Thermal Gradient Cycling Research Articles

Early-Detection of EVA Encapsulant Degradation in PV Modules Based on Vibration Frequency Analysis

In engineering applications, the frequency analysis represents a first and practical step to collect relevant parameters for structural and mechanical diagnostics. Any possible material / component degradation and deterioration can be prematurely detected by frequency modifications that exceed a certain alert value. In this paper, the attention is given to the dynamic mechanical analysis of commercial photovoltaic (PV) modules, in which the solar cells are typically encapsulated in thin viscoelastic interlayers made of Ethylene-Vinyl Acetate (EVA), which are primarily responsible for the load-bearing capacity of the sandwich PV system. As a major effect of ageing, ambient conditions, non-uniform / cyclic thermal gradients, humidity and even extreme mechanical / thermal loads, the rigidity of these films can largely modify and decrease, thus possibly affecting the mechanical capacity of the PV module, and even exposing the solar cells to fault. Knowledge of the effective bonding level is an important step for diagnostic purposes. In this regard, the present study is based on a preliminary but extensive parametric Finite Element (FE) numerical investigation of full-scale commercial PV modules of typical use in buildings. The attention is given – for PV module arrangements of technical interest – to the effect of EVA stiffness in terms of vibration modes and especially frequency sensitivity. As shown, when compared to newly installed PV modules, any kind of stiffness decrease is associated to major frequency modifications for the composite system, and in the worst configuration, such a frequency scatter can decrease down to -40% the original condition. Such a marked stiffness decrease would be implicitly associated to a weak mechanical performance of the sandwich section, with major stress peaks and deflections in the PV system, even under ordinary loads. The presented results, in this sense, suggest that major consequences can be prevented and minimized by monitoring the vibration frequency of PV modules.

Non-destructive vibration-based monitoring analysis of PV modules with encapsulant degradation by frequency change

In engineering applications, the frequency analysis represents a first and practical step to collect relevant parameters for structural and mechanical diagnostics. Any possible material / component degradation and deterioration can be prematurely detected by frequency modifications that exceed a certain alert value. In this paper, the attention is given to the dynamic mechanical analysis of commercial photovoltaic (PV) modules, in which the solar cells are typically encapsulated in thin viscoelastic interlayers made of Ethylene-Vinyl Acetate (EVA), which are primarily responsible for the load-bearing capacity of the sandwich PV system. As a major effect of ageing, ambient conditions, non-uniform / cyclic thermal gradients, humidity and even extreme mechanical / thermal loads, the rigidity of these films can largely modify and decrease, thus possibly affecting the mechanical capacity of the PV module, and even exposing the solar cells to fault. Knowledge of the effective bonding level is an important step for diagnostic purposes. In this regard, the present study is based on a preliminary non-destructive experimental analysis, and on an extensive parametric Finite Element (FE) numerical investigation of full-scale commercial PV modules of typical use in buildings. The attention is given – for PV module arrangements of technical interest – to the effect of EVA stiffness in terms of vibration modes and frequency sensitivity. As shown, especially compared to newly installed PV modules, any kind of stiffness decrease is associated to major frequency modifications for the composite system, and in the worst configuration, such a frequency scatter can decrease down to −40% the original condition. Such a marked stiffness decrease would be implicitly associated to a weak mechanical performance of the sandwich section, with major stress peaks and deflections in the PV system, even under ordinary loads. The presented results, in this sense, suggest that major consequences can be prevented and minimized by monitoring the vibration frequency of PV modules.

