Papers by Keyword: Thermal Fatigue

Abstract: Hot-working tools are frequently exposed to high cyclic temperature and mechanical loads, usually resulting in thermal fatigue cracking, which is observed as a network of fine cracks on the surface. The crack network degrades the performance of the tool and even invalidates it. Deep rolling is a surface strengthening treatment generally applied to improve the fatigue performance of metal components. But few attentions are paid on its influence on thermal fatigue cracking of the components, considering the relaxation of residual stresses and work hardening, which are deemed to mainly account for the improvement of fatigue performance. In this paper experiments were carried out to investigate the influence of deep rolling on thermal fatigue behavior of hot-work tool steel AISI H13. The experiment results show that deep rolling can improve the thermal fatigue behavior of AISI H13. To explore the mechanism of the effect, the changes of the residual stresses, the microhardness of the samples are also presented.

Abstract: Thermal fatigue occurred in many engineering constructions made of polymeric composites subjected to the intensive loading and vibrations. During this process the mechanical energy is dissipated in the form of heat due to the hysteretic behaviour of the material, which introduces the self-heating effect. In some cases the self-heating effect dominates the fatigue process and intensifies much structural degradation of composite elements. The paper presented a survey of engineering applications in which the self-heating effect and thermal fatigue occurred and recent advances in theoretical and experimental research in this area. Selected results of experimental studies were presented and discussed.

Abstract: Microstructural evolution of a nickel-base directionally solidified superalloy during thermal fatigue was investigated. Three kinds of phase transformation have been observed: μ phase precipitated due to the relatively increasing of the concentration of forming μ phase elements as a result of the depletion of Al and Cr; Script-like carbides and γ phase dissolved into the matrix. Also, thermal fatigue was strongly influenced by oxidation.

Abstract: The microstructures and properties of 2024 aeronautical aluminum alloy subjected to thermal cycling were investigated in simulated Low Earth Orbit space environment. The experimental results demonstrate that the microhardness of aluminum alloys changed with the increasing of thermal cycles. The results of X-ray diffraction (XRD) and transmission electron microscopy (TEM) show that the property changes of the 2024 aluminum alloys during the thermal cycling period are closely related to the microstructural evolution. The relationship between the microstructural evolution and the thermal fatigue was also discussed.

Abstract: ncreasing functionality accompanied with device miniaturization in microelectronics has led to increased market demand for packages with small form factor. Over the years, embedded wafer level packaging (EWLP) has become an attractive option since it allows a reduction in package size and height. In the EWLP approach, the singulated dies are embedded within the molding compound through the wafer level compression molding process. For this study, critical mechanical challenges such as die shift and thermal cycling performance of a multi-chip embedded wafer level package (MCEWLP) are addressed through numerical modeling. For improved accuracy in die shift predictions, both mechanical effects and fluidic effects need to be taken into account. Mechanical effects account for around 75% of the die shift while fluidic effect contributes to the remaining 25%. It is shown that reducing the die size and the inclusion of UBM as a buffer layer can effectively increase the fatigue life of the packages.

Abstract: Thermal fatigue behavior of a nickel-base directionally solidified superalloy was investigated using the notched flaky specimen. The da/dN K curve and da/dN-a relationship has been applied to analyze the thermal stress.

Abstract: With macrographic examination, SEM observation and fracture quantitative analysis technology, the fracture fractography of a spray tube with both the mechanical stress and the thermal stress in a boiler desuperheater of a power plant was interpreted, and the failure mode and the cause were researched. Results showed that rupture of the spray tube was due to the fatigue, and further, the thermal mechanical fatigue. The alternating bending stress and thermal stress were main factors of the fracture failure. The shallow surface thermal fatigue cracks in tube inner wall were induced by thermal stress. When the thermal mechanical fatigue crack grew up steadily to the shallow surface cracks zone, fast growth occurred and the local fast fracture zone generated which had different fractographies with final fast fracture zone of the normal fatigue fracture.

