Thermal Exposure Time Research Articles

Mechanical behavior of composites reinforced with fibers caroa

Composites were prepared with 13, 23 30 and 40 % fiber and evaluated the mechanical performance in tensile, flexural and impact. The mechanical properties of these composites were also evaluated function of time at 110 °C thermal exposure. Caroa fibers were characterized by techniques such as thermal gravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the best mechanical properties were achieved for composites containing 23 to 30 % fiber. The incorporation of 23 % fiber caroa increased both the modulus of elasticity in the tensile test as the flexural strength and impact, the composite with 30 % fiber caroa showed higher tensile strength. The results show that the tensile and flexural strength of the composite decreased with time of thermal exposure. The thermal aging at 110 °C caused a decrease in tensile properties of the composites.

Ablation mechanism and properties of silica fiber-reinforced composite upon oxyacetylene torch exposure

The mechanism of mass loss and endothermic properties of silica fiber-reinforced phenolic composites during ablation were investigated in this paper. A theoretical prediction model combining the surface ablation theory and heat transfer theory of heat shield was developed to study the surface ablation behavior. In the formulation of the mathematical model, the effect of the moving boundary on the thermal response was considered, which results from the surface recession of the material in the thickness direction during ablation. The surface ablation recession rate and wall temperature of silica fiber-reinforced phenolic composite specimen were measured using an oxyacetylene torch experimental platform. Then, the efficiency of the model was verified by comparing calculation and experimental results. According to the principles of energy conservation on the ablated surface of the material, the proportion formulas of the heat absorption induced by individual endothermic mechanisms and the total heat absorption were derived. Similarly, the proportions of the mass loss caused by individual mass loss mechanisms were also given. Finally, variations of the ablation properties of the silica fiber-reinforced phenolic composites versus thermal exposure time were calculated and analyzed.

Assessment of segregation kinetics in water-moderated reactors pressure vessel steels under long-term operation

In reactor pressure vessel (RPV) bcc-lattice steels temper embrittlement is developed under the influence of both operating temperature of ∼300 °C and neutron irradiation. Segregation processes in the grain boundaries (GB) begin to play a special role in the assessment of the safe operation of the RPV in case of its lifetime extension up to 60 years or more. The most reliable information on the RPV material condition can be obtained by investigating the surveillance specimens (SS) that are exposed to operational factors simultaneously with the RPV itself. In this paper the GB composition in the specimens with different thermal exposure time at the RPV operating temperature as well as irradiated by fast neutrons (E ≥ 0.5 MeV) to different fluences (20–71)·1022 m−2 was studied by means of Auger electron spectroscopy (AES) including both impurity and main alloying elements content. The data obtained allowed to trace the trend of the operating temperature and radiation-stimulated diffusion influence on the overall segregants level in GB. The revealed differences in the concentration levels of GB segregants in different steels, are due to the different chemical composition of the steels and also due to different grain boundary segregation levels in initial (unexposed) state. The data were used to estimate the RPV steels working capacity for 60 years. The estimation was carried out using both the well-known Langmuir-McLean model and the one specially developed for RPV steels, which takes into account the structure and phase composition of VVER-1000 RPV steels, as well as the long-term influence of operational factors.

Effect of Thermal Exposure Time on the Relaxation of Residual Stress in High Pressure Die Cast AM60

Effect of Thermal Exposure Time on the Relaxation of Residual Stress in High Pressure Die Cast AM60

Influence of thermal exposure on microstructure evolution and tensile fracture behaviors of compacted graphite iron

The mechanical properties of compacted graphite iron (CGI) may undergo degradation when it exposes at high temperature environment for a long period of time. In this study, the influences of thermal exposure conditions on microstructure evolution, tensile properties, especially in-situ tensile fracture behaviors of CGI were investigated. The experimental results show that the cementite surrounding the vermicular graphite (VG) gradually decomposes into ferrite and graphite with increasing the thermal exposure time. The increase of ferrite content with lower strength but higher ductility around VG particles, and the size and content of VG particles can lead to the easier initiation of crack, decrease of strain hardening rate, and the retard of the crack propagation rate in the matrix. All these factors often cause the decrease of strength but increase of elongation to fracture for CGI. Finally the relationship among yield strengths of CGI and matrix, and graphite content was discussed.

Evolution of mechanical properties of thermal barrier coatings subjected to thermal exposure by instrumented indentation testing

The evolution of mechanical properties, namely Young's modulus, hardness, fracture toughness and residual stresses of air plasma-sprayed 8wt% Y2O3-stabilized ZrO2 thermal barrier coatings (TBCs) subjected to thermal exposure at 1100°C, was evaluated based on an instrumented indentation testing system. It is found that the variation of Young's modulus and hardness with the increase of thermal exposure time can be divided into two regimes: a rapid initial increase, followed by a more gradual increase. Conversely, fracture toughness decreased from the initial value of 1.48–1.05MPam1/2 after 350h, indicating the degradation of TBCs. And the corresponding residual stress in top coat varied from the deposition stress of −29.9MPa to a maximum of −56.0MPa within 100h, and then decreased to −15.0MPa after 350h. The evolution of mechanical properties was correlated with microstructural changes in the top coat. Microstructure observations revealed that thermally grown oxide (TGO) formed at the interface between ceramic coat and bond coat, and was predominantly comprised of Al2O3 layer, along with some oxide clusters of (Cr, Al)2O3, (Co, Ni)(Cr, Al)2O4 and NiO. The growth of TGO followed a parabolic law with a rate constant of 0.13236μm2/h. After 350h, porosity in 8YSZ coatings was reduced from 8.39% to 5.79% as a result of activated diffusion-controlled sintering.

Evolution behavior of superlattice phase with Pt2Mo-type structure in Ni–Cr–Mo alloy with low atomic Mo/Cr ratio

Abstract

A mesomechanical model for predicting the degradation in stiffness of FRP composites subjected to combined thermal and mechanical loading

The mechanical properties of Fiber Reinforced Polymer (FRP) composites decrease with increasing thermal exposure temperature and time. A mesomechanical model was presented to predict the degraded behavior of FRP composites supporting a static compressive loading under high temperatures. The thermal softening, thermal decomposition of the matrix material and phase transition of the reinforced fibers were considered in the developed model, which adversely affect the stiffness properties of the composite material. Also, in order to evaluate the effect of high internal pressure on stiffness property, the bulk modulus was applied in the formulation of the mathematical model. High temperature compression experiments were conducted to measure the temperature-dependent elastic modulus. The accuracy of the model was further assessed by comparing simulated and experimental modulus. The reduction in stiffness properties of FRP composites at high temperatures can be roughly divided into three stages by analyzing the predicted temperature-modulus curve.

Degradation and biodegradation of polyethylene with pro‐oxidant aditives under compost conditions establishing relationships between physicochemical and rheological parameters

ABSTRACTIn the present work, an analysis is carried out to provide a relationship between the Molecular Weight (Mw) of degraded LDPE films (containing Mn stearate as pro oxidant (MnSt‐LDPE) and changes in viscosity, elongation at break (EB %) and carbonyl index (CI) occurring during thermal degradation in the thermophilic phase of the compost process. The thermal treatment comprised various temperatures (50°C, 60°C, and 70°C) and exposure times, and was characterized through a so‐called Energy‐Time Factor (the product of thermal energy and exposure time). Changes in viscosity, EB %, and CI were correlated to this factor. A modified Mark‐Houwink equation was used to relate the zero shear‐rate viscosity and Mw of the degraded LDPE films. Results indicate that the EB %, Mw and viscosity decrease simultaneously with an increase in the CI as the Energy‐Time Factor augments, allowing the assessment of the variation of these properties with Mw. Calculations of the percentage abiotic degradation (%D) of LDPE films indicate that a Mw of 6 kg mol−1 corresponds to a maximum abiotic degradation degree of 91.85%, which is henceforth susceptible to biodegradation. The film treated with Energy‐Time Factor of 2.79E+09 J s mol−1 reached a 74% of biodegradation in 90 days (average time of the composting process). Results exhibit clearly the correlation between abiotic and biotic degradation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42721.

Effect of water subcooling to the saturation temperature on boiling crisis characteristics during a rapid increase in the heat release power

The experimental results on the effect of water subcooling to the saturation temperature and the thermal exposure time on the maximum allowed energy removed by coolant without the boiling crisis onset are presented. The boiling crisis under subcooling conditions results in cooled surface damage. The results of this study can find application to calculate cooling systems of equipment operating in pulsed mode.

