Through-thickness Temperature Gradient Research Articles

Carbon nanotube fibers as efficient p- and n-type thermoelements within geopolymers: A route for large-scale thermal energy harvesting from building structures

This study reports for the first time, the thermoelectric (TE) performance of continuous p- and n-type carbon nanotube fiber (CNTF) filaments integrated within a sustainable, fine-grained geopolymer (GP) based concrete matrix for the complete replacement of cement with supplementary cementitious materials towards CO2 emissions reduction policy. Moreover, CNTF as CO2 negative, conductive, durable and lightweight material could multidimensionally contribute on the achievement of sustainability goals. Herein, pristine CNTF filaments were molecularly modified with p- and n-type aqueous-based dopants in order to stably tune their intrinsic semi-conducting properties via a facile and scalable dip-coating process. The achieved Seebeck coefficient values of the optimized p- and n-type CNTF were +45.2 μV/K and −38.4 μV/K, respectively. Thereafter, the p- and n-type CNTF filaments were employed as pillar-like thermoelectric elements incorporated within a GP mixture for the assembly of a structural thermoelectric generator (TEG) device. The obtained TEG-enabled GP-based concrete demonstrator comprising of 8 p-n serially interconnected junctions is capable of generating a power output of 0.3 μW at an applied through-thickness temperature gradient (ΔT) of 50 Κ. Subsequently, the experimental work was validated using coupled field of TE simulations. Self-powered infrastructures are forecasted to efficiently benefit from available ΔT on large-scale upon their operational lifetime, further promoting future net-zero energy consumption concepts.

Heat-assisted friction stir incremental sheet forming of thermoplastics

Incremental sheet forming (ISF) has been attracting increasing interests for customised manufacture of thermoplastics, but the application in high-performance and hard-to-form thermoplastics is still a challenge. This paper develops a new heat-assisted friction stir incremental sheet forming (HAFS-ISF) process to form polyether ether ketone (PEEK) and polymethyl methacrylate (PMMA, acrylic). Flexible heating tapes and tool rotational friction heat the thermoplastic sheet on both bottom and top surfaces, which maintain the sheet material at high temperature and reduce the through-thickness temperature gradient. Results are given for the effect of key parameters on forming temperature, force, accuracy and thickness distribution. Spindle speed has the most significant effect on all the studied aspects, followed by the pre-heating temperature, while feed rate and step size have less effect. With lower spindle speed of 100 rpm, the through-thickness temperature gradient is improved. Compared with PEEK parts, more uniform forming temperature and smaller forming force can be observed for PMMA parts. But the material brittleness causes worse formability, and the severer twist and stretching effect affect the forming accuracy of PMMA parts. Forming accuracy, fracture and twist are discussed by developing analytical models. The model for forming accuracy considers the initial bending effect and springback. The fracture is discussed by modelling the stress triaxiality and equivalent strain distribution under contact. While the model for twist considers effect of the applied torque on shear strain energy and continuous accumulation of twist along the tool path. By comparing analytical models and testing observations, the effectiveness of the models is confirmed to understand the underlaying mechanisms and to address a few challenges.

Effects of temperature gradients on thermomechanical fatigue of nickel-based superalloy

Temperature-gradient-mechanical fatigue (TGMF) is a significant concern in internal cooling turbine blades. In the present study, the through-thickness temperature gradient was determined by the inverse heat conduction method and investigated in TGMF. Using the continuum damage mechanics, the environmental impact was identified as the primary contributor to the in-phase TGMF damage. A damage accumulation model encompassing fatigue, creep and environmental impact was established and provided satisfactory accuracy in TGMF life assessments. The main conclusion is that temperature gradients do not change the fracture mode but significantly increase stress levels by at least 20%, accelerating oxidation damage and resulting in a TGMF life ca. 10 times shorter than that of TMF.

