Heat cycle test of ceramics-sprayed type 304 stainless steel was carried out. Surface strain was measured using the laser speckle strain/displacement gauge (SSDG) during the heat cycle test. Fatigue processes were discussed with result of both surface strain measurement and microscopic observation in the longitudinal section of the specimen. It was found that heat cycle fatigue life of LPPS-bond-coated specimen was longer than that of APPS-bond-coated specimen.
Fatigue crack growth under multiaxial loading is of importance in engineering practice. In order to examine the effect of stress biaxiality on the fatigue crack growth rate, torsional (biaxial) and uniaxial bending fatigue tests were carried out in this study by using a tough-pitch copper. The experimental results showed that in the case of torsional fatigue the compressive non-singular stress acting parallel to the direction of the initial cracked plane considerably accelerated the mode I crack growth rate in comparison with the result of the uniaxial bending fatigue test.
The fatigue crack growth (FCG) behavior under cyclic loading in TiAl intermetallic and Ti-6Al-4V alloy was investigated at elevated temperature. Crack opening displacement (COD) was measured by using the Laser speckle strain/displacement gage (SSDG) during the fatigue tests. It was found that the FCG rate in TiAl intermetallic at 800℃ was controlled by the effective stress intensity factor range ⊿K_, while the HCG rate in Ti-6Al-4V alloy above 500℃ was controlled by the maximum stress intensity factor K_ as well as ⊿K_. It was also found that thresholds ⊿K_ in TiAl intermetallic at 800℃ were higher than those in Ti-6Al-4V alloy above 500℃
Fatigue tests were conducted at both 893K and room temperature for ZrO2-8Y2O3-sprayed and Al2O3-sprayed type 304 stainless steel. Surface strain during the fatigue test was monitored to examine the delamination of a subsurface layer using a laser speckle strain gauge. Surface strain was found to decrease when subsurface delamination occurred. The number of cycles to delamination was decided by the surface strain behavior. It was found that delamination lives of a ZrO2-8Y2O3-sprayed specimen with an Atmospheric Plasma Sprayed (APS)-bond-coating and that with a thermal aged Low Pressure Plasma Sprayed (LPPS)-bond-coating were longer than those of a ZrO2-8Y2O3-sprayed specimen with a non-thermal aged LPPS-bond-coating and an Al2O3-sprayed specimen with an APS-bond-coating. The strength of an APS-bond-coating was so low that multiple cracks occurred, while the strength of a LPPS-bond-coating was so high that a single crack occurred near the fracture surface. In case of the specimen with an APS-bond-coating, the fatigue fracture life of an Al2O3-sprayed specimen was longer than that of a ZrO2-8Y2O3-sprayed specimen. The reason was that the crack initiated at a bond-coating penetrated into a substrate with no delamination between a bond-coating and a substrate for a ZrO2-8Y2O3-sprayed specimen. In case of the specimen with a LPPS-bond-coating, the fatigue fracture life of a specimen with a thermal aged bond-coating was longer than that with a non-thermal aged bond-coating and the fatigue lives of these specimens were longer than those with an APS-bond-coating. The improvement of the fatigue fracture life was caused by the restriction of a crack initiation into the substrate due to the LPPS-bond-coating with a relatively higher strength. It was concluded that a bond-coating with high strength and high adhesive strength was effective on the improvement of fatigue fracture lives of plasma sprayed materials.
Experimental investigations of sensitivities for defect detection in pipes by guided waves were described. An efficient method of transduction with a reflector was used and its sensitivity was evaluated in comparison to that of conventional transduction without a reflector. An efficient transduction could be realized by placing the reflector at an appropriate axial position, which was briefly described in the paper. The experiments were carried out using a 60.5-mm-outer-diameter and 4-mm-thick aluminum pipe and a piezoelectric ring-shaped sensor system. It was confirmed that the sensitivity in the case with the reflector was more than twofold that in the case without the reflector. The detection limits and sensitivities in a noisy environment were compared between the transductions with and without the reflector.
