Oh-Yang Kwon

Microstructure and bonding strength of WC reinforced Ni-base alloy brazed composite coating

Abstract A tungsten carbide (WC) reinforced NiCrBSi(Co) composite coating was produced on mild steel by high temperature vacuum brazing. The microstructure, phases present and the interface diffusion have been investigated. The cobalt coated on the surface of WC particles dissolved into the liquid nickel-base alloy but the WC was not decomposed severely during the vacuum brazing process. Microanalysis showed that the composite coating matrix was composed of γ-Ni solution and γ-Ni+(Ni,Co) 3 B lamellar eutectic. Diffusion and metallurgical reaction occurred at the WC/NiCrBSi(Co) interface and composite coating/substrate interface. A 6-μm thick brazing seam or interfacial bonding zone was found between the coating and the substrate as the coated sample cooled down. Bonding strengths of the WC/NiCrBSi(Co) interface and the composite coating/substrate interface reached 100–140 MPa and 300–360 MPa, respectively, which are far higher than those of thermal spray coating. The abrasive wear resistance of brazed composite coating also appears to be better than that of the flame overlaid at the same composition.

Comparison of nondestructive microfailure evaluation of fiber-optic Bragg grating and acoustic emission piezoelectric sensors using fragmentation test

Nondestructive evaluation of microfailure mechanisms in two-diameter SiC fibers/epoxy composites is investigated using a directly embedded fiber-optic sensor attached with an acoustic emission piezoelectric (AE-PZT) sensor. Interfacial shear strength by fragmentation test, and optical failure observation inside microcomposite can contribute to analyze two sensors quantitatively. Although fiber Bragg grating (FBG) sensor exhibits sudden wavelength shift due to plastic deformation by larger diameter SiC fiber breakage, AE-PZT monitors much more precise microfailure process, such as the fiber break or matrix cracking. Since the FBG sensor can measure the strain at only a single point, whether it can detect a fiber break in single-fiber composite specimen depends on its proximity to the failure location. In addition, micro-strain measurement at one single point may not provide enough information on the whole microfailure process including multiple fiber breakage and matrix crack. It can be considered that FBG sensor can be somewhat effective in measuring the continuous micro-strain change due to the internal disturbance such as resin curing, whereas AE-PZT sensor can be effective in detecting the microfailure by elastic wave propagation through the composite materials.

Reverse engineering of compound surfaces using boundary detection method

This paper proposes an efficient reverse engineering technique for compound surfaces using a boundary detection method. This approach consists in extracting geometric edge information using a vision system, which can be used in order to drastically reduce geometric errors in the vicinity of compound surface boundaries. Through the image-processing technique and the interpolation process, boundaries are reconstructed by either analytic curves (e. g. circle, ellipse, line) or parametric curves (B-spline curve). In other regions, except boundaries, geometric data are acquired on CMM as points inspected using a touch type probe, and then they are interpolated on several surfaces using a B-spline skinning method. Finally, the boundary edge and the skinned surfaces are combined to reconstruct the final compound surface. Through simulations and experimental works, the effectiveness of the proposed method is confirmed.

Dynamic Characteristics of a Damaged Plate

It is very important to well understand the dynamic characteristics of damaged structures to successfully develop or to choose a most appropriate structural damage identification method (SDIM) as the means of non-destructive testing. In this paper, the dynamic equation of motion for damaged plates is derived by introducing a damage distribution function, which may characterize the effective state of structural damages. It is found that structural damages may induce the coupling between modal coordinates. The effects of damages on the vibration characteristics of a plate depending on their locations, sizes, and magnitudes are numerically investigated in a systematic way. The numerical investigations are also given to the effects of damage-induced modal coupling on the changes in vibration characteristics and to the minimum number of natural modes required to predict sufficiently accurate vibration characteristics of damaged plates.

Acoustic emission monitoring of short fatigue cracks

The behavior of short fatigue cracks of about 100 μm in length has been monitored by using acoustic emission (AE). By introducing wavelet de-noising technique, AE signals due to short fatigue cracks appeared to be separable from those due to various sources of background noise. With its extremely high sensitivity and the real-time capability, AE can be a unique means to detect the onset of fatigue damage, which is known to be otherwise impossible or extremely difficult to be accomplished.