Thermal Shock Cracking Research Articles

An advanced ceramifiable organic-inorganic hybrid polysiloxane coating with superior moisture-resistant property

A ceramifiable organic-inorganic hybrid polysiloxane, prepared via a straightforward solution blending method, was successfully coated on the surface of silicon dioxide ceramic via a scrapping method. A uniform and dense lithium aluminum borosilicon (LABS) vitrified layer in situ formed on the surface of silicon dioxide ceramics was successfully prepared through liquid phase filling method to fulfill the moisture resistance requirements under high temperature cycle conditions. Effects of aluminum content on the microstructure change process and moisture resistance properties of the ceramifiable organic-inorganic hybrid polysiloxane have been systematically researched. With the increase of aluminum content, the ceramic yield of the hybrid polysiloxane and the heat loss capacity of LABS glass-ceramics increase. The crack-free and high temperature fluid vitrified layer endows the LABS glass-ceramic with excellent thermal shock resistance cycle ability, self-healing ability and moisture resistance. Results showed that the LABS vitrified coating with an aluminum-boron ratio of 1:2 presents the most appealing thermal shock cycle resistance and crack self-healing ability after 20 thermal cycles, as well as a water absorption rate of 0.3814 %. By refining the application of modified polysiloxane moisture-resistant coatings in high-temperature circulation environments, this work shed lights on the importance of in situ growth LABS vitrified coating in accelerating surface reconstruction and improving moisture resistance, incenting additional innovative breakthroughs in the LAS-based glass-ceramic materials design and fabrication within the aviation field.

Phase-field modeling of thermal shock fracture in functionally graded materials

In this paper, the thermo-elastic fracture problems of functionally graded materials (FGMs) are thoroughly investigated based on a modified phase field model. In this model, the material constants and fracture toughness vary along a specified direction, the thermal conductivity and stiffness constants in the damaged regions are degraded by the phase-field variable, and the crack evolution is driven by the variation of elastic energy induced by the thermo-mechanical loading. Therefore, the temperature, mechanical and damage fields are coupled with each other. The validation of the present approach has been checked by comparing the present results with the analytical, numerical, and experimental results in literature. The examples of thermo-elastic tensile fracture and thermal shock cracking of FGMs are performed to investigate the influence of material heterogeneity and external thermal loading. Furthermore, the experimental phenomenon of crack deflection in FGMs has been captured by the present simulations. The present study provides a guidance for understanding the fracture mechanism, material design and safety assessment of FGM structures in engineering practice.

Artificial thermal shock cracks in WRe – A proof of concept study

Thermal shock cracks are frequently observed in environments where extensive thermal cycling at high amplitudes is common. Typical cases for such are first wall materials in proposed fusion reactors and rotating anodes for computed tomography scanners. The formation of these cracks is driven by significant tensile stresses during the cooldown phase, following prior plastic deformation at elevated temperatures.This work proposes an experimental approach, where artificial thermal shock-like structures are generated via femtosecond laser ablation in WRe as a model material. Following a detailed laser parameter study in a W sheet material, patterns were introduced to WRe samples and investigated by different microscopy techniques. Their morphology as well as their response to thermo-cyclic loads was investigated and compared to conventionally induced thermal shock cracks.The introduction of artificial cracks by femtosecond laser ablation that mimic the morphology and behaviour of naturally induced thermal shock cracks is feasible within certain boundaries. Cuts, comparable to thermal shock cracks in width and depth, can be ablated. The cuts show an effect on the surrounding microstructure in line with the thermal shock cracks. Therefore, comparability could be proven concerning morphological and microstructural aspects. The conducted thermo-cyclic fatigue tests revealed an analogous effect on subsequent damage accumulation between the thermal shock cracks induced by thermal loading and artificial laser ablated cuts, proving the comparability under dynamic loading conditions. The extent of the effect is adjustable by tailoring the geometry of the lasered structures.The presented work suggests the possibility of studying the influence of thermal shock cracks on further fatigue mechanisms in exceptionally demanding applications, such as first wall materials or rotating anodes, by practical experimentation. Based on the resulting in-depth understanding of underlying interactions, the lifetime of these components might be prolonged by a deliberate introduction of crack networks or laser-ablated structures.

