3D FEM SIMULATIONS OF LASER THERMOGRAPHY Mohammed ...

More Info at Open Access Database www.ndt.net/?id=15220

3D FEM SIMULATIONS OF LASER THERMOGRAPHY Mohammed Basheer Chalil1,a, Parag Ravindran2,b and Krishnan Balasubramaniam1,c 1

Center for Non Destructive Evaluation, Indian Institute of Technology Madras, India.

2

Department of Mechanical Engineering, Indian Institute of Technology Madras, India. a

[email protected],[email protected], [email protected]

Keywords: Laser Thermography, Surface Crack, Finite Element Simulation, COMSOL.

ABSTRACT Laser Thermography is a novel method for detecting surface breaking cracks in metals which uses a powerful laser beam. It produces a highly localized heating spot from which heat diffuses radially. A pulsed laser source is used for producing a heat spot on the sample near to a surface breaking crack. The crack that is perpendicular to the surface and close to this hot spot will delay the lateral heat flow and this disturbance is observed by an IR camera, which will reveal the crack. The hot spot is then scanned over a region to map the crack. This allows for a remote imaging of crack morphology, even in elevated temperatures. The present study involves a 3D finite element simulation using COMSOL as a tool to simulate the thermal flow from the laser heated spot in the proximity of a crack. The modeling helped us to understand the various parameters affecting the thermal images of laser heated spots which utilized to develop a strategy for extracting the details of the crack.

INTRODUCTION Conventional NDE thermography has been very successful in detecting in-plane defects such as coating adhesion failures, delamination or impact damage in carbon fibre composites and cracks parallel to its surface. In conventional method heat applied by a flash lamp, eddy current source etc, across the broad area of a sample surface and the defects will cause high resistivity to the flow of heat, which will reduce cooling rate. This produces a thermal contrast at the surface the can be recorded by an infrared camera to reveal the presence and extent of a sub-surface defect. But near surface cracks in metals, such as fatigue cracks, grow predominantly perpendicular to the surface, hence conventional method is not suitable as they do not block the flow of heat applied to the surface. Pulsed laser spot imaging is a technique that uses a high power focused pulsed laser spot to produce a highly localized heating spot from which heat diffuses radially. A crack developed perpendicular to the surface that is close to the heated spot will delay the lateral heat flow and the thermal image by IR camera will reveal the perturbation caused by the crack and this can be used to detect its presence [1, 5]. A 3D finite element simulation using COMSOL as a tool to simulate the thermal flow from the laser heated spot in

the proximity of a crack helped us develop an efficient image processing strategy for extracting the scanned cracks. COMSOL MODELING A 3D numerical heat transfer model has been developed that has the ability to deal properly with heat flow though air filled cracks with openings in the micrometer range. This is achieved by balancing thermal fluxes flowing into the crack and through the crack, with those flowing out of the crack. The interaction between a laser heat source and a crack was studied using the heat diffusion pattern that was developed in COMSOL for studying laser spot thermography [1-4].These patterns will guarantee correct thermal gradients in the bulk material either side of the crack, when it is meshed in fine size. The voids or crack needs very fine mesh spacing that is necessary to deal with real cracks that often have openings of only a few micrometers or at least 10 to 15 percentages less than that of the crack size. Modeling and analyzing in COMSOL is very easy compared with other heat transfer 3D models based on the finite difference method, and it serves computation time and memory.

a

b

c

d

Fig. 1 shows the heat distribution on a slab of 30*30*3mm3, after it is heated by laser. (without Crack) a

b

c

d

Fig.2 shows the heat distribution on the slab with a crack 1.5mm away from Laser spot center (Crack coincides with tangent of laser spot) a) Heat distribution after 5e-4s, b) Heat distribution after 0.005s, c) Heat distribution after 0.1s, d) Heat distribution after 0.25s The parameters that affect the thermal images or heat distribution pattern include: host material, crack opening, length depth and geometry; laser power and pulse duration; spot imaging time and spot distance from the crack. Firstly, a pulsed laser spot was chosen to excite the thermal transient because a bigger temperature difference will be produced with respect to surrounding and from diffuses radially. If the pulse is long, heat will start to dissipate in the sample whilst the laser is still on. This

phenomenon is more obvious in metal samples. Fig.1 shows the modeled surface temperature image of a laser beam incident on a stainless steel metal block obtained from this 3D model. These thermal images ware obtained when the block was irradiated by a 1.18 watt, 3.67ns laser pulse with a spot radius of 1.5mm at the right hand side of a crack. The laser spot center was 1.5mm away from the crack. A 10mm long through notch was embedded in the slab with opening was 1µm. Thermal flow in the image is shown in Fig.2as a ‘D’ shape rather than a round shape in specimen without crack because of the heat blockage by the crack. The difference in the heat flow for different crack length and position are compared. After 0.05s

After 0.1s

After 0.2s

After 0.3s

Fig.3 shows the heat distribution on slab at different time, when crack is 1 mm from Laser spot.


