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.     

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.     

Fracture behaviour of magnesia and magnesia–spinel composites before and after thermal shock

Microstructural changes, as a consequence of the thermal expansion mismatch between magnesia and spinel phases, and fracture behaviour of magnesia and spinel composites have been investigated as a function of spinel content (10, 20, 30 wt.%). Fracture surfaces of magnesia showed mostly transgranular fracture; for the composites, however, the amount of intergranular fracture increased with increasing spinel content. This change in fracture behaviour is thought to be the main reason for the increase in the work of fracture, γ_WOF. The 30% spinel composite was found to exhibit both the greatest resistance to crack propagation, and the greatest resistance to thermal shock damage, with the highest retained strength after quenching.     

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.     

Observation and quantification of the fracture process zone for two magnesia refractories with different brittleness

In this paper, the formation of the fracture process zone (FPZ) of industrially produced magnesia spinel and magnesia refractories was analysed using digital image correlation (DIC). Compared to pure magnesia materials, the magnesia spinel materials exhibited a higher amount of microcracks, causing a larger FPZ. A critical displacement, where the cohesive stress between the crack faces decreases to zero, is determined by analysing the development of the localized zone. Critical displacement determined from the changes of the FPZ width and length is used to determine the onset of macro-cracking and locate the crack tip. The development of the fracture process zone for a magnesia spinel initiates before reaching the maximum load, and the onset of the macro-crack is in the post-peak region. The FPZ size increases until the formation of a macro-crack takes place, but decreases afterwards. For the magnesia refractory, no pronounced FPZ could be detected.     

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.     

R-curves determination of ordinary refractory ceramics assisted by digital image correlation method

As a figure-of-merit, the rising ratio of crack propagation resistance to fracture initiation resistance indicates a reduction of the brittleness and enhances the thermal shock resistance of ordinary refractory ceramics. The significant nonlinear fracture behaviour is related to the development of a fracture process zone (FPZ). The universal dimensionless load–displacement diagram method is applied as a promising graphical method for the determination of R-curves for magnesia refractories showing different brittleness. By applying digital image correlation (DIC) together with the graphical method, the problems arisen with accurate determination of the fracture initiation resistance and the crack length are overcome. Meanwhile, the R-curve is subdivided with respect to the fracture processes, viz the fracture initiation, the development of FPZ and the onset of traction free macro-crack. With the simultaneous crack lengths evaluated from DIC, the contribution of each fracture process to the crack propagation resistance at certain loading stage is quantitatively presented.     

Investigation of microstructure-property relantionships of magnesia-hercynite refractory composites by a refined digital image correlation technique

Industrial magnesia-spinel bricks destined for thermal shock applications often show more flexibility and improved crack growth resistance. Components from the spinel structure group are usually added to promote microcracking coming from thermal expansion mismatch. This leads to the development of toughening mechanisms that are very effective in improving the crack propagation resistance.Magnesia-hercynite composites were investigated in order to highlight their fracture process, with regard to their microstructure, by using Digital Image Correlation (DIC). The direct measurement of displacement fields between digital images of the reference state and the deformed one has provided valuable information on material deformation during loading. The aim of this work was to investigate the fracture behaviour of refractories through the coupling of the Wedge Splitting Test (WST) and DIC. By using a refined DIC process transformation taking into account a discontinuity of displacement, called 2P-DIC, a more effective characterisation of the fracture behaviour was achieved.     

Combination of Brazilian test and digital image correlation for mechanical characterization of refractory materials

Flexibility of refractories is an interesting property to improve thermal shock resistance of refractories. It can often be obtained by promoting a nonlinear mechanical behaviour which reduces brittleness. Among industrial refractory materials developed in this aim, magnesia hercynite is of a particular interest for cement industry. As linings for rotary kilns, magnesia hercynite can be submitted to tensile and compressive stresses. Since tensile strength is usually much lower than compressive one for brittle materials, the mechanical characterization is, in such case, more significant in tension than in compression. To overcome difficulties involved by direct tensile test, an indirect tensile test (Brazilian test) has been applied here and combined to digital image correlation in order to measure kinematic fields on the surface of the sample during loading. This combination has allowed to accurately measure initial elastic properties (Young’s modulus and Poisson ratio), to detect crack initiation and to analyze fracture process.     

Effects of Chrome Oxide Concentrate Powder on Properties of Magnesia Composite Materials

The magnesia composite materials were prepared using fused magnesia,used magnesia chrome bricks, magnesium aluminate spinel, alumina powder and chrome oxide powder as starting materials. In order to improve the performance of magnesia composite material,the influence of chrome oxide concentrate powder on the properties of magnesia composite material was researched. The apparent porosity,bulk density,cold crushing strength,linear expansion and thermal shock resistance of the specimens were determined. The results show that adding chrome oxide concentrate promotes the formation of the composite phases of three kinds of spinel,which is beneficial to the thermal shock resistance. The magnesia chrome spinel can not be formed at high temperatures theoretically,but can dissolve in the magnesia composite material. Comprehensively,the optimal addition of chrome oxide concentrate is 10. 0 mass% for the magnesia composite materials.     