Mechanisms and experimental study of directional thermal shock fracture of granite under bidirectional horizontal loading

The study of the mechanism of thermal shock directional fracture of rocks under bidirectional horizontal stress is important for the application of directional thermal shock fracture technology. With the engineering background of the thick igneous roof overlying the coal seam, we conducted high temperature thermal shock directional fracture tests on granite under different horizontal loads to investigate the fracture mechanism. The results show that during the directional thermal shock of granite, the heating rate of borehole surrounding rock experienced three stages of rapid increase, rapid decrease and slowly decrease. AE tests were used to characterize the typical features of rocks during thermal shock fracture: the appearance of macrocracks in the specimen was accompanied by sharp increases in the cumulative AE count and the sudden drops in b-value. The experimental results show that thermal shock can create macroscopic directional fractures within the rock. Within a certain range of horizontal stress difference, the expansion direction of thermal shock cracks could be released locally from geological stress control, i.e. expanding along the direction of the minimum horizontal dominant stress. This provides completely new thinking for the cutting of hard roof and the directional fracturing of rock. In addition, directional thermal shock caused modifications in the distribution of stress in borehole surrounding rocks. We have established a model for stress distribution around the borehole rock and given the calculation formula for the initiation stress of the rock. The studies provide significant theoretical guidance for the industrial application of directional thermal shock fracturing technology.

Towards 3D-printed alumina-based multi-material components with enhanced thermal shock resistance

A novel architectural design is introduced which utilizes the layer-by-layer capabilities of the vat photopolymerization 3D printing process to fabricate multi-material ceramic components with improved thermal shock resistance. The combination of 3D-printed alumina-zirconia (ZTA) with alumina (A) layers generates compressive residual stresses in the embedded alumina regions during cooling down from sintering. Thermal shock tests in water are performed on samples at different maximum temperatures and the strength degradation of the multi-material design is investigated and compared to the reference monoliths. Experimental results show that the retained strength of the multi-material ceramic after thermal shock is twice as high as that of the monoliths, associated with the crack arrest capability of the embedded layers. The concept is demonstrated on 3D-printed multi-ceramic blades for potential high temperature applications, showing enhanced “damage-tolerance” against thermal shock cracks. These findings open the path for fabricating reliable ceramic components using the vat photopolymerization process.

Experimental study on acoustic emission characteristics of high temperature thermal shock directional fracturing of granite with different heating conditions

High temperature thermal shock directional fracturing of hard rock has tremendous promise in the mining and tunneling industries, but the utilization mechanism is unclear and must be resolved before taking it to the field application stage. In this study, the granite specimens were subjected to bidirectional horizontal loads using the real triaxial pressure testing machine, and the thermal shock fracturing tests were conducted utilizing the homemade thermal shock system. The AE (acoustic emission) system tracked the whole granite fracture process throughout the test in real time. The results showed that the granite‘s fracture time under the high temperature thermal shock was negatively related to the peak heating temperature and heating rate. The spectral analysis of the AE was employed to consider a large number of 20–40 kHz AE signals as a sign of thermal shock damage. By spatial localization of AE events, it was determined that the thermal shock cracks sprouted at the borehole edge in the specimen’s center. In the horizontal direction, the cracks extended from the center to the specimen edge; in the vertical direction, the cracks extended from the specimen surface to the deeper portion of the specimen. The RA-AF value analysis indicated that the granite thermal shock damage mode was a mixed tensile-shear damage mode dominated by tensile damage. In addition, the higher the heating rate, the higher the percentage of tensile cracks. Increasing the heating temperature promoted the formation of shear damage inside the specimen when the heating rate was equal. Analyzing the b-value and βt-value indicated that at low heating rates (10 and 60 °C/min), many micro-cracks formed inside the granite before merging into macro-cracks. However, with the higher heating rate (110 °C/min), the formation of micro-cracks and the macro-cracks within the granite proceeded almost simultaneously. The research results provided some experimental foundation and theoretical guidance for high temperature thermal shock directional fracturing of hard rocks.

