Showing papers on "Thermal shock published in 1996"

Stress intensity relaxation at the tip of an edge crack in a functionally graded material subjected to a thermal shock

Abstract: We analyze thermal stresses and the stress intensity factor in an edge-cracked strip of a functionally graded material (FGM) subjected to sudden cooling at the cracked surface. It is assumed that the shear modulus of the material decreases hyperbolically with the higher value at the surface exposed to the thermal shock and that the thermal conductivity varies exponentially. Volume fractions of the constituents in a ceramic-metal FGM are then determined with the assumed shear modulus gradient using a three-phase model of conventional composites. The differences between the other assumed material properties and those predicted by the three-phase model are delineated and the applicability of the assumed FGM is discussed. It is shown that the maximum tensile thermal stress in the strip without cracks is substantially reduced by the assumed thermal conductivity gradient and that the magnitude of the compressive stress is increased. A strong compressive zone just away from the thermally shocked surface is devel...

Induction of heat shock protein 70 protects intestinal epithelial IEC-18 cells from oxidant and thermal injury.

Abstract: The potential importance of inducible heat shock proteins (HSPs) in conferring protection to intestinal epithelial cells was investigated using IEC-18 cells. To establish optimal HSP induction, [35...

Thermal Shock Behavior of Two-Dimensional Woven Fiber-Reinforced Ceramic Composites

Abstract: The thermal shock behavior of three types of two-dimensional woven, continuous fiber-reinforced (Nextel{trademark} 312 (3M Co., St. Paul, MN) or Nicalon{trademark} (Nippon Carbon, Tokyo, Japan)) ceramic matrix (silicon carbide matrix that had been processed by chemical vapor infiltration or polymer impregnation and pyrolysis) composites was studied using the water-quench technique. Thermal-shock-induced damage was characterized by a destructive technique of four-point flexure and a nondestructive technique of Young`s modulus measurement by the dynamic resonance method. Compared with monolithic ceramics, the continuous fiber-reinforced ceramic composites were capable of preventing catastrophic failure that was caused by thermal shock. Analysis of the results that were based on the stresses that were generated by thermal shock and the mismatch of thermal expansion between fibers and matrices suggested possible mechanisms of the thermal shock damage. Preliminary results showed evidence of matrix cracking and delamination because of the thermal shock damage. The feasibility of using the nondestructive technique to detect thermal shock damage also was demonstrated.

Method of making thermal shock resistant sapphire for IR windows and domes

Abstract: Single crystal sapphire is conventionally employed for mid-wave IR windows and domes exposed to high aerodynamic heating environments. Single crystal sapphire, however, suffers a loss of compressive strength in the c-axis of 95% on heating from ambient to 800° C. This loss of compressive strength on heating results in poor thermal shock resistance. Poor thermal shock resistance jeopardizes the reliability of the windows and domes. The process of the present invention strengthens sapphire and improves its resistance to thermal shock by introducing magnesium (Mg) ions into the crystal lattice of sapphire to form a solid solution. The solid solution increases the strength of the sapphire. Additionally, a larger increase in strength results from converting the Mg into second phase precipitates comprising magnesium aluminate spinel. The formation of the second phase particulates results in precipitation-hardening or precipitation-strengthening due to the volume change on forming the magnesium aluminate spinel. The second phase particulates comprising magnesium aluminate spinel are held to such a small size that the second phase particulates impart increased fracture strength while not significantly degrading the IR transmission when compared with untreated sapphire. Consequently, precipitation-hardening with magnesium aluminate spinel improves the compressive strength of sapphire while retaining high thermal conductivity and IR transmission.

