|NDT.net May 2005 Vol. 10 No.5|
Non-contact damage monitoring by laser AE techniqueM. Enoki, S. Nishinoiria
Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
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© SPIE - The International Society of Optical Engineers.
ABSTRACTAE method is a well-known technique for in-situ monitoring of fracture behavior by attaching piezoelectric transducer. However, conventional AE transducer cannot be used at elevated temperature or severe environment. Laser based ultrasonic (LBU) technique has been developed to characterize materials properties and detect flaws in materials. We developed the AE measurement system with laser interferometer to apply this technique to microfracture evaluation in various materials. AE during sintering of alumina ceramics and thermal spying of alumina powder on steel substrates were successfully measured by laser interferometers. The effect of processing parameters on AE behavior was clearly observed by analyzing AE waveforms. One of the most advantages of this laser AE technique is to estimate the temperature where microcracks are generated. These results could give a feed back to control processing conditions in order to avoid damage in materials. It was concluded that the laser AE method was very useful to detect microcracks in ceramics during fabrication.
Keywords: Acoustic emission, laser interferometer, microfracture, ceramics, coatings
1. INTRODUCTIONRecently, ceramics is extensively applied to structural materials because of its strength, abrasion resistance, chemical stability and so on, and ceramics components become larger and more complex. Ceramic components are produced by heating and densifing green compact at elevated temperature. These components are sometimes easily fractured during sintering due to large size and complex shape of ceramic components, so it becomes a problem that this fracture interrupts manufacturing process. It is important to control the crack initiation and propagation in green compact during sintering to resolve this problem. Furthermore, it is necessary to understand the evolution of mechanical properties during sintering to control the crack behavior [1, 2].
As the operation temperature of gas turbines has been rapidly increased to achieve high efficiency in recent years, materials are required improve their stability and mechanical properties at elevated temperature above 1773 K. Thermal barrier coating (TBC) with ceramic coating layer has been developed to shield heat from the outside and increase thermal stability of the surface . As these coatings are subjected to both external stress and internal thermal stress constantly or periodically, the evaluation of mechanical properties and failure process of TBC at elevated temperature is desired to ensure the integrity. A health monitoring of structural components in service is important to assess the integrity of TBC, and a process control is also very important to provide uniform coating thickness and maximize the coating properties, as well as an estimation of the mechanical properties of coatings such as thermal shock resistance,thermal fatigue resistance and creep resistance .
Acoustic emission (AE) technique is a promising tool for reliability assessment of materials because it can monitor the generation and growth of microcrack in real time. However, conventional contact AE technique has a limit in application at elevated temperature, because a conventional piezoelectric transducer cannot be used above about 800 K. We have investigated the non-contact AE measurement technique using laser interferometer as a AE sensor [4-11]. This laser AE technique has several advantages such as non-contact measurement, absolute velocity measurement of AE signals, and applicability for severe environment. The purpose of this study is to investigate the influence of processing parameters on the generation and growth process of defects during fabrication process of ceramics by means of an in-situ monitoring system based on laser AE technique.
2.1. In-situ monitoring during plasma spray coatings
Some dead time for AE measurement was required to adjust the focus of laser beam on the measuring surface. Total dead time before the start of an acquisition was about 1 min. Low noise type demodulator (AT-3600S, Graphtec Corp.) was used to measure an out-of-plane surface velocity on a sample with range of 1 mm/s/V. In order to reduce noise level, output signals were filtered with high pass filter (HPF) of 50 Hz and low pass filter (LPF) of 200 kHz. Detected AE waveforms were recorded by AE analyzer (DCM-140, JT-Toshi Corp.).
2.2. In-situ monitoring during sintering of alumina
3. RESULTS AND DISCUSSION
3.1. AE behavior during plasma spray coatings
Figure 4 shows the relationship between the amplitude and generation temperature of AE for different traverse speed of gun, where dBC was 70 µm, Tp was 773 K and dTC was 1 mm. AE generation temperature TAE in the sample of 0.1 m/s was higher compared with that in the sample of 0.2 m/s. The number of AE events in the case of 0.2 m/s was lager than that of 0.1 m/s, and the temperature range during AE generation of 0.2 m/s sample was wider than that of 0.1 m/s. Many interlamellar microcracks were observed in top coat of coating samples, and delamination was found near the interface between top coat and bond coat. It is difficult to quantitatively characterize the damage induced sample during coating. However, non-contact measurement of AE behavior enables to estimate the damage during coating process.
AE signals detected during cooling process could be classified into two types by the peak frequency, that is, Type-A and Type-B, respectively. Type-A signals have a peak around 75 kHz, on the other hand Type-B signals have several characteristic peaks in 100-200 kHz. Type-A signals were especially detected before the final delamination of specimens, and Type-B signals were broadly observed during cooling process.
3.2. AE behavior during sintering of alumina
Single-channel inverse analysis was applied to estimate the microcrack size using deconvolution method. In this study, the breaking of a pencil lead was used as the simulated AE signal source. With assumption of mode-I type penny-shaped cracking, the Green's function of the media was calculated from the simulated AE signal by the response waveform detected at the epicenter of the media and converted numerically to a dipole from the monopole source. The crack radius, a, was estimated as follows :
where D0 is the first peak of source function, s0 is the strength for microfracture and n is the Poisson's ratio, respectively. Crack radius was estimated by this equation. Properties used in calculation were that of fully-sintered alumina ( n= 0.23, s0 = 340 MPa).
Figure 7 shows the relation between generation temperatures of AE signals and crack radii estimated by the inverse analysis of AE. As shown in Fig. 6(a), generation temperature of the first AE of notched sample was higher than that of smooth one. In the case of the notched sample, no AE signal was detected after generation of the microcrack radius of over 1 mm, while some AE events were detected in the case of the smooth sample. Smooth samples with holding time of 120 min as shown in Fig. 7(b) and (c) showed same behavior. However, larger number of AE events than in case of 30 min, especially relatively smaller microcracks were detected. Most frequent crack size determined from Fig. 7(b) and (c) was around 250 and 300 µm, respectively. In case of the sample in Fig. 7(c), no surface crack was observed after firing while other samples in Fig. 7 cracked. Figure 8 shows the relation between crack generation time and crack radii. It could be seen that microcracks with radius of 150-1500 µm and generation time of 1.5-5.5 µs were generated during firing and larger crack had relatively longer generation time.
4. CONCLUSIONSWe developed a non-contact in-process monitoring system for ceramics and coatings with laser AE technique, and applied this system to the detection of microfracture during fabrication process. Conclusions of this study are as follows: