Cyclic Thermo-mechanical Stress Research Articles

Термические напряжения в изоляции тяговых электродвигателей локомотивов

Thermomechanical processes and cyclic temperature changes in the structure of insulation of traction electric motors lead to thermal gradient appearance. Increased temperature widens the elements of winding, damages insulation material (thermal fatigue, extreme deformation, creeping, etc.). Methods for the numerical study of stress-strain state of the material of the insulation of traction electric motors of diesel locomotives on different stages of static and cyclic loading have been developed. It is determined, that the main factor defining the intensiveness of insulation aging is the impact of alternating cyclic thermo-mechanical stresses, but not thermomechanical destruction of impregnating composition. Analytical expressions for the evaluation of stress-strain state of insulation of traction motor diesel locomotives are received and they are distinguished by the fact that they require a relatively small amount of basic data and let conduct accurate calculations taking into account the peculiarities of the electric drive system. An approximate predictive model for the evaluation of interphase thermal stresses in the assembly of the materials of winding slots of the traction electric motor of a diesel locomotive is developed. The insulation material is considered as linear elastic when the deformation level is less than the yield strength. The joining components can be considered from the view point of structural analyses as elongated rectangular plates suffering from linear elastic strains. The analyses results can be used for evaluation of thermomechanical stresses in the impregnating materials of traction electric motors of diesel locomotives and their analogues.

Adjusting Mechanical Properties of Forging Dies Produced by Ausforming

Due to high thermo-mechanical loads, tools used in hot forming operations need a high resistance to different damage phenomena, such as deformation, cracking and abrasion. They are exposed to cyclic thermo-mechanical stress conditions, which leads to tool failure and subsequent tool replacement during cost-intensive production interruptions. To increase wear resistance, forging tools can be produced in the metastable austenite area. Forming of steel below the recrystallisation temperature, also known as “ausforming”, offers the possibility to increase strength without affecting ductile properties. This is due to grain refinement during forming. In this study, the thermo-mechanical treatment ausforming will be used to form the final contour of forging dies. For this purpose, an analogy study was performed where a cup-preform is ausformed, which represents the inner contour of a highly mechanically loaded forging die. It is investigated to what extent a fine-grained microstructure generated in the last forming stage can be achieved and how it influences the tool’s performance. The hot-working tool steel X37CrMoV5-1 (AISI H11) was used as workpiece material. To achieve optimal properties, process routes with tempering temperatures from 300 °C to 500 °C and global true plastic strains of φ = 0.25 and φ = 0.45 were examined. The results were evaluated by pulsation tests, metallographic analysis and hardness measurements of the formed parts. Optimal ausforming parameters were derived to produce a high performance forging die.

Drivers for the cracking of multilayer polyamide‐based backsheets in field photovoltaic modules: In‐depth degradation mapping analysis

AbstractThere is a lack of understanding on the root cause of cracking of photovoltaic (PV) backsheets due to the challenge of multilayer characterization and the complicated failure modes at the submodule level. In this work, the in‐depth degradation mapping of field‐exposed polyamide‐based (PA‐based) PV module backsheets was studied, with the major focus on the identification of underlying drivers for the through cracking (in between the solar cells). PV modules were retrieved from five different locations, comprising a variety of climates, including humid subtropical, hot‐summer Mediterranean, tropical savanna climate, and hot arid. A suite of microscale cross‐sectional characterizations, including chemical changes, fluorescence intensity, and modulus as a function of distance from the air surface of the backsheet, was performed. Results showed more advanced signs of degradation of the inner layer than the outer layer in the cracking region. Increases in the modulus were identified as the major indicator for the cracking. Moreover, a rudimentary test by immersion in acetic acid, which forms during photodegradation of the ethylene‐vinyl acetate copolymer (EVA) encapsulant, showed the first‐time direct evidence that acetic acid can largely accelerate the chemical degradation and facilitate the cracking of PA inner layer. This study suggests that the field cracking of PA‐based backsheet can be attributed to the combined effects of chemical degradation and physical reorganization (chemi‐crystallization) under cyclic thermomechanical stresses. Therefore, it is important to consider effects of local microclimates and interlayer infection to understand the heterogeneous nature of backsheet failures.

Resistance change in on-chip aluminum interconnects under cyclic thermo-mechanical stress

On-chip metallization, especially in modern integrated BCD technologies, is often subject to high current densities and pronounced temperature cycles due to heat dissipation from power switches like LDMOS transistors. This paper continues the work on a sensor concept where small sense lines are embedded in the metallization layers above the active area of a switching LDMOS transistor. The sensors show a significant resistance change that correlates with the number of power cycles. Furthermore, influences of sense line layer, geometry and the dissipated energy are shown. In this paper, the focus lies on a more detailed analysis of the observed change in sense line resistance.

