Number Of Thermal Cycles Research Articles

Thermal Deformation of Granite under Different Temperature and Pressure Pathways

The thermal cracking of rocks is the phenomenon of expansion and deformation of mineral particles inside the rocks. The thermal deformation of the same granite sample under different heating pathways was meticulously analyzed, and the effect of the variation of external stress on the thermal expansion was carefully studied. The thermal deformation threshold temperature was low under triaxial stress, and a larger expansion was produced under uniaxial stress. The thermal deformation of the rock mass was found to be irreversible. Upon heating to the same temperature, the expansion was found to decrease with the increase in the number of thermal cycles. Besides, for each thermal cycle, the amount of deformation caused by cooling was always greater than the amount of deformation caused by temperature rise; this difference in deformation was especially obvious under a change from triaxial stress to uniaxial stress. In the process of elevated temperature which has the same heating rate, the thermal expansion coefficient was greater under uniaxial stress than it was under triaxial stress; under same external stress, the thermal expansion coefficient with a high heating rate was generally greater than the thermal expansion coefficient with a low heating rate.

Bond durability of CFRP laminates-to-steel joints subjected to freeze-thaw

The degradation mechanisms of bonded joints between CFRP laminates and steel substrates under severe environmental conditions require more durability data and studies to increase the database and better understand their causes. Studies on bond properties of double-strap CFRP-to-steel bonded joints with two different composite materials as well as adhesive coupons subjected to freeze-thaw cycles for 10,000 h were conducted to reduce that gap. In addition, the equivalent to the number of thermal cycles and their slips induced in the CFRP laminates was replicated by an equivalent (mechanical) loading-unloading history condition imposed by a static tensile machine. The mechanical properties of the adhesive coupons and the strength capacity of the bonded joints were only slightly changed by the artificial aging. It was confirmed that the interfacial bond strength between CFRP and adhesive is critically related to the maximum shear stress and failure mode. The interfacial bond strength between adhesive and steel degraded with the aging. However, the equivalent thermal cyclic bond stress caused no detectable damage on the bond because only the interfacial elastic regime was actually mobilized, which confirmed that pure thermal cycles aging, per se, at the level imposed, have a low impact on the degradation of CFRP-to-steel bonded joints.

On the Thermal Fatigue of a Room-Cured Neat Epoxy and Its Composite

An experimental investigation is conducted to evaluate the potential degradation in the mechanical properties of an epoxy resin and unidirectional glass fiber-reinforced epoxy (GFRE) as a result of exposure to fluctuating temperature. A commonly used room-cured epoxy resin and the GFRE are subjected to various numbers of thermal cycles (up to 1000 heating/cooling cycles). Mechanical tests are conducted to examine the influence of thermal cycles on the stiffness, ultimate strength and strain of the resin and its GFRE. The Fourier transform-Raman spectroscopy (FT-Raman) is conducted to investigate the influence of the thermal cycles on the resulting chemical changes and curing degree of the resin. In addition, the Differential Scanning Calorimetry (DSC) analysis is conducted to investigate the variation in the glass transition temperature (Tg) of the resin as a function of the applied thermal cycles.

Interrelation of microstructure and thermodynamic parameters in TiNi alloy during multiple martensitic transformations

The effect of multiple martensitic transformations on the microstructure and thermodynamic parameters of the alloy of the TiNi system was investigated. It was shown that in the Ti50Ni50 alloy, with an increase in the number of thermal cycles with rapid heating and cooling up to n = 100, a consistent increase in the dislocation density occurs, and a decrease in the width of martensitic plates is observed. And also, that in TCs with fast heating and cooling of the Ti50Ni50 alloy in a coarse-grained state, a change in the trend in the temperatures of martensitic transformations is observed — with an increase in the number of thermal cycles to n = 50, they decrease, and at n> 50 the temperatures increase.

Thermal cycling behaviour of plasma sprayed NiCr-Al-Co-Y2O3 bond coat in thermal barrier coating system

The aim of this study was to investigate the thermal cycling behaviour of NiCr-Al-Co- Y O bond coating in thermal barrier coating system with ZrO -MgO as a top coating. 2 3 2 The coatings were deposited by atmospheric plasma spraying on stainless steel X15Cr13 (EN 1.4024) substrate. The used composite powder NiCr-Al-Co-Y O was mechanically 2 3 cladded, and the steel substrates were preheated to 160-180 ?C. The thermal cycling performance of the obtained bond coat and the effect of formed complex ceramic oxides of the Al O -Y O system were tested by heating to 1200 ?C and cooling in air to 2 3 2 3 160-180 ?C. The number of performed thermal cycles was 7, 32, and 79. The quality of the obtained coating, as well as its thermal cycling behaviour, was assessed through the microstructural analysis, microhardness and tensile bond strength measurements, and change in chemical composition and microhardness. The obtained results showed that the steel substrate, bond coat oxidation and interdiffusion at bond coat/substrate interface have a significant influence on changes in chemical composition and microhardness of the bond coat. The correlation between oxidation behaviour of NiCr- Al-Co-Y O bond coat and number of thermal cycles was also discussed.

Correlation between thermal history and high-temperature order–disorder phase transition in Fe–18Ga alloy

The internal friction (IF) method combined with high resolution XRD was used to systematically study the correlation between thermal history and high-temperature order–disorder phase transition in Fe–18Ga alloy. In the first and second heating processes, results show that the IF peak (Ptr) position related to order–disorder phase transition is independent of thermal history, but thermal history has a significant effect on the peak height. With the number of thermal cycles increasing, the peak height of Ptr become more pronounced. Combined with high resolution XRD results, the mechanism of this phenomenon is mainly due to the fact that more D03 phase is formed as the number of cycles increases. Furthermore, the elimination of internal dislocation in the Fe–18Ga alloy will also contribute to the formation of the Ptr peak.

Thermal reliability of typical fatty acids as phase change materials based on 10,000 accelerated thermal cycles

The thermal reliability of phase change materials (PCMs) is an important factor for the successful application of latent heat thermal energy storage. In this work, 10,000 thermal cycles have been firstly carried out to investigate the thermal reliability of four typical fatty acids (lauric acid, myristic acid, palmitic acid and stearic acid) as PCMs. The thermal properties before and after cycles were tested by differential scanning calorimeter. It was found that with increasing number of thermal cycles, both the phase transition temperature and the latent heat tended to decrease. In the experiment, the maximum change in the melting temperature was −0.37 °C and change rate of latent heat was −7.57%. The freezing temperature decreased obviously with increasing number of thermal cycles, which led to a maximum increase of 4.15 °C in the degree of supercooling. In addition, the specific heat and thermal conductivity of the PCMs after 10,000 thermal cycles had no significant change. FT-IR showed that the chemical structure of fatty acids had not changed after thermal cycles. The experimental results showed that these fatty acids can function as admirable PCMs with high thermal reliability for long-term applications.

Thermal fatigue behaviour of Fe-1.7C-11.3Cr-1.9Ni-1.2Mo roller steel in temperature range 500–700 °C

Thermal fatigue resistance of a high Cr hot work roller steel is evaluated at 500, 600 and 700 °C. Surface layer degradation was investigated and characterised after a finite number of thermal cycles. Degradation mechanisms of the cooled surface layer in relation to the size, shape, orientation, distribution of carbides and crack oxidation progress were elucidated. Complex phenomena of crack growth is discussed where internal cracks nucleate due to stress at crack tip and oxidation along carbides follows from crack tail. Quantitative evaluation of cracks shows increased crack length with increasing temperature and number of thermal cycles.

Evolution of microstructure of Lead free cu/Sn solders and copper oxide phase precipitation in Cu3Sn intermetallic during thermal cycling

Intermetallic layers were formed after Solid Liquid Interdiffusion (SLID). Cu-Sn interdiffusion leads to the formation of two intermetallic layers: Cu6Sn5 and Cu3Sn. In this work, Sn solder was subjected to thermal cycling from −65 to +250 °C at 10 °C/min. After few cycles we noticed the appearance of a new –pillar– shaped phase made of copper oxide. The Intermetallic layers were mechanically tested using die-shear testing. The die-shear resistance appears to be increasing when the number of thermal cycles increases. The microstructure was observed using Backscatter Electron detector (BSD) and Scanning Electron Microscopy (SEM). The composition of the phase was measured using Energy Dispersive x-ray spectroscopy (EDX). It appears that a new copper oxide phase forms after few cycles. As the number of cycles increase, the size and the number of the copper oxide phases increase.

