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

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 (Δ T ) 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 Δ T values, only small matrix cracks on the surface were observed, and they were related to initial decreases in Young's modulus. At higher Δ T 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.     

炸药热冲击损伤破坏及超声波特性参量检测

进行了 JOB- 90 0 3炸药试样的热冲击试验和热冲击前后的超声波特性参量检测试验 ,观察并检测到试样在热冲击试验的不同阶段出现损伤破坏及超声波特性参量的具有一定典型性的变化。试验初步表明 ,初始损伤或裂纹对炸药的力学性能有十分明显的影响 ,超声波特征参量对炸药的损伤及破坏进一步表现出一些可识别的特征     

Thermal shock of ground and polished alumina and Al2O3/SiC nanocomposites

The thermal shock behaviour of sintered alumina and alumina/SiC nanocomposites with 1, 2.5 and 5 vol.% SiC was studied. The thermal shock testing was carried out by means of quenching into water from high temperatures (ΔT in the range 0–750 °C). Both single shocks and repeated shocks were used. The damage introduced by thermal shock was characterised by degradation of strength in four-point bending and by changes in Young's modulus. The effects of the surface finish of the test specimens (either ground or highly polished surfaces) on the thermal shock resistance were also studied. In both alumina and nanocomposite materials, specimens with ground surfaces showed a better resistance to thermal shocks than specimens with polished surfaces. However, the resistance of the nanocomposite material to single and repeated thermal shocks was no better than that of the pure alumina.     

JOB-9003炸药"激热"冲击损伤破坏及超声特征

进行了未经热处理的JOB-9003塑料粘结炸药(PBX)标准压缩试样的"激热"冲击损伤破坏试验,对试样热冲击试验前后的超声波特性参量进行了检测,试验显示出JOB-9003炸药存在一个明显的"激热"冲击损伤破坏临界温度差,并获得了试样热冲击损伤破坏的超声波参量特征.     

The Effects of Water Entry Postures on the Thermal Shock Behavior of Alumina

The effects of water entry postures on thermal shock behavior during quench tests were investigated for the first time using alumina ceramic. The differences in the crack patterns and residual strength for specimens with different water entry postures after the quench tests were shown. At the initial thermal shock temperature of 310°C, the average residual strength of the lateral posture specimens is 330.4 MPa, but the longitudinal ones' is 118.4 MPa. The results showed that water entry posture strongly affects the quenching thermal shock behavior; to better experimentally characterize the thermal shock resistance, the unified water entry posture during testing must be considered.     

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

The thermal shock behavior of three types of two-dimensional woven, continuous fiber-reinforced (Nextel^TM 312 (3M Co., St. Paul, MN) or Nicalon^TM (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.     

某些碳化硅材料的内耗和热震损伤

研究了某些碳化硅材料的力学行为，考察了抗折强度与弹性模量和内耗的关系。根据损伤力学，引进了热震损伤变量，并考察了碳化硅材料热震损伤与弹性模量和内耗的关系。由于弹性模量和内耗均可用非破坏实验方法测得，因此，在确定了强度和热震损伤与弹性模量和内耗的关系之后，可用非破坏实验来估计材料的强度和热震损伤。此外，还研究了常温强度和高温强度均高、抗热震性均优的碳化硅材料。该材料适用于制造陶瓷窑具。     

Alumina/NiCu Laminate under Thermal Shock up to 1000°C: I, Experimental

The mechanical behavior of an alumina/NiCu laminate under thermal shock loading was investigated. The maximum thermal shock temperature was 1000°C. The laminate architecture was the cause of a basic change in the cracking mechanisms, manifested in a dramatic increase in the mechanical residual strength over that of monolithic alumina. The laminated system was constructed by alternating alumina layers coated with copper films with nickel interlayers and joining them by a combination of liquid-state (brazing) and solid-state (diffusion) bonding. The material system was tested by water quenching square-shaped laminated specimens initially at temperatures of up to 1000°C. Three-point bending tests revealed the mechanical strength before and after thermal shock, and SEM analysis described the damage mechanisms and the extent of debonding at the alumina/NiCu interfaces.     

