Computer Simulation of Metal Surface Micro-crack Inspection Using Pulsed Laser Thermography

Transcription

1 Vol.37 (UCMA 26), pp Computer Simulation of Metal Surface Micro-crack Inspection Using Pulsed Laser Thermograph Tang Qingju, Bu Chiwu 2, Liu Yuanlin, Yu Fengun, Zhao Yawei 3 School of Mechanical Engineering, Heilongjiang Universit of Science and Technolog, Harbin, 522, P.R. China 2 College of Light Industr, Harbin Universit of Commerce, Harbin, 528, P.R.China 3 College of Engineering and Information Technolog, Universit of Chinese Academ of Sciences, Beijing, 49, P.R.China tangqingju@26.com Abstract. Surface micro cracks are eas to produce in the preparation and service process of metal material, which impacts on the safe operation of metal components. Pulsed laser spot ecitation and infrared thermal imaging technolog are combined to detect metal surface micro-cracks. The working principle of laser infrared thermal imaging detection technolog was described. The three dimensional heat conduction model of pulsed laser ecitation flu transfer in metal plate was established, and calculated using finite element method (FEM). The results showed that, thermal flow in the image is a D shape. There are temperature differences between the sound regions and defective regions, and the defects eperiences the process of obscure, graduall clear, and graduall obscure. Pulsed infrared thermograph sequence was processed b polnomial fitting method, and the coefficient images effectivel improve the contrast between defective and non-defective areas, which is beneficial to the determination and of recognition micro cracks. Kewords: Micro-crack; Thermograph; Pulsed laser; FEM; Polnomial fitting Introduction Surface micro cracks are eas to produce in the metal preparation process, due to the processing technique, heat treatment environment, etc. And in the installation and service process of metal parts, surface micro cracks are also eas to produce, due to a variet of factors such as temperature, chemical corrosion, load. Microcracks tend to epand and merge, forming a macroscopic crack, ma eventuall lead to the fracture of metal parts. Therefore, in order to ensure the safet and reliabilit of metal components, whether in the manufacturing or service period, it is necessar to monitor the metal status [-5]. ISSN: ASTL Copright 26 SERSC

2 Vol.37 (UCMA 26) 2 Presentation of the Heat Transfer Model In this section, the heat transfer model for crack defects of stainless steel detection is established. The geometric mesh model of the test specimens are shown in Fig.. Given (,, z) the Cartesian coordinates and (l, L, e) the length, width and thickness of the studied sample, as shown in Fig. 2, and under the assumption that transverse and longitudinal conductivities are uniform in the sample, the following sstem (balance equation, boundar and initial conditions) is obtained [6]. Fig.. Geometric mesh model of the studied samples L = 2 2 z c 2 z o e T T T T () z t The front face net heat pulse flu and the rear face heat flu are established with regard to the convection heat transfer. T (,, z, t) z ze Q hf Tam T (,,, t ) z T (,, z, t) z z hrt am T (,,, t ) z l (2.a) (2.b) Other boundaries are assumed to be insulated. T (,, z, t) T (,, z, t) l T (,, z, t) T (,, z, t) L (2.c) (2.d) The initial condition epresses the temperature distribution in the whole domain at the time t=. T(,, z, t) T(,, z,) T where,, = = t= am (3) are the thermal conductivities in the,, and z directions, Q z is the heat distribution of the laser beam, is the Dirac function, h is the convective heat transfer coefficient, and T is the ambient temperature. am The thermal phsical properties parameters of stainless steel specimens and air gap 48 Copright 26 SERSC