Interaction of air plasma-sprayed spinel and yttria coatings with ultra-high temperature ceramic-metal hot-melt

Air Plasma Sprayed (APS) ceramic coatings are conventionally used for protecting metallic parts of hot section components in aero- and land-based gas turbines. Owing to their capability to withstand very high temperatures high specific heat capacity, and thermal shock resistance, such APS coatings are also investigated for the potential to protect the core catcher in nuclear reactors during the high-temperature core-meltdown accidents resulting in corium hot-melt. In this study, candidate plasma-sprayed spinel and yttria coatings on SS 316LN substrates were subjected to an ultra-high temperature (UHT) ceramic-metal hot-melt at ∼2500 °C. Uncoated steel substrates showed melting, whereas coated substrates were found un-attacked at the expense of degradation of the coatings such as surface melting, topcoat sintering, columnar grain growth and thermal shock cracks. The evolution of interfacial fusion was observed in the case of yttria coatings due to the formation of primary YAG and Al2O3-YAG eutectic at the interface. Spinel coatings with a comparatively lower thermal conductivity could generate a higher ΔT across the topcoat thickness with limited grain growth in the substrate. Evidence of local melting at the interface and evolution of temperature gradient between the hot-melt and substrate are comprehensively illustrated. Out of the two coatings tested, spinel was found to be more protective to the steel substrate.

Enhanced thermal isolation in porous thermal barrier coatings by the formation of pore guided thermal-shock cracks

Enhanced thermal isolation in porous thermal barrier coatings by the formation of pore guided thermal-shock cracks

Phase-Field Simulation of Temperature-Dependent Thermal Shock Fracture of Al2O3/ZrO2 Multilayer Ceramics with Phase Transition Residual Stress.

Compared with single-phase ceramics, the thermal shock crack propagation mechanism of multiphase layered ceramics is more complex. There is no experimental method and theoretical framework that can fully reveal the thermal shock damage mechanism of ceramic materials. Therefore, a multiphase phase-field fracture model including the temperature dependence of material for thermal shock-induced fracture of multilayer ceramics is established. In this study, the effects of residual stress on the crack propagation of ATZ (Al2O3-5%tZrO2)/AMZ (Al2O3-30%mZrO2) layered ceramics with different layer thickness ratios, layers, and initial temperatures under bending and thermal shock were investigated. Simulation results of the fracture phase field under four-point bending are in good agreement with the experimental results, and the crack propagation shows a step shape, which verifies the effectiveness of the proposed method. With constant thickness, high-strength compressive stress positively changes with the layer thickness ratio, which contributes to crack deflection. The cracks of the ceramic material under thermal shock have hierarchy and regularity. When the layer thickness ratio is constant, the compressive residual stress decreases with the increase in the layer number, and the degree of thermal shock crack deflection decreases.

Measurement of ceramics cracking during water quenching by digital image correlation

Measuring the thermal shock crack growth process is crucial for revealing ceramic materials and structures’ thermal shock failure mechanisms and evaluating their reliability. We used a self-made water quenching system to conduct thermal shock tests on alumina and zirconia ceramics. The thermal shock process was recorded by high-speed digital image correlation (DIC) during the test. The process of thermal shock crack initiation and propagation in two kinds of ceramics was determined by analyzing the speckle image change on the sample’s surface. It is found that the crack growth rate of alumina is faster than that of zirconia, which is caused by different material parameters. This paper presents an in-situ measurement method for the initiation and propagation of thermal shock cracking in ceramic materials. It can provide a measurement method to identify and predict the thermal shock damage of ceramic components.

Simulation of crack patterns in quasi-brittle materials under thermal shock using phase field and cohesive zone models

Complex crack patterns are usually formed on the surface of brittle and quasi-brittle solids under thermal shock. In this paper, the phase field coupled cohesive zone model is established to simulate the brittle and quasi-brittle fracture processes in three-dimensional solids, especially to reproduce the complex crack patterns under thermal shock. Several examples are given to illustrate the effectiveness of the model to complex fracture problems under thermal shock: from uniaxial tensile and thermal shrinkage fracture of a one-dimensional rod to nucleation, propagation, and crack pattern formation of thermal shock crack of a three-dimensional ceramic bottle. In addition, the formation mechanism of crack patterns in ceramic bottle is discussed, especially the dominant role of the difference in thermal expansion coefficients between the ceramic matrix layer and the glaze layer in the formation of crack patterns and the final crack density.

