Ichiro Maekawa

Damage induced in sic by a particle impact

Abstract Impact damage induced in silicon carbide ceramics was studied by using a SiC particle or a steel particle. Ring cracks, radial cracks and lateral cracks were observed on the specimen surface, and cone cracks were observed on the fracture surface. The outermost diameter of ring cracks increased with impact velocity; however, the innermost diameter was almost constant. The effect of temperature on these ring crack sizes could not be seen clearly. In order to evaluate the effect of the damage on the strength of a specimen, the residual strength was measured by four-point bending tests. The degradation in residual strength caused by an SiC particle impact was larger than the degradation in the case of steel particle impact, although larger ring cracks were observed on the surface in the case of steel particle impact compared with the case of SiC particle impact. For oblique impact, the degradation in residual strength of the specimen was large when it was impacted transversely with regard to specimen axis compared with longitudinal impact.

The influence of non-metallic inclusion on the fracture behaviors of 25Mn-5Cr-1Ni austenitic steel

Abstract The influence of inclusion on fatigue life, threshold stress intensity factor, elasticplastic fracture toughness and Charpy absorbed energy of high manganese steel were studied for the temperature range of 298 ~ 77 K. For this purpose, two series of steel, which contain different inclusion quantities, were used. Specimens are prepared to be S-L or S-T orientation with regard to the rolling direction. Experimental results showed that reduction in above strengths depend on both quantity and the orientation of inclusion. Crack growth behavior was discussed by taking into account the linkage of a main crack with a micro crack formed at the inclusion tip.

Damage in CFRP Laminates due to Repetitive Low-Energy Impacts.

A simple experimental method of repeated low-energy impact was devised to provide some insights into the impact damage behavior of CFRP laminates. The macroscopic failure mode and the internal damage as a consequence of repeated impacts were investigated, at some different levels of incident impact energy, with the laminated specimens prepared from different stacking sequences. The results indicated that there existed a critical incident impact energy which led to the delamination and the critical size of delamination which produced the degradation of residual strength. The delamination behavior in the laminated specimens depended largely upon the level of incident impact energy. The stiffer the specimen, the larger the delamination.

Improvement of Impact Fatigue Life of Sheet Valve.

Sheet valves used in a reciprocal compressor are subjected to a pulsation of impact force which k results in cracks forming at the tip during prolonged use, and sometimes chipping also occurs due to their connection with the cracks. In this study, the effect of geometrical improvement of the sheet valve on the impact fatigue life was investigated. The impact fatigue life was defined by the number of pulsations to crack initiation at the tip. Impact fatigue experiments were conducted using commercial steel sheet valves and three improved types of valves. A small upward bend in the valve tip increased the fatigue life considerably. An inclined cut at the valve tip did not result in a clear increase in fatigue life. On the other hand, scatterings in fatigue lives of improved valves were somewhat increased compared with that of commercial valves. Dynamic stress states are also investigated by FEM analysis.

Parametric representation of fatigue threshold and dislocation free zone

ABSTRACT Fatigue threshold stress intensity factor based on dislocation dynamics was formulated as a simple function of temperature and grain size. This theory corresponds well with experimental results for steel under not so high temperature, and was related to surface roughness effect on ΔKth. Furthermore representation of ΔKth by dislocation free zone was discussed.

The onset of crack growth in aluminium alloy due to impact tension.

The purpose of this paper is to confirm the effect of specimen length and the microstructure on the onset of precrack growth when an impact load is imposed on it. Impact, impact under static pretension and static tensile experiments were carried out using high strength aluminium alloy at 288 K and 77 K. The effect of specimen length on the critical condition for crack growth was discussed, based on the static stress intensity factor (K1)pre and the impact fracture stress σcd defined by the onset of crack growth. A linear relation between (K1)pre and Df which was a summation of (K1)pre and σcd√(πa), where a was a precrack length, could be seen and this relation was independent of specimen length and temperature. Microstructural aspects near the crack tip region were also observed on the fracture surfaces. The results of microscopic deformation measured by the X-ray micro beam diffraction method and the dislocation distribution observed by TEM showed that they were almost the same at the same temperature and were independent of loading type and specimen length. Comparing above results with our previous ones for steel specimens, it is concluded that the effect of specimen length on crack growth is negligiable in the case of impact fracture when the microscopic deformation behavior at a crack tip is insensitive to a rapid stress rise, which depends on specimen length.

Estimation of Corrosion Fatigue Lives Based on the Variations of the Crack Lengths Distributions During Stress Cycling

Many small distributed cracks have been observed on the specimen during corrosion fatigue process, and the damage of corrosion fatigue is related to the behaviour of these distributed cracks. The distribution of crack lengths during corrosion fatigue was approximated well by the three parameter Weibull distribution under plane-bending fatigue tests of carbon steel in salt water. A method of estimation of corrosion fatigue lives was proposed. The crack initiation, crack growth behaviour and the variation of the distribution of crack lengths during corrosion fatigue process were taken into consideration in the method. Comparing the estimated results with experimental results, it was concluded that this method could estimate the corrosion fatigue lives with good accuracy.