of steel bolted joints at high temperatures and under complex loading conditions.

Failure analysis on outer tube breaks of a double-sleeve rapid cooling heat exchanger

Failure analysis of a double-sleeve rapid cooling heat exchanger used for ethylene cracking process is presented in this study. The outer tubes of the heat exchanger break four times in succession. The inner tube and the outer tube materials of the heat exchanger are 13CrMo4-5 and SA106Gr.B, respectively. Based on the analysis of fracture appearance, chemical composition, mechanical properties, oxidation products and metallographic microstructure, it is found that short-time local overheating resulted in the four outer tube breaks. The observed pearlite spheroidization of SA106Gr.B indicate the temperature at the break positions exceed the phase transition critical temperature AC1（735℃）. Based on the analysis of the heat transfer process and boiler water circulation, it is found that the heat transfer deterioration of in a single set of double-sleeve, i.e., water is completely converted into superheated saturated steam, is the root cause of short-time local overheating. In view of the above failure mechanism, several countermeasures were proposed including the verification of the circulation ratio to ensure that the heat absorption per unit volume of cooling water is within the threshold value should be conducted in the design process, considering the uneven distribution of cooling water and cracking gas at the inlet and the influence of shock vibration caused by water and cracked gas, an appropriate safety factor should be considered for the design of gap / water volume during operation, as well as promoting the monitoring of vital parameters (e.g., the temperature during operation).

Structure and properties of fine diameter continuous SiC ceramic fiber reinforced Al matrix composites

The polymer-derived-SiC ceramic fibers reinforced 7075 aluminum alloy composites were prepared by hot pressing method. The effect of an in-situ carbon coating on the fiber surface was investigated. The results showed that the uncoated SiC fibers reacted with the Al matrix after hot pressing at 630 °C for 6h. The interfacial bonding was too strong to deflect micro-cracks, leading to a pseudo-brittle fracture with a flexural strength of only 170.7 MPa. When an in-situ carbon coating of 400 nm was prepared on each SiC fiber, the carbon coating reacted with the Al matrix instead of the SiC fiber to form a complex multi-layer interface of 160 nm. The composite had an enhanced strength of 879 MPa. When heat treatment in air, the interfacial bonding strength of the two composites both gradually decreased, making the fracture mode and mechanical properties change finally.

Fracture mechanics behavior of coarse-grained MgO–C at room and high temperature

The fracture mechanical properties of coarse-grained MgO–C were investigated in four-point bending tests at single-edge V-notched beams (SEVNB) at room and high temperature. For measurement of the crack mouth opening displacement (CMOD) an optical system was used. Room temperature tests were accompanied with microstructural observations of the crack path with the use of digital image correlation to investigate the mechanisms of crack propagation and to determine the crack length. MgO–C showed stable crack propagation at room temperature as well as up to 1500 °C. Increased fracture toughness KIc and work of fracture were observed in the temperature range from 700 °C to 1200 °C. This behavior was correlated with crack closure, quasi-plastic deformation as well as the activation of visco-plastic creep mechanisms above 1200 °C. Additionally, the results were discussed in terms of Hasselman's thermal shock parameters.

Developments to the Bonded Block Modeling technique for Discrete Element simulation of transversely isotropic rocks

In this research, a novel three-dimensional Bonded Block Model (BBM) for the Discrete Element Method is proposed for the simulation of transversely isotropic rocks. The model stands on three new developments proposed to the conventional BBM: the adoption of Rigid Body Spring Network (RBSN) bonds combined with a fictitious stress method for the realization of transversely isotropic elastic behavior; a replaceable particle scheme for the explicit representation of cleavage fracture; and a dual Cohesive Zone Model for the simulation of fracture propagation on cleavage and rock matrix according to non-linear fracture mechanics theory. The model is verified by comparing numerical unconfined compression, direct tension and three point bending tests’ results with analytical and finite element solutions, and it is validated against laboratory data reported in literature for three transversely isotropic rock types. The results show that the improved BBM provides for mesh objective numerical representation of the anisotropy in deformability, peak strength and mode-I non-linear fracture mechanics properties of transversely isotropic rocks. In addition, the new model manifests realistic failure modes and the macroscale properties can be defined directly, without trial-and-error calibrations.

