Chitoshi Masuda

Variation of fractographic appearance for different microstructures in welded joints having the same fatigue crack propagation properties

Abstract Fatigue fracture surfaces were examined with a scanning electron microscope to investigate the influence of the different microstructure between weld metal and heat affected zone. The specimens were centre-cracked type transverse butt welded joints. The relationship between macroscopic fatigue crack propagation rate and the stress intensity factor range is the same in spite of the difference in microstructure for both materials. It is shown that the fractographic appearance changes with microstructure even in the very low growth rate region near fatigue threshold. This suggests that fractographic appearance is not necessarily a guide to the rate of fatigue crack growth.

Fatigue crack propagation mechanism for SiC whisker or SiC particle reinforced aluminum matrix composites

Fatigue crack propagation properties and fracture mechanisms were examined for three commercially fabricated aluminum matrix composites containing SiC whiskers (SiCw) and SiC particles (SiCp) under rotating bending condition. Fatigue crack propagation rates for SiCw/A2024 and SiCp/A356 composites were lower than those for unreinforced alloys at a given stress intensity factor near fatigue threshold, while the fatigue thresholds for those composites were higher than those for matrix alloys. For SiCp/A357 composite the fatigue crack propogation rates were higher than those for SiCp/A356 composite. Fractography revealed that the fatigue crack would propagate to the whisker/matrix interface following the formation of dimple patterns in the whisker rich zones or formation of striation patterns in the whisker poor zones for SiCw/A2024 composites, while for SiCp/A356 and SiCp/A357 composites the fatigue crack would propagate in the matrix near fatigue threshold. The near final failure crack would be linked to th...

Noncontact ultrasonic imaging of subsurface defects using a laser-ultrasonic technique

Noncontact ultrasonic imaging of subsurface defects using a laser-ultrasonic technique was demonstrated. Pulsed laser irradiation was used for ultrasonic generation, and optical heterodyne interferometry was used for detection of ultrasonic vibration on the specimen surface. Bonded thin plates with character-shaped hallmarks with 1.5 mm width on bonding layer were used as specimens. Imagings in ultrasonic transmission and reflection modes were performed, and imagings using not only XY-scanning of specimens but also a laser beam scanner were demonstrated. Ultrasonic images of hallmarks located 0.2 mm below the specimen surface were obtained through the experiments.

Development of Three-Dimensional Ultrasonic Simulation and Its Application.

A computer simulation technique for 3-dimensional ultrasonic waves was developed for visualization and investigation of ultrasonic propagation in isotropic and anisotropic solids with defects. The technique is similar to a finite-difference method (FDM) and a mass-particle model method, but uses a new nodal calculation method based on fundamental consideration of an elastic wave equation. The new method simplifies calculations on boundary nodes, so that it requires no pseudonodes as in the FDM, and is applicable to not only anisotropic but also complex materials including defects. A program has been developed for 3-dimensional calculation, but it requires the huge memory and high speed of supercomputers. Due to the limitation in computer performance, we present here conventional applications under the 2-dimensional condition including 3-dimensional motion. Some simple applications are shown for ultrasonic propagation in anisotropic (hexagonal) materials and layered materials, and wave scattering at a crack.