Acrylamide content and antioxidant capacity in thermally processed fruit products

Acrylamide as a known processing contaminant was determined in various heat-treated plum products purchased from a local market using LC/ESI-MS-MS. The highest level of acrylamide in the range up to 60 μg/kg was detected in a plum stew known as a "povidla", and in prunes, respectively. These products typically undergo intensive heat treatment that may take from several hours to days. Using a fruit dehydrator in home production of prunes, a low level of acrylamide under LOQ (15 μg/kg) was detected in comparison to most commercial products. Only in one of the prune samples from the market was the acrylamide content near to LOQ. The highest content of acrylamide (46 μg/kg) was detected in the Slovak sample of prune originated in Nitra region. High acrylamide content, in the range from 23 to 45 μg/kg, was observed in prunes from South America. In the rest of analysed heat-treated plum products such as plum juice, plum compote or baby food with plum puree, acrylamide was not detected due to moderate conditions during thermal processing: temperature below 120 °C and a shorter time of thermal exposure. The total phenolic content and antioxidant capacity of prunes were analysed using a UV-VIS-NIR spectrophotometer and an electron paramagnetic resonance (EPR) spectroscopy. Home-prepared prunes were characterized by the highest content of phenolics (4780 mg GAE/kg) and antioxidant capacity (14.6 mmol TEAC/kg). Commercial samples of prunes reached phenolics in the range from 1619 to 3461 mg GAE /kg, and antioxidant capacity was observed between 6.1 and 12.1 mmol TEAC/kg. Antioxidant capacity of prunes strongly correlated with total phenolic content and yellow and red colours measured in a CIELab system. However, no significant correlation between the acrylamide and antioxidative or organoleptic properties of prunes was observed. Moreover, it was noticed that bio production of plums did not demonstrate any positive impact on final acrylamide content or antioxidant capacity in comparison to conventional technology.

Microstructural evolution and mechanical properties of Inconel 718 after thermal exposure

Inconel 718 was subjected to various heat treatments, i.e., solution heat treatment, standard ageing treatment and standard ageing plus 700°C thermal exposure. The mechanical properties of the alloys were determined using tensile tests and Charpy pendulum impact tests at 650°C and room temperature, respectively. The highest yield strength of 988MPa was attained in the standard aged specimen, whereas a maximum impact toughness of 217Jcm−2 was attained in the solution-treated specimen. After thermal exposure, the mechanical properties of the specimens degrade. Both the yield strength and impact toughness decreased monotonically with increasing thermal exposure time. Subjected to a 10000-h long-term thermal exposure, the yield strength dramatically decreased to 475MPa (almost 50% of the maximum strength), and the impact toughness reduced to only 18Jcm−2. The microstructures of the specimens were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Coarsening of γ′ and γ″ and the transformation of γ″ to δ-Ni3Nb was observed after thermal exposure. However, a complete transformation from metastable γ″ to δ-Ni3Nb was never accomplished, even after the 10000-h long-term thermal exposure. Based on the obtained experimental results, the effects of the microstructural evolution on the mechanical properties are discussed.

Design and synthesis of polyimide – Gold nanofibers with tunable optical properties

Aromatic polyimides possess high thermal stability, flexibility, good chemical resistance, mechanical strength, enhanced dielectric properties, being thus widespread used in electronic and microelectronic industry or as a support in high temperature applications. Herein, a soluble functional polyimide with pendant phenol groups was synthesized and its potential to stabilize gold nanoparticles was highlighted by comparison with an aromatic polyimide with similar backbone but lacking the phenol units. Furthermore, composite nanofibers based on the polyimide bearing phenol units and gold nanoparticles were prepared by using an in-situ e-spinning approach. Electrostatic spinning revealed homogenous fiber formation with diameters in the range of 0.28–0.77μm for pure polyimide, whereas for gold containing polyimide beaded fiber morphologies with diameters in the range of 50–60nm were determined. A clear influence of the solution properties was measured regarding fiber diameters. Thermal treatment of the composite nanofibers yielded highly dispersed embedded metallic gold particles exhibiting diameters in the range of 9–22nm, with an increase in particle diameters by enlarging the thermal exposure time. The thermal stability of the composite nanofibers was evaluated by thermogravimetric analysis (TGA) and only a slight decrease in the decomposition temperature in comparison to the pristine polyimide-based nanofibers was observed. Finally, we investigated the effect of thermal treatment on the optical properties of the new nanofibers and we revealed the possibility of tuning the reflective characteristics of the nanofibrous surfaces in function of the annealing time.

