PSI - Issue 65

Akhmetkhanov R. S. et al. / Procedia Structural Integrity 65 (2024) 1–5 Akhmetkhanov R.S. / Structural Integrity Procedia 00 (2024) 000–000

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1. Introduction

In particular, the present work is devoted to the detection of damages of different nature in various materials by analyzing the obtained raw thermal thermography images. Tests based on this methodology belong to the category of active thermography are given in Nesteruk D.A., Vavilov V.P. (2007) and Moiseychik A.E., Moiseychik E.A.(2014). Temperature difference between the defect and the base material required for singularity detection. The released energy causes a local temperature rise and is displayed by the camera as a hot spot. Heat tends to propagate in the material equally in all directions (assuming homogeneity of the material) the presence of a crack or impact damage impedes lateral heat flow, thus changing the circular shape of the hot spot. An experienced operator or program can detect this asymmetry of the warmer region. The thermal inspection method is widely used for diagnosing materials and technical systems, and for evaluating the physical properties of materials. Examples of such use are given, for example, in the works of the authors U. Galietti, D. Palumbo (2010), Rajic N., Rowlands D (2013), P. Servais, N. Gerlach.(2005), Muflih Alhammad, Nicolas P. Avdelidis and Clemente Ibarra-Castanedo (2022). Let us now turn to the consideration of thermograms for spot heating. To evaluate the local thermal effect on structural materials, let us consider a thermal profile (a slice in a plane). It consists in point step-by-step heating of the object by a focused laser beam with simultaneous high-speed registration of the unsteady temperature field pattern by a thermal imager. At point heating of a material the thermal field slice looks as follows (Fig.1). The distribution of the temperature heating zone depends on the time of influence t on this area ( t 1<< t 7). In the presence of defects or inhomogeneity, the slice will be distorted. 2. Main results

Fig. 1. Experimental dependence of temperature T on the distance to the center of the heating spot r in a sheet of stainless steel 12Х18Н9Т 1 mm thick for different moments of time t.

Figure 2 shows thermograms at spot control in the zone without defect ( a,b ) and defect ( c,d ). In case of defect, the temperature in the heating zone is higher. It can be seen visually that the heat spots are not symmetrical. The non-symmetry of the heat spot is caused by the inhomogeneity of the material and, as a consequence, by the inhomogeneity of the thermal conductivity of the material in different directions.

Fig. 2. Thermograms of material heating (a and b) in defect-free material and (c, d) defective material.