Seok Hwan Ahn

BENDING STRENGTH OF Si3N4 MONOLITHIC AND Si3N4/SiC COMPOSITE CERAMICS AND ELASTIC WAVE CHARACTERISTICS BY WAVELET ANALYSIS

This paper describes the investigation of bending strength and elastic wave signal characteristics of Si3N4 monolithic and Si3N4/SiC composite ceramics with crack healing ability. The elastic wave signals, generated during the compression load by a Vickers indenter on the brittle materials, were recorded in real time, and the AE signals were analyzed by the time-frequency analysis method. The three-point bending test was performed on the Si3N4 monolithic and Si3N4/SiC composite ceramic specimens with/without crack-healed. Consequently the bending strength of the crack-healed specimens at 1300°C was completely recovered up to that of the smooth specimens. And the frequency properties of crack-healed specimens tended to be similar to the distribution of the dominant smooth specimens frequency. This study suggests that the results of the signal information for the anisotropic ceramics show a feasible technique to guarantee structural integrity of a ceramic component.

Cracked-Healing and Bending Strength of Si3N4Ceramics

Crack-healing behavior of Si3N4 composite ceramics has been studied as functions of heat-treatment temperature and amount of additive SiO2 colloidal. Results showed that optimum amount of additive SiO2 colloidal and coating of SiO2 colloidal on crack could significantly increase the bending strength. The heat-treatment temperature has a profound influence on the extent of crack healing and the degree of strength recovery. The optimum heat-treatment temperature depends on the amount of additive SiO2 colloidal. Crack healing strength was far the better cracked specimen with SiO2 colloidal coating on crack surface. After heat treatment at the temperature 1,273 K in air, the crack morphology almost entirely disappeared by scanning prob microscope. At optimum healing temperature 1,273 K, the bending strength with additive SiO2 colloidal 0.0 wt.% without SiO2 colloidal coating recovered to the value of the smooth specimens at room temperature for the investigated crack sizes 100 ㎛. But that with SiO2 colloidal coating increase up to 140 %. The amount of optimum additive SiO2 colloidal was 1.3 wt.% and crack healed bending strength with SiO2 colloidal coating increase up to 160 % to smooth specimen of additive SiO2 colloidal 0.0 wt.%. Crack closure and rebonding of the crack due to oxidation of cracked surfaces were suggested as a dominant healing mechanism operating in Si3N4 composite ceramics. 1. 서 론

Crack opening behavior of penetrated crack under fatigue load

The leak-before-break (LBB) behaviors of a structural component under high and low fatigue loads are an important problem in nuclear power plants, liquid nitrogen gas tankers and chemical plants. This paper is an experimental study to evaluate the crack opening behavior after penetration for plate and pipe specimens. Crack opening displacement after penetration under low fatigue load could be satisfactorily determined at the center of the plate thickness regardless of the specimen size. In the case of high fatigue load, it is shown that the crack opening displacement at the center of a penetrated crack can be derived using the gross stress,σC, and the front surface crack length, as, together with the back surface crack length, ab.

Fracture Behaviors and Acoustic Emission Characteristics of Pipes with Local Wall Thinning

Abstract. Fracture behaviors of pipes with local wall thinning are very important for the integrity of power plant piping system. The locally wall thinned phenomenon of pipes is simulated as metal loss due to erosion/corrosion (E/C). Acoustic emission (AE) has been widely used in various fields because of its extreme sensitivity, dynamic detection ability and location of growing defects. In this study, monotonic bending tests without internal pressure are conducted on 1.91 inch diameter fullscale carbon steel pipe specimens. Locally wall thinned shapes were machined to be different in size along the circumferential direction of straight pipes. We investigated failure modes of locally wall thinned pipes and evaluated AE signals by AE counts, accumulative counts and time-frequency analysis during the bending test.

The Bending Strength of Brittle Materials and the Characteristics of the Elastic Wave Signal by Vickers Indentation

Four kinds of brittle materials were used to evaluate the bending strength under three-point bending and the characteristics of the elastic wave signal by Vickers indentation. The bending test was carried out under room temperature and high temperature. A crack was made at the tension side of the specimen with a Vickers indenter to investigate fracture strength. Fracture wave detector was used to evaluate characteristics of waveform and frequency of the elastic wave signal.

