Key Engineering Materials Vols. 321-323

Abstract: Three-dimensional cohesive zone model was used to simulate the fatigue crack growth of a weld specimen. Damage accumulation was accounted for using a constitutive model. A surface along which the fatigue crack would grow was assumed along the symmetric plane of the specimen and the cohesive elements were applied on the surface. Tensile tests of weld and parent specimens were simulated by the finite element method. Results of fatigue crack growth for the weld and parent specimens were compared and the effect of changing a constitutive parameter on the fatigue crack growth behavior was also checked.

Abstract: Low cycle fatigue cracks are mainly detected at discontinuous welded locations with high stresses under repeated cyclic static loads due to cargo loading and unloading. Theoretical and analytical methods have been used to estimate the local stress and strain, which affect the prediction of fatigue life, but these methods have difficulties considering stress concentration at notched locations and complicated material behavior of welded joints or heat affected zones. Electronic speckle pattern interferometry(ESPI) system is a nondestructive and non-contact measurement system, which can provide relatively accurate full field strain at critical positions such as welded zones and structurally discontinuous locations. In this study, local strain was measured by ESPI system at the welded cruciform joint, and then low cycle fatigue test was performed. Effect of local strain on low cycle fatigue life was examined by using the strain value measured by ESPI. In order to verify the relations between local strains and fatigue lives, after theoretical local strains and stresses were calculated by using Neuber’s rule, the measured local strains corresponding the experimental fatigue lives were compared with the results of Neuber’s rule and established codes of the British standard and DNV curve.

Abstract: Fatigue and fractures such as blades and discs, which originate in the centrifugal force caused in the rotor, become factors that often cause serious accidents. The enlargement of machine and structure improves the possibility that do the latency of minute cracks and inclusion in materials in manufacturing processes. Severe working conditions also promote the propagation of the crack while operating. Thus, it is very important for the improvement in the safety of materials to analyze the interference problem of inclusion and crack. In this study, an inclusion in the rotating disc and the interference problem of the crack were examined. The influence of the mechanical property of the inclusion and the distance to the crack tip was examined. The stress intensity factor was determined by using photoelasticity and method of caustics. As a result, the stress intensity factor was decreased when Young’s modulus of inclusion increased. Moreover, the stress intensity factor decreased as the distance from the crack tip to the inclusion boundary became longer. The stress intensity factor of the crack on the rotation center side increased more than the cracks on the circumference side of the inclusion.

Abstract: The stress analysis of the double column made of the dissimilar material is not enough reported the problems about the interface of the crack on the surface. These products are guaranteed for the high performance, and are composed of the dissimilar material or composted to the new material or composed to the new materials again by the compound of the material and the advancement of joint technology. The Stress Intensity Factor K
, Kand K usually act in single or mixed modes. The separation of K
, Khas already been achieved. In this paper, an experimental technique for determining the stress intensity factor Khas been presented by using the method of reflected caustics in combination with the stress freezing method. The experimental model is a cylindrical bar with w radial crack along the direction of the axis under the load of pure torsion. In order to obtain a caustic pattern, a slice cut from the frozen model is annealed, and evaluated it for the stress intensity factor in a double column of the different kind material by the experiment analysis.

Abstract: The tribological performance of multi-walled carbon nanotubes (MWNT) in mineral oils is investigated at ambient temperature. The frictional forces, wear amounts and cycles to scuffing of the oils with nanotubes and without those were measured using the ball-on-disk tester. It was found that there were little differences in the frictional forces and wear amounts of two oils. However, the scuffing times of oils with nanotubes were much longer than those of oils without nanotubes in sliding tests. The nanotubes were very effective on maintaining the oil gap and protecting the surfaces in boundary lubricated sliding.

Abstract: The theoretical residual stresses in the autofrettaged tube were calculated, considering Bauschinger effect caused by the reverse yielding while removing autofrettage pressure. Actual residual stresses were measured by using X-ray diffraction analysis, and compared with the theoretical calculations. From the tension-compression tests, however, the tube steel showed a significant Bauschinger effect, depending on the plastic strain prior to unloading. Fatigue crack growth life of the tube was evaluated by integrating the crack growth rate equation. Stress intensity factor of an inside crack at the tube due to pressure and residual stresses was determined from finite element analysis and superposition principle. Compared to the unautofrettaged tube, the extended fatigue life of the autofrettaged thick-walled tube was obtained depending on the autofrettage level and the Bauschinger effect.

Abstract: The initial crack under fretting condition occurs at lower stress amplitude and at lower cycles of cyclic loading than that under plain fatigue condition. INCONEL alloy 600 and 690 are high–chromium nickel alloy having excellent resistance to many corrosive aqueous media and high-temperature atmospheres. In this paper, the effect of fretting damage on fatigue behavior for INCONEL alloy 600 and 690 were studied. Also, various kinds of mechanical tests such as hardness, tension and plain fatigue tests are performed. Fretting fatigue tests were carried out with flat-flat contact configuration using a bridge type contact pad and plate type specimen. Through these experiments, it is found that the fretting fatigue strength decreased about 40~70% compared to the plain fatigue strength in two materials. In fretting fatigue, the wear debris is observed on the contact surface, and the oblique micro-cracks at an earlier stage are initiated. These results can be used as basic data in a structural integrity evaluation of heat and corrosion resisting alloy considering fretting damages.

Abstract: The operation mode of thermal power plants has been changed from the base load to duty cycle. From the changeover, fossil power plants cannot avoid frequent thermal transient state, for example, start up and stop, which results in thermal fatigue damage at the heavy section components. The rotor is the highest capital cost component in steam turbine and requires long outage for replacing the new one. For optimized power plant operation life and inspection management of rotor is necessary. It is known as general that start-up and shutdown operation greatly affect on steam turbine life. The start-up operation condition is especially severe because of the rapid temperature and rotational speed increase, which causes damage and reduction of main components life of steam turbine. The start-up stress of rotor which is directly related life is composed of thermal and rotational stresses. The thermal stress is due to the variation of steam flow temperature and rotational stress is due to the rotation speed of itself. In this paper, the analysis method of start-up stress of rotor which considers simultaneously temperature and rotation speed transition is proposed, which includes a case study regarding 500MW fossil power plant steam turbine rotor. Also, the method of damage quantitative estimation of fatigue damage to operation condition is described. The method can be applied to find weak points to the fatigue damage. Using the method, total life consumption can be obtained, and can be also used for determining future operation mode and the life extension of old fossil power units.

Abstract: Cold expansion method retards the crack initiation due to the compressive residual stress developed on a hole surface. Most previous researches have shown only the beneficial distribution of residual stresses in the retardation of the crack initiation at the stress concentration area. Also, there have been only few studies on the relation between crack growth and residual stress around other adjacent holes. A few fastener holes of aircraft structures is a shot distance which is less than 20mm between holes. The purpose of this study is to provide better understanding of the residual stress effect around a hole in a structure as crack growth starts from another hole. By finite element method, this study showed that residual stress in a CT specimen is redistributed by cold expansion process and that tensile stress increases in proportion to the cold expansion ratio in the vicinity of the crack. Stress intensity factor increases as the cold expansion ratio increases.

Abstract: This study examines the performance of adhesive and cohesive interfacial crack model in assessing the brittle shear behavior of reinforced concrete bridge columns. The quarter-scale reinforced concrete columns tested at the University of California at San Diego by Xiao et al. (1993) to explore the ductility of reinforced concrete columns under cyclic loading. The three columns R-1, R-3 and R-5 with different transverse reinforcement are considered for finite element failure analysis.