Effects of Damage on Thermal Shock Strength Behavior of Ceramics

When subjected to severe thermal shock, ceramics suffer strength degradation due to the damage caused by the shock. A fracture-damage analysis is presented to study the effects of damage on the thermal shock behavior of ceramics. It is assumed that a narrow strip damage zone is developed at the tip of a preexisting crack after a critical thermal shock and the damage behavior can be described by a linear strain-softening constitutive relation. Damage growth and strength degradation are determined based on fracture and damage mechanics. Numerical calculations are carried out for two ceramic materials, and the strength degradation agrees quite well with experimental results. The effects of bridging/damage stress, the fracture energy of the bridging/damage zone, and specimen size on thermal shock strength behavior are studied. A higher fracture energy can enhance the residual strength of thermally shocked ceramics and, for a given fracture energy, a higher bridging stress is needed to reduce the strength degradation. It is also shown that the thermal shock strength behavior is size-dependent, and a high value of ( K _IC/O_b)², where K _IC is the intrinsic fracture toughness and O_b is the bending strength, can improve significantly the residual strength.     

Effect of SiC content on electrical,thermal and ablative properties of pressureless sintered ZrB2-based ultrahigh temperature ceramic composites

The effects of SiC content (10–40 vol.%) on electrical, thermal and ablation properties of pressureless sintered ZrB₂-SiC composites showing interfacial segregation of W-rich phases have been studied. The electrical resistivity was measured by four-probe method, whereas thermal diffusivity and coefficient of thermal expansion (CTE) were determined using laser-flash method and thermo-mechanical analyzer, respectively. Whereas thermal conductivities calculated from experimentally obtained thermal diffusivity values are found to be the highest for the ZrB₂-20 SiC composite, both electrical conductivity and CTE decrease with increasing SiC content. The specimens were subjected to thermal shock by soaking at 800–1200 °C, followed by water-quenching. Further, some specimens were exposed to oxyacetylene flame (2200 °C) for 10 min. The damage was estimated from changes in mass, Young’s modulus, and hardness. The highest thermal shock and ablation resistance have been observed for the ZrB₂-20 SiC composite, as thermal properties and formation of protective oxide scale play key role.     

Elastic Energy at Fracture and Surface Energy as Design Criteria for Thermal Shock

The physical properties which affect the propagation of cracks in specimens fractured by thermal shock are discussed. The driving force for crack propagation is provided by the elastic energy stored at fracture. The mechanism of energy dissipation which will tend to arrest the propagating cracks is provided by the "effective surface energy" required to produce the newly formed crack surfaces. An expression is derived applicable to a body of spherical shape for the mean area traversed by cracks nucleated by thermal shock. Three numerical examples are given for materials with widely different physical properties, and their fracture behavior is predicted. Good agreement with experiment was obtained. Thermal shock damage resistance parameters suitable for the relative comparison of the "degree of damage" to be expected in materials fractured by thermal shock are proposed. The criteria for a low degree of damage are high values of Young's modulus of elasticity, Poisson's ratio, and effective surface energy and low values of strength. Recommendations are made for the selection of materials for severe thermal shock, where the best materials available are known to fail.     

Cyclic thermal shock behaviors of carbon fibers reinforced SiCN ceramics with bio-inspired Bouligand-like architectures

Ceramic matrix composites (CMCs) are commonly used for high temperature components in aircrafts. However, thermal shock, as a typical loading case, will cause high thermal stresses in CMCs resulting in brittle fracture failure, and material cracking caused by thermal shock can further reduce the effectiveness of thermal protection function. In the present paper, we propose a bionic hierarchical fiber preform design method to improve the thermal shock resistance of ceramics. The effect of architectures of fiber preforms of continuous carbon fiber-reinforced CMCs on the thermal shock resistance was investigated to understand its importance and the related mechanical mechanisms. Thermal shock (cycling) tests were performed with continuous carbon fibers reinforced SiCN ceramic matrix composites (C_f/SiCN) prepared by PIP. 3D micro-CT scan and three-point bending tests were also conducted to evaluated the resultant damage. The results showed that smaller internal damage and higher thermal shock resistance can be obtained in comparison to pure SiCN ceramics, and the underlying mechanism can be explained by the fact that smaller pitch angle can resist the through-thickness crack propagation via promoting diffused in-plane damage. The present study offers a possibility in developing biomimetic C_f/SiCN ceramics with excellent thermal shock behavior.     