Effect of Thermal Cycle Loadings on Mechanical Properties and Thermal Conductivity of a Porous Lead-Free Solder Joint

This paper demonstrates to what extent the number of thermal cycles affects the mechanical properties as well as the thermal conductivity of a porous solder joint in an insulated-gate bipolar transistor discrete. The blind mode voids were used for a finite-element method (FEM) simulation to obtain the results close to the actual conditions. The FEM results indicate that the concentration of creep strain at the interface of the solder/chip is in its maximum value, and it slightly decreases along with the depth of the solder layer. FEM also reveals that the boundaries of voids act as critical regions for strain concentration. An upward in the number of thermal cycles also leads to the void growth and coalescence process. The enhancement of void volume from 0 to 15 cycles is about 4% volume of the solder layer. Moreover, scanning electron microscope micrographs approve the FEM results and show the void growth and the damage accumulation during thermal cycling. The thermal analyses indicate that the increase in thermal cycles (creep strain) leads to a significant rise in thermal impedance. This event can be due to the increase in the void volume of the solder layer leading to the decrease in the effective area of heat path.

Thermally induced microcracks and mechanical property of composite honeycomb sandwich structure: Experiment and finite element analysis

Microcracking in composite honeycomb sandwich structure and its effect on mechanical properties are studied in this paper. A methodology is presented to study the extent of mechanical strength degradation of composite sandwich structure, subjected to thermal fatigue. The material under study is used for spacecraft structural applications. The test coupons were exposed to thermal cycling at elevated temperature as high as +150°C inside the oven and cryogenic temperature of −190°C by dipping in liquid nitrogen, which is comparable to the thermal environment experienced by spacecraft structures. After each thermal cycle, coupons were inspected for microcracks under an optical microscope at the cross section. The microcracks were then quantified using parameters like crack length and crack density with increase in the number of cycles. Flatwise tensile test was conducted on the coupons after every 10 thermal cycles, up to 60 cycles, to make a correlation between crack density and mechanical strength. It was observed that by increasing the number of thermal cycles, the crack density increases and the flatwise tensile strength decreases up to a specific number of cycles. Finite element analysis was performed to predict the possible location of microcracks formation and compared with experimental observation. Good correlation was observed.

Temporary Stability of Compressive Strength of Flow and Universal Type LC PMCCS Materials

Abstract This paper reports the results of compressive strength and elasticity studies of light-cured polymer matrix ceramic composites (LC PMCCs). The main purpose was to obtain new data on experimental composites and compare them with commercial composites from the world’s leading manufacturer. The objective was to investigate the relationship between the content of reinforcing components in the composites studied and the stability of their strength in time, expressed as the number of fatigue thermal cycles.

Effect of thermocycling aging on the flexural strength of feldspathic ceramic

Objective: The aim of the study was to evaluate the effect of aging through different thermocycling protocols on the flexural strength of a feldspathic ceramic. Material and Methods: Fifty ceramic bars, Vitablock Mark II (VITA), with dimensions of 18X4X2mm were prepared. The bars were randomly distributed to the groups (n = 10), which were defined according to the number of thermal cycles (TCy): G0 - no TCy; G500 - 500 cycles of TCy; G6000 - 6000 TCy; G10000 - 10000 TCy; G15000 - 15000 TCy. After aging, the specimens were subjected to the three point bending test in a universal testing machine (EMIC DL 1000), under 0.5mm/min speed and loading of 50kgf, until catastrophic failure. A sample of each group was evaluated for topographic morphology under Scanning Electron Microscopy. Results: For mechanical testing, the following mean values and standard deviation (MPa) were obtained: G0 (99.78 ± 5.07); G500 (101.64 ± 5.59); G6000 (98.13 ± 4.95); G10000 (91.77 ± 9.68); G15000 (101.51 ± 4.22). An analysis of variance by regression equation (p = 0.387) was performed, demonstrating a weak and non-significant correlation between flexural strength and number of thermal cycles. Conclusion: It can be concluded that aging in water solely by different numbers of temperature cycles did not influence on the flexural strength of a feldspathic ceramic. Keywords Material resistance; Aging; Ceramics.

