Key Engineering Materials Vols. 297-300

Abstract: A relationship between fatigue crack opening behavior and the reversed plastic zone size is studied. An elastic-plastic finite element analysis (FEA) is performed to examine the opening behavior of fatigue crack. The contact elements in this analysis are adopted in the mesh of the crack tip area. The smaller element size than reversed plastic zone size is used for evaluating the distribution of reversed plastic zone. In the author’s previous results, the FEA could predict the crack opening level, which the size of crack tip elements was in proportion to the theoretical reversed plastic zone size. It is found that the calculated reversed plastic zone size is related to the theoretical reversed plastic zone size and crack opening level. The calculated reversed plastic zone sizes are almost equal to the reversed plastic zone size considering crack opening level obtained by experimental results. It is possible to predict the crack opening level from the reversed plastic zone size calculated by the FEA. We find that the experimental crack opening levels correspond with the opening values of crack tip contact nodes on the calculated reversed plastic zone.

Abstract: Thirty three SENB and nine TPB specimens, which were machined from an 8.1mm thick 7075-T7351 aluminum stock plate, were fatigued in the as-received or shot peened conditions. The SENB specimens were then fractured by overload and were recorded. The crack growth rate of the tunneling crack profiles were measured by fractography at various life cycles. SN curves of the as received and shot peened TPB specimens were also constructed. Peened surface roughness, subsurface microstructure and micro-hardness profiles were examined. X-ray diffraction technique was used to measure the residual stresses in selected shot peened SENB and TPB specimens. The fatigue life under a severe shot peening intensity (Almen scale 0.016A) of the SENB specimens was slightly larger than that of the as received SENB specimens at low stress ratio of R = 0.1 but the difference was found to be smaller at R = 0.8. The SN curves of the as received and severely shot peened TPB specimens were nearly identical. Grain distortion due to peening was observed to a depth of 410µm and was correlated with the measured residual compressive stresses. The compressive residual stress had little effect on the crack growth rates in shot peened SENB and TPB specimens.

Abstract: This study performs dynamic analysis of underground structures on multi-layered half planes in frequency domain by using the coupled finite and boundary element method. The near field including underground structures is modeled with onventional finite elements, while the far field is modeled with boundary elements which satisfies radiation conditions. In evaluating the dynamic fundamental solutions, semi-analytical solutions due to line loads are employed. Therefore, the range of wavenumber integration can be reduced significantly. These solutions satisfy the reflection and transmission conditions of waves at each layer interface, so that the multi-region problem can be analyzed. Numerical examples are given to demonstrate the accuracy and efficiency of the proposed method.

Abstract: In general, the S-N curve in railway axles was mainly carried out under 107-108 cycles, while the service area of a railway axle is 108-109 cycles. The strain gages using electrical resistance have been used to measure stresses in railway vehicle wheelsets. However, there are some problems with strain gages using electrical resistance for railway axles. For example, the measured data is for special or limited intervals only. Strain gage installation is complicated, that is, it requires lead wires for measurement. The design of railway axles makes use of data that was obtained many years ago. The applied stresses in wheelsets running for a long time and in new railway vehicle wheelsets have not been studied clearly yet. It is necessary to carry out stress monitoring for more than 108 cycles to evaluate the safety of railway wheelset. Therefore, it is necessary to develop new stress monitoring techniques that can easily measure the working stress of the wheelset. In the present paper, the stress measurement technique of copper electroplating is considered because of its high potential for this purpose.

Abstract: This paper is an overview of our previous works [1-15]. A “micro-meso-process theory” [1, 3, 7] for quantitative analyses of the critical conditions of fatigue source evolution in metals and metallic parts has been established under comprehensive considerations of the processes occurring in individual weak grains (called as “micro-processes”) and the cooperative processes occurring in their surrounding grains (called as “meso-processes”), as well as the macro-scopical features of the fatigue behaviour. All these processes have stochastic and harmonizing characters, then it is concluded, that the most important process for fatigue source evolution is the formation of “cyclic meso-yielding zones” (CMYZs) under the cyclic loading, in which dislocation motions occur during loading not only within some weak grains, but also within rather many grains surrounding them and, whereafter, during unloading or reverse loading, back motions of dislocations can occur along intersected slip plans in individual weak grains. The critical condition for fatigue source evolution is that the CMYZs get over some critical sizes and the apparent fatigue limit of metal or metallic part is a stress / load needed for formation of such critical CMYZs at the position of weakest link. According to this consideration, a concept of surface / internal fatigue limit of metal is put forward [5, 6, 11, 12]; furthermore, the relationships of internal fatigue limit of metals to their small strain flow stress [8] are established and the procedures for prediction of the apparent fatigue limits of un- and surface-hardened smooth/notched specimens are proposed [9-15].