Correlating pipe dimensions and success of local heat treatment: Developing nomograms to deduce heat treatment parameters

Local post-weld heat treatment (PWHT) is the only option for heat treating field welded joints. Quite often, the success of local PWHT in alleviating the residual stress and tempering the material within the soak band (SB) is dependent on the ability to achieve the required heat treatment temperature and maintain through-thickness temperature gradient (TTG) within the specified limits at the end of heating cycle, whence soaking begins. Field observations reveal the inadequacy of AWS D 10.10 specified parameters viz. rate of heating (ROH), heat band (HB) and insulation band width in not achieving the required TTG for certain pipe dimensions. Although prior works have attempted to address this issue by widening the HB, the capacity of the heating equipment often pose a limitation. In such cases, reducing ROH is a plausible alternative. With, no such prior studies seen in literature, an exhaustive finite-element analysis simulating the local PWHT on 81, SA106GrC pipe samples (diameter and thickness varied in 9 levels each) was performed, thrice. First as per AWS recommendations, second by halving the code deduced ROH and third by doubling the code deduced HB. The trend of important outcomes (TTG, power source rating and energy consumption) with great significance to the heat treatment industry were also compared and analysed. Two nomograms were developed to serve as a ready reckoner for field heat treater in not only assessing the adequacy of heat treatment parameters but also with possible alternatives in achieving the desired TTG using field-available power source.

Temperature Monitoring of Through-Thickness Temperature Gradients in Thermal Barrier Coatings Using Ultrasonic Guided Waves

Ultrasonic guided waves offer a promising method of monitoring the online temperature of plate-like structures in extreme environments, such as aero-engine nozzle guide vanes (NGVs), and can provide the resolution, response rate, and robust operation that is required in aerospace. Previous investigations have shown the potential of such a system but the effect of the complex physical environment on wave propagation is yet to be considered. This article uses a numerical approach to investigate how thermal barrier coatings (TBCs) applied to the surface of many components designed for extreme thermal conditions will affect ultrasonic guided wave propagation, and how a system can be employed to monitor through-thickness temperature changes. The top coat/bond coat boundary in NGVs has been shown to be a temperature critical point that is difficult to monitor with traditional temperature sensors, which highlights the potential of ultrasonic guided waves. Differences in application method and layer thickness are considered, and analysis of through-thickness displacement profiles and dispersion curves are used to predict signal response and determine the most suitable mode of operation. Heat transfer simulations (COMSOL) have been used to predict temperature gradients within a TBC, and dispersion curves have been produced from the temperature dependant material properties. Time dependant simulations of wave propagation are in good agreement with dispersion curve predictions of wave velocity for the two lowest order modes in three thicknesses of TBC top coat (100, 250, and 500 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \\hbox {m}$$\\end{document}). When wave velocity measurements from the simulations are compared to dispersion curves generated at isotropic temperatures, the corresponding temperature represents the average temperature of a gradient system well. Such a measurement system could, in principle, be used in conjunction with surface temperature measurement systems to monitor through-thickness temperature changes.

Investigation on the Through-Thickness Temperature Gradient and Thermal Stress of Concrete Box Girders

Bridges are generally affected by thermal loads which include the daily cycle, seasonal cycle and annual cycle. Thermal loads mode and thermal effects on bridges, especially for concrete girders, are quite essential but complicated. To investigate the temperature field and thermal stress in the thickness direction of a concrete box girder, the temperature field of a prestressed concrete continuous box girder bridge is monitored, and the temperature distribution in the thickness direction of the concrete box girder is analyzed. Finite element simulation, utilizing air elements specifically designed for concrete box girders, is employed to analyze the temperature field and thermal stress profiles along the thickness of the slab. The findings indicate a variation in temperature along the thickness of the concrete box girder slab. The most significant temperature differential, reaching up to 10.7 °C, is observed along the thickness of the top slab, followed by the bottom plate, with the web exhibiting relatively smaller differentials. Temperature in the full thickness range has a significant impact on the top plate, while the web plate and bottom plate are greatly influenced by temperature ranging from the outer surface to the center of the plate thickness. The temperature difference between the center of the plate thickness and the inner surface is approximately 0. The variation in temperature due to the variation in thickness direction is a temporal factor, wherein the outer layer of the roof is primarily compressed, while the inner layer is subjected to tension. The external surface of the web is mainly compressed. The stress exerted by the internal surface temperature is minimal. The internal and external surface effects of the floor are similar, and as time passes, tensile and compressive stresses appear.

Residual stresses in thick composites considering through-thickness temperature gradients during manufacturing

In the composite materials of wind turbine rotor blades, residual stresses form during manufacturing due to the different thermo-chemo-mechanical behaviors of the fibers and the matrix. These residual stresses decrease the ultimate and fatigue strength of the blade because the failure stress and cycles to failure are reached at lower levels of external loading. It is thus of utmost importance to consider residual stresses in the design stage to obtain reliable blade designs.This work deals with the prediction of residual stresses in a thick composite representative for a spar cap in a wind turbine rotor blade. A model is formulated for the thermo-chemo-mechanically coupled problem. An emphasis is given on the non-uniform and time-variable temperature distribution across the thickness of the composite. The different sources of residual stresses (e.g., chemical shrinkage, non-uniform distribution of different thermal expansion coefficients of fibers and matrix, exotherm, etc.) are analyzed, quantified, and compared. The impact of the time-dependent temperature distribution on the formation of residual stresses is highlighted by means of a representative numerical example.

Hermeticity of SiC/SiC composite and monolithic SiC tubes irradiated under radial high-heat flux

Demonstration of hermetic SiC fiber–reinforced SiC matrix composite cladding under normal operating environments has been identified as one of the most critical feasibility issues for accident-tolerant fuel cladding in light-water reactors. This study provides critical experimental data needed for understanding the effects of irradiation on hermeticity. SiC composite and monolithic tubes were neutron-irradiated to 2 displacements per atom with and without a nominal radial heat flux of 0.6 MW/m2 to produce a simulated in-pile stress state for the normal operation of a light-water reactor. The through-thickness temperature gradient under irradiation results in a gradient in swelling, which causes a significant stress buildup. Such irradiation-induced stress was modeled using a commercial finite element analysis code. The radial heat flux–irradiation synergism was experimentally investigated by constructing a special irradiation capsule and evaluating the helium hermeticity of the specimens. The simulated stress state exhibited a near equi-biaxial tensile axial and hoop stress of ∼150 MPa at the inner surface of the SiC composite tube. This stress level is potentially beyond the matrix cracking stress. Degradation of hermeticity of the SiC composite tubes was observed after irradiation, indicating irradiation-induced cracking, whereas the irradiated monolithic SiC tubes remained hermetic. The results indicate that loss of hermeticity caused by radiation-induced microcracking is a potential issue for SiC composite cladding, depending on the magnitude of the temperature gradients. Coating the outer surface of the cladding was identified as a mitigation strategy that might overcome the cracking issue.

Low frequency photothermal excitation of AFM microcantilevers

Photothermal excitation at frequencies below the mechanical resonance of the atomic force microscopy (AFM) microcantilever can be utilized in force modulation microscopy, fast force displacement curve acquisition, and tip-based mass spectroscopy. To understand the microcantilever bending response in these modes, accurate models of the thermoelastic response of the AFM microcantilever are needed. We study the sub-resonance photothermal vibrational response of coated and uncoated AFM microcantilevers as a function of laser modulation frequency and spot location. The sub-resonance microcantilever response shows distinct thermoelastic regimes. Below the microcantilever's thermal roll-off frequency, the vibration amplitude is mostly constant. Past this frequency, the vibration amplitude decreases with increasing frequency. At modulation frequencies below the thermal roll-off frequency, the most efficient photothermal laser spot to excite harmonic motion is near the free end of both coated and uncoated microcantilevers. For the tested coated microcantilevers, the most efficient photothermal laser location migrates from near the free end of the microcantilever to near the fixed end as modulation frequency increases. For the tested uncoated microcantilever, the most efficient photothermal laser location remains unchanged at the tested frequencies. To predict the bending response of the coated microcantilever, a bilayer bending model is implemented. At low frequencies, this model underpredicts the bending response compared to experiments by up to 90%. This may be due to neglecting microcantilever bending contributed by a through-thickness temperature gradient. Our results illustrate different aspects of the frequency-dependent photothermal laser spot optimization that can guide users to maximizing microcantilever response to a given input power.

Active control of cure-induced distortion for composite parts using multi-zoned self-resistance electric heating method

High dimensional accuracy of the carbon fiber reinforced polymer (CFRP) composite parts has been a long-term challenge for manufacturers in the aerospace industry. Traditionally, the uneven curing temperature could aggravate the cure-induced distortion (CID) and should be avoided as much as possible. But even under an ideally uniform temperature field, the CID of curved parts still cannot be eliminated because of the anisotropic shrinkage nature of the CFRP laminate. In this work, a novel CID control method based on the multi-zoned heating methodology is proposed, which actively applies an optimized non-uniform temperature field in different zones of the part to counteract the CID influenced by geometric structures. The counteracting principle of the proposed method lies in that using the actively formed through-thickness temperature gradients in different zones generates numerous small local thermal deformations, and their combination can significantly reduce the overall CID of the CFRP part. A co-optimization framework combined with the numerical model and genetic algorithm is established to rapidly compute the optimized multi-zoned temperature field for the CFRP parts with arbitrary complex structures. A good agreement between the numerical and the previous experimental results shows the effectiveness of the numerical model, and the multi-zoned temperature field optimization case of a scaled air-intake CFRP part demonstrates the performance of the co-optimization framework. Finally, by using the multi-zoned self-resistance electrical heating method, the CID reduction of the proposed method is experimentally validated on a complex CFRP part, where the average displacement reduces by >60 % compared to that of the traditional oven process.

