Thermal Fatigue Process Research Articles

Thermal Fatigue of Ceramics: Theory, Life Predication and Characterization Methods

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.

Discussion on modeling of thermal fatigue cracks in numerical simulation based on eddy current signals

This study evaluates modeling of thermal fatigue cracks by the finite element method from the view point of eddy current testing. Five artificial thermal fatigue cracks introduced into type 304 stainless steel plates were prepared for the research. Eddy current signals were gathered by a differential type plus point probe and subsequent destructive tests were performed to confirm the true profiles of the cracks. Numerical simulation based on the results of eddy current testing and destructive tests were carried out to show how the thermal fatigue cracks should be modeled in numerical simulations. The results of the numerical simulations revealed that thermal fatigue cracks tend to be much less conductive than stress corrosion cracks if they are assumed to have uniform conductivity inside. The results also imply that taking consideration of magnetization induced by the thermal fatigue process enables eddy current signals to be analyzed more quantitatively.

Methods of detailed thermal fatigue evaluation of nuclear power plant components

AbstractThe fatigue check of components has to be considered as an essential module of the safety concept as well as the ageing and lifetime management of nuclear power plants. It is based on special safety requirements in the design phase as well as in the framework of the ageing and lifetime management. Considerable efforts of load identification (fatigue monitoring) as well as the strict code conformity are characteristic features. The predominance of thermal cyclic loadings as well as relevant stress/strain amplitudes in the low cycle fatigue regime (LCF) are peculiarities compared to other technical domains. The code based fatigue concept is explained in the first part of the contribution. Furthermore, potentials for the quantification of existing margins are shown. On this, a new approach of mechanistic simulation of the thermal cyclic fatigue process based on short crack fracture mechanics is described in detail in the second part.

Formation and Thermal Fatigue Properties of Fine-grained Heat Affected Zone on Cast-hot-working-die Steel after Electropulsing Stimulation with High Current Density

The fatigue crack propagation behaviors of cast-hot-working die (CHWD) steel untreated and treated by electropulsing stimulation with different current densities were investigated in the present study. The elliptical heat affected zone (HAZ) was formed ahead of specimen notch tip due to the concentrating heat release induced by current bypassing and concentrating effect. The acreages of specimen HAZs were gradually enlarged because of the more heat energy input by elevating current density of electropulsing stimulation. The grain refinement and dislocation density increase in the specimen HAZs were enhanced by elevating the current density of electropulsing stimulation, which inevitably resulted in the improvement on the HAZ mechanical properties and the enhance of fatigue resistance. In addition, the induced compressive stress by electropulsing stimulation could decrease the peak value of tensile stress forming in the thermal fatigue process, which also enhanced the thermal fatigue resistance. Consequently, the resistance to thermal fatigue of CHWD steel was enhanced by grain refinement, dislocation density increase and induced compressive stress after electropulsing stimulation with high current density.

Magnetic testing methods of high-temperature steels

In this work the deterioration of the base materials of boiler pressure parts, steam pipes of power generators were investigated by magnetic testing methods. The coercive force and the magnetic Barkhausen noise were measured during the thermal shock fatigue process. Strong correlation was found between the above mentioned parameters and the remnant life of power plant materials.

The Unlubricated Sliding Wear Behavior of a Wrought Cobalt-Chromium Alloy Against Monolithic Ceramic Counterfaces

The unlubricated sliding wear and friction behavior of a wrought cobalt-chromium alloy against sintered Al2O3, ZrO2-toughened Al2O3, MgO-partially-stabilized ZrO2, and hot-pressed Si3N4 ceramic counterfaces has been investigated. Both untreated (baseline) and surface-borided conditions of alloy 6B disks were investigated. The dominant wear and/or material degradation mechanism for surface-borided alloy 6B was a combination of abrasion and plastic deformation; in addition, adhesive transfer of the cobalt alloy to monolithic Al2O3 counterfaces was a significant wear mechanism for the as-solution-treated alloy 6B disk. For the partially stabilized ZrO2 counterface, the degradation mechanism involved thermal and/or mechanical fatigue processes, resulting in the formation of microcracks in the 6B disks transverse to the sliding direction. This microcracking has been ascribed to the low thermal conductivity of the partially stabilized ZrO2 and the activation of a martensitic, fcc-to-hcp transition with each passing of the ZrO2 ball. For borided alloy 6B against hot-pressed Si3N4, a transition from negligible to severe wear was noted with increasing sliding speeds.