Tensile and long-term creep properties of structurally and functionally integrated carbon fiber reinforced plastic at room temperature

Carbon fiber reinforced plastic (CFRP) based on phenolic resin have been widely used in aerospace and pressure vessel fields because of their high stiffness to weight ratio and excellent mechanical properties. In this paper, carbon fiber reinforced plastic were prepared by molding method using phenolic resin as the matrix. The fiber distribution inside the material and its influence on tensile fracture mechanism and creep properties were investigated. The specimens were prepared along horizontal and vertical directions. The test results show that the fibers tend to be distributed horizontally within the material and are influenced by the mold. Horizontal specimens with a large number of fibers parallel to the tensile direction exhibit more excellent tensile properties. The vertical specimen is loaded by the resin and interface with weaker properties inside. The fracture mechanism of horizontal specimens is fiber breakage, fiber pullout and resin fracture, while the fracture mechanism of vertical specimens is interfacial debonding and resin fracture. The difference in tensile fracture mechanism of horizontal and vertical specimens affects the creep properties of CFRP. The creep resistance of fiber is better than that of resin. Therefore, horizontal specimens exhibit better creep resistance in long-term creep tests due to the loads borne by the fibers. Modified Time Hardening model can accurately fit all experimental data. Creep curves can be predicted based on the model parameters.

Fracture of porcine aorta—Part 2: FEM modelling and inverse parameter identification

The mechanics of vascular tissue, particularly its fracture properties, are crucial in the onset and progression of vascular diseases. Vascular tissue properties are complex, and the identification of fracture mechanical properties relies on robust and efficient numerical tools. In this study, we propose a parameter identification pipeline to extract tissue properties from force-displacement and digital image correlation (DIC) data. The data has been acquired by symconCT testing porcine aorta wall specimens. Vascular tissue is modelled as a non-linear viscoelastic isotropic solid, and an isotropic cohesive zone model describes tissue fracture. The model closely replicated the experimental observations and identified the fracture energies of 1.57±0.82 kJ m−2 and 0.96±0.34 kJ m−2 for rupturing the porcine aortic media along the circumferential and axial directions, respectively. The identified strength was always below 350 kPa, a value significantly lower than identified through classical protocols, such as simple tension, and sheds new light on the resilience of the aorta. Further refinements to the model, such as considering rate effects in the fracture process zone and tissue anisotropy, could have improved the simulation results. Statement of significanceThis paper identified porcine aorta’s biomechanical properties using data acquired through a previously developed experimental protocol, the symmetry-constraint compact tension test. An implicit finite element method model mimicked the test, and a two-step approach identified the material’s elastic and fracture properties directly from force-displacement curves and digital image correlation-based strain measurements. Our findings show a lower strength of the abdominal aorta as compared to the literature, which may have significant implications for the clinical evaluation of the risk of aortic rupture.

Fracture mechanical behavior of fine-grained carbon-bonded alumina at room and high temperature

The fracture mechanical properties of fine-grained carbon-bonded alumina (Al2O3–C) were investigated in four-point bending tests at single edged V-notched beams (SEVNB) at room and high temperature. Graphite served as a reference material. For measurement of the crack mouth opening displacement (CMOD) an optical system was used. The fracture behavior of Al2O3–C changed from brittle at room temperature to a crack resistance behavior at 1400 °C due to visco-plastic phenomena. The fracture toughness KIc was determined as 0.69 ± 0.07 MPa m1/2 at room temperature and 0.55 ± 0.21 MPa m1/2 at 1400 °C. At room temperature, the work of fracture γwof was determined as 26.3 ± 6.3 J/m2, and Hasselman's thermal shock parameters Rst and R'''' were 8.6 K m1/2 and 0.9 mm, respectively. Although a quantitative analysis of the work of fracture at 1400 °C was hindered by heat shimmer within the chamber, the results indicated significant increases of this parameter as well as Hasselman's thermal shock parameters up to 1400 °C.

Gradient changes in fiber bundle content and mechanical properties lead to asymmetric bending of bamboo

Natural biomaterial bamboo has excellent strength and toughness. The content of fiber bundles and the gradually increasing tensile strength of fiber bundles from the inside to the outside in the radial direction of the bamboo wall are the two main factors leading to significant differences in the fracture behavior and mechanical properties of bamboo under loading from different sides of bamboo strip. In this study, the flexural strength, toughness, and bending fracture energy of the bamboo loaded from the inside of the sample (Mode B) were found as 28%, 191%, and 174%, respectively, higher than those of the bamboo loaded from the outside of the sample (Mode A). The mechanical properties of samples loaded radially (Mode C) were higher than Mode A and lower than Mode B. The fracture process of the sample and the corresponding fracture morphology showed parenchyma fracture (matrix failure), fiber fracture, and interfacial debonding (fiber–fiber, fiber-parenchyma, parenchyma-parenchyma) occurring in all samples. Tearing of the cell wall, fiber debonding, fiber bridging, fiber pull-out, and crack deflection are the major fracture-toughening mechanisms of bamboo. This study reveals the relationship between the gradient changes in the mechanical properties and content of bamboo fiber bundles and the bending and fracturing behavior of bamboo, which provides theoretical basis and ideas for the development of new bamboo engineering materials, the bionic design of bamboo structures, and the structural optimization of fiber-reinforced composite materials.