Thermal Barrier Coatings in burner rig experiment analyzed through LAser Shock for DAmage Monitoring (LASDAM) method

This study investigates failure mechanisms in a typical thermal barrier coating (TBC) system comprising an EB-PVD columnar top coat, an aluminide bond coat, and a Ni-based single crystal superalloy substrate, simulating gas turbine operating conditions using a burner rig. TBC degradation, initiated by interfacial defects from the LASAT method, was studied during thermal gradient cycling under fast and slow cooling. In-situ optical and infrared imaging, along with ex-situ SEM cross-sectional analysis, monitored failure mechanisms. The Laser Shock for Damage Monitoring (LASDAM) technique provided insights into gradient and cooling rate impacts on columnar TBC damage. Results showed significant effects of cooling rate on delamination and localized failure at blister sites, with LASDAM revealing significant overheating at damage sites. Analysis included full-field temperature and damage assessment, emphasizing blister-driven delamination under severe thermal gradients. Discussion focused on elastic stored energy effects, noting that fast cooling induced transient conditions where reversed temperature gradients increased damage, limiting TBC lifespan.

Experimental and numerical analysis of CMCs mechanical properties under high-temperature thermal gradient environment

This paper carried out a high-temperature thermal gradient cycling test of ceramic matrix composites (CMCs), investigating the failure characteristics of CMCs in non-uniform high-temperature environments. The result shows that the damage characteristics of CMCs under thermal gradient environments are significantly different from those under uniform temperature environments. The SiC fiber pull-out lengths at different yarn fractures have obvious differences in the thermal gradient direction. The thermal stress damage propagation pattern of CMCs under the thermal gradient environment is revealed by numerical analysis, and its residual mechanical properties under different thermal gradient environments are predicted. The numerical results could be adopted as a theoretical basis and suggestions for temperature control of CMCs components in practical applications.

Thickness-related failure behaviors of the thermal barrier coatings under thermal gradient cycling

The ZrO2–7 wt% Y2O3 (7YSZ) is the most commonly used ceramic layer in the thermal barrier coating (TBC) system due to its low thermal conductivity. However, the thermal stress-induced coating failure would damage its performance and limit its application. In the present study, a burner rig is employed to attain the operating temperature field and conduct the thermal gradient cycling tests of the coating. Moreover, the failure modes of the YSZ coating with different thicknesses (100, 200, and 400 μm) are investigated. The results show that the coating with 200 μm thickness has the most extended cyclic lifetime (>340 cycles), while 100 μm and 400 μm thick coatings have cyclic lifetimes of 240 and 80 cycles, respectively. The coating microstructure evolution, thermal growth oxides (TGO) thickness, and coating sintering behaviors are considered to explain the phenomenon. Furthermore, a TGO growth/sintering competitive failure model is proposed to describe the failure behaviors and elucidate the failure mechanisms of the coating with different thicknesses. The coatings with different thicknesses have different temperature distributions in the same external conditions. It results in different TGO growth rates and different degrees of coating sintering. The balance of TGO growth and coating sintering makes the 200 μm thick coating the most extended cyclic lifetime.

Effect of bitumen type, temperature and aging on mixed I/II fracture toughness of asphalt binders-experimental and theoretical assessment

The overall characteristics and fracture behavior of asphalt pavements and overlays is significantly controlled by the mechanical behavior of bitumen used in the mixture of asphalt concrete. The bitumen behaves like a dominantly brittle and elastic medium under subzero temperatures. In practice, cracked asphalt pavements experience mixed mode tensile-shear deformations due to moving traffic loads or cyclic thermal gradients. On the other hand, aging phenomenon can affect significantly the overall strength and load bearing capacity of pavements and bituminous asphalt mixtures. Different binders with different penetration grades and performance characteristic are often used for paving the roads. In order to investigate the effect of binder type and aging condition on the mixed mode I/II cracking response of bitumen materials, an experimental and theoretical study is performed in this research. Three binders: namely 30/40 bitumen, 60/70 bitumen and modified bitumen with SBS polymer (i.e. polymeric binder) are used to conduct a full set of mixed mode I/II fracture toughness experiments using an inclined edge notched bend beam specimen. The experimental results showed significant influence of mode mixity, bitumen type and aging on the fracture toughness data. The fracture toughness is reduced for the whole tested binders by moving from pure mode I to pure mode II. While aging has negative effect on the crack growth resistance of 30/40 and 60/70 binders, the mixed mode I/II fracture toughness of polymeric modified bitumen was increased after aging. The morphology of fracture surface and direction of fracture was studied for the tested binders under different environmental and loading conditions. The mixed mode I/II fracture toughness data were also predicted using two generalized tangential stress/strain based fracture criteria.