Characterization of Ceramic Thermal Shock Cracks Based on the Multifractal Spectrum

Ceramics are commonly used as high-temperature structural materials which are easy to fracture because of the propagation of thermal shock cracks. Characterizing and controlling crack propagation are significant for the improvement of the thermal shock resistance of ceramics. However, observing crack morphology, based on macro and SEM images, costs much time and potentially includes subjective factors. In addition, complex cracks cannot be counted and will be simplified or omitted. Fractals are suitable to describe complex and inhomogeneous structures, and the multifractal spectrum describes this complexity and heterogeneity in more detail. This paper proposes a crack characterization method based on the multifractal spectrum. After thermal shocks, the multifractal spectrum of alumina ceramics was obtained, and the crack fractal features were extracted. Then, a deep learning method was employed to extract features and automatically classify ceramic crack materials with different strengths, with a recognition accuracy of 87.5%.

Effect of material parameters on thermal shock crack of ceramics calculated by phase‐field method

AbstractBased on alumina ceramics, we employ the phase‐field method to study the effects of thermal conductivity, specific heat, density, thermal expansion coefficient, Young's modulus, and fracture toughness on thermal shock cracks. The results show that increasing thermal conductivity and fracture toughness will reduce thermal shock damage. That is, the long crack length becomes shorter, or the crack density becomes smaller. However, increasing the thermal expansion coefficient and Young's modulus will increase thermal shock damage. It is consistent with the previous thermal shock theory. The effect of material parameters on crack propagation speed was also considered. In addition, we carried out a thermal shock test of the zirconia. The results of the phase‐field calculation are the same as the thermal shock results of the zirconia. This paper verifies that the phase‐field method is suitable for simulating thermal shock cracks in other ceramics.

Investigation of the thermomechanical behavior of nonhomogeneous, layered materials with mixed-mode cracks subjected to rapid thermal shock loading

For mixed-mode, thermal shock crack problems with thermal lagging behaviors, it is difficult to obtain the transient thermal stress intensity factors (TSIFs). This paper aims to develop a method combining the Newmark method and the transient interaction energy integral method (IEIM) to extract the transient, mixed-mode TSIFs of nonhomogeneous, layered materials with mixed-mode cracks. The transient temperature field obtained by non-Fourier heat conduction theory is discretized in the spatial and temporal domains through the finite element method (FEM) and Newmark method, respectively. The effect of the continuity of thermomechanical parameters at the dual interface on the transient, mixed-mode TSIFs of nonhomogeneous, layered materials is discussed, then the fracture resistance of nonhomogeneous, layered materials is evaluated. Moreover, the maximum hoop stress criterion is utilized to obtain the crack growth angle and the equivalent, transient TSIFs of nonhomogeneous, layered materials. The influences of the interface on crack growth angle and equivalent transient TSIFs of nonhomogeneous layered materials are also studied. The present work is beneficial for the thermal design of nonhomogeneous, layered materials with mixed-mode cracks subjected to rapid thermal shock loading.

Investigation of the fracture problem of functionally graded materials with an inclined crack under strong transient thermal loading

In this paper, a special method for solving transient thermal stress intensity factors (TSIFs) in functionally graded materials (FGMs) with inclined cracks under strong, transient thermal shock loading is established based on the hyperbolic heat conduction equation. First, the Newmark method is employed to obtain the strong transient temperature field of FGMs containing inclined cracks. Then, the transient interaction energy integral method (IEIM) is used to solve the TSIFs for inclined cracks. In addition, the influences of the distribution patterns of the thermal relaxation parameter, the crack length and the crack angle on the TSIFs, and the crack growth angle are investigated. Moreover, we discuss the difference between using the hyperbolic heat conduction theory and the classical, Fourier heat conduction theory in calculating the TSIFs of FGMs with inclined cracks. The results show that the present, combined IEIM-Newmark method can be used to solve strong transient thermal shock crack problems with high efficiency. This work can be used for the thermal design, evaluation, and engineering applications of FGM structures.

Thermal shock resistance and toughening mechanism of W/Ta and W/TiN/Ta laminated composites

The brittleness of tungsten (W) is a significant limitation on its widespread applications as a high-temperature structural material. Laminated composite technology proved to be an effective method to improve the toughness of W. In this work, W/Ta and W/TiN/Ta laminated composites were prepared by spark plasma sintering (SPS) to overcome the brittleness of W. The toughening properties of the W/Ta and W/TiN/Ta laminated composites were evaluated by electron beam thermal shock and laser thermal diffusivity testing. The toughening mechanism was clarified by scanning electron microscopy, electron probe micro-analysis, and transmission electron microscopy. After the three-point bending test, interfacial debonding was observed predominantly at the incoherent W-TiN rather than at the semi-coherent Ta-TiN interfaces. At 0.30 GW/m2 of electron beam thermal shock, the average width and depth of thermal shock cracks in the W/Ta laminated composite were ~ 9.4 μm and ~ 30.0 μm, respectively, which were much smaller than those in the W/W laminates and W/TiN/Ta laminated composite.