Fracture response of double cantilever beam subject to thermal fatigue

When adhesively bonded joints undergo thermal fluctuations, they become fatigued and suffer mechanical degradation. A comprehensive experimental investigation is conducted to examine the performance and degradation of adhesively bonded joints, in the form of the standard double cantilever beam specimen, subject to various levels of cyclic thermal loadings. Results indicate that regardless the number of applied thermal cycles, the joint experiences degradation of its interface adhesive, and that the degradation evolves as the number of thermal cycles increases, leading to the gradual degradation of the overall interface strength. Also, a computational framework, using the cohesive zone modeling technique, is developed for predicting the growth of a crack under the thermal and subsequent mechanical loadings. The good agreement observed between the computational and experimental results validates the integrity of the developed computational framework.

Critical Current Degradation of Coated Conductors Under Soldering Conditions

The critical current degradation of coated conductors was investigated at 77 K s.f., after heating the samples in a solder bath to temperatures between 200 °C and 300 °C for durations between 30 s and up to 2 h to simulate possible soldering conditions. We found that the critical current degradation is strongly dependent on temperature and time, but independent from the number of thermal cycles. At 225 °C for 10 min a degradation of about 5% was measured. At 300 °C for 10 min the degradation was about 45%. Slight variations among different manufacturers were observed. Across production batches of the same manufacturer no variations were found for tapes of the same structure. Slight variations of tapes with different stabilizer thicknesses were detected after heat treatment.

Experimental investigation of thermal cycling effect on physical and mechanical properties of bedrocks in geothermal fields

High temperature associated with geothermal fields affects the performance of bedrocks. Evaluation of physical and mechanical behavior of rocks in the process of thermal cycling at high temperature is one of the main issue in this application, which is also the main topic of the present study. In this study, microscopic observation and uniaxial compression tests with acoustic emission (AE) monitoring were conducted on two bedrocks (i.e., marble and granite) after treatment with different thermal cycles at high temperature. It is found that the P-wave velocity decreases as the number of thermal cycle increases. The characteristic stress levels and Young’s modulus decrease with the increase of the number of thermal cycle in the treatment. The peak strain and the maximum volumetric strain show an increasing trend as the number of thermal cycle increases. After failure, more fragments are observed in specimens treated with more thermal cycles and the integrity is also found to be lower than specimens treated with less thermal cycles. The AE technique is able to capture the failure process and the associated micro-cracking behavior during loading. The degradation of macro-properties of the rocks is to a large extent attributed to the generation of grain boundary and intra-grain micro-cracks inside the rock specimens due to the applied thermal stress. Overall, the thermal cycling weakens the mechanical properties of rocks; however, the weakening effect will become not pronounced with the increase of the number of thermal cycle in the treatment if a high temperature is applied as in this study (i.e., 600 °C).

Effects of applied current density and thermal cycling on the degradation of a solid oxide fuel cell cathode