Thermal Fatigue Cycle Shock Effects on Physical and Structural Properties of H13 Tool Steels before and after Heat Treatments

Any tool that is subjected to repetitive thermal service will undergo degradation in loss of various properties such as, strength, structures, thermal stability, and so on. These natural changes lead to huge premature failures and unexpected break downs during production by causing uneconomical and undesirable situations. Therefore in this present research, the raw, heat treated, and heat treated and nitrided H13 tool steel specimens were subjected to a thermal shock cycle condition similar to a real industrial application using a unique in-house built thermal shock cyclic fatigue (TSCF) testing machine to impose the thermal gradients. All the TSCF tested samples were then characterized by physical and structural tests, including, hardness, (Rockwell, HRc), X-ray diffraction (XRD), and field emission scanning electron microscope (FESEM). The interesting changes in hardness, distorted crystal structure, and crack initiation due to the imposed cyclic thermal gradients by TSCF process up to 2000 thermal shock cycles were found to be different for differently treated H13 tool steel specimens. Therefore, this present investigation specifically would help in predicting the design parameters and to fabricate the proper mould components of various casting products.

In situ S/TEM Heating Experiments to Study the Effects of Cyclic Thermal Gradients in Additive Manufacturing Build Processes

<i>In situ</i> S/TEM Heating Experiments to Study the Effects of Cyclic Thermal Gradients in Additive Manufacturing Build Processes

In-situ TEM Heating Experiment to Study Effects of Cyclic Thermal Gradients on Ti-6Al-4V under Additive Manufacturing Growth Conditions

In-situ TEM Heating Experiment to Study Effects of Cyclic Thermal Gradients on Ti-6Al-4V under Additive Manufacturing Growth Conditions

Failure Analysis of an 18% Cr Ferritic Stainless Steel in a Simulated Exhaust Condensate Containing Urea

Exposure in the laboratory simulating a selective catalytic exhaust system component using urea injection for NO x reduction and an artificial exhaust condensate in a diesel application has been performed. The hot, cyclic corrosion test was designed to simulate the conditions seen during use in a commercial vehicle application. The sample was subjected to a cyclic thermal gradient from ambient to a high of 500 °C and a 5% solution of urea containing sulfates, chlorides, and nitrates. During material evaluation for a selective catalytic exhaust component, a perforation was observed in a 439 ferritic stainless steel component. Examination of the sample found multiple modes of failure and multiple types of deposits from the urea test solution.

Multi-layered thermal barrier coatings on BMI polyimide matrix composite

Al/YSZ and (Cu-6Sn)/YSZ as the double layer thermal barrier coatings (TBCs) and Cu-6Sn/NiCrAlY/YSZ as the triple layer TBC with 100μm thick for bonding layers and ceramic top coat were deposited on bismaleimide (BMI) matrix composite substrates. The results showed the coherent and clean substrate/coating interface in the triple layer coating system. On the other hand, significant pores, cavities and cracks were appeared throughout substrate/bond coat interfaces when double layer coating systems were created on the polymeric substrate. Fourier Transform Infra-Red (FTIR) spectroscopy showed that decomposition of polymeric substrate took place for double layer coating system in spite of that for the triple layer coating system. The results of laser flash analysis showed that lower thermal diffusivity value of Cu-6Sn/NiCrAlY bonding layer in the triple layer coating system than the two other bonding layers in double layer coating systems may result in lower heat flux transmitted to the substrate during ceramic top coat deposition. NiCrAlY intermediate layer reduced the differences of microhardness and coefficient of thermal expansion (CTE) values between YSZ top coat and metallic inner layer resulting in reduced mismatch in the thermal and mechanical properties at the two interfaces of the triple layer coating system as compared to double layer coating systems. It can be inferred that the triple layer coating system with 300μm thick YSZ might increase the working temperature of polymeric substrate due to significant reduction in its thermal conductivity value. Thermal gradient cycling test at the coating surface temperature between 300°C to 450°C showed that the increase of surface temperature is followed by the significant decrease of TBC lifetime. The failure mode was the crack formation and delamination at the bond coat/substrate interface.