熱衝撃疲労試験による岩石の物性変化の評価に関する研究

We assembled a thermal shock fatigue testing machine that repeats the cycles of rapid heating by furnace and rapid cooling by water bath. Using this test machine, we investigated the damages of thermal shock fatigue process, in four kinds of rocks, observing the extension of rocks by thermal shock fatigue and the change of strength and physical properties. From the experimental result, we examined the relationship between the strength and other physical properties of the rocks.The results are;1. The process of crack extension by thermal shock fatigue could be divided into three stages, that is crack initiation, crack coaleScence and fracturing of the rocks. Let T denote the testing temperature and N the number of repetition cycles of heating and cooling, and we could approximately express the relation as follows;T=AiN-κwhere κ is the gradient and Ai is the stage of the crack initiation (A1), the crack coaleacence (A2) and the f raturing of the specimen (A3).By examining the constants in the above expression, we can deScribe the properties of the thermal shock fatigue of the rock.2. As the repetition cycles of heating and cooling increase, the early part of the stress-strAin curve began to show the non-linearity. This is caused by the initiation and the coaleScence of cracks.3. Since the non-linear part of the stress-strAin curve is considered to show the damages of thermal shoch fatigue, we examined the validity of √E′/ρ with the velocity of the primary wave, where E′ is the secant modulus at 100kgf/cm2, which is correspond to the slope of early part of stress-strAin curve. The results show that the degree of Vp or by examining the non-linear part of the stress-strAin curves.4. Contrasting the uni-axial compressive strength (Sc) agAinst the 50% secant modulus (E50) or the velocity of primary wave (Vp) shows that the increase or decrease, respectively, but the better correspondence is seen between Sc and E50 than between Sc and Vp.

Statistical mechanics of fracture and fatigue of compacted copper fibre cement composites

Using experimental data on flexural strength, fracture statistics of compacted copper fibre cement composites is studied by means of Weibull's Statistical Theory on brittle materials. The diagrams of cumulative probability of fracture versus breaking stress for fibre cement and for cement paste are plotted both in the moist and dry states. Further, size influence on the cumulative probability of fracture is analysed. Since copper and cement have different thermal-expansion coefficients, the fibre cement and the cement paste were subjected to thermal shock between 10°C and 100°C, completing 3000 cycles that cause no rupture but reduction in mean strength. Fatigue curves and local probability of fracture diagrams are drawn for each 500 cycles. It is shown that the inclusion of short ductile copper fibres randomly distributed in a brittle cement matrix is able to block the thermal fatigue process. This property is attributed to the fact that the different expansion coefficients of cement and copper produce microcracks.

Some aspects of thermal fatigue in stainless steel

This paper is concerned with the analysis of failures in the moderator circuit branch piping of the ATUCHA-1 pressurized heavy water reactor (PHWR), which is made of austenitic steel to DIN 1·4550 specification (similar to AISI 347). These failures are considered to result from thermal fatigue processes induced by fluctuations in a zone where stratified temperature layers occurred, the fluctuations being associated with variations in moderator flow. The first section evaluates the possibility of cracking due to thermal fatigue phenomena and concludes that under service conditions a crack may initiate and grow through 7 mm thickness of the branch pipe. In laboratory thermal fatigue tests that simulated the thermomechanical conditions for such a component, the number of cycles required to initiate a thermal fatigue crack in a notched modified standard fatigue specimen was about 10 3. This value may be used to give a conservative prediction of the number of thermal cycles for crack initiation in actual branch pipes, including those subject to the cold plug condition which is produced in some emergency shut-down and valve testing situations. It was also demonstrated that beyond a crack depth of 7 mm stress corrosion cracking is the main process in further crack propagation. The relevance of this prediction is confirmed by microfractographic observations, since the brittle nature of the fracture surfaces under service conditions appears very different from the transgranular ductile striations found in both thermal and mechanical fatigue test specimens as a result of interacting environmental effects.

