Papers by Keyword: Stress Gradient

Abstract: The purpose of this study is to develop a general bending fatigue strength design method that can be applied to various geometries. Considering the conditions for fatigue crack initiation and crack arrest, the bending fatigue strength was evaluated using the actual stress at the critical point where the maximum stress occurs, and the relationship between the bending fatigue strength and the geometry close to the critical point was investigated. The results indicate that the bending fatigue strength evaluated using the actual stress depends on the shape close to the critical point. A higher stress concentration leads to a higher fatigue strength, which is defined as the apparent bending fatigue strength. The apparent bending fatigue strength decreased with a decreasing stress gradient. The lowest value of the apparent bending fatigue strength was observed at a zero stress gradient, which corresponded to the tensile fatigue strength of a smooth test piece. Therefore, the tensile fatigue strength of a smooth material can be used as a criterion for estimating the apparent bending fatigue strength. Moreover, the stresses at the critical point were much larger than the yield stresses of the test pieces, indicating the necessity of considering small-region yields in the evaluation of bending fatigue strength.

Abstract: Strength of solder joints is usually evaluated by a shear test and a pull test. The reliability of the solder joint is evaluated by the repetitive pull tests of solder bulk specimens. However, the stress and strain field that caused by thermal load on the solder joint of the product model for estimating the reliability is different from these tests. Therefore, we proposed a repetitive bending test as a reliability test of solder joints producing the stress and strain field caused at the solder joint of product model. We proposed a repetitive multi-point bending test as a method to predict the fatigue life of the solder joint in the thermal cycle test in a short period of time. The influence of strain gradient on the inelastic strain amplitude used for lifetime evaluation is estimated. The controllability of the strain gradient by the three-point bending test parameters is investigated. The effect of residual stress on inelastic strain amplitude during sample preparation for thermal cycle test is also evaluated.

Abstract: The limit elastic state of an infinitely large half-plane under the action of concentrated vertical and horizontal forces has been considered. The gradient yield condition specifying the elasto-plastic material yielding initial moment at non-uniform stress state is used as the limit state criterion. The analytical expressions allowing to interpret the result in the singular point have been derived.

Abstract: Metallic materials usually have microscopically heterogeneous structures, such as polycrystalline structures, affecting macroscopic mechanical characteristics. Both macroscopic and microscopic non-uniform deformations of polycrystalline pure copper under a moderate stress gradient were investigated. In this study, macroscopic and microscopic non-uniform deformations under higher stress gradients are investigated. Uniaxial tensile tests using three-curve specimens with different curvatures and grain sizes were performed. In order to evaluate the heterogeneous strain field in the specimen surface, the development of the displacement field was measured using the digital image correlation method (DIC). The stress field was evaluated by coupling the DIC and finite-element methods. In smaller-grain specimens, a strong strain concentration was generated in the minimum cross-section area. Although a strong strain concentration was also confirmed in a larger-grain specimen, the strain field depended not only on the specimen shape but also on the microscopic heterogeneity. This microstructure-driven non-uniform deformation was also observed in the specimen with a larger curvature radius. These results indicated that the macroscopic non-uniform deformation should be estimated by the material parameter related to the microscopic heterogeneity.

Abstract: Residual stress measurements were successfully performed on the representative IN718 fatigue specimens by X-Ray Diffraction. All surface residual stresses were found to be compressive. A stress gradient normal to the surface was observed on all specimens. The residual stresses tended to become less compressive with increasing depth into the parts. Residual stress measurement is the special requirement for NADCAP CRITERIA AC 7101/7. In this paper, residual stress measurements were successfully performed on two IN718 low cycle fatigue test specimens.

Abstract: The work presents non-local line method by which the equivalent fatigue zones were designated. These zones are one-dimensional efficient lengths in which operating stress variables cause initiation of fatigue cracks. The algorithm of the presented method considers the issues of multi-axial stress state, critical plane and weighting function. Calculations of stresses in the test element were performed by using the FEM assuming cyclic material properties, which are described with the model for multi-linear hardening. The empirical lifetime of elements with a notch was assumed to define length of the crack amounting to 0.1 mm. As the result of calculations, the dependence of effective length on nominal stress and radius of the notch was determined.

Abstract: X-ray stress analysis suffers from homogeneity limitations of the stress field in the analyzed volume. When this homogeneity is not fulfilled, it is possible to reduce the irradiated volume down to stress homogeneity achievement. New limitation however occurs : the diffracting sites become too few for stress homogenization. We show that the diffractometry analysis corresponds to a spatially convoluted stress field. The inverse convolution problem is posed. An example of regularization method is given.

Abstract: A fabrication process to manufacture SU8 photoresist microstructures is presented in which BP212 positive photoresist was used as sacrifice layer and SU8 was used as structure layer. No crack has been observed in the obtained microstructures. The relation between PEB temperature and stress gradient in SU8 film has been studied by measuring radii of released SU8 cantilevers made at different PEB temperatures.

Abstract: In order to improve the radial strength of energy storage flywheel, this paper design and analyses the multi-ring flywheel rotor in different conditions by finite element method. From the results, we can confirm that interference condition is critical influence on the stress gradient of the rim. The results can provide a guide for high energy density multi-ring composite flywheel rotor design.

Abstract: The S-N curves of medium carbon steel(MCS) are determined by the ultrasonic symmetric bending fatigue system (20 KHz). The results show that two S-N curves of MCS display the characteristic of “continually decreasing type” up to 109 cycles and exhibit no traditional horizontal plateau beyond 106 cycles. When the fatigue cycles are over 109, fatigue failure do occurs. By comparison of the two curves of 2mm thick and 5mm thick MCS, the results show that in the high-cycle stage(107cycles), size effect has no significant impact on fatigue Properties. This can also be explained by the theory of crack initiation, in high cycle stage, fatigue crack initiation exists in the sample surface, and in ultra-high-cycle stage, fatigue crack initiation exists in internal initiation.