NT-32 * TISSUE ABLATION DYNAMICS DURING MAGNETIC RESONANCE-GUIDED LASER INDUCED THERMAL THERAPY FOR INTRACRANIAL TUMORS

BACKGROUND: Magnetic Resonance-guided Laser Induced Thermal Therapy (MRgLITT) is a minimally-invasive procedure used in the treatment of intracranial tumors, epilepsy and pain. Thermal damage is estimated using real-time MR thermometry but little is known about the dynamics of the process and how various intracranial tumors affect overall ablation. OBJECTIVES: Determine the relationship between thermal energy delivery and the time to maximal estimated thermal damage in the setting of previously untreated glioblastoma multiforme (GBM), recurrent GBM, and metastatic tumors or radiation necrosis. We aim to understand whether differences exist between these intracranial malignancies and how the ablation process is affected. METHODS: We used real-time ablation data from 24 patients across five unique intracranial pathologies. Ablations were performed using the Visualase Thermal Therapy System (Visulase Inc.) using a 980-nm diffusing tip diode laser. Thermal damage area was plotted against time for each ablation. Subsequently, the duration required to reach 50% (t50) and 97% (t97) of maximum damage was estimated. Comparisons were then made between different intracranial pathologies. RESULTS: The average (mean ± SEM) t97 for previously untreated GBM (n = 7) recurrent GBM (n = 6) and tumor metastasis/radiation necrosis (n = 7) are 167 ± 18, 165 ± 22, 97 ± 22 seconds, respectively. The mean t97 is significantly shorter for tumor metastasis/radiation necrosis when compared with previously untreated GBM (p < 0.05). CONCLUSIONS: While optimal duration of thermal ablation varies between intracranial tumors, we found that metastatic tumor/radiation necrosis requires significantly shorter duration of ablation. In most cases, maximal ablation was reached long before the industry recommended ablation time. Understanding the duration required to reach maximal ablation may reduce thermal exposure times during the ablation process. Future studies are needed to examine the relationship between ablation power, irrigation speed, and the effect of prior therapies on the ablation dynamics.

Microstructural evolution and compressive deformation of a new Ni–Fe base superalloy after long term thermal exposure at 700 °C

Abstract A new Ni–Fe base superalloy for applications of advanced ultra-supercritical (A-USC) boiler tubes was heat treated to three different statuses, i.e. aging treatment, aging treatment plus 1000 h and 7000 h thermal exposure at 700 °C. The specimens with various heat treatments have been compressed at 700 °C. The yield strength first increased with thermal exposure time and decreased after further thermal exposure. After aging treatment and 1000 h thermal exposure, deformation was heterogeneous, and dislocation slip combing climb was dominant process. However, after 7000 h thermal exposure, deformation became homogeneous. Stacking faults shearing the γ′ precipitates were also observed. The coarsening of γ′ precipitates and α-Cr precipitation occurred during long term thermal exposure. The influence of microstructural evolution on the yield strength and deformation mechanisms is discussed.

Elevated-temperature mechanical properties and thermal stability of Al-Cu-Mg-Ag heat-resistant alloy

The elevated-temperature mechanical properties and thermal stability of Al-Cu-Mg-Ag heat-resistant alloy were studied by tensile test, transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. The results show that with the increase of Ag content, the tensile strength and yield strength increase, which is attributed to the increase of the precipitations number and the decrease of the size. The same conclusions are drawn in the study of increasing Mg content. The alloy possesses excellent thermal stability. At 100–150 °C, the strength of the under-aged alloy increases at the initial stage, and after reaching the peak strength, it remains the same. The secondary precipitation of the under-aged alloy occurs in the process of exposure at 150°C, and it distributes diffusely after thermal exposed for 20 h. Then, the tensile strength decreases gradually with increasing the thermal exposure time at 200–250 °C. The strength of the peak-aged alloy decreases gradually, and the precipitation grows up, but the number decreases gradually with prolonging the exposure time at 100–250 °C. The strength of two kinds of alloys decreases with elevating of exposure temperature.