Combining nonlinear acoustics and physico-chemical analysis of aggregates to improve alkali–silica reaction monitoring

Very few chemical, physical and mechanical parameters appear suitable to monitor progressive damage caused by alkali–silica reaction (ASR) in concrete with the proper level of sensitivity and/or specificity. The purpose of the experimental work presented in this paper is to handle this limitation by proposing a non-conventional approach based on the combined use of nonlinear acoustics and physico-chemical analysis to assess the damage caused by ASR.The study was first carried out on laboratory concrete specimens kept in conditions promoting ASR (100% R.H. and 38 °C), as well as on laboratory concrete specimens submitted to thermal damage (thermal shock). For both types of damage, nonlinear acoustics allowed detecting and tracking the early evolution of micro-cracking in concrete with a higher sensitivity than other non-destructive parameters (i.e. dynamic Young's modulus or ultrasonic pulse velocity). The physico-chemical approach allowed distinctions to be made between types of damage by assessing granular swelling in the case of ASR, while no physical change in aggregates was detected regarding thermal damage.The procedure was then applied to concrete cores extracted from a large concrete field structure affected by ASR and submitted to residual expansion tests. Results confirmed the sensitivity of the nonlinear parameter to track residual damage in concrete and confirmed (with physico-chemical analysis) that ASR specifically contributed to the residual expansion and damage observed.     

THERMAL SHOCK EFFECT ON THE TRANSVERSE STRENGTH OF CLAY BODIES1

Test specimens of twenty commercial shale and fireclay bodies, ranging in softening point from cones 7 to 30, were fired in laboratory kilns to cones 04, 2, and 6. Comparisons of transverse strength before and after thermal shock were made, thermal shock h a i h g been produced by eight cycles of heating in a furnace at 1100°F for 15 minutes and cooling over an air blast for an equal length of time. Results indicate that as initial strength increases, per cent reduction in strength increases and strength after thermal shock increases to a maximum and then declines. Curves are fitted to the data and the mathematical relationships are shown. The results of other investigators are discussed.     

Study of the castable selection for blast furnace blowpipe

The contradictory properties required of castable refractories makes selecting castable refractories for industrial applications challenging. This paper seeks to describe the material selection for a blast furnace blowpipe application that is subjected to sudden temperature changes and must prevent heat loss. Three commercial high alumina castables containing andalusite or mullite from different manufacturers were characterized. Thermal shock damage resistance was evaluated using thermal shock damage resistance theory and experiments. The castables’ coefficient of thermal expansion was estimated using quantitative X-ray diffraction. Crack propagation resistance was measured using the work-of-fracture technique. Thermal shock damage was experimentally evaluated by measuring the modulus of elasticity and rupture prior to and after thermal cycles. Ultimately, the microstructure of the castables was related to the thermal shock damage behavior by estimating the aggregate size and the fracture toughening mechanisms using light optical and scanning electron microscopes. Heat loss was evaluated by calculating the blowpipe shell temperature using a one-dimensional steady-state heat conduction model. The best commercial castable refractory for blowpipe showed high thermal shock damage resistance and low thermal conductivity. The results in this study agreed with thermal shock damage resistance parameters and showed a correlation between coarse microstructure with large aggregate and higher thermal shock damage resistance.     

Behavior of silicon carbide/cordierite composite material after cyclic thermal shock

In the present work Mg-exchanged zeolite and silicon carbide were used as starting materials for obtaining cordierite/SiC composite ceramics with weight ratio 30:70. Samples were exposed to the water quench test from 950 °C, applying various number of thermal cycles (shocks). Level of surface deterioration before and during quenching was monitored by image analysis. Ultrasonic measurements were used as non-destructive quantification of thermal shock damage in refractory specimens. When refractory samples are subjected to the rapid temperature changes crack nucleation and propagation occurs resulting in loss of strength and materials degradation. The formation of cracks decreases the density and elastic properties of material. Therefore by measuring these properties one can directly monitor the development of thermal shock damage level. Dynamic Young's modulus of elasticity and strength degradation were calculated using measured values. Level of degradation of the samples was monitored before and during testing using Image Pro Plus program for image analysis. The capability of non-destructive test methods such as: ultrasonic velocity technique and image analysis for simple, and reliable non-destructive characterization are presented.     

