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Residual stress less than tensile yield stress of the material is obtained for usual groove welds of high-strength steels (See Fig. 1). In the present research attention was focused on the difference in thermal stress cycle between mild steel and high-strength steel welds. Experiments were made in such manner that a round bar specimen (Fig. 3) set in a rigid frame (Fig.2) was subjected to a thermal cycle simulating weld heat-affected zone. The thermal cycle was given by high frequency induction heating and air-cooling or controled-cooling. Thermal stress developed in the specimen was measured by a load-cell (capacity 2 tons) connected to the specimen. A mild steel and HY-80 steel were used for the experiments (See Table 1.)Thermal cycles measured at several points on the specimen are shown in Figs. 4 and 5. Free expan-sions as shown in Figs. 6 and 7 were obtained between end chucks of the specimen during thermal cycles. Uniform temperature distribution along parallel part of the specimen was not expected because of high frequency induction heating. Peculiar shape of the free expansion curves is obtained as the result of nonuniform temperature distribution (See Fig. 8.).Figs. 9 thru 11 show thermal stress cycles plotted for surface temperature at the center of the specimen. It was found that factors by which residual stress is lowered are the decrease in Ar transformation temperature range over which yield stress of the material is rather small and the increase in expansion during the transformation. Therefore, rapid cooling from higher temperature than Ac1 point lowers residual stress level in high yield-strength steel welds (See Fig. 12).Degree of restraint for the rigid frame used was investigated by an analogical system shown in Fig. 13. The restraint was about one-third as large as a bar with uniform temperature fixed at the ends.

In this work, a protocol for evaluating the long-term thermal and chemical stability of capric acid is presented. Capric acid is a phase change material (PCM) employed as a latent heat thermal storage material to store/retrieve heat during the phase transition from solid to liquid or vice versa. The nature of the materials utilized for latent heat thermal batteries plays a vital role in their long-term performance for the different applications. Therefore, precisely investigating these material thermal and chemical stability is significant. The protocol consists of two tests (accelerated thermal cycling and thermal stress) in which the performance of the PCM will be investigated. A benchmark experiment for accelerated thermal cycling was performed where the impact of different parameters (heating/cooling rate and sample volume) was investigated for the initial 50 thermal cycles. This was followed by long-term accelerated thermal cycling for a total of 1500 thermal cycles (melting followed by crystallization). A thermal stress where the PCM stays in a liquid state was performed for the capric acid to investigate the impact of the higher temperature on the stability of the PCM. The thermal properties reliability over time was measured with differential scanning calorimetry (DSC), which indicated a variation of 3.2 % in onset phase change equilibrium temperature and 10.45 % in phase change enthalpy after 1500 thermal cycles. Gas chromatography (GC) and nuclear magnetic resonance spectroscopy (NMR) were performed to reveal that by-products are formed during capric acid thermal treatment. These results provide valuable guidance for the research community to design adapted thermal cycling protocols and select appropriate characterization methods, thereby enabling more accurate and reliable investigations of the long-term stability of organic PCMs. • Accelerated thermal cycling and Thermal stress test for capric acid • Investigated the impact of heating/cooling rate and volume of sample • After 1500 thermal cycles the variation in T m(onset) and ∆ fus H was 3.2 % and 10.45 % respectively • By-products formed during the thermal cycling were detected with NMR and GC. • The by-products were identified as Octanoic acid, heptly ester, and Heptadecane.

In order to improve the work efficiency and save resources in the process of laser cladding on the H13 steel surface, based on COMSOL, by combining computer simulation and experiment, a plane continuous heat source model was used to simulate and analyze the temperature and stress field. The optimal power and scanning speed were obtained. It is found in the simulation process that the thermal sampling points stress increases with the increase of laser power and scanning speed. Because of the existence of solid–liquid phase variation in the laser cladding process, there are two peaks in the maximum thermal stress cycle curve of the sample points located in the molten pool, and the starting and ending time of each sample point’s peak value is basically the same. When the sample point is outside the molten pool, because the metal at the corresponding location is not melted, so there is no obvious peak value in the thermal stress cycle curve. With the increase of cladding layer depth corresponding to each sample point, the variation range of the two alternating thermal stress peaks increases first and then decreases, while the duration increases. According to the peak value of alternating thermal stress at the sampling point, the molten pool depth can be predicted. The residual stress analysis of the cladding layer is carried out according to the analysis results of temperature field and stress field. Through the actual cladding experiment, it is found that the depth of molten pool in the simulation results is basically consistent with the experimental results. All simulation results are verified through actual cladding experiments.

