Hisashi Izumi

Fatigue Strength of SiC Whiskers Reinforced Aluminum Composite

Fatigue strength of aluminum reinforced by SiC whiskers was studied under rotating bending moment at elevated temperature in air . The tests on five kinds of whisker volume fractions (0, 5, 10, 15 and 20) were carried out. The results obtained were summarized as follows;(1) The tensile strength of the composite material becames higher as the volume fraction of whiskers increases. And the tensile strength decreases with increasing the test temperature.(2) The fatigue strength at room temperature increases proportionally with the increase of the volume fraction of whiskers. The fatigue strength at elevated temperatures follows the relatonship with the volume fraction as well as the tensile strength.(3) The typical striation on the fatigue fracture surface becomes obscure and the surface becomes rougher as SiC volume fraction increase.

Effect of Cold and Hot-Working on Stress Corrosion Cracking Behaviour of 18-8 Stainless Steel

An experiment was made to examine the effect of cold and hot-working on stress corrosion cracking behaviour of 18-8 Cr-Ni stainless steel. After drawing, a wire specimen of 1mm in diameter was heated at 1050°C and then cooled in water. The heat treated wire was cold-worked from 10% to 35% in elongation at room temperature, using the Amsler universal machine, and hot-workings were also done at 400°C, 650°C and 750°C for the heat treated specimen. To investigate the effect of carbide precipitation upon the stress corrosion cracking, cold worked specimens were heated for 50hr at 400°C, 650°C and 750°C.A hot concentrated aqueous magnesium chloride solution (42wt% MgCl2) was employed as a corrosive medium, and tensile stresses were imposed on the specimen by a direct weight. The time of fracture was measured, making clock stop by a falling weight when the specimen fractured.For all the specimens, with or without working, the increase of applied stress caused a decrease in time to fracture, tf. When the log tf was plotted as a function of the stress imposed on the specimen, a linear relationship was obtained. This straight line had a knee at which the slope of log tf-stress curve changed. It seems that the two parts of this curve corresponded approximately to crackings initiated before and after film breakdown. The location of the knee was almost unchanged by cold or hot-working and also by carbide precipitation, even though the mechanical properties of specimen changed significantly.Resistance against the stress corrosion cracking of the specimen decreased by cold-working when the applied stress was below the knee, while it slightly increased by cold-working when the applied stress was higher than the stress corresponding to the knee. It was true as well with the hot-worked specimens.It appeares that carbide precipitation did not reveal any appreciable effect on the stress corrosion cracking behaviour of the specimen.Microscopic examinations confirmed that the cracks propagate across the individual grain and not intergranular crackings.