Thermal shock behaviour of mullite–cordierite refractory materials

Abstract

The characterisation of thermal shock damage in cordierite–mullite refractory plates used as substrates in fast firing of porcelain whiteware has been investigated. Two different refractory compositions (termed REFO and CONC), characterised by different silica to alumina ratios, were studied. Thermal shock damage was induced in as received samples by water quenching tests from 1250°C. Thermal and mechanical properties were measured at room temperature by means of standard techniques and then the thermal shock resistance parameter R was calculated. The fracture toughness of selected samples was measured before and after thermal shock by the chevron notched specimen technique. The reliability of this technique for evaluation of small differences in fracture toughness after a given number of thermal shock cycles was investigated. The suitability of K _Ic measurements by the chevron notched specimen technique to characterise the development of thermal shock damage in refractory materials was proved in this investigation.     

Role of fatigue in damage development of refractories under thermal shock loads of different intensity

Refractories insulation of industrial furnaces often fail under repetitive thermal shock. Degradation of silica refractories under thermal shock loads of different intensity was studied. The load variation was achieved by utilisation of geometrically similar samples of different dimensions. Finite element method modelling predicted loads developing during the test. Resulting damage was determined by the ultrasound velocity and crack patterns. Tests involving up to 150 cycles demonstrated the role of fatigue in enabling sub-critical crack formation and countering the crack arrest. Repetitive cycles reduce crack wake friction and intensify loading due to crack debris re-location. Damage saturation, sigmoidal and near-exponential damage growth was typical for low, intermediate and high loads, respectively. Similar trends of damage accumulation were observed in mechanical displacement controlled cyclic fatigue tests performed in wedge splitting set-up. Strain and strain energy based criteria of thermal shock intensity seem to have complimentary value in predicting the crack formation and growth. Thermal shock damage after the first cycle seems to be an effective parameter to predict overall resistance to the degradation in the sample. Load reduction due to previous crack formation related to the fatigue potential for subsequent crack development can explain the crack size variation typically observed in refractories after multiple thermal shocks. For thermal shock tests, the variation of sample size, instead of the temperature interval, is a suitable alternative for refractories with strongly temperature dependant material properties.     

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.     

Resistance to Thermal Shock and to Oxidation of Metal Diborides–SiC Ceramics for Aerospace Application

Two SiC-containing metal diborides materials, classified in the ultra-high-temperature ceramics (UHTCs) group, were fabricated by hot-pressing. SiC, sinterability apart, promoted resistance to oxidation of the diboride matrices. Both the compositions, oxidized in air at 1450°C for 1200 min, had mass gains lower than 5 mg/cm². Slight deviations from parabolic oxidation kinetics were seen. The resistance to thermal shock (TSR) was studied through the method of the retained flexure strength after water quenching (20°C of bath temperature). Experimental data showed that the (ZrB₂+HfB₂)–SiC and the ZrB₂–SiC materials retained more than 70% of their initial mean flexure strength for thermal quenchs not exceeding 475° and 385°C, respectively. Certain key TSR properties (i.e., fracture strength and toughness, elastic modulus, and thermal expansion coefficient) are very similar for the two compositions. The observed superior critical thermal shock of the (ZrB₂+HfB₂)–SiC composite was explained in terms of more favorable heat transfer parameters conditions that induce less severe thermal gradients across the specimens of small dimensions (i.e., bars 25 mm × 2.5 mm × 2 mm) during the quench down in water. The experimental TSRs are expected to approach the calculated R values (196° and 218°C for ZrB₂+HfB₂–SiC and ZrB₂–SiC, respectively) as the specimen size increases.