Issue 38

I. N. Shardakov et alii, Frattura ed Integrità Strutturale, 38 (2016) 331-338; DOI: 10.3221/IGF-ESIS.38.43 333 step a 5–10-minute pause was made to record temperature on the stretched surface of the beam. Simultaneously, crack patterns and widths were obtained. To excite heat transfer processes, the beam was subjected to external heat pulse (magnitude of 926 W and duration of 10 sec) and then cooled. Temperature recorded along the whole surface of the CFRP layer using infrared imager FLIR T620 ([9-11]). Shots were taken “through the mirror”, which ensured the safety of people and equipment at loads close to the destruction of the beam (Fig. 2b). (a) (b) Figure 2 : Load testing machine (a) and infrared shooting arrangement (b) . The details of experimental techniques were determined based on the analysis of the results of numerical solution of nonstationary heat conduction problem in a system of "carbon sheet - epoxy - concrete - delamination - concrete". Difference in surface temperature of the multi-layered system with and without debondings at corresponding instants of time at heating and cooling is called here a temperature response to the presence of delamination. Numerical simulations enabled us to assess the conditions at which the temperature response will be the most. It appears that on heating of the beam by the heat source of 926 W for 10 seconds, the temperature response should be measured at the stage of its cooling, namely 8 seconds after its start (i.e. 18 seconds after the beginning of observation) [12]. Thermography images of the composite surface were obtained at each loading step. The initial thermograms for each j -th step (Fig.3a) is a two-dimensional array of differential temperature values ( , ) j T x y determined at 19th and 0th seconds at points with coordinates   , x y . The index 0, j N  specifies the number of loading step; loading is absent at 0 j  . The obtained initial thermograms were processed using an algorithm specifically designed using Matlab programs. In the first step of the algorithm, we calculate the normalized thermograms         * * * * 0 , , , / , j j j TN x y T x y T x y T x y  where   , j T x y is the initial temperature difference at the j -th loading step at the point   , x y ,   * * 0 , T x y and   * * , j T x y are the initial temperature differences at the 0-th and j -th loading steps at the point * * , x y where no debonding is known to be present. Normalized thermograms for successive loading steps are given in Fig.3b. Next the temperature contrast   , j C x y (Figure 3c) is determined: