Key Engineering Materials Vols. 321-323

Abstract: This paper shows the experimental study to evaluate the fatigue strength of a bogie frame for Korean tilting train. The various load cases were established based on the international standard (UIC615-4) and the special tilting load case, which is unique in the tilting train, was derived by dynamic analysis and logical assumption of operation under worst tilting condition. The experimental study was conducted by means of two ways, both static and fatigue loading test. Through the static load test, the safety against fatigue was assessed using Goodman diagram of the material used. The fatigue load test was carried out up to 1x107cycles. The fatigue strength was assessed by fatigue crack inspection by means of NDT test at the stage of 6x106cycles and 1x107cycles.

Abstract: The use of linear and second order stress extrapolation to obtain KI and KII in two-dimensional finite element models of a thick plate containing an edge crack was examined. Three loading cases were studied, including classical Mode I and Mode II problems and a problem of tribological contact. Linear extrapolation was observed to yield accurate predictions of KI in cases of dominant Mode I loading. In Mode II situations, notably where the crack faces experienced compressive normal stresses, second order extrapolation was observed to improve estimates of KII

Abstract: In this research, fatigue crack propagation tests of magnesium alloy AZ31B and aluminum alloy 2024T3 were conducted under conditions of biaxial and uniaxial loading by using a cruciform specimen in a biaxial fatigue machine, in order to investigate the effect of non-singular stress cycling. From these comprehensive experiments, in the magnesium alloy, the remarkable effect was found in the specific biaxial load stress ratio RB (= σx 0/σy 0) on
KI-da/dN relation. On the other hand, in the aluminum alloy, it was confirmed that there is no influence of a RB on
KI-da/dN relation.

Abstract: This study has performed the experimental study to assess the structural safety of a bolster frame that is applied to the bogie system of Korean tilting train. In order to achieve this goal, firstly, loading conditions imposed on the bolster frame were investigated. Based on the international standard and dynamic analysis, the loading conditions were derived. In this study, the dynamic load cases that consider the carbody tilting effect were established. The structural safety of the bolster frame was evaluated by static test under these static loads and the fatigue strength was assessed using Goodman diagram. From the assessment, the bolster frame has satisfied the structural safety. In addition, fatigue test was carried out up to 1x107cycles under tilting load condition established in this study. The inspection of fatigue crack using NDT method was conducted at 6x106 and 1x107 cycles.

Abstract: Austenitic stainless steel has a large problem to decrease its corrosion resistance and mechanical properties by sensitization in the welding process. Thus, in this paper, corrosion fatigue characteristics of artificially sensitized STS304 were investigated. According as heat treatment period for sensitization increases, the Cr-carbide deposition in the grain boundary and degree of sensitization (Ia/Ir) increased. From the results, corrosion fatigue strength of sensitized STS304 was remarkably reduced compare to non-sensitized ones.

Abstract: CF8A steel is a material of the piping system in nuclear power plant. In order to evaluate its fracture characteristics and degradation by corrosion, electrochemical corrosion and corrosion fatigue characteristics were evaluated. CF8A steel was artificially degraded at 400°C for 3 months. Corrosion rate of degraded CF8A steel in NaCl solution of room temperature increases with concentration of NaCl solution increase. However, concentration of NaCl solution will be more than 4.0wt.%, it shows decreasing tendency. Crack growth rates of degraded and not-degraded CF8A steel in air condition do not show remarkable difference. However, in 3.5wt.% NaCl solution, it showed higher than that of in air.

Abstract: Multiple collinear surface cracks distributed that are randomly along a weld toe have a strong influence on the fatigue crack propagation life of welded joints. This issue is investigated using statistical approaches based on series of systematic experiments, in which initial crack numbers, their locations and crack sizes, i.e. depth and length, are taken into account. The number of initial cracks follow a normal distribution, and the probability of initial crack depths and lengths can be accurately described by the Weibull distribution. These characteristics are used to calculate the fatigue crack propagation life, in which the mechanisms of the mutual interaction and the coalescence of multiple cracks are considered as well as the Mk-factors. The automatic calculation of fatigue crack propagation life is achieved by the application of NESUSS, where parameters such as the number, location and size of cracks are all treated as random variables. The random variables are dealt through a Monte-Carlo simulation with sampling random numbers of 2,000. The results of the simulation provide the statistical characteristics of the fatigue crack propagation life for welded joints as a function of the number of initial cracks. The sum of the simulation results and the fatigue crack initiation life referred from a previous paper is in good agreement with the experimental results.

Abstract: The results of cavity inspection in uniaxial creep tests and creep crack growth tests were compared to determine the cavity effect. The quantity of the cavities increased with increasing strain and crack growth rate as stress and temperature were increased. This resulted in the change of da/dt vs Ct. The increased rate in the number and size of the cavities in P92 steel with temperature was relatively fast, resulting in the high rate of da/dt against Ct. In this study, to determine the characteristics of defects in structure, the sub surface defects of P92 and P122 steels were investigated by an intelligent phased array ultrasonic inspection system. The system was found to be capable of detecting the creep crack length and the formation of cavities at the crack tip.

Abstract: The automation of fatigue durability analysis for welded bogie frames according to the UIC-code is proposed by using the ModelCenter, which enables several tools used in fatigue durability analysis to be integrated, i.e. I-DEAS, ANSYS and BFAP, and an iterative analysis of geometrical changes in transom support bracket to be performed. In additions wrapping programs to control I/O-data and interfaces of these tools were developed. The developed automation technique brings not only a significant decrease in man-hour required for durability analysis, but also provides a platform for multidisciplinary engineering activities.

Abstract: Fatigue cracks and SCC (Stress Corrosion Cracking) on a back surface were detected, and the shape of cracks were evaluated using a surface wave generated by focusing angle beam ultrasound. Internal ultrasonic images showed a little shorter length than optical images. Cross section of the fatigue crack and internal distribution of complicated shaped SCC were estimated.