Effect of Microstructure on Thermal Shock Cracking of Functionally Graded Thermal Barrier Coatings Studied by Burner Heating Test

Abstract: The thermal shock fracture mechanism of metal/ceramic functionally graded thermal barrier coatings was studied by bumer heating tesL Discussions were made on the basis of fracture mechanics with special reference to the effect of microstructure on crack extension behavior. Two types of FGM coatings, having the same graded structure with different microstructures, were fabricated by slurry dipping and HIP sintering process: PSZ/IN 100 FGMs having finely mixed microstructure and PSZ/Inco 718 FGMs having rather coarse microstructure. The fracture toughness of each composition was determine by conventional vickers indentation method on uniform nonFGM specimens. It has been shown that the fracture toughness depends strongly on the microstructure following from the mixing conditions and the particle size of the raw material powders. In PSZ/IN 100 FGMs, the fracture toughness increased with increased in the metal phase content, while in PSZ/Inco718 FGMs it was fairly lower than that of PSZ/IN 100 FGMs, owing to roughly dispersed metal phase in the PSZ matrix. The results of bumer heating test revealed that the crack formation was always observed on the ceramic surface during cooling. By comparison between the fracture toughness and mode I stress intensity factor, the initiated vertical cracks in PSZ/ Inco718 FGMs were considered to extend into the interface of FGM/substrate without deflection. This crack extension behavior was confirmed by observing the cross-section of the tested sapmles. Although vertical cracks in PSZ/IN 100 FGMs tend to be arrested in the FGM coating, with the extension of the cracks into the graded layer, they deflected toward the direction parallel to the surface. The depth of the parallel cracks beneath the surface may correspond to a location of mode II stress intensity being equal to zero.

A mathematical model of high-temperature synthesis of nickel aluminide Ni3Al by thermal shock of a powder mixture of pure elements

Abstract: Within the framework of model concepts of the structure of a nickel-aluminum powder mixture a mathematical model was constructed for the high-temperature synthesis of nickel aluminideNi3Al by thermal shock according to the equilibrium state diagram of the aluminum-nickel system and using the equations of the diffusion kinetics of intermetallic-compound formation at the interface of the components under continuous heating by an external energy source. The main characteristics of thermal shock in the aluminum-nickel system were calculated numerically as a function of the size distribution of the nickel component of the starting stoichiometric powder mixture.

Theoretical and experimental considerations on the thermal shock resistance of sintered glasses and ceramics using modelled microstructure-property correlations

Abstract: The thermal shock resistance of brittle materials such as glass and ceramics is one of their weaknesses. Pores and other incorporated second phases in these materials alter these properties which are decisive for thermal shock behaviour, and may therefore increase this behaviour in a precalculable manner. It has been theoretically demonstrated when and why porosity leads to an improvement in thermal shock resistance. The thermal shock resistance for porous borosilicate sintered glass and porous eutectic calcium titanate ceramic have been calculated and compared to experimental values. The results confirm that low porosities lead to an improvement in thermal shock resistance, that the thermal shock resistance has a maximum at a certain porosity, and that above certain porosities, the presence of pores deteriorates the thermal shock resistance. If porous materials are considered as a special case of composite materials, then relations valid for porous materials can be transferred to composite materials and vice versa (“composite concept”). This has been investigated using the examples of borosilicate sintered glass with incorporated antimony particles and eutectic calcium titanate ceramic with incorporated paladium particles. In the case of the glass-antimony composite material, improvements in thermal shock resistance of about 15% with 10 vol % antimony incorporation, were calculated and confirmed experimentally, while for calcium titanate-palladium composite materials, a 15% improvement in thermal shock resistance was already achieved with about 5 vol % metallic phase.

Thermal Shock Damage in a Two-Dimensional Woven-Fiber-Reinforced-CVI SiC-Matrix Composite

Abstract: Thermal shock damage in a two-dimensional woven-Nicalon(tm)-fiber-reinforced-CVI SiC-matrix composite was induced by water quenching and characterized by optical microscopy as a function of quench temperature difference (ΔT) and number of quench cycles. Mechanical damage generated in flexure on quenched and unquenched specimens also was characterized and compared to the thermal shock damage. The observed thermal shock damage consisted of small matrix cracks and fiber-matrix interfacial debonding on the surface, and large interior cracks in the matrix that formed between and parallel to the fiber cloths. At low ΔT values, only small matrix cracks on the surface were observed, and they were related to initial decreases in Young's modulus. At higher ΔT values, larger cracks between the fiber cloths in the specimen interior were observed and related to decreases in the ultimate strength. Cyclic quenching resulted in progressive thermal shock damage that was consistent with Young's modulus measurements.