Optimization of Temperature Field by Differentiating Number of Circumferential Control Zones in Local Post Weld Heat Treatment on 9%Cr Heat-Resistant Steel Pipe

Abstract This paper reports the optimal use of control zones to achieve required uniformity of temperature distribution in local postweld heat treatment of welds in 9% Cr heat-resistant large pipe system. In this research, local post weld heat treatment tests on temperature distribution with different control zones were carried out on 9%Cr steel pipes (OD 710 mm × 35 mm and OD 575 mm × 35 mm), which was further used for the development of the thermal analysis of the postweld heat treatment model via abaqus. The research results revealed (1) the effect of number of control zone on the uniformity of temperature distribution; (2) the effect of size of pipe diameter on the circumferential temperature and the through-thickness temperature gradients. This article discusses the possible reasons for the temperature difference at various positions of the pipe caused by the air flow inside the pipe with different control zones. Based on the obtained results, a practical method was designed for the selection the number of circumferential control zones on 9%Cr heat-resistant steel pipeline according to the required degree of temperature distribution uniformity. This paper contributes to the specific knowledge and the generic methodology.

Estimate of Temperature Gradients of Thin-Walled Structures Under Thermomechanical Fatigue Loading

Temperature gradients are significant in the fatigue life assessment. Thermal loading conditions were suspected to affect temperature gradients and thermal stresses in structures. In the present work, thin-walled plates and tubes were experimentally investigated to quantify the effects of thermal loads on the through-thickness temperature gradients, and finite element models were developed based on the experiments and used to generate more general correlations. It is confirmed that both transient peak temperature gradient and steady-state values are linear to heating power and cooling conditions, and are insensitive to plate thicknesses. By introducing the nondimensional temperature gradient, the temperature distribution in thin-walled structures can be estimated with reasonable accuracy.

Influence of kerosene flame on fire-behaviour and mechanical properties of hybrid Carbon Glass fibers reinforced PEEK composite laminates

This work examines the influence of kerosene flame exposure on the residual mechanical behavior (tension and compression) of hybrid quasi-isotropic composite laminates consisting of carbon/glass fibers and a PEEK thermoplastic matrix. The influence of a kerosene flame exposure (116 kW/m2 and 1100 °C), on the composites structural integrity was examined as a function of exposure time (5–10-15 min). The changes in the tensile and compressive properties (axial stiffness and strength) were compared with respect to the virgin materials (experiencing no prior flame exposure). The discussions on fire- and mechanically-induced damage mechanisms are supported by fractographic analysis of specimens. It is therefore possible to better understand how the fire-induced damages within the laminates micro- and meso-structures modify the mechanical behavior of flame-exposed laminates.Regardless the exposure time, the kerosene flame exposure involves in-plane and through-thickness temperature gradients, which ultimately create two well defined areas within the laminates: an extensively delaminated one (the one close to the exposed surface) and a relatively well preserved one (close to the back surface). From the present work, it is possible to conclude that the mechanical properties in tension are severely affected (-50% in stiffness and −70% in strength) by prolonged exposures to kerosene flame (15 min) with respect to as-received specimens. When compared to the 5 min case, flame exposure time (ranging from 5 to 15 min) seems to have little very influence on tensile properties and the effect is moderate on compressive properties. The barrier formed by an extensive thermally-induced delamination contributes to relatively preserve the structural integrity of the plies close to the back surface. The mechanical loading is taken up by the 0° fibers in non-delaminated areas of the specimens, resulting in preserving the residual mechanical in tension and in compression.

