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.     

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.     

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。     

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.     

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.     

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 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.     

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     

Post-peak cyclic behaviour of concrete in uniaxial tensile and alternating tensile and compressive loading

Deformation controlled uniaxial tests have been carried out on cylindrical concrete specimens in order to establish the complete stress-deformation curve in tensile loading. Furthermore, cyclic tests have been performed under three types of loading: cycling from a tensile low stress, cycling from a compressive lower stress, and cycling between a lower and an upper stress to the envelope curve, respectively. The results seem to show that the tensile envelope curve is unique and the residual strength is the more affected the lower the stress at unloading is.     

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.     

Rubber-like tensile behaviour of yielded high-impact polystyrene

The tensile stress-strain properties of yielded high-impact polystyrene (HIPS) have been investigated at room temperature. Two HIPS samples were considered, having different rubber-phase particle sizes and morphologies. Specimens have been subjected to two subsequent tensile test runs, the first one to produce the yielded matter and the second one to test it. Different amounts of yielded matter content in the specimens and different strain rates for yielded matter production were examined by varying the test conditions in the first run. Yielded HIPS exhibits a rubber elastic stress-strain behaviour, which has been interpreted by assuming that a rubber-like material, proportional in amount to the plastic deformation, is present in the specimens as a consequence of the yielding process.     

Deformation behaviour of agglomerates under tensile stress

A new instrument has been developed in which the stress—strain behaviour of agglomerates can be measured in uniaxial tension. The instrument permits tension to be measured under predetermined stresses, and further enables the tensile stress—strain behaviour to be determined in the agglomerate in the unstable region.Agglomerates with different bonding mechanisms were studied. In the case where agglomerates are held together by liquid bridges, a model has been developed which permits the stress—strain behaviour to be predicted. This is confirmed by experiments. For agglomerates with a high degree of liquid saturation, the tensile stress—strain behaviour can be predicted from the capillary pressure relation. The tensile stress—strain curves of dry agglomerates and of agglomerates with solid bridges between particles are shown and compared. From the different stress—strain characteristics, the physical nature of the bonds can be elucidated.     

Experimental relationship between splitting tensile strength and compressive strength of GFRC and PFRC

This paper describes an experimental investigation into the relationship between the splitting tensile strength and compressive strength of glass fiber reinforced concrete (GFRC) and polypropylene fiber reinforced concrete (PFRC). The splitting tensile strength and compressive strength of GFRC and PFRC at 7, 28 and 90 days are used. Test results indicate that the addition of glass and polypropylene fibers to concrete increased the splitting tensile strength of concrete by approximately 20-50%, and the splitting tensile strength of GFRC and PFRC ranged from 9% to 13% of its compressive strength. Based on this investigation, a simple 0.5 power relationship between the splitting tensile strength and the compressive strength was derived for estimating the tensile strength of GFRC and PFRC.     

Analysis of a castable refractory using the wedge splitting test and cohesive zone model

A cohesive zone approach is applied to the wedge splitting test (WST) using the finite element code Abaqus to obtain the tensile strength, the fracture energy and insight about the crack wake region. A finite element model updating (FEMU) method, with a cost function based on the measured load (FEMU-F), is used to calibrate the sought parameters. Digital image correlation (DIC) provided the kinematic boundary conditions, and the images were also used to define the geometry for the finite element analysis. Besides the fracture energy analysis and the experimental load, gray level images and displacement fields are analyzed in order to validate the results. The cohesive region is active in the whole analyzed test as confirmed by estimates using the cohesive length.