Cryogenic operation of CMOS circuits is essential for scalable quantum computing and other high-performance applications. Although much research has focused on the behavior of devices at low temperatures, the long-term effects of repeated thermal stress cycles remain largely unexplored. This work investigates the impact of consecutive cooling-heating cycles between 4K and 280K on the electrical characteristics of 65nm CMOS polysilicon resistors with varying sizes and doping types. Using a fully automated cryogenic measurement setup, eight full thermal cycles were applied to three different resistor structures. Resistance vs. temperature (R–T) characteristics were measured in the full temperature range for each cycle. The results reveal the appearance of hysteresis in the curves R–T after thermal cycling, with the most pronounced effects observed in larger devices. In particular, the maximum resistance variation remained below 2% of the nominal value, a trend that is maintained after repeating the thermal cycle 8 times. These findings suggest that while 65nm CMOS resistors are suitable for cryogenic electronic systems, care has to be taken when designing high-precision systems depending on the value of resistors such as comparators or data converters.

Failure location and mechanism analysis about a microsystem fault is presented in this work. The fault phenomenon of abnormal low output power amplitude appears only at low temperature during thermal cycle test. A series of analysis including internal fault location and replacing parts for validation help to find the fault origin: a line winding inductor. The failure mechanism of the failed inductor is analyzed by optical microscope, X-ray inspection, scanning electron microscope, energy spectrum analysis and so on. It is found that the enameled wire of the abnormal inductor is damaged during the installation process and the crack develops under thermal cyclic stress. By summarizing the analysis process and the test result, the investigation can provide reference for the failure analysis of similar microsystems.

Research findings in the past have demonstrated that the heat of hydration can induce thermal cracking in early age concrete. The disproportionate thermal expansion can cause considerable tensile stresses at concrete surfaces. The minimum size of mass concrete has remained unsolved due to the complexity of the problem. In this study, a way to predict early age concrete temperature and associated thermal stresses is described, including the needed experimental testing and finite element modeling technique.;In thermal analysis, an adiabatic calorimeter was designed and built to measure the adiabatic temperature rise of the concrete. The measured adiabatic temperature rise was converted into a mathematical formulation for finite element modeling. Thermal properties including specific heat and thermal conductivity were considered to be degree of hydration dependent and determined individually for each element in the model. The interface conductance between concrete and steel formwork was determined experimentally and put in place to account for the thermal resistance from the concrete-formwork interface. External surface convection for formwork-air interface was used to calculate surface heat loss.;In order to calculate the thermal stresses, the degree of hydration was utilized to estimate the elastic modulus development. Concrete tensile creep and compressive creep behaviors were included using a step-by-step incremental calculation algorithm. Influences from loading age and temperature effect were considered in each time increment of the creep models. Based on test results from concrete cubes, the modeling method and experimental procedures used in this study were validated to exhibit reasonable accuracy in terms of temperature and strain predictions. The thermal and stress analysis models can be used to calculate the temperature profile and thermal stress development histories for early age concrete.

Results of the conducted research aiming to demonstrate the methodology of optimization of dam monolith length (distance between contraction joints), through monitoring the thermal tensile stresses during construction and service life of a concrete gravity dam that is built using the block method, are presented in this paper. A 3D space–time numerical model for phased thermal stress analysis is employed in a large concrete gravity dam case study. For the adopted block dimensions, schedule, and dynamics of construction and material parameters, the thermal stress analysis is conducted, taking into account the following: thermal physical properties of the material, the cement hydration process, heat exchange with the external environment and the reservoir, and self-weight of the structure. The main advantage of the proposed methodology is the possibility of controlling the cracks resulting from thermal tensile stresses in the monolith of a concrete gravity dam, by optimizing the monolith’s length to minimize the zones in which the tensile capacity of concrete is exceeded. The results obtained from the temperature field analysis show that the maximum temperature increase in the dam’s body results from the cement hydration process in combination with summer air temperatures in the construction phase. The aforementioned factors account for the increase in temperature of up to 45.0 °C, while during winter cooling of the structure occurs due to lower temperatures, especially in the surface zones. The results of the stress field analysis show that the extreme values of thermal tensile stresses are present in the process of a sudden or gradual cooling of the concrete when shrinkage occurs. Finally, it is shown that the reduction of the monolith length by 5.0 m (from 20.0 m to 15.0 m) results in a decrease in the extreme thermal tensile stress values by an average of 0.70 MPa (up to 12.0%) in winter and an average of 1.10 MPa (up to 20.0%) in summer; while for the entirety of the analyzed time period, results in a decrease in the extreme thermal tensile stress values by an average of 16.0% (0.93 MPa).

Thermal cycling stress is one of the most potential factors challenging the reliability/lifetime of semiconductor devices and cause of their failures. This paper analyzes and investigates the impacts of various control strategies on thermal cycling stress of the semiconductor devices of the Modular Multilevel Converter (MMC) for drive applications. Various control strategies based on injecting a common-mode voltage and a circulating current are investigated and their impacts on power loss distribution and thermal cycling of the semiconductor devices of the MMC-based drive systems are evaluated. Simulation results in the MATLAB/SIMULINK software environment are presented to evaluate the accuracy of the analysis and impacts of three control strategies on power losses and thermal distribution of semiconductor devices of the MMC-based drive system.