Thermal shock behaviour of a coarse grain porous alumina

Abstract: An apparatus of thermal shock by air cooling in which experiments can be precisely modelled is presented. The different physical parameters have been measured as a function of the temperature. The heat transfer coefficient has also been determined by two methods, on a copper specimen and directly on the alumina thermal shock specimen. A precise knowledge of the transient temperature field is therefore available.

Thermal stress intensity factor for functionally gradient half space with an edge crack under thermal load

Abstract: The aim of this work is to investigate the thermal stress intensity factor of a functionally gradient half space with an edge crack under a steady heat flux. All material properties of the functionally gradient half space, except for the coefficient of linear thermal expansion, are exponentially dependent on the distance from the boundary of the plate. The coefficient of linear thermal expansion is assumed to be two-dimensionally dependent. The problem is reduced to a singular integral equation by using the Fourier transform. The thermal stress intensity factor versus the nonhomogeneous material parameters is calculated and represented in figures. The numerical results show that thermal stress intensity factor is dramatically decreased when the material nonhomogeneous parameters are appropriately selected.

The laboratory simulation of thermal shock failure

Abstract: Infrared transparent windows on aircraft and missiles can be subjected to rapid heating which can cause thermal shock failure. A commercially available device for simulating such thermal shock environments has been assessed and used in a study of a range of materials. The material ordering resulting from these experiments has been compared with that obtained by considering a thermal shock figure of merit.

Fracture behavior of al2o3/sic-platelet composites

Abstract: Mechanical behavior of hot-pressed SiC platelet reinforced alumina composites has been analyzed as a function of SiC platelet content for two different alumina matrix powders. Fracture toughness and flexural strength at temperatures ranging from 25 to 1200 °C, R-curve behavior, and thermal shock resistance have been determined. Small differences in the impurity content of the starting Al2O3 powders strongly determine the microstructure and the mechanical behavior of Al2O3/SiC-platelet composites. Low alkali content alumina led to composites with large matrix grain size which presented spontaneous microcracking. At high temperature, a high viscosity liquid phase is formed that shields cracks enhancing mechanical properties and R-curve behavior. A small amount of impurities reduced Al2O3 matrix grain size and avoided spontaneous microcracking. Enhanced fracture toughness (up to 30%) at room temperature, R-curve behavior, and thermal shock resistance were achieved for these materials.

Synthesis of functionally gradient aluminium titanate/alumina composites

Abstract: By virtue of its very low thermal expansion coefficient and low thermal conductivity, aluminium titanate (A12TiOs) is highly suitable for applications where thermal insulation and thermal shock resistance are required. However, AlzTiO5 does present two important problems, which have restricted its wider industrial use. The first relates to the extensive microcracking, which results in low thermal expansion but also in low strength. In view of this, composite materials like AlzTiOs-mullite [1], AI2TiOs-ZrO 2 [2], AlzTiO5-TiZrO4-ZrO 2 [3] and A12TiOs-mullite-ZrO2 [4] have been developed. Thermal decomposition of the material at temperatures below 1280 °C is the second problem [5]. On the other hand, alumina is a very versatile engineering material with relatively high strength, high hardness, and excellent oxidation and chemical resistance. However, like most ceramics, it has an intrinsically poor thermal shock resistance and low fracture toughness, which restricts its use in structural applications. It follows that the marriage of A12TiO5 and alumina should lead to a layered composite that will display all of the advantages of both materials. This concept has been successfully exploited by Marple and Green [6-8] and Low and colleagues [9-11] in synthesizing, respectively, layered mullite/alumina and mullite/ZTA with improved mechanical properties such as strength and fracture toughness. The method involved encasing an alumina or ZTA host body with a mullite (3A1203"2SiO2) protective layer [6, 9]. This was done by infiltrating the host body with a silica-rich solution (e.g. ethyl-silicate), followed by firing the composite at an appropriate temperature. Due to thermal expansion mismatch between mullite and the host body, desirable macroscopic compressive stresses can be induced in the casing. The protective layer inhibits crack growth, thereby improving the effective fracture toughness and strength. In this letter, we report the preliminary results of functionally-gradient A12TiOs-alumina composites by encasing an alumina host with an A12TiO5 layer using an infiltration technique. The microstructureproperties relationship is discussed. The procedure for sample preparation was based on an infiltration method adopted by Marple and Green [6] and Low et al. [9] in their processing of layered mullite-ZTA composites. High purity alumina (A1000G, Alcoa, USA) powder was used to make the ceramic preform for infiltration. The powder was

