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.     

Tensile behaviour of magnesia carbon refractories

The determination of the Young's modulus and the tensile strength of heterogeneous refractories are the subjects of this paper. Great differences have been observed for a similar material according to both the usual tests performed and the interpretation proposed to define these properties. The causes of the discrepancies of the Young's modulus under compression and tensile loading are examined in detail. Then, it is shown that (i) the accuracy measurement of the deflexion in the bend test with a particular device and (ii) the integration of the shearing distorsion in the calculation of the deflexion by the classical beam theory, allow for finding the appropriate value of the Young's modulus. The classical definition of the modulus of rupture (M.O.R.) is also examined. Considering a nonlinear behaviour of the refractory, it is shown by finite element analysis of the beam, that the M.O.R. overestimates the tensile strength.     

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.     

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.     

On the calibration of cohesive parameters for refractories from notch opening displacements in wedge splitting tests

Cohesive elements are commonly used to describe crack propagation in heterogeneous materials with toughening mechanisms. This work aims to provide a guideline on how these fracture parameters can be calibrated using notch opening displacements (NODs) measured via digital image correlation and force data from wedge splitting tests (WSTs). Weighted finite element model updating was applied to calibrate material and boundary condition parameters in the same framework. The influence of each parameter on force and NOD data are given together with uncertainties for the calibrated parameters. Numerical results were in very good agreement in terms of splitting force, NOD, displacement and gray level residual fields. It is shown that images obtained during WSTs focusing on the crack path (i.e., hiding the loading region) can be used to drive numerical simulations and obtain cohesive parameters.     

Thermomechanical and deformation properties of periclase and magnesia-spinel refractories

Conclusions The strength of periclase and magnesia-spinel refractories at normal temperatures varies in wide limits and is determined by the quantity, shape, dimensions, and distribution of the pores and microcracks. At high temperatures the degree of influence of the pores and microcracks on the strength is reduced.In the region of medium temperatures (600–1200°C) there is a temporary strengthening and an increase in E_d of the magnesia-spinel refractories, due to the internal stresses caused by the difference in the coefficient of linear thermal expansion of the spinels and periclase.With further increase in temperature the articles' strength and E_d are reduced as a result of the softening of the silicate bond. The abradability of the refractories depends slightly on the porosity but strongly on the compressive strength.The least creep is typical of refractories made of fused materials (PKhVP) with a comparatively low concentration of silicate phase, the viscous flow of which at temperatures above 1450°C determines the plastic deformation of the periclase and magnesia-spinel articles made industrially.Periclase refractories containing 2.5–3% silicates in the form of montichellite are brittle up to 1400°C and above this temperature they are elastoplastic, while articles and tiles containing up to 0.5% silicates in the form of Ca₂SiO₄ may be brittle up to 1600°C. They possess high thermomechanical properties and wear resistance in service in gate valves for steel ladles.The wear under friction of periclase tiles at various temperatures is determined by the crystal structure, the change in strength of the materials, and the state of the rubbing surfaces, and also by the adhesion and deformation reactions.Translated from Ogneupory, No. 1, pp. 7–14, January, 1983.     

Influence of thermal damage occurrence at microstructural scale on the thermomechanical behaviour of magnesia-spinel refractories

Many refractory materials exhibit high thermal shock resistance, which is often mostly due to their high flexibility. Understanding the microstructure key points allowing to develop a non-linear mechanical behaviour is of great relevance for future material improvements. The present work aims at optimising the processing of magnesia-spinel refractory materials close to industrial ones with simplified microstructures. The final goal is the investigation of the relationship existing between microstructure evolutions and induced thermomechanical properties. The thermal expansion mismatch which exists between the two phases (spinel inclusions and magnesia matrix) is expected to generate, during cooling, radial microcracks around the inclusions. The development of such microcracks network, closely related to the inclusions content, has been studied and the damage occurrence has been confirmed by several high temperature characterisation techniques. The influence of this thermal micro damage on the evolution of stress-strain law in tension of such materials has also been investigated.     

Combined damaged elasticity and creep modeling of ceramics with wedge splitting tests

During the crack propagation in common refractory ceramics at high temperatures, creep may occur in the wake of a process zone and in front of a crack tip. To account for this phenomenon, an integrated material constitutive model was developed by combining the mechanical behavior following isotropic damaged elasticity concept and Norton-Bailey creep. The post peak fracture behavior followed the bilinear softening law and a simple criterion was defined to consider the creep asymmetricity in uniaxial tension and compression. The material constitutive model was applied to inversely identify mode I fracture parameters with wedge splitting tests of an alumina spinel material at 1200 °C. It showed that the mean ratio of the nominal notch tensile strength to the actual tensile strength was 1.93 and the mean pure fracture energy was 297.6 N/m. In addition, the creep contributed 12.9% on average into the total fracture energy.     

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

The 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.     