Thermal fatigue and cracking behaviors of asphalt mixtures under different temperature variations

Effect of weld thermal cycle, stress and helium content on helium bubble formation in stainless steels

The tensile thermal stress generated by laser irradiation with forced cooling is critical in the cleavage processing of thin plate glass. In this study, we predicted the conditions for generating tensile thermal stress in laser-induced cleavage of thin plate glass using numerical models from the viewpoint of the cooling and heating areas. An unsteady two-dimensional model was used to predict the temperature distribution and an unsteady plane stress model was used to predict the thermal stress. To generate tensile thermal stress, a cooling area is required behind the heating area. A specific scanning speed is required to yield the maximum tensile stress between the heating and cooling areas. A weak heat transfer coefficient in the cooling area generates tensile thermal stress only in the direction perpendicular to (y direction) the scanning direction of the heat source (x direction). A strong heat transfer coefficient generates tensile thermal stress in both the x and y directions. These tensile thermal stresses are surrounded by horseshoe-shaped compressive thermal stress. The tensile thermal stress can be controlled by selecting an appropriate cooling method for the cooling area.

The objective of this study was to investigate the effect of thermal cycles, encountered by an Super Sonic Transport (SST) in service, on the cumulative frequency of microcracks and the degradation of compressive strength in carbon fiber/polyimide resin composite materials.One thermal cycle was designated as a sequence from room temperature (RT) to -54°C, and then to +177°C, finally back to RT. Transverse microcracks initiated on the sectional free edge surface of the laminates were observed and counted by using an optical microscope. Approximate 10000 thermal cycles were given for five kinds of carbon fiber/polyimide composite material: IM7/PIRA, IM600/PIXA-M, IM600/PIXA-MT3, IM7/K3B, IM7/R1-16, as well as a Ti/Gr (polyimide CFRP) fiber metal laminate. For IM7/R1-16 thermal cycling tests were conducted up to 40000 cycles.Transverse microcracks were observed not only on the sectional free edge surface but also on the inside cross section of specimens cut after thermal cycling tests for IM7/R1-16. Static open-hole-compressive (OHC) strength was measured at RT before and after thermal cycling tests on the five kinds of material except for IM7/PETI-5. Static nonhole-compressive (NHC) strength was measured at RT on IM600/PIXA-M and IM600/PIXA-MT3. In addition, in order to know the estimates of thermal stress generated by the thermal cycles, the thermal stresses were calculated by the classical lamination theory (CLT) using basic lamina data given in a reference.

We investigated the thermal stress in silicon carbide metal-oxide-semiconductor field effect transistors (SiC MOSFETs) by Raman spectroscopy. We performed the thermal cycle tests of the SiC MOSFET. The horizontal compressive stress increased after the thermal cycle tests. The increase of compressive stress largely depended on the temperature conditions, and the compressive stress became large as the maximum temperature of the cycle test increased. The stress increasing didn't observe from the sample which was removed the solder and Cu plate. This result suggest that the improvement of die-attach materials is important to reduce the thermal stress in SiC MOSFETs. The quantitative stress evaluation by Raman spectroscopy can be used as an indicator of the deterioration process of the devices.

Lead-free solders were never an industry choice until government legislation, their wide spread use is still in its infancy due to long term reliability issues. A specific SAC (Tin-Silver-Copper) family of solder alloys has emerged as the favourite to offer technical advantages as well as meeting those legislative requirements. This paper investigates accelerated life behaviour of lead-free solder joints and printed circuit boards using thermal and electrical stress cycling. The aim is to understand the degradation of these materials in a practical operating environment. Whilst corrosion and debris deposits have been found, no significant evidence has been obtained for tin whiskering. EDX analysis has shown the presence of high concentrations of elements considered to arise from the packaging material. Thermal cycling tests have presented an aggressive environment to the samples and the effect on them has been supported by microscopic and macroscopic observations of debris and corrosion. The electrical behaviour, i.e., the joint resistance, has not however, significantly degraded.

With the proliferation of area array packages in consumer, enterprise, automotive, medical, and military electronics, a wide range of board level underfills and edgebond adhesives, reworkable and non-reworkable, have been developed to ruggedize these components. While these materials are used to enhance drop test, bend test, vibration test and Tumbling test performance, their use can affect thermal cycle performance, negatively or positively. The trade-offs between process performance, rework, and reliability, along with advances to overcome the trade-offs are discussed. In this study, we have experiments to compare the drop results between the PCBA with underfill and without underfill. For thermal cycle experiments, one underfill shows positive while another one exhibit negative. There are also the experiment to show the tradeoff between reliability and reworkability. Both thermal cycle experiment and stress calculation illustrated the higher the Tg and the lower the CTE, the longer the thermal cycle life it will be. But too high of Tg, it will be non-reworkable. We also shows a low-CTE, highly crack resistant reworkable underfill encapsulant designed for flexible and bendable application. Finally, the alternative edge bond material is good for the choice of easier rework or reducing the consumption of undefill. The experiment shows that the reworkable edge bond adhesive enhances thermal cycle performance very much.