Effect of Crack Orientation in Aluminium Plate using Tone Burst Eddy Current Thermography

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1 ISSN (ONLINE): , ISSN (PRINT): Volume-6, Issue-2, March-April 216 International Journal of Engineering and Management Research Page Number: Effect of Crack Orientation in Aluminium Plate using Tone Burst Eddy Current Thermography Rajeev VR 1, Ramjith Krishnan R 2 1 Assistant Professor, Department of Mechanical Engineering, Archana College of Engineering Alappuzha, INDIA 2 Assistant Professor, Department of Mechanical Engineering, Vidya Academy of Science and Technology Technical Campus, Trivandrum, INDIA ABSTRACT If determining the size and orientation of cracks, safe life of structural members can be evaluated. Mainly aircraft industries use Tone Burst Eddy Current Thermography (TBET) technique was used for the evaluation of crack damage in Aluminium plate or pipe like structures. This paper mainly describes about how the crack parameters mainly orientation affects distribution of temp produced by TBET heating. Parameter considered in this study is effect of crack orientation. 3D simulation results have been achieved by using COMSOL multi-physics with AC/DC module and general heat transfer. In TBET Technique at crack edges, induced current is seen concentrated thus indicating a localized high heating in those areas relative to other regions. It was determined that this technique has some advantages for the inspection aircraft structural components compared to other modalities, particularly in cracked regions. Keywords TBET, COMSOL, Aluminium Plate I. INTRODUCTION TBET is an evolving non-contact, nondestructive evaluation method with applications especially in aircraft industries for surface and subsurface crack detection under paints [3]. TBET technique combines both Eddy Current Testing and Thermographic non-destructive evaluation techniques to provide efficient method for defect detection and characterization over a relatively wide area in very short time. Eddy currents are developed on them conducting material by electromagnetic induction and generation of heat by Joule heating. Defect detection is based on the changes of induced eddy current flow and IR camera is used for detecting and characterizing surface and sub-surfaces defects. Heat diffusion into the material, when perturbed by the presence of a subsurface defect, causes a local temperature contrast (between the defective and the non-defective regions) on the measuring surface. This temperature contrast is detected by the thermal imaging equipment. Recorded images over several time frames are processed for extracting the internal flaw information [3]. The main difference between eddy current thermography and TBET is the type of signal that is applied to the coil. In TBET a burst of signal having high frequency is applied to the coil for short period of time which for a few microseconds for conductors and a few seconds for semiconductors. The time for which the signal is applied to the coil is called excitation time and the time between two successive signals is called observation time. 31 Copyright 216. Vandana Publications. All Rights Reserved. II. SIMULATION MODEL The simulations were performed on an aluminium plate of 22mm 15mm 2mm. A circular coil made of copper with 2mm diameter is used for induction. The liftoff of the coil from the plate was kept at 2mm. It is placed parallel to the specimen (X-direction). A peak current of nearly 1A was passed through the coil and the temperature history of the surface was monitored. The simulation was done with various excitation frequencies ranging from 25 to 15Hz and the optimum frequency is taken as 12Hz.[2] The Simulation of TBET technique requires a multi-physics electro-thermal approach involving, a) An electromagnetic model for the eddy-current generation, b) An electro-thermal model for the conversion of eddycurrents into heat, and c) The heat transfer model for the heat conduction from the heating surface into the material [3].

2 ISSN (ONLINE): , ISSN (PRINT): A 3D Finite Element Modelling (FEM) approach has been selected so that the model can be later extended to more complex and arbitrary configurations. All models were developed using the Multiphysics COMSOL package version3.4. Figure 1: 2 D view of the model Table 1; Material properties used for the simulation Material Property Air Aluminium Copper Relative permeability, μr Electrical x x1 Conductivity, s(s/m) Thermal Conductivity,k (W/m. K) Density, r(kg/m 3 ) Specific Heat, C p (J/kg. K) Figure 2;3-D model of TBET system The Boundary conditions used for the electromagnetic induction were 1) Axial symmetry at r= 2) Magnetic insulation at the air boundaries (Aj=) 3) Continuity of magnetic fields at the interior boundaries And for the heat transfer, 1) Axial symmetry at r= 2) Temperature boundary condition at the air boundaries (T=Ta =3 K) 3) Heat flux at the other boundaries. The loads applied where 1) Excitation frequency 2) External current density 3) Excitation time and observation time Table 2; Constants used for the simulation Constants Air Aluminium Copper Convective Coefficient,h (W/m 2. K) Emissivity, e Ambient Temperature(K) III. FINITE ELEMENT BASED TBET MODEL A 3-D model of TBET system is made on COMSOL multiphysics software. The model consists of an aluminum plate having 21mm 15mm 2mm size and a copper coil having 2mm diameter is placed at a lift-off distance of 2mm above the plate. This whole set up is enclosed in air having 27mm 18mm 5mm size. Figure 2 shows the model of the test environment made [2]. Figure 3: Model after applying loads 32 Copyright 216. Vandana Publications. All Rights Reserved.