TiO2对电熔合成镁铝尖晶石的影响

以轻烧氧化镁粉、工业氧化铝为原料合成电熔镁铝尖晶石材料,重点研究二氧化钛为添加剂对电熔合成镁铝尖晶石的物相组成,显微结构的影响,通过XRD,SEM分析试样的物相组成、晶胞参数和断口的微观形貌.研究发现:二氧化钛为外加剂可以增大镁铝尖晶石的晶胞参数、提高材料的体积密度进而改善材料的烧结性能,并且随着二氧化钛加入量的增加镁铝尖晶石的晶胞参数和晶胞体积呈现先增大后减小的变化规律.分析认为由于二氧化钛、钛酸镁固溶到镁铝尖晶石中促使镁铝尖晶石的晶胞参数和晶胞体积增大,而过量的钛酸镁位于晶界阻碍镁铝尖晶石的长大,导致镁铝尖晶石的晶胞参数和晶胞体积减小.在电熔法制备镁铝尖晶石时,二氧化钛的加入量不宜超过5wt%.     

Fracture behavior of magnesia refractory materials under combined cyclic thermal shock and mechanical loading conditions

To investigate the effect of cyclic thermal shock and mechanical loading on the fracture behavior of magnesia refractories showing different brittleness, the as-received and cyclic thermally shocked specimens are subjected to monotonic and cyclic wedge splitting test. The whole duration of test is monitored by digital image correlation and acoustic emission. Both thermal and mechanical fatigue resistance increase with the reduction of brittleness. Repetitive thermal shock results in pronounced reduction of strength. However, the specific fracture energy and nonlinearity increase after exposure to thermal shock due to the expanded micro-crack network inducing the development of a significant fracture process zone. Periodic loading mainly leads to the decrease of strain bearing capacity, as the fatigue loads favor the extension of crack tip instead of fracture process zone expansion. The combined application of periodic thermal shock and mechanical loads gives a new insight into the progressive damage behavior of refractory under critical conditions.     

A novel lightweight periclase-composite (Mg8-xFex+yAl16-yO32) spinel refractory material for cement rotary kilns

This study presents novel lightweight periclase-composite (Mg_8-xFe_x + yAl_16-yO₃₂) spinel refractories (LPSR) for the high temperature zone of cement rotary kilns. The LPSR was prepared by using microporous magnesia aggregates instead of sintered magnesia aggregates in traditional periclase-composite spinel refractories (TPSR). Hercynite-corundum composite aggregates, as well as microporous magnesia aggregates with a median pore size of 3.50 μm and a 20.1% lower bulk density than those of the sintered magnesia aggregates were used as raw materials. The microstructures, fracture behavior and strength of the LPSR in contrast with those of the TPSR were determined by SEM and three-point bending tests. After substituting the microporous magnesia aggregates for the sintered magnesia aggregates, a rougher surface of the microporous aggregates and wider transition-layer containing a solid solution spinel phase at the microporous magnesia aggregate/composite spinel aggregate interfaces were observed. Thus, a better bonding at the microporous magnesia aggregate/matrix interfaces as well as of the microporous magnesia aggregate/composite spinel aggregate interfaces was achieved. The wider transition-layer and better interfaces impeded crack propagation along the aggregate/matrix interface and increased the percentage of crack propagation within the aggregates. Thus, the mechanical strength of the LPSR was significantly enhanced. Compared with the TPSR, the LPSR had a lower bulk density of 2.56 g/cm³, but also a higher apparent porosity of 27.8% and a higher compressive strength of 46.4 MPa.     

Three bond modes of basic refractories used for Ruhrstahl Heraeus degassing process

Magnesia–spinel brick and unburnt periclase–spinel–Al brick are being employed as a substitution of traditional magnesia–chrome brick in the chromium-free campaign of lining materials in Ruhrstahl Heraeus (RH) degasser. These three materials are investigated, in terms of physical properties, corrosion resistance and flexibility by wedge splitting test. Tracking their physical alterations and chemical reactions through burning or heating, three bond modes are discovered. Magnesia–chrome brick is subject to a series of phase transformation with rising temperature to yield a liquid envelop around chromite-ore particles, to further form porous rim while liquid is gradually absorbed by surrounding magnesia and eventually to precipitate secondary chromite spinel lied between magnesia particles by thoroughly dissociating chrome ore. The precipitated chromite spinel functions as the featured bond that enhances hot strength and corrosion resistance to slag, and additionally liquid coexistence improves the flexibility. The direct bond mode of magnesia particles in magnesia–spinel brick endures slag penetration by immanent character of MgO. Spinel incorporation in magnesia effectively improves thermal shock resistance. Due to minor negative value of permanent linear change after reheating, further sintering (densifying) in using at high temperature would bring a risk of loosening and open joints of magnesia–spinel lining. While used in RH degasser, unburnt periclase–spinel–Al bricks undergo a miraculous process of metallic Al melting, gaseous AlN and AlON formation, MgAlON whiskers germination combined with gaseous Mg reduced, and micron-size whisker network bond domination in their matrix. Such a whisker-network bond renders the material a successful eco-friendly alternative to magnesia–chrome refractory.     

Effects of different particle size of spinel and Y2O3 additive of the periclase-spinel refractory on the sintering densification and corrosion resistance to copper smelting slag

In order to analyze the sintering densification and copper smelting slag corrosion resistance of periclase-spinel refractories, the periclase-spinel refractories were prepared with fused magnesia, magnesia-rich spinel, industrial alumina, and yttrium oxide as the main raw materials. The different particle sizes of spinel in material and with or without Y₂O₃ additive were studied. The study demonstrated that: (1) The different particle sizes of spinel in periclase-spinel refractories can result in different effects. Adding particle spinel to the refractory can improve the strength and corrosion resistance of the periclase-spinel refractories. The addition of spinel and magnesia powders in the matrix resulted in cracks due to the great difference of coefficient of thermal expansion between magnesia and spinel. The reduction in bulk density and strength of the material decreased slag penetration resistance because of its poor sintering properties. While adding the alumina in the matrix can further fill the crack and prevent slag penetration by the volume expansion of in-situ reaction to form spinel. (2) The periclase-spinel refractories can be reacted with Cu slag to form a Mg₂FeO₄ insulating layer as the iron ion becomes oxidized. Adding Y₂O₃ in periclase-spinel refractories can result in grain boundary phase reconstruction, which can promote sintering densification, improve the slag physical infiltration resistance, and improve the chemical corrosion resistance of materials.     

A Retrospective Review of Alumina-magnesia-carbon Refractories

The residual expansion of in-situ spinel formation in using of alumina-magnesia-carbon(AMC)bricks monolizes the lining of steel-making ladles with the closure of their joints,which has been an effective solution avoiding washing out of the joints in ladle lining by the reduction of the penetration of liquid slag and molten steel.Alumina-magnesiacarbon refractories are overall reviewed,in terms of major raw materials,thermal evolution,corrosion and oxidation,and thermomechanical behavior,as well as type,addition and fraction of magnesia used.General commercial products contain 5%-10%MgO and 5%-10%C with a certain amount of metallic aluminum powder,which is believed to facilitate spinel formation at early stage of heating-up,although high magnesia containing AMC bricks are studied and used sometimes.With low ratio of Al₂O₃/C=12.9 and the carbon content of 6.4%C,AMC brick exhibits the highest corrosion resistance.It is important to determine the type,addition and fraction of magnesia used in AMC refractories for demonstrating high corrosion resistance and superior thermomechanical behavior.     

Fractographic and Fracture Mechanical Investigations of Refractories under Different Loading Conditions

Crack initiation and propagation have been investigated under tensile and shear loading in ceramically and carbon bonded refractories.A wedge splitting test procedure and a modified shear test have been applied.Test results have been used for material characterization especially with respect to brittleness.Furthermore a microscopic fractographic test procedure was developed and applied on fractured test specimens.In order to explain brittleness dependence on structure properties correlation of fractographic and fracture mechanical results has been evaluated.Frequently brittleness reduction is achieved by a lower amount of transgranular crack propagation associated with a strength decrease while maintaining specific fracture energy unchanged.Deviations from pure linear fracture mechanics increase with decreasing brittleness and contribute to specific fracture energy.Shear specimens may show two generations of cracks,a first one initiated by tensile loads （stable propagation） and a second one by shear loads （unstable propagation）.     

Effect of the phase transformation on fracture behaviour of fused silica refractories

In this paper, the influence of phase transformation on the properties and fracture behaviour of fused silica refractory was investigated. The virgin fused silica refractory is amorphous, and possible failure is attributed to the propagation of a single crack in the structure. Due to the crystallization and phase transformation of low-/high- temperature cristobalite subpolymorphs occurring during the heat treatment, microcracks are formed especially in the matrix and at the grain boundary. This microcracking enables the development of sizable fracture process zone, which is responsible for the increase of specific fracture energy even with the decrease of strength. Therefore, the heat-treated specimens exhibit lower brittleness and higher strain tolerance before failure compared with the virgin fused silica refractory. All of these properties represent a better thermal shock resistance. Furthermore, microcracking causes a characteristic temperature dependence of Young’s Modulus due to phase transformation and partial crack closure at increased temperatures.     

MgO对原位烧结含锆莫来石材料性能影响的研究

初步研究了氧化镁对原位烧结含锆莫来石陶瓷性能的影响,经XRD分析表明:由于氧化镁的引入,在材料烧结过程中,有部分镁铝尖晶石的形成,同时在原位状态下也能形成部分Mg稳定的ZrO2,这对于形成氧化锆/镁铝尖晶石/莫来石热梯度晶相组成以减缓热应力十分有利,而且随着氧化镁加入量的增大,镁铝尖晶石、四方相氧化锆形成的量增多;并研究了材料抗热震系数达到最佳时的氧化镁加入量。     

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.