Effect of microporous aggregates and spinel powder on fracture behavior of magnesia-based refractories

The influences of microporous aggregates and spinel powder on the properties and fracture behavior of magnesia-based refractories were investigated by the three-point bending test and wedge splitting test with the digital image correlation method. With microporous aggregates instead of dense ones, lower thermal conductivity, higher cold modulus of rupture and compressive strength were observed for lightweight magnesia-based refractories. Besides, the results indicate that the strengthened interlocking interface between microporous aggregates and matrix in lightweight magnesia refractories decreased the proportion of crack propagation along the aggregate/matrix interface (P_AM). This reduced the tortuosity of crack propagation as well as increased the brittleness. With the addition of spinel powder in the matrix, the pregenerated microcracks by thermal mismatch increased the P_AM, which increased the tortuosity of crack propagation, improved fracture energy and reduced the brittleness. Lightweight magnesia spinel refractories merely showed a slightly higher brittleness than dense ones.     

Mechanical and fracture investigation of magnesia refractories with acoustic emission-based method

In this study, the fracture behaviour of magnesia, magnesia chrome and magnesia spinel (MgAl₂O₄ and FeAl₂O₄) refractories under wedge splitting test are qualitatively and quantitatively investigated with the acoustic emission (AE) and digital image correlation (DIC). First of all, the concepts of characteristic widths are proposed for estimating the brittleness of refractory materials according to the shape of load-displacement curve and validated by their good correlation with the characteristic length. Besides, the AE data are analyzed with AE parameter-based approaches and offer new insight into the fracture behaviour of refractory materials, including the classification of the cracking events in grains and in matrix, the distinction between the tensile mode and shear mode damage, and the visualization of the fracture process zone development. It confirms that the pre-existing micro-crack networks in refractories are favourable for the brittleness reduction, which enhance their nonlinear fracture behaviour and thermal shock resistance.     

Mixed-mode fracture behaviour of refractories with asymmetric wedge splitting test. Part II: Experimental case study

The asymmetric wedge splitting test for performing mixed-mode loading and its numerical evaluation has been presented in a companion paper (Part I). In this work (Part II), the influences of various levels of mode II loading on damage behaviour of refractories with different brittleness were experimentally investigated by comparing mode I and mixed-mode fractures under symmetric and asymmetric wedge splitting loading with seven different wedge angles. The digital image correlation technique was also used for strain maps visualization as well as the deformation parameters acquisition.With the increase of asymmetric wedge angle, the fracture behaviour becomes unstable what is associated with steeper load-displacement curves, more instantaneous energy release and restrained fracture process zone development. The in-plane shear loading contributes to the accelerated extension of the crack tip and its deviation from central plane. Meanwhile, the co-existing local shear stresses caused by the refractory's heterogeneity lead to crack path deflection as well.     

Determination of the fracture behaviour of MgO-refractories using multi-cycle wedge splitting test and digital image correlation

Refractories with reduced brittleness show a pronounced deviation from linear elastic behaviour and an enhanced thermal shock resistance. This paper aims to study the influence of microstructure on the fracture behaviour of magnesia refractories. The wedge splitting test(WST), which enables stable crack propagation for quasi-brittle materials, was used to identify the fracture behaviour and evaluate the energy dissipation. The evaluation of the crack lengths of the magnesia and magnesia spinel materials during the entire cyclic WST is based on the localized strain evaluated using the digital image correlation (DIC). A significant fracture process zone develops in the magnesia spinel material. The relationship between the dissipated energy and the actual crack length, which was used to characterize the crack growth resistance, was determined. The refractory materials that showed reduced brittleness consume a small amount of energy for fracture initiation but a large amount of energy for further crack propagation.     

Investigation of fracture behavior of cement-bonded corundum castables using wedge splitting test and digital image correlation method

To investigate the fracture behavior of cement-bonded corundum castables, various cement contents and pre-treating temperatures have been comparatively studied using the wedge splitting method and the digital image correlation technique. The results show that the microstructure enhances the mechanical properties, so the fracture energy and the maximum load as well as the fracture modes are affected correspondingly. The castables demonstrate the highest fracture energy and maximum load at 1600 °C with cement content of 10 wt% due to an appropriate amount of CA6. At the temperatures of 110 and 1100 °C, the crack propagation within the matrix and along the interface are dominated whereas within the aggregates significantly increased at 1600 °C, leading to the brittleness of materials. However, increasing the cement content can reduce their brittleness, caused by the maximum strain in thex-direction, largest length of the main crack, and high ratio of crack propagation in the matrix.     