Thermal-mechanical damage in railroad wheels due to hot spotting

As a railroad wheel is heated during a brake application, a compressive stress is developed in the tread surface. Free thermal expansion is partially or completely constrained by the cooler underlying material. If yielding of the surface material occurs, a residual tensile stress could be developed. Constraint of thermal expansion can cause stresses which may eventually initiate and propagate fatigue cracks. Constraint of free thermal expansion is a necessary ingredient in the thermal fatigue process. The number of cycles that produces cracks would classify this process under the more general heading of “low cycle fatigue”. It has been observed that a phenomenon known as “hot spotting” develops on the tread surface as braking occurs. These areas of contact, roughly 20–40 mm in diameter, experience high localized stresses as a result of the severe temperature gradients associated with them. Thermal strains of these hot spots were calculated and in the case of total constraint all this strain is converted to mechanical strain. Thermomechanical fatigue testing provided the necessary data for the fatigue analysis. The result of this work provides a life prediction of railroad wheels experiencing hot spotting.

岩石の熱疲労に関する実験的研究

It is well known that rocks are more or less deteriorated by sudden cooling after being heated. Furthermore, by repeating the cycle of heating-cooling, rocks might be expected to be weakened more severely.In this paper, measuring the changes of the physical and mechanical properties of rocks including apparent specific gravity, P-wave velocity, Young's modulus and uniaxial compressive strength, the thermal fatigue process of rocks is characterized for four kinds of rocks, namely, OGINO tuff, EMOCHI welded tuff, AKIYOSHI marble and INADA granite. The cylindrical specimens are suddenly submerged into water after being heated and the cycle is automatically repeated in the testing machine.The maximum temperature and the maximum cycles in the experiment are 600°C and 54, respectively.The main results obtained are as follows:(1) The main failure mechanism is different between the crystalline rock and the sedimentary rock. The failure of the former takes place by the thermal interaction between minerals and that of the latter by the transient thermal stresses. As the result, crystalline rocks collapse to be particles or powders and sedimentary rocks are fractured initiating regular thermal cracks (Fig.6).(2) The strengths of the rocks except welded tuff decrease remarkably within 5 cycles if the temperature is sufficiently high and the cooling time is larger enough (Fig.3).(3) The strengths of the rocks except marble decrease as the cooling time increases. However, the additional effect is very small if the cooling time is larger than that needed for the specimens to be perfectly cooled (Fig.4).(4) The cycles at which the specimens collapse exponentially increase as the temperature decreases (Fig.5).

Reliability and thermal impedance studies in soft soldered power transistors

This paper discusses the reliability of soft soldered silicon power transistors under various thermal cycling conditions. A thermal cycling system is described which provides electrical testing and thermal impedance measurements of the devices until they fail. The thermal impedance measurement technique and the cycling parameters of the devices tested are discussed. Differences between computed and measured steady-state thermal resistance indicate interface problems associated with the fabrication of soft soldered devices. The study is focused on 2N3055 power transistors from a variety of manufacturers. It is shown that the thermal impedance is sensitive to changes in the Si-header joint and will increase upon cycling due to the thermal fatigue process. Information on mechanical failure of joints is obtained from scanning electron microscopy studies on the failed devices. The important parameters that affect the lifetime of the devices are indicated, and their effects are shown quantitatively. It is concluded that thermal impedance measurements can provide a reliable probing technique for imperfections in the soft soldered contacts found in many power devices.

Strength of the materials used for pistons of internal-combustion engines on subjection to fatigue and thermal fatigue loading

1. The strength characteristics of the piston alloys under consideration, measured under conditions of isothermal and thermal fatigue approaching those encountered in actual practice, are considerably lower than the static strength characteristics. This indicates the importance of allowing for the factors in question when calculating the strength of interual-combustion-engine pistons. 2. Of the alloy studied in the present investigation, the lowest thermal-fatigue strength characteristics corresponding to approximately normal working conditions are those of aluminum alloy AL25. For temperature cycles of 55⇌330°C the breaking stress on a base of 2·104 cycles of heat exchangers falls to 1 kgf/mm2, as compared with the isothermal fatigue limit (6.1 kgf/mm2) and the static tensile strength (12 kgf/mm2) at the same temperature. 3. Under conditions of combined loading, the fatigue life of alloy AL25 falls more rapidly than would follow from the linear hypothesis of damage summation, allowing for both mechanical and thermal fatigue. A vital factor in this case is the damage associated with interaction between the fatigue and thermal fatigue processes due to multiple-factor effects. 4. Criteria for the failure of piston materials under multiple-factor conditions call for urgent and more extensive investigation.