Influence of grain size on the toughness and thermal shock resistance of polycrystalline YBa2Cu3O7−δ

Abstract: The toughness and thermal shock resistance (quenching from RT in air into liquid nitrogen) have been investigated on four grades of polycrystalline ‘YBaCuO’ of almost equal porosity using the Vickers indentation technique. The mean grain sizes varied from 1.5 to 10 μm. The change in toughness with increasing grain size is described by a bell-shaped curve as already published by Rice et al. for the case of other non-cubic ceramics. For the determination of the thermal shock resistance, an original method, based on the relative increase of the length of Vickers indentation radial cracks, as the sample is quenched, has been used. The ranking of the four grades can only be explained if the increase of the thermal residual mismatch stresses, as the samples are put into liquid nitrogen, is taken into account such as to predict a shift of the bell-shaped change of toughness with increasing grain size as the temperature reaches that of liquid nitrogen. The proposed methodology delivers a powerful tool for investigating non-cubic polycrystalline structural or functional ceramics in the R&D stage.

Thermal shock resistant ceramic material and production thereof

Abstract: PROBLEM TO BE SOLVED: To obtain a compact ceramic material having a low thermal expansion characteristic, excellent in thermal shock resistance and obtainable by burning at a low burning temperature by forming a sintered material containing each specific composition and main crystalline phase. SOLUTION: This thermal shock resistant ceramic material contains 1.5-6.5wt.% Li2 O, 1.0-10wt.% MgO, 14-30wt.% Al2 O3 and 58-83wt.% SiO2 based on oxide compositions, and is a sintered material containing a low expansion petalite and/or spodumene and cordierite as a main crystalline phase. The ceramic material is obtained as a sintered material by preparing a base material obtained by blending 30-75wt.% lithium silicate raw material (e.g.; a petalite raw material and a spodumene raw material), 20-55wt.% kaoline raw material, 1.5-10wt.% MgO raw material (e.g.; talc) based on MgO, and 0-15wt.% SiO2 compound (e.g.; quartz) based on SiO2 in addition to the SiO2 content supplied by the above raw materials, forming and burning the base material at a maximum temperature of 1,300 deg.C.

Thermal Shock Fracture Behavior and Fracture Energy of β'-Sialon-BN Composites

Abstract: Fracture energy and R-curve behavior of β′-Sialon-BN composites were investigated to elucidate the strength degradation caused by thermal shock. The fracture energy and R-curve behavior were obtained from compliance analysis on the load-displacement curves of the quasi-static fracture of 3-point bend specimen with chevron-notch. The critical temperature difference of strength degradation (ΔTc) for the composite containing 30mass% BN was 700K and 900K for Z=1.0 and 2.0 (Si6-zAlzOzN8-z), respectively. These ΔTc results agree well with the values calculated from Hasselman's equation. The steep reduction of strength by thermal shock and the disappearance of R-curve behavior were observed on the same thermal shock temperature difference ΔT. The relationship between the ratio of effective fracture energy to elastic fracture energy (γeff/γi) and retained strength was found to be roughly linear. It was presumed that the residual stress from thermal shock influences the crack-face contact shielding mechanism in the composites and is a possible inhibition factor to forming of the R-curve effect.

