Junichi Katsuta

Image Based Displacement Vector Estimation Method around the Fatigue Crack Tip

To clarify the instantaneous strain field during the fatigue crack propagation test, an attempt is made to estimate the displacement vector of the small area of the crack tip from the high speed movie. The key theory of the estimation is Particle Image Velocimetry (PIV) which is frequently use in the fluid dynamical data analysis. PIV means the area based correlation matching between the sequential image pair. At first the earlier image divided into the small subareas, and on the later image most correspondent position to the subarea is searched. The each displacement of the subarea position between earlier and later image means the local deformation of the crack tip. Through the numerical simulation, the subarea size and search area size are defined to get the realistic result. To make the instantaneous deformation within a cycle of the fatigue crack propagation test, high-speed camera with high-power camera is used. The frame rate is beyond 1000Hz and shutter speed is 1/8000 second. By using the such camera and optimized PIV algorithm, the instantaneous strain field can be estimated and this will analyze the phenomenon of fatigue crack propagation.

Evaluation of propagation considering elasto-plastic behavior at fatigue crack tip

Publisher Summary The chapter discusses the evaluation of propagation considering the elasto-plastic behavior on a fatigue crack tip. It emphasizes on the generation of the tensile and compressive plastic zones near the crack tip during fatigue propagation. The relationships between generation and acceleration and deceleration and crack stop phenomena are investigated respectively. The role of each zone of the hysteresis loop denoting the relationship between loads and strains near the crack tip on fatigue propagation is studied by the fatigue crack propagation tests and the Kth test. The chapter clarifies that the elasto-plastic behavior at the crack tip has a great influence on the fatigue crack propagation because the crack closure phenomenon during the cyclic loading is confirmed. The chapter aims to consider the hysteresis loop at the crack tip in detail and clarify the physical role of each zone of the loop.

Detection and Evaluation of Infinitesimal Fatigue Crack in the Welded Joints

Fatigue cracks of more than several hundred millimeters in length are found and repaired upon maintenance, which is now executed by visual check. These fatigue cracks are detected in and through a secondary phenomenon, as local corrosion, water leakage, and abnormal vibration. The presence of fatigue cracks, however, cannot be directly detected.This paper proposes a method to detect fatigue cracks from the early phase of infinitesimal fatigue cracks, in which present visual maintenance judges that there is no fatigue damage. A crack size is estimated according to results of crack detection and by FEA. The judgment of fracture possibility and the method to estimate residual strength and remaining fatigue life are considered in accordance with the crack size. The results obtained from this study are as follows:1) Lowering rate of strain, calculated as the ratio of strain after crack initiation to strain at the time of no damage, is proposed as a monitoring parameter.2) Monitoring lowering rate of strain from the time of no damage allows a fatigue crack in and after the phase of the infinitesimal crack to be detected.3) In cases of the changes in cyclic loading, cracks can be detected by lowering rate of strain concentration.