Abstract: A temperature sensor with a thermocouple placed at ~0.5 mm from roll surface is used in hot rolling conditions to evaluate by inverse calculation heat transfers in the roll bite. Simulation analysis in industrial hot rolling conditions with short contact lengths (e.g. short contact times) and high rolling speeds (7 m./sec.) show that the temperature sensor + inverse analysis with a high acquisition frequency (> 1000 Hz) is capable to predict with a good accuracy (5 to 10% error) the roll bite peak of temperature as well as the roll surface temperature evolution all around the roll rotation. However as heat flux is more sensitive to noise measurement, the peak of heat flux in the bite is strongly under-estimated (40% error) by the inverse calculation and thus only an average roll bite heat flux could be expected from the sensor (these simulation results will be verified with an industrial trial that is being prepared). Rolling tests on a pilot mill with low rolling speeds (from 0.3 to 1.5 m./sec.) and strip reductions varying from 10 to 40% have been performed with the temperature sensor. Analysis of the tests by inverse calculation show that at low speed (<0.5 m="" sec="" and="" large="" contact="" lengths="" reduction:="" 30="" to="" 40="" the="" roll="" bite="" peak="" of="" heat="" flux="" reconstructed="" by="" inverse="" calculation="" is="" correct="" at="" higher="" speeds="" 1="" 5="" smaller="" reduction="" :="" 10-20="" reconstruction="" incorrect:="" in="" under-estimated="" though="" its="" average="" value="" analysis="" reveals="" also="" that="" transfer="" coefficient="" htc="" sub="">roll-bite (characterizing heat transfers between roll and strip in the bite) is not uniform along the roll bite but is proportional to the local rolling pressure. Finally, based on the above results, simulations with a roll thermal fatigue degradation model in industrial hot rolling conditions show that the non uniform roll bite Heat Transfer Coefficient HTC_roll-bite may have in certain rolling conditions a stronger influence on roll thermal fatigue degradation than the equivalent (e.g. same average) HTC_roll-bite taken uniform along the bite. Consequently, to be realistic the roll thermal fatigue degradation model has to incorporate this non uniform HTC_roll-bite.

Abstract: Cylinder cover is an important component of the engine. It is damaged by low cycle thermal fatigue, high cycle thermal fatigue and creep in operation. Thus, the life and reliability of the cylinder cover are important criteria for engines. The temperature field and stress field of the cylinder cover are computed by the sequentially coupled analysis method: the elastic strain, plastic strain and creep strain of the dangerous point on the cylinder cover are analyzed in ten startup-working-shut down cycles; and it is proved in theory that low cycle thermal fatigue is the primary factor affecting the service life of the cylinder cover and the startup time of the engine is an appropriate parameter to estimate the life of thecylinder cover. The creep has little influence on the stress amplitude, and makes the mean stress increased. The relaxation is coexisting with creep in high temperature; it reduces the increase speed of mean stress. After finite cycles, the mean stress tends to constant gradually. Therefore, the creep-fatigue interaction can equal to thermo-mechanical fatigue whose stress amplitude and mean stress are constant. At last, the mean service life of cylinder is predicted.

Abstract: Thermal fatigue is a common problem when ceramics are used at high temperature. Typically, the mechanic properties of ceramics decrease after either long service times at high temperatures or cycles of temperature changes. The thermal fatigue process, the factors influencing the thermal fatigue and the prediction of the thermal fatigue life of ceramics are concerned topics. The thermal fatigue of ceramics is mainly explained by the critical stress fracture and thermal shock damage theories. The thermal fatigue tests include the traditional strength method, the quench-indentation method and the non-destructive detection such as the acoustic emission technique. Based on the thermal fatigue theories, the thermal fatigue life can be predicted using built models. The establishment of the standards for the testing of ceramic thermal fatigue will enhance the comparability of experimental data and further promote the development of analysis theory of thermal fatigue, benefiting the design of engineering ceramics.