Effects of applied current density and thermal cycles on the durability of a solid oxide fuel cell (SOFC) cathode have been studied. SOFC half-cells with and without a gadolinium-doped ceria (GDC) interlayer were fabricated and tested for 1000 h at 900 °C under various current densities and thermal cycles. Performance degradation of the half-cells was assessed by increment of the area specific resistance (ASR). Initially, the ASR of the half-cells without the GDC interlayer decreased for around 150 h due to cathode activation and thereafter increased. A rapid increase in the ASR was observed at higher applied current density, which is attributed to delamination of the electrolyte/cathode interface due to the formation of Sr zirconates, and microstructural change in the cathode. However, these adverse effects were prevented by the GDC interlayer. The half-cells with the GDC interlayer exhibited a smaller degradation rate as compared to that without the GDC interlayer. During the thermal cycling test, ASR values of all GDC interlayer thickness cells increased with an increasing number of thermal cycles. The thermally cycled cell with a GDC interlayer thickness of 3.4 μm showed a lower degradation rate due to the dense GDC interlayer, which resulted in less interfacial resistance and prevented elemental diffusion towards the electrolyte. However, the half-cells with GDC interlayer thickness of 2.4 and 4.5 μm showed a higher increase in the ASR due to relatively higher Sr diffusion and delamination of the cathode/GDC interlayer interface, respectively.

Performance investigation of a micro-tubular flame-assisted fuel cell stack with 3,000 rapid thermal cycles

Solid oxide fuel cell research and development has faced challenges with slow startup, slow shutdown and a limited number of thermal cycles, which hinders the technology in areas like micro-combined heat and power. A novel micro combined heat and power system, based on a boiler/hot water heater with integrated micro-tubular flame assisted fuel cells (mT-FFCs), is proposed which requires rapid startup, shutdown and thousands of thermal cycles. A 9 cell mT-FFC stack is developed and operated in a two-stage combustor. Rapid startup and shutdown of the fuel cells is demonstrated. The first-stage combustor is ignited, turned off and re-ignited for a total of 3000 on/off, thermal cycles. A maximum heating rate of 966 °C.min−1 and a maximum cooling rate of 353 °C.min−1 is achieved while thermal cycling. Despite the presence of CO in the exhaust, the anode remains porous and crack free after ∼150 h of thermal cycling testing. The mT-FFC stack continues to generate significant power, even after completing the cycling test, and a low voltage degradation rate is reported.

Improving plasticity of the Zr46Cu46Al8 bulk metallic glass via thermal rejuvenation

In this paper, effects of cryogenic thermal cycling on deformation behavior and thermal stability of the Zr46Cu46Al8 bulk metallic glass (BMG) were studied. The results show that with the increase of the number of cryogenic thermal cycles (CTC), thermal stability remains almost unchanged, while the plasticity is increased, indicating that the cryogenic thermal cyclic treatment is an effective way to improve plasticity of metallic glasses without distinctly deteriorating thermal stability. Our analysis suggests that the increase in the defect density resulted from the cryogenic thermal treatments are responsible for the plasticity increment. Variation of yield strength can be well interpreted from microstructural percolation which affected by both density and characteristic volume of the defect sites.

Effects of Thermal Shock Cycling on Mechanical and Thermal Properties of Carbon/Basalt Fiber-Reinforced Intraply Hybrid Composites

In this study, the effect of thermal cycling on the mechanical and thermal properties of carbon/basalt fiber-reinforced polymer (CBFRP) composites is investigated. Carbon and basalt fibers are woven in Twill 2/2 pattern in the structure of intraply hybrid composite and fabricated by vacuum-assisted resin infusion molding method. Fabricated composite samples were exposed to 20, 40, 60, and 80 cycles between − 40 and + 120 °C. To evaluate the effect of thermal cycling on the mechanical and thermal properties of composites, tensile, bending, short beam shear and dynamic mechanic analysis tests were carried out. The extracted results such as tensile modulus and strength, flexural modulus and strength, interlaminar shear strength, storage modulus, loss factors and glass transition temperature of RT and samples that were exposed to different number of thermal cycles were compared with each other. It is concluded that increasing the cycle numbers in the beginning of thermal cycling leads to improvement of the mechanical properties of composites. This phenomenon occurs due to the post-curing in the structure of epoxy and enhancement of cross-links at the interface of the fibers and matrix. Increasing the cycle numbers leads to mismatch in the interface of the composite due to different coefficients of thermal expansion between the composite components and emergence of cracks and delamination in the structure of the composite that causes reduction in the mechanical and viscoelastic properties of the CBFRP composites. According to scanning electron microscopy, the fiber pullout, bundle breakage, fiber buckling, debonding and delamination were the most important failure modes in the mechanical tests.