Design of header and coil steam generators for concentrating solar power applications accounting for low-cycle fatigue requirements

Concentrating solar power plants are experiencing an increasing share in the renewable energy generation market. Among them, parabolic trough plants are the most commercially mature technology. These plants still experience many challenges, one of which is the cyclic daily start-up and shut-down procedures. These pose new challenges to industrially mature components like the steam generator system, as frequent load changes might decrease their lifetime considerably due to cyclic thermo-mechanical stress loads. In this context, the header and coil design is a promising configuration to minimize the stresses.This paper presents a method to design the header and coil heat exchangers of the steam generator, taking into account low-cycle fatigue requirements, by defining minimum allowable heating rates for the evaporator and superheater. Optimal designs were obtained by minimizing the total water pressure drops and purchase equipment costs. A comparison with a sizing routine without accounting for low-cycle fatigue constraints was also conducted.The model was validated against the component data of a 55 MWe power plant, with a maximum deviation on the total area estimation of +2.5%. The results suggest that including the heating rate constraints in the design routine substantially affects the optimal design configuration, with a 41% cost increase for a 1 bar pressure drop. The optimal design for maximizing the lifetime of the components uses tube outer diameters of 38 mm and 50 mm and a low number of tubes per layer (4–10) for the superheater.

Aging sensors for on-chip metallization of integrated LDMOS transistors under cyclic thermo-mechanical stress

LDMOS transistors in integrated power technologies are often subject to thermo-mechanical stress, which degrades the on-chip metallization and eventually leads to a short. This paper investigates small sense lines embedded in the LDMOS metallization. It will be shown that their resistance depends strongly on the stress cycle number. Thus, they can be used as aging sensors and predict impending failures. Different test structures have been investigated to identify promising layout configurations. Such sensors are key components for resilient systems that adaptively reduce stress to allow aggressive LDMOS scaling without increasing the risk of failure.

Cyclic thermo-mechanical stress, strain and continuum damage behaviors in light alloys during fatigue lifetime considering heat treatment effect

In this article, thermo-mechanical fatigue behaviors in light alloys have been investigated to find the effect of heat treatments. For this objective, thermo-mechanical fatigue tests were performed on the A356.0 aluminum alloy and the AZE911 magnesium alloy, with and without typical T6 heat treatments. Obtained results demonstrated no significant difference in thermo-mechanical fatigue lifetime of the A356.0 alloy between non-heat-treated and heat-treated test specimens at 250°C of the maximum temperature, which was attributed to the over-ageing phenomenon. As a consequence, this low effect showed that the heat treatment could be eliminated for cylinder heads. However, the thermo-mechanical fatigue lifetime of the AZE911 alloy was significantly affected by the heat treatment. The explanation for the mentioned behavior could be found in the material micro-structure, which was affected by dissolving the brittle intermetallic phase in the matrix of the AZE911 alloy. However, the magnesium alloy still requires more improvements in the fatigue lifetime for a possible substitution in cylinder heads. Continuum damage behaviors showed that higher damage values occurred in the aluminum alloy, in comparison to the magnesium alloy, both for heat-treated and non-heat-treated specimens.

Improving the Efficiency of Carbide End Mills by Deposition of Nano-Scale Multi-Layered Composition Coatings

The study has considered the challenge of improving the efficiency of carbide end mills through modification of the physical and mechanical properties of tool by forming on its working surfaces nanostructured multi-layered composite coatings with use of filtered cathodic vacuum arc deposition (FCVAD). The technique of deposition of three-component multi-layered composite coatings with nanosized grain structure and thickness of sublayers on working surfaces of end mills has been developed. The developed coatings contain three key components, each of which has a strictly functional purpose specifying enhanced wear resistance, strong adhesive bonding with tool material and barrier properties with regard to heat flows and diffusion, and that allows improving the efficiency of carbide end mills. The study has found out that when carbide end mills with developed coatings are used at cutting, the cyclic thermomechanical stresses acting on the cutting part of the tool during the cutting stroke and idle run of cutting edge of end mills are smoothed, and that leads to the decrease in the rate of tool wear and improves the efficiency of carbide end mills. The study was supported by grant No. 9.251.2014/K, project code 251.

AC Loss Before and After Cyclic Mechanical Loading in the ITER RF CICCs

AC loss of Nb3Sn cable-in-conduit conductors (CICC) decreases after cyclic mechanical loading, but remains almost the same in case of Nb-Ti ones. It was observed during previous ITER CS, TF, and PF inserts tests in JAEA. The same phenomena have been detected on numerous ITER CS, TF, and PF short samples tested in the SULTAN facility. Experimental methods of ac loss investigation in JAEA-Naka and SULTAN were different, but the final results showed a good agreement. Analysis and numerical investigation of the present work were based on RF PF&TF CICCs SULTAN test results, theory of superconductivity, and our own physical and engineering understanding of evidence. Cyclic mechanical loading in both JAEA and SULTAN was initiated by electromagnetic force. In this paper, we attempt to explain why hysteresis, coupling, and mechanical loss decrease dramatically after cyclic mechanical loading and thermo-mechanical stress caused by WUCD (warm-up and cool-down) in Nb3Sn CICC.