Radial variations in fracture and acoustic emission of a thermic graphite electrode

For an extruded electrode graphite, grain orientation increases and porosity decreases with increasing radial distance from the centre of the log. Radial variation in grain orientation is produced during the extrusion stage of manufacture. The radial porosity gradient may be influenced partly by orientation of grains and also by the impregnation and baking stages of manufacture. Dynamic elastic modulus, flexural strength and critical stress intensity factor increase with distance from the centre of the log; these trends are related to the radial porosity gradient. Fracture toughness and the total number of acoustic emissions at fracture, σ_AE, are insensitive to graphite microstructure. Fracture of the graphite is non-linear and deviations from linear elastic fracture mechanics increase with distance from the centre of the log, a trend which is related to the radial porosity gradient. An empirical correlation between a non-ideal fracture parameter and σ_AE is demonstrated for the electrode graphite and three nuclear graphites.     

Fracture energy evaluation of refractories in wedge splitting tests from notch opening displacements

The work of fracture of refractories is commonly calculated from crack mouth opening displacements (CMODs) in wedge splitting tests (WSTs). This paper proposes a methodology for estimating the fracture energy from notch opening displacement (NOD) measurements, which is useful for setups where CMOD is not accessible. NODs and CMODs are calculated for both faces of two WSTs experiments on a castable refractory via digital image correlation (DIC) and finite element simulations. A quadratic function fits well the non-linear CMOD vs. NOD behavior in the crack initiation regime, while an affine trend describes the propagation regime. Although the nonlinearity associated with crack initiation is more complex, the crack propagation energy can easily be estimated from NOD data when CMODs cannot be measured.     

Mechanical characterization and damage assessment of thick adhesives for wind turbine blades using acoustic emission and digital image correlation techniques

Adhesives play a key role in the structural integrity of the Wind Turbine Blades as they are one of the main load carrying materials. A deep knowledge of the adhesives' mechanical behaviour in terms of failure mechanisms and damage processes enhances the attempt to optimize the blade design. Therefore, a comprehensive experimental programme was performed in order to determine the static mechanical properties of the adhesives. Ultimate tensile strength, ultimate compression strength, ultimate shear strength and the elastic properties of the adhesive specimens were determined through tensile and compression tests on dogbone specimens and single-lap bonded joints. The Acoustic Emission (AE) technique was used to relate the acoustic activity in the specimens to their damage state. More specifically, a frequency-based methodology, analysing the AE data, was used for the identification of the different damage mechanisms into the material during the loading. In addition, Digital Image Correlation technique, as a full-field technique, was used to measure displacements and deformations.     

基于声发射监测的岩石水压致裂实验研究及数值模拟分析

采用岩石破坏过程渗流-应力耦合分析软件,考虑岩石岩性、节理裂隙状态、注水孔形状及大小、岩石强度、密度、渗透率及加载方式等影响因素,模拟研究了岩石水压致裂过程中的岩石损伤演化过程及破裂规律.同时结合室内物理实验和水压致裂数值模拟,对两种条件下的模拟实验结果及其应力—应变曲线进行对比分析,揭示了岩石水压致裂过程的裂隙岩体失稳规律,得出深部矿井采动渗水和突水过程中,覆岩裂纹起裂、扩展、渗水、突水,失稳破裂的全过程规律,数值模拟结果和实验结果具有较好的一致性,对矿山防治突水灾害有一定的理论指导意义.     

Mixed-mode fracture behaviour of refractories with asymmetric wedge splitting test. Part I: Numerical implementation and validation

The suitability of asymmetric wedge splitting test (WST) for mode I/II mixed-loading was validated by FE simulation with a three-dimensional heterogeneous continuum FE model. The unsymmetrical strain and stress patterns were observed for mixed-mode loading. Compared with mode I loading of symmetric WST, the introduction of in-plane shear accelerates the crack extension and deviation from symmetry plane with a smaller fracture process zone. Additionally, the asymmetric WST with small, medium and large wedge angles were simulated for sensitivity analysis. With the increasing of asymmetric wedge angle, the recorded vertical load-displacement curves turn from “mild” to “steep”, and the ratio of mode I to mode II fracture energy G_I/G_II decreases accordingly while the symmetric one has the highest G_I proportion. The asymmetric WST with large wedge angle deforms the most at same applied vertical loading displacement, while all WSTs show similar damaged elements amount at the same crack mouth opening displacement.     

Fracture behaviour of magnesia refractory materials in tension with the Brazilian test

In this work, the tensile failure of magnesia, rebound magnesia-chrome and chrome-containing magnesia-spinel refractories under the Brazilian test were investigated. The digital image correlation and acoustic emission were applied simultaneously for ensuring the validity of Brazilian test and studying the fracture process. The brittle refractories fail abruptly while reaching their load peaks because of the unstable crack propagation. However, the chrome-containing magnesia-spinel refractory shows a reduced brittleness due to the pre-existing microcracks, which promotes quasi-stable crack propagation evidenced by the nonlinearity in the pre-peak region and the softening in the post-peak region. Besides, the thickness-to-diameter ratio has a great influence on the fracture behaviour, which also shows brittleness dependence. The fracture behaviour of rebound magnesia-chrome refractory varies from brittle to less brittle while the thickness increasing from 10 mm to 50 mm. The quasi-stable crack propagation favors the central crack initiation and ensures the tensile failure under the Brazilian test.     