3 Vol.37 (UCMA 26) which simulate the crack defects are shown in Tab.. Table. Thermal phsical properties parameters Parameter Component Densit ρ [kg/m 3 ] Specific heat c [J/(kg )] Thermal Conductivit λ[w/(m )] Thermal diffusivit α 6 [m 2 /s] Stainless steel Air FEM Simulation 3. Temperature Distribution Caused b Laser Spot Source The heat transfer FEM simulations have been done for stainless steel specimen size of 3mm 3mm 3mm with crack size of length 3mm, depth mm and width.mm, laser spot to the crack distance of 5mm, laser spot radius of 5mm, and laser beam power of 2 W.Fig. 2 shows the normalized temperature distribution of specimen surface at some moments after the laser pulse ecitation. It can be seen that thermal flow in the image as a D shape, which is because of the heat blockage b the crack. It is found that there are temperature differences between the sound regions and defective regions, and the heat flu transverse diffusion occurs graduall with the proceeding of heat conduction, so the defects eperiences the process of obscure, graduall clear, and graduall obscure (a) (b) (c) (d) Copright 26 SERSC 49

4 Vol.37 (UCMA 26) (e) (f) Fig.2. The normalized temperature distribution of specimen surface: (a) t=.5s, (b) t=.5s, (c) t=.s, (d) t=.5s, (e) t=2.5s, and (f) t=3.s. 3.2 The Effect of Laser Beam power Fig. 3 shows the temperature rise at the laser spot center caused b laser beam power of W, 2 W, 3 W, 4 W, 5 W and 6 W. Temperature/ºC W 2W 3W 4W 5W 6W t/s Fig. 3. The temperature rise at the laser spot center caused b different laser beam power From Fig. 3, it can be seen that, the temperature rise more with the rise of laser beam power. It means that the cracks can be more easil detected when using big laser beam power. However, over heat ma lead to the ablation of the specimen surface. Usuall, when small laser beam power is used, in order to improve the signal to noise of defects, some processing algorithms can be used to the pulsed thermograph sequence. 4 Conclusion Laser infrared thermal imaging detection technolog is used to detect metal surface micro-cracks. The working principle of laser infrared thermal imaging detection technolog was described. The three dimensional heat conduction model of pulsed laser ecitation flu transfer in metal plate was established, and calculated using FEM. The results showed that, thermal flow in the image is a D shape, and there are temperature differences between the sound regions and defective regions, and the 5 Copright 26 SERSC

5 Vol.37 (UCMA 26) defects eperiences the process of obscure, graduall clear, and graduall obscure. Pulsed infrared thermograph sequence was processed b polnomial fitting method, and results show that the coefficient images effectivel improve the contrast between defective and non-defective areas, which is beneficial to the determination and of recognition micro cracks. Acknowledgments. This project is supported b Young Talent Fund Project of Heilongjiang Universit of Science and Technolog (Grant No. Q234), Youth Innovation Talent Training Program of Heilongjiang Province Regular Institutions of Higher Education Stud on CFRP laminate defects detection using infrared thermal wave nondestructive testing technolog under modulated laser eitation, and Harbin Special Funds for Scientific and Technological Innovation Talents (Grant No. 25RAQXJ69). References. An, Y.-K., Kim, J.H., Yim, H.J:. Lamb Wave Line Sensing for Crack Detection in a Welded Stiffener, J. Sensors. 4, (24) 2. Israr, A.: Model for Vibration of Crack Plates for use with Damage Detection Methodologies. Journal of Space Technolog., 7-25 (2) 3. Navarrete, M., Villagrán-Muniz, M., Ponce, L.: Photoacoustic detection of microcracks induced in BK7 glass b focused laser pulses, J. Optics & Lasers in Engineering. 4, 5- (23) 4. Lura, P., Weiss, J., Jensen, O.M.: Detection and Analsis of Microcracks In High- Performance Cementitious Materials. Advances in Construction Materials (26) 5. Fujinawa, Y., Noda, Y., Takahashi, K.: Field Detection of Microcracks to Define the Nucleation Stage of Earthquake Occurrence. International Journal of Geophsics. (23) 6. Charunetratsamee, S., Poopat, B., Jirarungsatean, C.: Feasibilit Stud of Acoustic Emission Monitoring of Hot Cracking in GTAW Weld, J. Ke Engineering Materials.545, (23) Copright 26 SERSC 5