Investigation of the resistance of open-column-structured PS-PVD TBCs to erosive and high-temperature corrosive attack

In modern gas turbines, highly loaded components are internally cooled and furthermore covered with thermal barrier coatings (TBCs) to withstand the harsh operating conditions with temperatures exceeding the application limit of such coatings. Under realistic operating conditions, siliceous minerals, of a calcium-magnesium-aluminum-silicate (CMAS) composition, are ingested into the turbine and deposited on the TBCs. Besides erosion, this also leads to degradation by chemical interaction.The plasma spray-physical vapor deposition (PS-PVD) process is an advanced method for manufacturing TBCs, which fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). Due to the unique plasma conditions, coatings with columnar microstructures exhibiting high strain tolerance can be created. However, because of the high amount of open porosity the resistance of such structures to CMAS and erosion attack was expected to be low.In the present work, PS-PVD TBCs were investigated in a burner rig facility under thermal gradient cycling conditions and simultaneous CMAS attack. The interactions of the PS-PVD-deposited YSZ and the CMAS melt were studied by means of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS) and compared to EB-PVD coatings. Additionally, the resistance of PS-PVD TBCs to erosion is compared to APS TBCs by means of room temperature tests according to ASTM G76-13.

Lifetime and failure modes of plasma sprayed thermal barrier coatings in thermal gradient rig tests with simultaneous CMAS injection

Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources such as sand, volcanic ash, fly ash, or variable quality fuels has been a tenacious issue during the recent years. This follows from the fact that engines are required to operate under increasingly harsh conditions in all kind of gas turbine applications following the demands for higher efficiency and operational flexibility. While the understanding of the mechanism of CMAS induced degradation of TBCs as well as approaches for mitigation of CMAS attack by means of advanced TBC compositions have grown remarkably, most of the reported results have been obtained from lab testing at isothermal conditions or from evaluation of ex-service components, either. The isothermal tests are not reproducing important thermomechanical effects from service conditions, and it may be hard to figure out the thermal history of the ex-service examples.In this study a burner rig facility has been used for the evaluation of TBC performance, where a thermal gradient is applied across the TBC at cyclic load and CMAS is continuously injected to the combustion environment to simulate the conditions closer to actual service in an engine while allowing control of important degradation limiting parameters. Tests have been performed on the state of the art material YSZ while loading parameters as high temperature dwell time and CMAS deposition rate have been varied. Tests have been evaluated in terms of chemical degradation, failure mode and TBC spallation lifetime by means of SEM, EDS and XRD. Test conditions and relevance to in-service operating conditions are discussed. Major impacts on thermal gradient cycling lifetime and similarities and discrepancies related to reported failure mechanism are reviewed.

The Investigation of Structure Heterogeneous Joint Welds in Pipelines

Welding joints of dissimilar steels don’t withstand design life. One of the important causes of premature destructions can be the acceleration of steel structural degradation due to cyclic mechanical and thermal gradients. Two zones of tube from steel 12H18N9T, exhibiting the structural instability at early stages of the decomposition of a supersaturated solid austenite solution, were subjected to investigation. Methods of x-ray spectral and structure analysis, micro hardnessmetry were applied for the research. Made the following conclusions, inside and outside tube wall surfaces of hazardous zones in welding joint have different technological and resource characteristics. The microhardness very sensitive to changes of metal structure and can be regarded as integral characteristic of strength and ductility. The welding processes are responsible for the further fibering of tube wall structure, they impact to the characteristics of hot-resistance and long-term strength due to development of ring cracks in the welding joint of pipeline. The monitoring of microhardness and structural phase conversions can be used for control by changes of mechanical properties in result of post welding and reductive heat treatment of welding joints.

On-Line Temperature Measurement Inside a Thermal Barrier Sensor Coating During Engine Operation

Existing thermal barrier coatings (TBCs) can be adapted enhancing their functionalities such that they not only protect critical components from hot gases but also can sense their own material temperature or other physical properties. The self-sensing capability is introduced by embedding optically active rare earth ions into the thermal barrier ceramic. When illuminated by light, the material starts to phosphoresce and the phosphorescence can provide in situ information on temperature, phase changes, corrosion, or erosion of the coating subject to the coating design. The integration of an on-line temperature detection system enables the full potential of TBCs to be realized due to improved accuracy in temperature measurement and early warning of degradation. This in turn will increase fuel efficiency and will reduce CO2 emissions. This paper reviews the previous implementation of such a measurement system into a Rolls-Royce jet engine using dysprosium doped yttrium-stabilized-zirconia (YSZ) as a single layer and a dual layer sensor coating material. The temperature measurements were carried out on cooled and uncooled components on a combustion chamber liner and on nozzle guide vanes (NGVs), respectively. The paper investigates the interpretation of those results looking at coating thickness effects and temperature gradients across the TBC. For the study, a specialized cyclic thermal gradient burner test rig was operated and instrumented using equivalent instrumentation to that used for the engine test. This unique rig enables the controlled heating of the coatings at different temperature regimes. A long-wavelength pyrometer was employed detecting the surface temperature of the coating in combination with the phosphorescence detector. A correction was applied to compensate for changes in emissivity using two methods. A thermocouple was used continuously measuring the substrate temperature of the sample. Typical gradients across the coating are less than 1 K/μm. As the excitation laser penetrates the coating, it generates phosphorescence from several locations throughout the coating and hence provides an integrated signal. The study successfully proved that the temperature indication from the phosphorescence coating remains between the surface and substrate temperature for all operating conditions. This demonstrates the possibility to measure inside the coating closer to the bond coat. The knowledge of the bond coat temperature is relevant to the growth of the thermally grown oxide (TGO) which is linked to the delamination of the coating and hence determines its life. Further, the data are related to a one-dimensional phosphorescence model determining the penetration depth of the laser and the emission.

Thermo-mechanical cyclic loading analysis of pipes with different type of defects: Temperature dependent properties

Shakedown and ratcheting behaviors of pipes under thermo-mechanical loading have an important role in their design. In this study, Plastic, Elastic shakedown and ratcheting limit of the pipelines with different types of defects were analyzed. The pipelines were subjected to the constant internal pressure and the cyclic thermal gradient. Different dimensions of these defects were also studied. Nonlinear kinematic hardening model of Armstrong–Frederick coupled with continuum damage mechanics was used and material properties were considered temperature-dependent. The unified Lemaitre’s model was used in continuum damage mechanics. Interaction diagrams (Bree's diagram) of the defective pipelines were obtained and parametric studies involving different types and dimensions of part-through slot in the defective pipeline were investigated. Continuum Damage Mechanics was used to predict life time of some defected pipelines.

Propagation feature of cracks in plasma-sprayed YSZ coatings under gradient thermal cycling

The cracking behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) during thermal gradient cycling test was investigated. The relationship between the size of the bright spot observed on YSZ coating surface and the size of the crack in thermal barrier coatings (TBCs) was examined by introducing pre-cracks in TBCs assisted with finite element analysis (FEA). The cracking behavior during thermal cycling, and the relationship between the residual lifetime of TBCs and the crack size were examined. Results show that the bright spot observed on YSZ surface corresponds to the crack in the coating and thus the size of bright spot increases with crack size. It was found that the crack size increases linearly with thermal cycle number when the relative thermal cycle number is less than 85% of thermal cyclic lifetime. The crack propagation acceleration at the later stage of lifetime is attributed to coalescence of numerous cracks. The obvious coalescence of cracks in TBCs begins to take place when the residual lifetime becomes approximately 15%, which leads to a rapid decrease of the residual lifetime of YSZ coating with the increase of thermal cycle number.