Abstract: An extensively published and correlated solder joint fatigue life prediction methodology is incorporated by which finite element simulation results are translated into estimated cycles to failure. This study discusses the analysis methodologies as implemented in the ANSYSTM finite element simulation software tool. Finite element models are used to study the effect of temperature cycles on the solder joints of a flip chip ball grid array package. Through finite element simulation, the plastic work or the strain-energy density of the solder joints are determined. Using an established methodology, the plastic work obtained through simulation is translated into solder joint fatigue life [1]. The corresponding results for the solder joint fatigue life are used for parametric studies. Artificial Neural Network (ANN) has been used to consolidate the parametric studies.

Abstract: A new simulation methodology using the finite element method (FEM) was proposed to predict the fatigue life of tires. In the FEM simulation the cracking energy density (CED) was calculated, and the virtual crack closure technique (VCCT) was used to calculate the strain energy release rate (SERR) for a crack. First, a plane in an element on which CED had its maximum was determined, and a crack was created on the plane. Once a crack was introduced, another plane on which CED had its maximum was again determined, and the crack was further elongated along the plane. In addition, SERR was calculated for every crack increment by using VCCT, and it was represented as a function of the crack length. Then, the fatigue life of a tire was determined using Paris law. For Paris law, the initial crack length and the final crack length as well as the material constants should be known. In this paper, the initial crack length was set in a way that the fatigue life predicted by using Paris law became the same as the test data for a tire model (Tire A), and the final crack length was determined from the cross-section views of failed tires which showed cracks grown about halfway toward the carcass from the carcass turn-up edge. Finally, the fatigue life for another tire model (Tire B) was predicted by using Paris law, and the predicted fatigue life was compared with the test data.

Abstract: In the shrink or press-fitted shafts such as railway axles, fretting can occur by cyclic stress and micro-slippage due to local movement between shaft and boss. When the fretting occurs in the press-fitted shaft, the fatigue strength remarkably decreases compared with that of without fretting. In this paper fretting fatigue life of press-fitted specimens was evaluated using multiaxial fatigue criteria based on critical plane approaches. An elastic-plastic analysis of contact stresses in a press-fitted shaft in contact with a boss was conducted by finite element method and micro-slip due to the bending load was analyzed. The number of cycles of fretting fatigue and the crack orientation were compared with the experimental results obtained by rotating bending tests. It is found that the crack initiation of fretting fatigue between shaft and boss occurs at the contact edge and the normal stress on the critical plane of contact interface was an important parameter for fretting fatigue crack initiation. Furthermore, the results indicated that a critical plane parameter could predict the orientation of crack initiation in the press-fitted shaft.

Abstract: Railway wheels and axles belong to the most critical components in railway vehicles. The service conditions of railway vehicles became more severe in recent years due to the increase of speed. Therefore, a more precise evaluation of railway wheel life and safety has been requested. Wheel/rail contact fatigue and thermal cracks due to braking are two major mechanisms of the railway wheel failure. One of the main sources of the contact zone failure is the residual stress. The residual stress on wheel is formed during the manufacturing process which includes a heat treatment, and then, is changed in the process of braking which results in wheel/rail contact stress and thermal stress. In this paper, an evaluation procedure for the contact fatigue life of railway wheel including residual stress is proposed. Also, the cyclic stress history for fatigue analysis is simulated by applying finite element analysis for the moving contact load. As a result, a fatigue life estimation methodology is proposed for railway wheels which includes the effects of residual stresses due to heat treatment, braking and repeated contact load, respectively.

Abstract: Once assessment of material failure characteristics is captured precisely in a unified way, it can be directly incorporated to the structural failure assessment under various loading environments, based on the theoretical backgrounds so called Local Approach to Fracture. The aim of this study is the development of an expert system applicable for the assessment of structural integrity throughout crack initiation and structural failure based on the Local Approach to Fracture. The generalized elasto-visco-plastic constitutive equation, which can consider the internal damage evolution behavior, was developed and employed in the 3-D FEA code in order to numerically evaluate the material and/or structural responses. Explicit information of the relationships between the mechanical properties and material constants, which are required for the mechanical constitutive and damage evolution equations for each material, was implemented in an automatic system using genetic algorithm based on an inference system. The material constants selected from genetic search and constitutive equations are used directly in the failure assessment of material and/or structures. The performance of the developed system has been evaluated for the S-N relationship assessment of several materials as well as the crack initiation assessment of various weldments in steel structures.