Characterizing environment-dependent fracture mechanisms of ceramic matrix composites via digital image correlation

Here, we unveil a methodology for a novel assessment of the fracture mechanics of SiC/SiC ceramic matrix composites enabled by in situ stereoscopic digital image correlation to quantify in-process flexural strain and crack opening displacement measurements. This technique isolates individual cracks on the composite surface as discontinuities in the spatial displacement field and correlates key fracture characteristics with the flexural strain of composite specimens during coupled four-point bend / hermeticity testing. Fracture was observed along the specimen length, originating at the tensile underside and propagating around the circumference of the tubular specimens with generally uniform spacing. Multiple specimens were also tested after heat treatments to 1200°C in open air, in vacuum, and in helium for 48 h to evaluate the environmental effects on the fracture mechanisms of SiC/SiC composites, which revealed degradation of flexural properties after treatment in open air resulting in brittle failure. Indentation-based fracture toughness measurements were performed, which confirmed a 25% reduction in toughness after open air heat treatment relative to the other heat treatments. This assessment indicated that significant oxidation may occur within the composites from these heat treatments and suggested that further protection of the composites may be necessary for high-temperature applications.     

Mechanical properties of illite-based ceramics with controlled porosity studied by modern in situ techniques

Deformation tests combined with modern in situ acoustic emission (AE) and digital image correlation (DIC) techniques were applied to monitor low strain rate compressive behavior of illite-based ceramics with controlled porosity (P). A strong effect of porosity on the mechanical performance was observed: Young's modulus decreased linearly from 29.4 ± 1.1 GPa (P = 14 vol%) to 3.0 ± 0.5 GPa (P = 55 vol%) and compressive strength decreased from 307 ± 13.6 MPa (P = 14 vol%) to 27.7 ± 1.0 MPa (P = 55 vol%). The AE and DIC techniques revealed a transition from brittle fracture to gradual localized crushing with increasing porosity. The AE signals possessed high-energy burst-like characteristics typical of brittle fracture and (micro)cracking. The AE data showed continuous activity from the beginning of loading, suggesting that true elasticity does not occur in this material. The combination of mechanical tests with in situ techniques, therefore, proved to be particularly effective in providing additional information on the deformation dynamics in ceramics.     

初始缝高比对钢纤维混凝土断裂性能的影响

为研究初始缝高比对钢纤维混凝土断裂性能的影响,对三种不同初始缝高比(0.1、0.2、0.3)的钢纤维混凝土开展断裂韧性试验,计算相关的断裂参数,并借助声发射(AE)和数字图像法(DIC)分析了裂缝发展过程中AE能量和全场应变的变化规律。结果表明,钢纤维混凝土的断裂韧性随初始缝高比的增大呈减小趋势,当初始缝高比为0.3时钢纤维混凝土的断裂性能大幅下降。钢纤维混凝土的断裂过程可根据AE能量划分为弹塑性阶段、裂纹稳定扩展阶段和断裂阶段,随初始缝高比增加,弹塑性阶段持续时间减少。DIC分析结果表明,在整个加载过程中,预制裂缝尖端会产生较大的应力集中,裂缝的横向应变较大。研究结果揭示了钢纤维混凝土裂缝发展机理,可为其工程应用提供理论支持。     

Fracture mechanism of alumina-spinel castables containing ZrO2 by wedge splitting test and digital image correlation technique

To clarify the fracture mechanism of alumina-spinel castables, two kinds of alumina-spinel castables with or without fused zirconia-alumina (FZA) were prepared. The full-field strains and crack propagation process in the region of interest (ROI) of alumina-spinel castables were investigated by wedge splitting test (WST) and digital image correlation (DIC) technique. Fractographic methods were used to analyze the crack propagation path of the castables after the WST. The results indicated that the load-displacement curve of alumina-spinel castables containing FZA exhibits a non-linear fracture, demonstrated typical ductile fracture; while that of alumina-spinel castables without FZA is linear, showed typical brittle fracture. The characteristic length reaches to 258.9 mm in FZA containing castables, more than 4 times that of the castables without FZA. In contrast, castables containing FZA has longer and more tortuous crack propagation path, larger damage zone length, which results in the increase of the dissipated energy. Crack branching can be observed around the main crack in castables containing FZA, meaning that microcracks toughening is the main mechanism for flexibility improvement of the alumina-spinel castables containing FZA, formation of micro-cracks can be attributed to the martensitic transformation of zirconia.