Fatigue Life Evaluation Method Research Articles

Fatigue life evaluation accuracy of power devices using finite element method

Fatigue life evaluation of power devices is closely related to the temperature distribution on the solder layer. During thermal cycling, the solder layer reaches a uniform temperature. In case the power cycle becomes short, this state is different from the real active state of the power device. Because the real temperature distribution on the solder layer is non-uniform. Therefore, there is a difference between a fatigue life evaluation that uses power cycling and one that uses temperature cycling. To accurately predict the fatigue properties of power devices, evaluation techniques based on a coupled electrical-thermal–mechanical analysis are proposed. This method is based on the finite element method. Therefore, the calculation accuracy of the fatigue life evaluation depends on the mesh resolution. The accuracy of the prediction is expected to indicate the accuracy of the numerical solution procedure, or modeling techniques (the accuracy of the physics captured, the electro-thermal-structure coupling and plasticity law etc.). To verify the calculation accuracy of this fatigue life evaluation method for a power device, we investigate how the mesh resolution affects the fatigue life evaluation.

Study on Fatigue Life Evaluation Using Small Specimens for Testing Neutron-Irradiated Materials

The effect of specimen size and shape on the fatigue life of reduced activation ferritic/martensitic steel “F82H IEA-heat” was investigated to develop a fatigue life evaluation method using a small specimen for testing neutron-irradiated materials. The fatigue test was carried out at room temperature in air using three kinds of round-bar specimen (minimum diameter cross sections of 1, 4, and 7 mm) and one hourglass specimen (minimum diameter cross section of 1.25 mm). The effect of specimen size on fatigue life from a level of 103 to 104 cycles was almost negligible for the round-bar specimens. The shorter fatigue life at the total strain range below 0.6% and the slightly longer fatigue life at the total strain range above 0.8% of the hourglass specimen relative to that of the standard specimen were observed from a level of 102 to 104 cycles.

Evaluation of Fatigue Life of Semiconductor Power Device by Power Cycle Test and Thermal Cycle Test Using Finite Element Analysis

To accurately predict the fatigue life of a power device, a fatigue life evaluation method that is based on the power cycle is presented in terms of an algorithm based on a combination of electrical analysis, heat analysis, and stress analysis. In literature, the fatigue life of power devices has been evaluated on the basis of the thermal cycle. This cycle is alternately repeated within a range from a high temperature to a low temperature. In an actual operating environment, however, a power device works in a power cycle that consists of being switched ON and OFF. To accurately predict the fatigue life cycle of a device, then, the evaluation should take account of this important aspect of the power cycle. To verify the utility of the evaluation method presented in this study, the results for a power cycle based on the combined use of electrical analysis, heat analysis, and stress analysis are compared to the results based on the thermal cycle, as found in the literature. Our conclusion is that the fatigue life cycle as estimated by the thermal cycle test is higher than that estimated by the power cycle.

1608 14MeV中性子照射材の評価に向けた微小疲労試験技術開発の現状(OS16-3 核融合炉-多種多様な力学的環境における強度・設計問題)

The effect of specimen size and specimen shape on the fatigue life of reduced activation ferritic steel, F82H IEA-heat, was investigated to develop the fatigue life evaluation method using small specimen for testing 14 MeV neutron irradiated materials. The fatigue test was carried out at room temperature in air using three kinds of round-bar specimen (the minimum diameter of the cross section: 1, 4, and 7 mm) and one hourglass specimen (the minimum diameter of the cross section: 1.25 mm). The effect of specimen size on the fatigue life was almost negligible for the round-bar specimens. The shorter fatigue life at relatively low strain range and slight longer fatigue life at relatively high strain range for the hourglass specimen than the standard specimen were observed.

High temperature bending fatigue behavior of stainless steels for automotive exhaust

Abstract Continually increasing working temperatures and a growing need for greater efficiency and reliability of automotive exhaust system require immediate investigation into the high temperature fatigue properties of heat resisting stainless steels. In this study, evaluation method of high temperature bending fatigue life has been established and S–N curves for ferritic stainless steels, such as 409L, 436L and 429EM STS, produced by POSCO have been evaluated at the temperatures of 600 and 800 °C, which are typical temperatures of exhaust system exposed during operation of a vehicle. Endurance limit of 429EM was found to be higher than the other two stainless steels and significantly reduced with increasing temperature, from 150 MPa at 600 °C to 65 MPa at 800 °C. Thickness effect on the fatigue life has also been investigated by varying the thickness of specimen and was found insensitive within the thickness range from 1 to 2 mm.

Evaluation of low cycle fatigue life in AZ31 magnesium alloy

Magnesium alloys are attracting engineers for their practical application to structural components. Here fatigue properties, which is essential for structural use, have been examined on extruded AZ31 bar under uniaxial cyclic loading by both strain and stress controlled conditions. Adding fatigue tests with mean stresses under stress controlling conditions, fatigue life evaluation method has been discussed along with the analysis of cyclic stress–strain behavior. The specimen is easy to yield in compression by twinning. This leads to the asymmetric hysteresis curves. It also tends to deform quasi-elastically during unloading from compression; this makes the plastic strain amplitude smaller to the maximum one in the hysteresis curve. These asymmetric features fairly disappear at half-life in the stress controlled tests. The fatigue lives and deformation characteristics can be expressed nicely by Manson–Coffin type equation. On the contrary, the strain controlled tests retain the asymmetry till the end and produce tensile mean stresses. The fatigue lives are unsuccessfully evaluated by the above equation. Various mean stress correction models for cubic metals are not operative in magnesium alloys. A new model has been devised adding a correction term of − σ m/2 E to the above mentioned Manson–Coffin type equation. Strain controlled test, as it retains pyriform shape till the end, could be evaluated more accurately with the maximum plastic strain amplitude in the hysteresis curve.