Numerical and experimental investigations on the temperature field in local post weld heat treatment of 9 % Cr heat resistant steel welded pipes

Local post weld heat treatment (PWHT) is usually employed in the field fabrication of large-sized 9% Cr heat resistant steel welded components, such as power plant boilers. Temperature field in local PWHT plays critical roles in both achieving PWHT purposes and avoiding detrimental effects of local heating. The objective of this research is to quantify the effects of local PWHT parameters and pipe dimensions on the temperature field in local PWHT of 9% Cr heat resistant steel pipes and finding the optimal process window to engineer desired weld properties. Finite element (FE) models were developed to simulate the temperature field of local PWHT and carefully verified by eight experiments using four 9% Cr heat resistant steel pipes with typical dimensions employed in ultra-supercritical (USC) boilers. The results show that the through-thickness temperature gradient is severely aggravated with the increase of pipe diameter and wall thickness, which will lead to inadequate tempering in the weld metal close to the inside surface of the pipe. To enlarge heated band (HB) width is an efficient way to mitigate the through-thickness temperature gradient, and the axial temperature gradient can be effectively relieved by augmenting gradient control band (GCB) width. The through-thickness temperature gradient is the major factor in implementing successful local PWHT for 9% Cr heat resistant steel pipes. A much smaller process window is found in 9% Cr heat resistant steel pipes due to their higher PWHT temperatures and narrower range of permissible PWHT temperatures.

Double Point Temperature Measurements Under Rib and Channel in an Operating PEFC Using MEMS Sensors

Introduction For more spread of polymer electrolyte fuel cells (PEFCs), investigating the temperature distribution inside an operating PEFC is important. Last year, correlation between local through-thickness temperature gradient and the PEFC performance was investigated [1]. In this case, temperature gradient was critical to the PEFC performance under certain operating conditions. Moreover, local in-plane temperature gradient should be suggested as an important factor to the PEFC performance. Therefore, in this study, two local temperature measurement on the interface of the cathode catalyst layer (CL) and the microporous layer (MPL) in a PEFC was performed while it was operated. Experimental method Thin-Film Thermocouple (TFTC) For the local temperature measurement inside a PEFC, in-house thin-film thermocouple (TFTC) was fabricated using micro electromechanical system (MEMS). The thermocouple was made from Au and Ni. Its measurement point was about 30 µm width and about 6 µm thick, which was so fine and thin that it wouldn’t disturb a reaction inside a PEFC.Cell Configuration An active area of 1 cm2 PEFC cell was used, and is has five 1mm width gas channels and five 1mm width ribs with parallel flow configuration. 15 µm thick reinforces type PEM was used. TGP-H-060 (with MPL 2.0 mg/cm2) was used and in both anode and cathode side. TFTCs were inserted between interface of cathode CL/MPL, and two TFTCs were set on rib and channel, respectively.Operation Conditions Power generation was performed by sweeping the cell voltage from OCV to 0 V at a rate of 1 mV/s. K-type thermocouples were used to measure the bi-polar plates with gas channels and sensor terminal temperatures. An impedance analyzer was used to measure the cell resistance. O2 and air were used as a cathode supply gas, respectively. Pure H2 was used as an anode supply gas. The flow rates of anode gas / cathode gas were set in two conditions, 500/500 mL/min, and 30/75 mL/min. Results and Discussion Here, results in one operating condition is being shown, when the high flow rate has been chosen, and O2 has been chosen as a cathode supply gas. Fig.1 (a) shows the cell voltage and cell resistance transition with current density. In this case, the cell voltage decrease due to diffusion overvoltage didn’t occur. And Fig.1 (b) shows the temperature increase from the beginning operating the PEFC on the CL/MPL interface under rib and channel. The temperature on the channel was higher than that on the rib. Due to the condensed water which was generated with the PEFC operation, the effective thermal conductivity got decreased. The amount of the condensed water on the rib was larger than that on the channel. Therefore, total amount of the condensed water might affect on the effective thermal conductivity and caused the temperature difference on the rib and channel. Reference [1] K. Shigemasa, et al., ECS Trans., 92(8),161-4 (2019) Figure 1

Investigating the vapour transport phenomena inside the cathode gas diffusion layer media by controlling local temperature gradient inside an operating proton exchange membrane fuel cell

Water management is critical for the function of the proton exchange membrane fuel cell (PEMFC). The local through-thickness temperature gradient of the cathode gas diffusion layer (GDL) affect water management for a PEMFC. That has been known and been targeted for the discussion for several years. In this study, the effects of temperature gradient inside PEMFC was investigated by controlling it externally with a fine (testing diameter about 30 μm) and thin (thickness about 6 μm) thin-film thermocouple (TFTC), which has been developed using micro electromechanical system (MEMS) technology. In addition, X-ray computed tomography (CT) has been chosen as a method to visualise inside of an operating PEMFC. The simultaneously measurement of temperature gradient and liquid water distribution was firstly accomplished. In conclusion, in the case where the temperature of the cathode catalyst coated membrane (CCM) side is higher than that of the channel side, the PEMFC performance drastically decreased. The difference in PEMFC performance was caused by a difference the preferential sites for water condensation.