Prefabricated crack propagation in translucent alumina ceramic sheets during flame thermal shock

The prefabricated cracks propagation of ceramic under flame thermal shock was studied by real-time observation. The experimental results show that wing cracks are generated from both tips of the prefabricated crack, and the complete failure of the samples is a high-speed process. The crack propagation speed is fast at first and then gradually slows down. The maximum propagation speed of the near flame tip increases with the prefabricated crack distance to the heated surface, while an inverse trend is observed for the far flame tip. The effect of prefabricated crack angle on thermal shock crack propagation was also considered. In addition, numerical simulations based on dynamic fracture mechanics were carried out to reproduce the entire cracking process. To this end, the relation between the dynamic stress intensity factors and the crack growth speed in the speed range of 100 m/s to 2600 m/s was identified by combing experiments and numerical simulations. The simulation results and the above-mentioned relation are consistent with the experimental observations. The numerical model can faithfully reproduce the crack evolution process of the thermal shock, which is difficult to observe in an experiment due to the high speed of the cracking process.

Numerical Analysis of Thermal Shock Cracking Behaviors of Ceramics Based on the Force-Heat Equivalence Energy Density Principle

Thermal shock is one of the main causes for the fracture of ceramic materials due to their inherent brittleness. Aiming to explore the mechanism of thermal shock cracking behavior of ceramics under different thermal shock conditions, a novel temperature-dependent failure criterion of thermal shock fracture of the ceramic materials was deduced based on the force-heat equivalence energy density principle. Combining this failure criterion and the finite element method, the thermal shock cracking behaviors of the thin circular and rectangular ceramic slabs under different thermal shock initial temperature were simulated. Results show that the morphology, periodicity, hierarchy, and number of thermal shock cracks obtained by numerical simulation are in good agreement with the experimental results. These essential characteristics verify the validity of the temperature-dependent failure criterion for thermal shock fracture. Furthermore, the strain energy of tension produced by thermal shock is proved to be the dominated mechanism for thermal shock-induced fracture.

Thermal Shock Crack Propagation of Alumina Simulated With the Phase-Field Method Under Temperature-Dependent Damage Criteria

The mechanical properties of alumina ceramic materials are significantly affected by temperature, so it is necessary to consider the effect of temperature on the damage criterion during the simulation of thermal impact crack propagation by the phase-field method. Based on the existing thermodynamic phase-field model, the governing equation of the phase-field model was modified by introducing the temperature-dependent damage criterion. Then the revised phase-field model is used to simulate the thermal shock experiment of alumina ceramics, and the simulation results were compared with the experimental results and the finite element simulation results without considering the effect of temperature on the damage criterion. The results show that introducing the temperature-dependent damage criterion can simulate more reasonably in the initiation and propagation process of thermal impact crack.

Combining the K-bubble strengthening and Y-doping: Microstructure, mechanical/thermal properties, and thermal shock behavior of W-K-Y alloys

Nano‑potassium bubble strengthened tungsten (W-K) alloy with excellent overall performance has been proven to be one of the most desirable plasma facing materials (PFMs). However, some key properties, e.g., thermal heat resistance, ductile-brittle transition temperature (DBTT), strength and toughness need to be further improved to meet the requirements of fusion reactor. Herein, pure yttrium (Y) was doped in W-K alloy to achieve multiple strengthening effects. The grains of W-K alloy have been significantly refined, and the tensile strength was improved along with the introducing of Y. Thermal shock cracking threshold has increased to exceed 0.62 GW/m2. Interestingly, the existing form and size of the precipitates differed with the change of doping amount, leading to the nonlinear evolution of DBTT. The W-K alloy with 0.1 wt%-Y doping content (W-K-0.1Y) showed the best match of strength and toughness. Furthermore, coupled with relatively good thermal conductivity, W-K-0.1Y alloy displayed prominent thermal shock resistance. This work may pave a way for designing PFMs with higher performance.