Effect of Stress Ratio on Fatigue Properties and Propsition of Fatigue Life Evaluation Method under High Stress Ratio for Ti-6Al-4V Alloy

Effect of Stress Ratio on Fatigue Properties and Propsition of Fatigue Life Evaluation Method under High Stress Ratio for Ti-6Al-4V Alloy

Residual fatigue life evaluation method for the cracked components under complex stress fields

This study reviews the influence function method(IFM) for calculating stress intensity factors (SIFs, K) and modifies it to apply for the estimating the residual fatigue life for the cracked components under complex stress fields. An IFM has been developed to analyze SIFs for surface cracks which are subjectedto nonuniformly distributed stresses. Through elastic superposition, the influence function method properly accounts for redistribution of stress as the crack grows through the component. This influence function is unique to the given geometry and independent of the loading. Some examples have been provided to show the effectiveness of the IFM including the distributions of K in a residual stress field. The significant effect of residual stress upon fatigue crack growth in a welded component has been demonstrated with the IFM.

Evaluation Method of Thermal Fatigue Life for Surface-Mount Solder Joints by Mechanical Fatigue Test.

In this study, we attempt to develop a more efficient method of thermal fatigue life evaluation for solder joints than the thermal cycle test. The mechanical fatigue tests for solder joints of surface mount devices are performed under several temperature and cyclic frequency conditions. In addition, thermal cycle tests are carried out in order to compare the results with the mechanical fatigue test. On the other hand, the strain range of solder joint in the mechanical fatigue and thermal cycles tests is evaluated by using the elasto-plastic finite element method, considering the effects of temperature and strain rate on the stress-strain curve of solder. From the experimental and analytical investigations, it is found that the relation between the elasto-plastic strain range and the fatigue life of the mechanical fatigue test agrees well with that of the thermal cycle test. Furthermore, an evaluation method of thermal fatigue life employing the mechanical fatigue test is proposed on basis of the investigations mentioned above.

Emerging technology for component life assessment

Operating plant component damage and failure experience is reviewed. Loading conditions such as thermal stratification and striping, turbulent flow and flow-induced vibrations are often found to limit useful life, even though such loadings were typically not considered when the components were designed. High cycle thermal and mechanical fatigue are identified as important damage mechanisms. A new method of correlating fatigue data and extrapolating to the very high cycle regime is described. The results of environmental degradation testing during the past fifteen years have shown that such effects are much more deleterious than previously assumed. Therefore environmental and aging effects must be taken into account in evaluating the reliability and dependability of components for extended periods of operation. Since most of the available data on environmental effects focus on measured crack growth rates, methods of developing improved fatigue life evaluation methods which include environmental effects on crack growth rates are now being developed. Fatigue tests on polished specimens are characterized by nominal stress amplitudes over yield, where linear elastic fracture mechanics ( da/ dn vs. ΔK ) methods, such as those used in the ASME Code, are not valid. The small plastic zone corrections used in the Code do not account for the plastic crack-driving energies encountered in low-cycle fatigue testing. J- integral solutions are used herein to evaluate the growth of cracks in these specimens. This approach can be shown to correlate the growth of cracks over the entire range of loading from elastic to grossly plastic conditions in widely different geometries and sizes, including the growth of very short cracks for materials of major interest in pressure vessels and piping. It can be used to correct S-N fatigue life evaluation, curves for known differences in crack growth rates whether they are due to corrosion-assisted fatigue or other variables. Environmental effects on the crack initiation phase of fatigue failure can be directly incorporated into S-N life evaluation curves. Once the crack propagation effects are included, the resulting improved S-N curves provide a means for plant operators to evaluate the current condition of these components and systems, taking into account the cumulative damage from operating transients and cycles which the plant has experienced. The safe residual life can then be evaluated using the S-N curves to include cumulative damage for the anticipated future period of operation. This plant life evaluation approach is applicable even where in-service inspections are not feasible. It provides a sound quantitative basis for making repair/replacement decisions.

実働状態のＰ‐Ｓ‐Ｎ曲線を用いた燃焼室壁部材の寿命推定と疲労寿命設計法

In this paper, the fatigue life evaluation method for the components of combustion chamber was considered on the basis of P-S-N curve obtained in service conditions. Also, the fatigue life design method for these components and some results of its application were described.First, the stress pattern in service load was modeled as the two step duplicated alternative stress pattern by applying Range Pair Mean Method for reading stress cycle frequency and Modified Goodman's Theory for evaluating mean stress.Then, the P-S-N curve in service condition was analysed for some examples of failure through calculation of fatigue damage by the linear damage theory and the non-linear damage theory. The values of the experimental constants and the relation between them were obtained in these fatigue damage calculation methods.According to the above mentioned results, the allowable stress ranges for several components were calculated under the condition that the allowable probability of failure was assumed to be 0.1% or 1.0% and the required life to be ten years.By comparing the obtained allowable stress ranges with those learned in experience, the present fatigue life evaluation method was confirmed to have sufficient reliability to be used as a fatigue life design method for these components.