Influences of Material Variations of Functionally Graded Pipe on the Bree Diagram

The present research is concerned with the elastic–plastic responses of functionally graded material (FGM) pipe, undergoing two types of loading conditions. For the first case, the FGM is subjected to sustained internal pressure combined with a cyclic bending moment whereas, in the second case, sustained internal pressure is applied simultaneously with a cyclic through-thickness temperature gradient. The properties of the studied FGM are considered to be variable through shell thickness according to a power-law function. Two different designs of the FGM pipe are adopted in the present research, where the inner surface in one case and the outer surface in the other are made from pure 1026 carbon steel. The constitutive relations are developed based on the Chaboche nonlinear kinematic hardening model, classical normality rule and von Mises yield function. The backward Euler alongside the return mapping algorithm (RMA) is employed to perform the numerical simulation. The results of the proposed integration procedure were implemented in ABAQUS using a UMAT user subroutine and validated by a comparison between experiments and finite element (FE) simulation. Various cyclic responses of the two prescribed models of FGM pipe for the two considered loading conditions are classified and brought together in one diagram known as Bree’s diagram.

Predictions of transverse thermal conductivities for plain weave ceramic matrix composites under in-plane loading

A coupled temperature-displacement steady-state analysis of plain weave ceramic matrix composites along through-thickness direction was reported in this paper. Based on the formulation of Binary Model, a meso-scale 2 × 2 unit cell model of plain weave composites was established. The periodic displacement boundary conditions and through-thickness temperature gradient were imposed. The non-linear strain-dependent thermal properties of constituent materials have been discretised by multi-linear curves, which have been implemented by the user defined subroutine, USDFLD of the finite element package, Abaqus. The degradation of the composite through-thickness thermal conductivity with uni-axial straining has been predicted. The predictions considering tow waviness have a good agreement with experimental results. The analysis indicates that the degradation of through-thickness thermal conductivities is due to the combination of shear failure and wake debonding mechanisms. The matrix/medium played an important role in the through-thickness heat flow of plain weave ceramic matrix composites.

Indentation failure of foam cored sandwich structures subjected to elevated temperatures

This research explores the detrimental effect of elevated temperatures on local indentation failure of polymer foam cored sandwich structures with laminated glass fibre reinforced epoxy face sheets. A simple analytical model to predict the critical indentation failure load at elevated temperatures is presented and validated against experimental observations. For this purpose, a sandwich beam loaded in three-point bending with an induced through-thickness elevated temperature profile was studied experimentally and by means of analytical and finite element models. In the experiment, the through-thickness temperature gradient was induced with an infrared lamp pointing on the top face sheet, while the local displacement and strain fields near the applied point load were recorded by digital image correlation. The analytical model proposed, which accounts for temperature degraded/reduced foam core properties, superimposes the local response, approximated by the classical Winkler foundation model, and the global response obtained by sandwich beam theory. The comparative study has shown that the critical load causing core crushing failure reduces significantly with elevated temperatures by as much as 50% at an elevated temperature of 90℃. It is shown that the simple analytical model can predict the local deflections and core stresses of foam cored composite sandwich structures subjected to simultaneous localized mechanical loading and elevated temperatures. Thus, the analytical model can be used as a preliminary design tool to determine the critical core crushing load at elevated temperatures.

An Experimental Apparatus and Procedure for the Simulation of Thermal Stresses in Gas Turbine Combustion Chamber Panels Made of Ceramic Matrix Composites

This paper presents an experimental procedure developed to simulate the behavior of ceramic matrix composites (CMCs) under the cyclic thermal stresses of a gas turbine combustion chamber. An experimental apparatus was assembled that produces a temperature gradient across the thickness of a CMC specimen while holding the specimen at its two extremities, which simulates the bending stress that would be observed at the center of a combustor panel. Preliminary validation tests were performed in which A-N720 oxide–oxide CMC specimens were heated to a surface temperature of up to 1160 °C using an infrared heater, which allowed for the calibration of heat losses and material thermal conductivity. The specimen test conditions were compared with predicted conditions in generic annular combustor panels made of the same material. Provided that a more powerful heat source is made available to reach sufficiently high temperatures and through-thickness temperature gradients simultaneously, the proposed experiment promises to allow laboratory observation of representative deterioration modes of a CMC inside an actual combustion chamber.