Erosion Testing of Thermal Barrier Coatings in a High Enthalpy Wind Tunnel

One of the major problems facing the users of aircraft engines and stationary gas turbines in dusty and dirty environments is erosion, causing engine performance deterioration. Thermal barrier coatings (TBCs) are often applied on metal engine components as combustor heat shields or tiles as well as turbine blades allowing enhanced operating temperatures and resulting in increased thermal efficiency of the turbine and also reduced fuel consumption and gaseous emission. Erosive attack by airborne dust or fly ash, coarse particles causes coating degradation resulting in lifting issues of engine components. In the present study, an erosion test facility was used to simulate the mechanisms of coating degradation expected in gas turbines in a more realistic way closer to real engine conditions. A loading situation combining thermal gradient cycling and erosive media was used. The experiments have been performed with an arc heated plasma wind tunnel (PWT total enthalpy up to 20 MJ/kg), which is available at the Institute for Thermodynamics at the University of the Federal Armed Forces in Munich, Germany. The experimental setup and the integration of the air jet erosion test rig into the existing PWT will be elucidated. Different plasma sprayed TBC materials, including the standard TBC material yttria-stabilized zirconia (YSZ), were investigated regarding their erosion resistance. For validation and verification, samples of nickel-based Mar-M 247 and INCO 718 alloys have been used.

Failure mechanisms of magnesia alumina spinel abradable coatings under thermal cyclic loading

Abradable coatings have been used in low- and high-pressure sections of jet engine compressors for more than 40 years. Today, they are also used in the high-pressure turbine of jet engines and are gaining more interest for applications in industrial gas turbines. They minimise the clearance between the rotating blade tips and the stationary liners. Aside from being abradable, the coatings have to be mechanically stable and withstand high thermo-mechanical loadings. A typical material used in engines today is yttria-stabilised zirconia (YSZ). This material advantageously combines a suitable thermal conductivity with a high thermal expansion coefficient, but shows a temperature capability limited to 1200 °C in long-term applications. Typical abradable coating thicknesses are above 1 mm. With increasing coating thickness and limited cooling efficiency leading to high surface temperatures, there is a risk of premature failure. As a result, new ceramic materials have been developed with better high-temperature capability. The present work investigates an atmospheric plasma sprayed ceramic double-layer coating system composed of 7YSZ as an intermediate layer and magnesia alumina spinel as a top layer. This double-layer system was sprayed onto disc-shaped Inconel 738 superalloy substrates, which were coated with a vacuum plasma sprayed MCrAlY bondcoat. The lifetime of the coating system was assessed via thermal gradient cycling testing with surface temperatures above 1400 °C. During cycling, the samples showed a typical failure mechanism with exfoliation of thin coating lamellae starting from the coating surface. This failure mechanism was not observed in thermal barrier or abradable coatings in the past. The failure mechanism was analysed and mismatch stress calculations were carried out.

On Shakedown, Ratchet and Limit Analyses of Defective Pipeline

In this study, the limit load, shakedown, and ratchet limit of a defective pipeline subjected to constant internal pressure and a cyclic thermal gradient are analyzed. Ratchet limit and maximum plastic strain range are solved by employing the new linear matching method (LMM) for the direct evaluation of the ratchet limit. Shakedown and ratchet limit interaction diagrams of the defective pipeline identifying the regions of shakedown, reverse plasticity, ratcheting, and plastic collapse mechanism are presented, and parametric studies involving different types and dimensions of part-through slot in the defective pipeline are investigated. The maximum plastic strain range over the steady cycle with different cyclic loading combinations is evaluated for a low cycle fatigue assessment. The location of the initiation of a fatigue crack for the defective pipeline with different slot type is determined. The proposed linear matching method provides a general-purpose technique for the evaluation of these key design limits and the plastic strain range for the low cycle fatigue assessment. The results for the defective pipeline shown in the paper confirm the applicability of this procedure to complex 3-D structures.