Issue 50

A. Kostina et alii, Frattura ed Integrità Strutturale, 50 (2019) 667-683; DOI: 10.3221/IGF-ESIS.50.57 672 Effect of excitation parameters on temperature contrast of the bi-layered sample with subsurface defects Results provided by Fig. 3 shows that model (1)-(5) can be used for the prediction of the temperature contrast and peak contrast time. Therefore, it will be used for the investigation of heating time and shape of the heating pulse on values of these parameters for the same specimen coated with ceramic layer. The physical properties of the ceramic layer (Yttria stabilized zirconia) are given in Tab. 2 [14]. Property Value Unit Density 5600 kg/m 3 Thermal conductivity 0.9 W/(m·K) Heat capacity 505 J/(kg·K) Table 2 : Thermophysical properties of the ceramic layer. The structure was considered as an object consisting of three materials: steel (the main detail), ceramics (top coating) and air (subsurface defects of various sizes representing delamination of the coating from the main detail). The sizes of the main detail and the defects correspond to the sizes presented in Fig. 1 (a). Fig. 4 shows a schematic representation of the cross-sectional area of the considered layered specimen. Subsurface defects have different thickness which varies from 0.14 mm to 3.5 mm (corresponds to the through defect). The following boundary conditions were applied: (3)-(5) to the coating and (4)-(5) to the lateral and rear surfaces of the object. Continuity conditions were used at the interface between the considered materials. The amplitude of the heating pulse o Q was equal to 4·10 5 W/m 2 in all cases. The finite-element model of the specimen with coating of 0.6 mm is shown in Fig. 5. As in the previous example, the mesh was considerably refined in the coating and in areas adjacent to the defects and sound area. The minimum element length in the coating was 0.07 mm. The maximum element length of the main detail was 10 mm. The complete mesh consisted of approximately 3·10 6 elements. Figure 4 : Schematic representation of the cross-sectional area of the layered specimen. All sizes are in mm. Figure 5 : Finite-element model of the specimen with the coating. Fig. 6 shows the effect of the heating duration on the temperature increase Δ T = T - T 0 obtained after 2 seconds of cooling. The impulse had a square shape with a smoothed descending part to simulate gradual decrease in the heating power. The smoothing interval was equal to 6.6 ms for all considered cases. The thickness of the coating was equal to 0.6 mm. It can be seen that all subsurface defects are visible. Defects of the smallest thickness (the first column) have minor values of