NON-DESTRUCTIVE EVALUATION AND STRENGTH CHARACTERISTICS OF CFRP WITH HEAT DAMAGE

In this study, the heat-damage process of a carbon fiber reinforced plastic (CFRP) under monotonic tensile loading was characterized by acoustic emission. Additionally, epoxy specimens and prepreg specimens were used to determine the characteristics of acoustic emission (AE) signals of epoxy and fiber, respectively. The AE characteristics of CFRP showed three types of distinct frequency regions. Time-frequency analysis methods were employed for the analysis of fracture mechanisms in CFRP such as matrix cracking, debonding and fiber fracture. To evaluate the cumulative counts of AE signals, it seems that the results can be applied usefully to guarantee structural integrity and/or to the survey of destruction of the structure with heat-damage, that was made to the composite materials.

Elastic Wave Signal Characteristics of SiC Ceramics with Crack Healing Ability by Wavelet Analysis

This paper reports for signal characteristics of before-and-after healing treatment SiC ceramics with crack healing ability. The elastic wave signals generated during the compress load by a Vickers indenter on the brittle materials were recorded in real time, and the waveforms of the individual signals were examined and classified based on their spectral characteristics. The compress loads were applied with the range from 9.8N to 294N. In a bulk SiC specimen, the AE signals occurred only when the load was compressive loading and unloading. But, in the after crack healing specimen of 294N only, even though the external compressive load was stopped and kept on holding constant load states, the AE signals occurred irregularly and continuously. The results of the WT and frequency analysis showed that these existed as the property of frequency in the limited range between 100kHz and about 200kHz. Three-point bending test was performed for the cracked and healed SiC specimens. Consequently the bending strength of the crack healed specimens was recovered completely, but most of the samples with the crack healed showed that the properties of the dominant frequency were comparatively lower than that of the bulk SiC samples. The classification of the wave signals can be used to develop algorithms for autonomous health monitoring systems of brittle material structures.

Nondestructive Evaluation in Materials Using Time-Frequency Analysis Methods

Application of signal processing techniques to nondestructive evaluation (NDE) in general has become a standard tool in determining the frequency characteristics of the signals and relating these characteristics to the integrity of the structure under consideration. The joint time-frequency analysis techniques are applied to analyze ultrasonic signals in degraded austenite stainless 316 steel, to study the evaluation of damage in this material. It is demonstrated that the nonstationary characteristics of ultrasonic signals could be analyzed effectively by these methods. WVD was found to be more effective for analyzing the attenuation and frequency characteristics of degraded materials through ultrasonic. It is indicated that the time-frequency analysis methods should also be useful in evaluating various damages and defects in structural members.

A Study on Experimental Evaluation for Fracture Behavior of Carbon Steel Pipes for High Pressure Service with Local Wall Thinning

The locally wall thinned phenomenon of pipes is simulated as metal loss due to erosion/corrosion. Therefore, fracture behaviors of pipes with local wall thinning are very important for the integrity of nuclear power plant. In this study, monotonic bending tests without internal pressure are conducted on 1.91-inch diameter Schedule 80 STS370 full-scale carbon steel pipe specimens. We investigated fracture strengths and failure modes of locally wall thinned pipes that welded and unwelded by four point bending test. From test results, we could be divided three types of failure modes.

Characteristics of Strength and Deformation of Inner/Outer Local Wall Thinned Straight Pipes

The experimental and the analysis for effects of circumferentially local wall thinning on the fracture behavior of pipes were carried out. Local wall thinning for experimental was machined on the outside of pipes in order to simulate the metal loss due to erosion/corrosion. In addition, the pipes with local wall thinning on the inside were carried out FE analysis, and it made a comparative study with that of outside. Failure mode could be classified into ovalization, local buckling and crack initiation depending on the thinned length and thinned ratio. Three-dimensional elasto-plastic analyses were able to accurately simulate fracture behaviors of inner/outer local wall thinning. Therefore the fracture behavior of the inner local wall thinning can be estimated with the outer local wall thinning.