Temperature difference setting method in thermal shock test

Abstract: PURPOSE: To set a proper temperature difference in a thermal shock test during the proof test of molded parts CONSTITUTION: In a process (step 1) for detecting thermal stress, a pre-crack is formed in the surface of a sample similar to molded parts using a Vickers indentation, and observing the progress of the pre-crack while applying thermal shocks is repeated with a temperature difference gradually increased, and the crack propelling stress required for the progress of the crack is calculated from the length of the advanced pre-crack and converted into a thermal stress corresponding to the temperature difference with the residual stress added thereto, and a coefficient of stress expansion is calculated In a process (step 2) for determining defective sizes, a plurality of test pieces equal in material to the molded parts are tested for bending strength to calculate breaking bending stress, the distribution of defective sizes present in the molded parts is estimated from the breaking bending stress, and the defective sizes to be eliminated during a thermal shock test are determined from that distribution In processes (steps 3, 4) for determining a temperature difference, an optimum temperature difference for use in the thermal shock test is calculated from the thermal stress and the defective sizes to be eliminated, which are calculated in the two processes

Thermal shock/fatigue evaluation of FGM by AE technique

Abstract: This study has been carried out to analyze the thermal shock and fatigue characteristics of functionally gradient material (FGM). The thermal shock/fatigue tests were conducted at a specific temperature and fracture patterns were studied by SEM and AE. Also, thermal fracture behavior of plasma-sprayed FGM and conventional coating material (NFGM) was examined by acoustic emission technique under heating and cooling. Furnace cooling and air cooling tests were used to examine the effect of temperature change under various conditions. The conventional and FGM coatings were compared to heat-resistant property and fracture surface of these materials for each temperature history. Based on these results, some critical temperature at the onset of coating failure can be evaluated to characterize the thermal resistance of the materials. It turned out to be that FGM gives higher thermal property compared with NFGM by AE signal and fracture surface analysis.

Layered ceramic capacitor and manufacture thereof

Abstract: PURPOSE:To provide a layered ceramic capacitor in which the generation of a crack can be prevented by relaxing an internal stress generated by a thermal shock. CONSTITUTION:Since an internal electrode 2 buried in a ceramic chip 1 uniformly has a number of very fine holes 2a, an internal stress generated by a thermal shock can be absorbed or relaxed by the deformation of the very fine holes 2a, thereby surely preventing the generation of a crack due to the internal stress to be able to maintain excellent quality and electrical characteristics. Also, since the area and the thickness of the internal electrode 2 are not changed with the existence of the very fine holes 2a. the same capacity as one which does not have the very fine holes 2a can be secured without changing the dimensions of parts, the area of an internal electrode or the like.

Method for predicting rapid cooling thermal shock fatigue life

Abstract: PURPOSE: To determine the strength, the probability of fracture, and the number of times of thermal shock being applied repetitively before fracture of a test piece with high accuracy by determining the relation between the stress enlarging coefficient of crack and a crack developing rate, together with a thermal conductivity, in a test environment just identical to that for thermal shock fatigue test for measuring the fatigue characteristics directly. CONSTITUTION: A test piece having an arbitrary shape is cracked previously and a stress enlarging coefficient K1 is calculated based on the shape of the crack and the test conditions where the crack begins to develop while assuming a thermal conductivity. The maximum value of the coefficient K1 thus determined is then compared with a critical value thus determining a thermal conductivity (h) satisfying the conditions of K1 . On the other hand, initial length of the previously formed crack is measured and thermal impact is applied repeatedly with a temperature difference lower than the critical temperature difference where the test piece is not fractured by applying the thermal impact once. The crack length is measured after N1 cycle and a crack developing rate V is determined. Thermal stress is then analyzed using the thermal conductivity thus determined and a multiple mode weibull distribution or the like is assumed based on the K1-V thus determining the relationship of the number of times of repetition SPT.