Post-fatigue fracture load, stress concentration and mechanical properties of feldspathic, leucite- and lithium disilicate-reinforced glass ceramics

ObjectiveTo evaluate the mechanical properties of different CAD/CAM ceramic systems and the post-fatigue fracture and stress distribution when used as cemented crowns. Materials and methodsSixty (60) CAD/CAM monolithic crowns were milled using three different ceramic materials (FD – Feldspathic [Vita Mark II]), LE - Leucite-based ceramic [IPS Empress CAD] and LD - Lithium Disilicate [IPS e.max CAD]) and adhesively cemented on resin composite dyes. Specimens were stored in distillated water (37 °C) for 7 days. After, half of the crowns were submitted to immediate fracture load test while the other half was submitted to fatigue cycling. The average cement layer of approximately 80 μm was assessed using scanning electron microscopy (SEM). The average thickness was used in the three-dimensional (3D) Finite Element Analysis (FEA). For each ceramic material, the density, Poisson ratio, shear modulus, Young modulus, fracture toughness, and true hardness were assessed (n = 3). The data was used to assess the Maximum Principal Stress throughout 3D-FEA according to each material during load to fail and post-fatigue. Data were submitted to two-way ANOVA and Tukey test (α = 0.05). ResultsLD showed the highest compression load, density, shear modulus, Young modulus, fracture toughness and true hardness values. While LE presented the lowest mechanical properties values. There is no difference in the Poisson ratio between the evaluated ceramics. ConclusionLD was susceptible to aging process but presented stronger physicomechanical properties, showing the highest post-fatigue fracture load and highest stress magnitude.

On fabrication of acrylonitrile butadiene styrene-zirconium oxide composite feedstock for 3D printing-based rapid tooling applications

The 3D-printed rapid tools are being used in finishing operations such as drilling, milling, broaching, roller burnishing, and other finishing operations that need anti-wear plastic composite materials. Zirconium oxide (ZrO2) is one of the ceramic materials which is highly appreciated due to its anti-wear properties. This study aims to develop the ZrO2 ceramic particles reinforced acrylonitrile butadiene styrene (ABS) thermoplastic composite feedstock filaments for 3D printing of rapid tools. In the first stage, the multiple numbers (as per Taguchi L9 orthogonal array (OA)) of ABS-ZrO2 feedstock filaments were developed by varying the loading of ZrO2 in ABS matrix (2 wt.%, 4 wt.%, and 6 wt.%), processing temperature (200, 205 and 210°C), and rotation speed of screw (4, 6 and 8 RPM). The optimum setting obtained for manufacturing ABS-ZrO2 composite feedstock filaments is the combination of 2% ZrO2 loading, 205°C processing temperature, and 6 RPM screw speed. In the next stage, fused filament fabrication (FFF) based 3D printing has been used to prepare the rapid tools. The wear test performed for 3D printed ABS-ZrO2 composites rapid tools shows only .62% weight loss which is lower as compared to virgin ABS (.91% weight loss). The results of the study are supported by fracture analysis, morphology, and mechanical properties.

Evaluation of fracture behavior of lightweight magnesia‐based refractories via wedge splitting test along with acoustic emission detection

AbstractThe effect of spinel powder on the fracture behavior and mechanical properties of lightweight magnesia‐based refractories containing microporous magnesia aggregates with high apparent porosity (37.4%) were investigated by the wedge splitting test (WST) with the digital image correlation and acoustic emission. With the addition of spinel powder, lightweight magnesia spinel refractories showed a higher cold compressive strength compared with lightweight pure magnesia refractories. From the WST, the addition of spinel powder increased the specific fracture energy and characteristic length of lightweight magnesia spinel refractories, which improved the crack propagation resistance. The increased tortuosity of main crack and a higher ratio of crack propagation along the aggregates/matrix interface were main reasons for reducing the brittleness of lightweight magnesia spinel refractories. Besides, acoustic emission (AE) signal activity indicated that the propagation of pregenerated micro‐cracks by the thermal mismatch and the development of fracture progress zone were primary ways to consume energy in lightweight magnesia spinel refractories. The reduced proportion of crack propagation within aggregates was also detected by the peak frequency of AE signals in lightweight magnesia spinel refractories. For microporous magnesia aggregates with high apparent porosity (37.4%), lightweight magnesia spinel refractories also showed reduced brittleness fracture behavior than lightweight pure magnesia refractories.