Fig.5 shows the heat distribution on slab at different time, when crack is 3 mm from Laser spot. After 0.05s

After 0.1s

After 0.2s

After 0.3s


The crack blockage effect can be quantified by the temperature difference or the gradient across the crack. Here we will use the temperature difference across the crack as a metric of the effect of a crack on the heating produced by a laser spot. The effect of changing the distance of the laser spot to the crack is shown in Figs. 2-6. It shows that the temperature difference across the crack reaches its largest value when the crack is in the very immediate vicinity of the heated spot. Maximum possible distance of the crack from the laser spot can be clearly understood from the plots 3-6, and is 3mm as in figure 5. If the distance of the crack is more than 3mm the probability of finding the crack is less. Hence we can say if the crack is in the immediate vicinity of the laser spot the crack can be easily identified from the contour itself, and the most probability is when it coincides with tangent laser spot circle. The effect on temperature distribution for different crack length, when crack is at 3mm from the Laser spot is also compared as shown in Figs. 7-11. After 0.1s

After 0.2s

After 0.25

After 0.4s

After 0.3s

Fig.7 shows the heat distribution on slab at different time, for a crack length 8mm and crack is 3 mm from Laser spot After 0.1s

After 0.2s

After 0.25

After 0.3s

After 0.4s

After 0.5s


After 0.3s

After 0.4s

After 0.6s

After 0.5s


After 0.95s

Fig.10

After 0.1s

Fig.11

After 0.2s

Fig.10 & 11 shows the heat distribution on slab at different time, for a crack length 1mm and crack is 3 mm from Laser spot in fig10 & at the radius of laser spot Based on 3D-FEM simulation results, the ‘optimum’ imaging distance of the laser spot center from the crack were found is 1.5mm. It is one radius of the spot size. The ‘optimum’ distance is where the temperature difference across the crack has the largest value. Thus the crack will block heat flow more obviously when the crack is in the vicinity of the spot, as crack causes more resistance to the heat flow. Fig.12 shows the temperature difference across the crack when the crack is inclined to both Z and Y axis[3,4]. The angles of the crack can be find out from the temperature changes at bottom surface as well as top surfaces from the heated spot, and is obtained by comparing the temperature distribution in slab with perpendicular crack at the same distance from the center of heated spot. At the position of 1.5 mm, which is the radius of the spot, it reaches the biggest value. After 0.5s

After 0.1s

After 0.2s

After 0.25s

Fig.12 shows the heat distribution on slab at different time, for a crack length 5mm and inclined at 450 with both Z & Y axis CONCLUSIONS The results presented in this paper are with respect to Nd: YAG laser power 1.18watt per pulse, with pulse width 3.67 ns. As long as the crack distance from the spot increases the observation increases, and 3.5mm is the maximum possible distance of detectability. Observation time required is also increases with increases in crack length. So for optimal results the spot should be in the immediate vicinity of the crack, it can be obtained by moving laser along the work piece while giving continuous laser pulses. The pulsed laser spot thermography technique has good crack detection sensitivity, especially cracks perpendicular to its surface, competitive with many of the established NDE techniques. The technique has the advantages of being non-contacting and of requiring no surface preparation. If the surface is polished one, we have to give very thin line coating of Graphite to reduce reflection, to increase the absorptivity of the laser radiation and emissivity of thermal radiations. Also it is known that for the technique to be successful, surfaces should be clean and free of deep scratches or indentations that would perturb heat flow in a similar manner to a crack. The paper also presents the results of an image processing method for extracting images of cracks after laser spot is irradiated.

REFERENCES [1] Crack Imaging By Pulsed Laser Spot Thermography, T. Li1, D. P. Almond1, D. A. S. Rees1, B. Weekes2,a UK Research Centre in NDE (RCNDE), Department of Mechanical Engineering University of Bath, Claverton Down, Bath, BA2 7AY, UK, b UK Research Centre in NDE (RCNDE), Imperial College, London, SW7 2AZ, UK [2] Crack Imaging By Scanning Laser Line Thermography, T. Li, D. P. Almond, and D. A. S. Rees Citation: AIP Conf. Proc. 1335, 407 (2011); doi: 10.1063/1.3591881 [3] COMSOL Assisted Simulation of Laser Engraving, Hamidreza Karbasi, Conestoga College Institute of Technology and Advanced Learning School of Engineering and Information Technology 299 Doon Valley Dr., Kitchener, Ontario, N2G 4M4, Canada [4] A COMSOL Model of Damage Evolution Due to High Energy Laser Irradiation of Partially Absorptive Materials,J.J. Radice, P.J. Joyce, A.C. Tresansky, R.J. Watkin, Mechanical Engineering Department, United States Naval Academy. [5] Flying Laser Spot Thermography for the Fast Detection of Surface Breaking Cracks, Joachim Schlichting 1,2, Mathias Ziegler 1, Christiane Maierhofer 1, and Marc Kreutzbruck