Papers by Keyword: Crack Growth Rate (da/dN)

Abstract: An automatic fatigue crack growth measurement system was developed, in which a special four-channel A-D acquisition board that could collect data in phase was used. The data collecting frequency is in the range of 4×(2~25600)Hz. The system is suitable for fatigue tests with the frequencies not higher than 250Hz. Eddy current transducers and standard load cell were used to measure displacement and load, respectively. The system can instantly calculate fatigue crack lengths, stress intensity factors and fatigue crack growth rates. As an application of the system, fatigue crack growth rates (FCG) and the thresholds of steels 42CrV and IR3Mo were presented.

Abstract: In the fatigue tests of specimens, sufficiently large tensile overload included in cyclic constant amplitude loading causes the retardation in fatigue crack growth. Crack retardation remains for some period of time after the overloading. The number of cycles in the retarded crack growth has been shown to be related to the plastic zone developed due to the overload. The magnitude of the elastic-plastic zone around the crack tip of DENT(Double Edge Notched Tension) specimen after a overloading was measured by ESPI(Electronic Speckle Pattern Interferometry) system. The fatigue crack growth rate also was measured by a traveling microscope. The relationship between the measured magnitude of plastic zone and the crack growth rate was compared with the equations proposed by Wheeler. Crack growth retardation model that was characterized by crack growth length and the size of plastic zone was proposed and compared with test result. From the research, the validity of proposed model is examined on crack growth retardation, and consequently fatigue life.

Abstract: The primary objective of this study is to develop a quantitative model to predict the effects of materials, environment and mechanics such as loading configuration on environmentally-assisted cracking (EAC) of stainless steels in high-temperature water. It has basically been accepted that crack propagation in oxygenated high temperature water is controlled by a slip-dissolution and/or deformation-oxidation mechanism. According to this mechanism, the crack-tip strain rate is an extremely important mechanical parameter for determining the crack growth rates. Based upon a formulation obtained by combining Faraday’s equation with an elastic plastic analysis of the strain singularity at a growing crack-tip in work hardening materials, a theoretical formulation of crack-tip strain rate has been derived for plane strain and plane stress conditions. The FEM analysis for 3D crack growth can be compared to the theoretical 2D analysis. In this paper, we first make a CCP (Center Crack Plane) model, and performed a 3-dimensional Finite Element Analysis (3D-FEA) to evaluate the crack-tip stain rate paying attention to the element mesh size and to the loading history. After optimization these parameters, the calculated crack-tip strain distribution, including its logarithmic singularity, was founded to agree well with the theoretical distribution. The significance of the crack-tip strain rate upon the crack-tip strain distribution and crack growth rate was demonstrated. The specimen size effects on crack growth rates were discussed from this point of crack-tip strain distribution. Finally, we focused on the importance of crack-tip strain rate as a unique mechanical parameter that controls the crack growth rate.