Prognostication of Residual Life and Latent Damage Assessment in Lead-Free Electronics Under Thermomechanical Loads

Requirements for system availability for ultrahigh reliability electronic systems such as airborne and space electronic systems are driving the need for advanced health monitoring techniques for the early detection of the onset of damage. Aerospace electronic systems usually face a very harsh environment, requiring them to survive the high strain rates, e.g., during launch and reentry, and thermal environments, including extremely low and high temperatures. Traditional health monitoring methodologies have relied on reactive methods of failure detection often providing little or no insight into the remaining useful life of the system. In this paper, a mathematical approach for the interrogation of the system state under cyclic thermomechanical stresses has been developed for six different lead-free solder alloy systems. Data have been collected for leading indicators of failure for alloy systems, including Sn3Ag0.5Cu, Sn0.3Ag0.7Cu, Sn1Ag0.5Cu, Sn0.3Ag0.5Cu0.1Bi, Sn0.2Ag0.5Cu0.1Bi0.1Ni, and 96.5 Sn3.5Ag second-level interconnects under the application of cyclic thermomechanical loads. The methodology presented resides in the prefailure space of the system in which no macroindicators such as cracks or delamination exist. Systems subjected to thermomechanical damage have been interrogated for the system state and the computed damage state correlated with the known imposed damage. The approach involves the use of condition monitoring devices which can be interrogated for damage proxies at finite time intervals. The interrogation techniques are based on the derivation of damage proxies and system prior-damage-based nonlinear least square methods, including the Levenberg-Marquardt algorithm. The system's residual life is computed based on residual-life computation algorithms.

Prognostication of system-state in lead-free electronics equipment under cyclic and steady-state thermo-mechanical loads

In this paper, a mathematical approach for interrogation of system-state under cyclic thermo-mechanical stresses has been developed for three-different lead-free solder alloy systems. Data have been collected for leading indicators-of-failure for alloy systems including, Sn1Ag0.5Cu, Sn3Ag0.5Cu, Sn4Ag0.5Cu second-level interconnects under the application of cyclic thermo-mechanical loads. Methodology presented resides in the pre-failure space of the system in which no macro-indicators such as cracks or delamination exist. Systems subjected to thermo-mechanical damage have been interrogated for system state and the computed damage state correlated with known imposed damage. The approach involves the use of condition monitoring devices which can be interrogated for damage proxies at finite time-intervals. Interrogation techniques are based on non-linear least-squares methods. Various techniques including the Levenberg–Marquardt Algorithm have been investigated. The system’s residual life is computed based on residual-life computation algorithms. Detection of system-state significantly prior to catastrophic failure can significantly impact the reliability and availability of electronic systems. Requirements for system availability for ultra-high reliability electronic systems are driving the need for advanced heath monitoring techniques for early detection of onset of damage. Traditional health monitoring methodologies have relied on reactive methods of failure detection often providing little on no insight into the remaining useful life of the system.

Deformation and Damage Analysis of a Hybrid‐Forming Tool under Thermal Shock Conditions

AbstractIn the hybrid–forming process for gradient structures [1] inhomogeneous cyclic thermo–mechanical stresses and strains lead to higher risks of failure of the forming tool. The main topic of this paper is the validation of finite element calculations for a tool–like specimen under complex thermo–mechanical loadings in order to predict the material behaviour [3]. To this end thermal shock experiments of tool–like specimens are performed. Optical measuring systems are used for three–dimensional digitalisation of the specimens to get a sufficient amount of data. Results of experimental optical measurings and results of finite element calculations are compared. Additionally, damage analysis using the eddy current method is performed to characterize the surface state of the cyclically thermal shocked specimens. This damage analysis provides data for lifetime prediction models under thermal shock conditions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The deformation behavior of Sn62Pb36Ag/sub 2/ and its implications on the design of thermal cycling tests for electronic assemblies

The introduction of new packages as well as the ongoing miniaturization in SMT make the evaluation of the reliability of solder joints a permanent task. Since solder joints fail due to low cycle fatigue caused by cyclic thermomechanical stress passive thermal cycling is an important test to evaluate the lifetime of solder joints. Since a reliability prediction with the thermal cycle encountered in reality would take years to complete the tests, methods to accelerate the test cycle are to be used. However, due to the viscoplastical deformation behavior of tin-lead solder it is mandatory to take the metallurgical behavior of the solder into account when designing accelerated tests. Two different deformation mechanisms occur, depending on the temperatures of the test as well as the temperature gradient: grain boundary sliding (GBS) and dislocation climb (DC) each one having its own influence on the damage occurring in the solder. Therefore, one is not free in choosing the parameters of a test cycle. In this paper the deformation behavior of tin lead solder is explained, A constitutive equation extracted from experiments is presented, describing the deformation behavior of Sn62Pb36Ag2. Results of simulations of the deformation behavior of Sn62Pb36Ag2 on the base of the constitutive equation are shown. Suggestions for the design of accelerated lifetime tests for solder joints are given.