3 ISSN (ONLINE): , ISSN (PRINT): Fig 4: Observation point IV. EFFECT OF CRACK ORIENTATION IN TBET METHOD (SURFACE-CRACK) A study is conducted to find the effect of crack orientation is TBET method. Cracks at various orientations to the axis conductor were modelled on Comsol, mainly four types of cracks were modelled for my study they are 1. At to the 2. At 9 to the 3. At 45 to the 4. At 135 to the Figure 5: Crack orientations at 9 and at 135 to the Figure 7: Temperature profile obtained for 135 crack Figure 8: Temperature profile obtained for crack After changing the crack orientation the problem is solved to get the temperature rise at it is plotted at the observation point. From the graph it is clear that the maximum temperature rise is produced for a crack placed perpendicular to the conductor (9ᴼ). This is because maximum hindrance to the flow of eddy current occurs when the axis of the crack is perpendicular to the Figure 6: Crack orientations at 45 and at to the Figure 9: Eddy current distribution on a plate a) with crack b) without crack Table 3; Temperature rise at different crack orientation (surface crack) Crack orientation Temperature rise for 1ms heating 33 Copyright 216. Vandana Publications. All Rights Reserved.

4 ISSN (ONLINE): , ISSN (PRINT): No crack Figure 12: Temperature distributions on a plate without crack Figure 1: Effect of crack orientation V. EFFECT OF CRACK ORIENTATION IN TBET METHOD (SUB-SURFACE CRACK) To find the effect of sub-surface crack on burst eddy current thermography method cracks having dimensions 2mmx5mmx1mm placed at a distance of.5mm from the surface of the plate at different crack orientation where modelled. An excitation frequency of 12Hz is applied to the coil. The temperature distribution is studied for both 1ms excitation time. Figure 11: Effect of crack orientation Figure 13: Temperature distribution on a plate with crack After changing the crack orientation the problem is solved to get the temperature rise and it is plotted at the observation point. From the graph it is clear that the maximum temperature rise is produced for a crack placed perpendicular to the conductor (9 ). This is because maximum hindrance to the flow of eddy current occurs when the axis of the crack is perpendicular to the. From the surface temperature plot it was found that there is no much temperature variation on the surface of the plate at different crack orientations but in the actual case it can be sensed by a high sensitivity infra-red camera having high frame rate because small temperature variation can be sensed by an IR camera but it is difficult to stimulate it in a software. Table 4; Temperature rise at different crack orientation (sub-surface crack) Crack orientation Temperature rise for 1ms heating No crack VI. CONCLUSION 34 Copyright 216. Vandana Publications. All Rights Reserved.

5 ISSN (ONLINE): , ISSN (PRINT): Tone burst eddy current thermography, a relatively new NDT technique which combines the advantages of eddy current testing and thermography, is being demonstrated well numerically. NDT techniques are mainly used for defect detection. But in the present work, the new NDT technique has been successfully demonstrated in estimation of both surface and sub-surface crack in a aluminium plate. A 3D finite element model was developed for simulation in the electro-thermal environment of COMSOL MULTIPHYSICS 3.5 software with aluminium plate as the test specimen and surface and subsurface cracks where modelled at different orientation to the to find the effect of crack orientation in tone burst eddy current thermography method. In my study I found out that maximum temperature rise is attained for perpendicular crack (9 ) and minimum temperature rise for parallel crack ( ) this is because maximum hindrance to the flow of eddy current is produced when the crack is perpendicular to the coil axis. TBET system a high sensitivity IR camera with high frame rate is preferred because such camera can pick up small change in temperature at a very high frame rate less than 5ms. In my analysis I came to a conclusion that images obtained with excitation time less that 5ms gives a clear picture about the crack. It was also found out that there will be a optimum frequency at which maximum temperature rise occurs for every model which depend on the lift off distance and the properties of the test specimen [1]. So while designing a TBET system finding out the optimum frequency is very important. This work can be extended to get a qualitative information about the crack by analysing the temperature profile obtained from analysis using suitable image processing software and algorithms. [5] G.Y. Tian and A. Sophian, Pulsed eddy current sensor, Encyclopedia of Sensors, Vol. 8, 26, pp [6] Tsopelas N. and N.J. Siakavellas (21) Eddy current thermography in circular aluminium plates for the experimental verification of an electromagnetic thermal method for NDT, Nondestructive Testing and Evaluation, 25, [7] Shull, P. J. (22). Nondestructive Evaluation - Theory Techniques and Applications, Materials Evaluation 58(1): [8] Ryerson Bar-Cohen, Y. "Emerging NDE Technologies and Challenges at the beginning of the 3rd Milennium Part II. " Materials Evaluation 58(2): REFERENCES [1] 2D Finite Element Analysis of Crack in Aluminium Pipe Using Tone Burst Eddy Current Thermography, International Journal of Engineering Studies and Technical Approach (IJESTA)(ISSN No ) Volume 2, No. 1, January 216,pp,1-11. [2] 3D Finite Element Analysis of Crack in Aluminium Plate Using Tone Burst Eddy Current Thermography, International Journal of Multidisciplinary Approach and Studies ( ISSN NO:: X) Volume 3, No.1, Jan - Feb 216 pp [3] N. Biju, N. Ganesan, C.V. Krishnamurthy, Krishnan Balasubramaniam Frequency optimization for eddy current thermography, NDT&E International 42 (29) [4] Sundararaghavan V., K. Balasubramaniam, N. R. Babu, and N. Rajesh (25) A multi frequency eddy current inversion method for characterising conducting gradients on water jet peened components, NDT&E Int., 38, Copyright 216. Vandana Publications. All Rights Reserved.