Cr_2O_3对方镁石－尖晶石耐火材料抗渣性的影响

采用电熔镁砂及预合成电熔ＭＡ砂为原料，在高温下烧成方镁石－尖晶石试样，当加入少量铬矿时可提高其热震稳定性及抗渣性。用坩埚法作富ＣａＯ转炉渣对方镁石－尖晶石系耐火材料的侵蚀试验后，采用ＸＲＤ、ＳＥＭ、ＥＤＡＸ、岩相分析等方法分析Ｃｒ_２Ｏ_３对方镁石－尖晶石耐火材料抗渣性的影响。     

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.     

Service tests of chrome-spinel-periclase refractories

Conclusions In chemical composition the experimental chrome-spinel-periclase refractories are characterized by a high degree of purity. The total content of impurity oxides of silicon, calcium, and iron does not exceed 2%. The parts possess increased density, porosity, and slag resistance but a lower heat resistance than the imported refractories with a silica content of 0.5%.Comparative tests of the experimental chrome-spinel-periclase parts (area of about 0.5 m²) and the imported refractories with a silica content of 0.5% in the slag belt of an ASEA-SKF installation ladle showed similar lives of them. For a final experimental test it would be desirable to produce a larger experimental production lot of the chrome-spinel-periclase refractories and to test them in the slag zone of ASEA-SKF ladles.Translated from Ogneupory, No. 1, pp. 44–47, January, 1985.     

Comparison of two full-field identification methods for the wedge splitting test on a refractory

Two full-field identification methods are applied to the Wedge Splitting Test (WST) to obtain crack tip positions, stress intensity factors (SIFs) and T-stress. The first method is based on Finite Element Model Updating (FEMU), and the second is integrated digital image correlation (IDIC). Both are applied to a simplified virtual experiment and then to a cyclic WST. The gray level residuals are used to assess which results are more trustworthy. Fracture energy analyses are performed to validate the estimated R-curves.     

Effects of Cr2O3 addition on property improvement of magnesia-spinel refractories used in RH snorkel

The Ruhrstahl-Heraeus (RH) snorkel is one of the most important parts of the RH furnace, which is the primary facility for producing high-quality steel. The magnesia-spinel refractory is the ideal replacement for the classic magnesia-chrome refractory in the RH snorkel. In this study, the effects of Cr₂O₃ addition (0–3 wt%) on properties of magnesia-spinel refractories were investigated. It was found that the addition of Cr₂O₃ results in the acceleration of the densification process and a decrease in the cold strength as well as a significant increase in the hot modulus of rupture. Thus, the solid solution formed with the spinel significantly improves the resistance to thermal shocks, with the greatest improvement being observed after the addition of 2 wt% Cr₂O₃.     

Prediction of splitting tensile strength of high-performance concrete

Splitting tensile strength (STS) is one of the concrete mechanical properties that are used in structural design. It can be related to numerous parameters, which include compressive strength, water/binder (W/B) ratio and concrete age. Until now, most researchers estimated the STS directly from compressive strength data. This paper suggests formulae that relate STS with that of compressive strength, W/B ratio and concrete age. The predicted STS can be obtained accurately using these formulae. It is proposed that the equation with the concrete age (t) parameter be used in predicting the STS of high-performance concrete (HPC).     

Tensile behaviour of homogeneous ethylene copolymers

The tensile behaviour of homogeneous ethylene–butene copolymer and heterogeneous ethylene–butene copolymer has been compared in uniaxial tension. The homogeneous copolymer exhibits a more marked double yield behaviour. Composition distribution and heat treatment greatly influence the double yield behaviour of ethylene copolymer. A more homogeneous composition distribution has the same effect as rising draw temperature and decreasing draw rate on the double yield behaviour of ethylene copolymers. It is found that the occurrence of the double yield phenomenon is distinguished by the difference between two yield strains. Metallocene‐based ethylene copolymer shows better tensile break properties than conventional ethylene copolymer. © 2002 Society of Chemical Industry     

Tensile drawing behaviour of polyethylene terephthalate

A wide range of polyethylene terephthalate fibres was prepared by melt spinning to different degrees of molecular orientation. The tensile drawing behaviour of these fibers was then studied, either by drawing over a heated cylinder at 85°C or by drawing over both a heated cylinder and a plate at 180°C. The mechanical properties and structure of the subsequent drawn fibres are discussed in terms of the network draw ratio, determined by matching true stress–strain curves for the drawn and initial melt spun fibres. It is shown that this procedure provides valuable insight into subtle differences in properties and structure that can arise from differences in processing routes.     

Compressive fatigue behaviour of refractories with carbonaceous binders

Furnace linings of magnesia-carbon and micro-porous carbon bricks experience cyclic compressive loads. An experimental programme has been carried out to assess the sensitivity of these materials to compressive fatigue failure. Next to room temperature tests, a number of high temperature tests have been performed. Results of the fatigue tests have been analysed together with the data of the monotonic stress–strain loading and creep tests. Compressive fatigue failure has been seen in both the materials. The less brittle material has shown lower fatigue life. The curve relating the fatigue strains with the amount of cycles has been of classical sigmoid shape with three phases. The strain rates of the secondary (linear) phase have shown good correlation with the number of cycles to failure. The grain–matrix interface has been found to play the critical role in the initiation and propagation of the fatigue cracks.