Capillarity Effect Controlled Precipitate Growth at the Grain Boundary of Long-Term Aging Al 5083 Alloy

A model was developed to predict thickness and continuity of β phase (Al3Mg2) formed at grain boundaries of long-term aged Al 5083 alloy. In this model, a variable collector plate mechanism was adopted at the early stage of aging, then, at about 1 month (\( 2\sqrt {D_{\text{b}} t} \approx 100\;{\text{nm}} \)), the model transitions to a constant collector plate mechanism. Two concentration profiles of Mg, one for a semi-infinite bulk at short diffusion distances and one for a finite slab at long diffusion distances (\( 2\sqrt {D_{\text{m}} t} \approx 20\,{\text{pct}} \) of the grain size), were applied to this model for different aging times. Capillarity effects were used to determine the morphology of β phase at the grain boundary. Combining different collector plate mechanisms and Mg concentration profiles, the whole β phase growth process was divided into three stages (short-term Mg concentration profile-variable collector plate, short-term Mg concentration profile-constant collector plate, and long-term Mg concentration profile-constant collector plate). Finally, the model was solved numerically. Experimental results of β phase length and thickness were obtained using transmission electron microscopy (TEM) images of Al 5083 aged at 343 K (70 °C) for different thermal exposure times. Modeling results of β phase thickness and continuity agree well with experimental observations.

Steady state flow of the FeCoNiCrMn high entropy alloy at elevated temperatures

Steady state flow behavior of the FeCoNiCrMn high-entropy alloy at temperatures ranging from 1023 to 1123 K was systematically characterized. It was found that the stress exponent (i.e., the reciprocal of strain-rate sensitivity) was dependent on the applied strain rate, and specifically the stress exponent is high (∼5) in the high strain rate regime, but decreases with decreasing strain rate. Microstructural examinations of the samples before and after deformation were performed to understand the interplay of the microstructures with the corresponding properties. Based on the observations, it was proposed that, at high strain rates, the deformation of the current high-entropy alloy was controlled by dislocation climb and the rate limiting process was the diffusion of Ni. At low strain rates, however, the deformation appeared to be controlled by the viscous glide of dislocations. Moreover, at the slowest strain rate (i.e., the longest thermal exposure time), new phases evolved, which caused elemental redistribution and weakening of the material.

A Study on Thermal Fatigue Life Variation According to Thermal Exposure Time

Thermal barrier coating. Thermal fatigue. Exposure time. Thermal fatigue test is one of the most widely used method to evaluate the durability of thermal barrier coating (TBC). However, thermal fatigue test can be concluded in totally different results according to the test variations. Especially, Exposure time of thermal fatigue test can affect the delamination life cycle of TBC. In this study, using the same test equipment which Kim et al. used, thermal fatigue tests were performed with different holding time at high temperature, and the test results by Kim et al. and those by this study were compared. In addition, delamination map was come to perfection from the test results to define more accurate thermal fatigue life.

The evanescent-wave cavity ring-down spectroscopy technique applied to the investigation of thermally grown oxides on Si(100)

We report about the contribution of thermally grown SiOx overlayer on the SiOx/Si interface (with oxidation states Sin+, where n = 1, 2, 3, 4) to the optical losses of a resonant spectroscopic cavity. The experiments on Si oxide thin films were performed in evanescent wave for Si samples in contact with a total internal reflection surface of a BK7 prism. The evanescent field can be exploited to investigate properties and processes such as the absorption of thin film or solid/air interface reactions. The results show that the oxide overlayer thickness grows with the thermal exposure time and is limited after more than 7 h of treatment. Transmission electron microscopy has been used for the native oxide thickness measurement and angle-resolved X-ray photoelectron spectroscopy used to determine the thermal oxide thickness. A change of absorption coefficient Δα in the range 100–200 cm−1 is obtained by evanescent-wave cavity ring-down spectroscopy (EW-CRDS) for thermal silicon oxide overlayer, in agreement with the general trend from literature. The evaluation from the EW-CRDS experiments presents the used setup as a competitive method for measuring the absorption properties of thin overlayer.