Issue 49

A. Vedernikova et alii, Frattura ed Integrità Strutturale, 49 (2019) 314-320; DOI: 10.3221/IGF-ESIS.49.31 317 Fig. 3 shows the infrared imaging of the specimen surface at the final stage of loading before its fracture and the time- temperature dependence during the test. At the beginning of the test, the average surface temperature decreases due to the thermoelastic effect, then the thermoplastic effect prevails, and the temperature of the specimen increases until the moment of necking and complete failure. Figure 3 : IR image of the specimen under quasistatic tensile conditions. According to Eqn. (4), the time dependence of the heat source field was estimated based on the change of the specimen temperature during the mechanical test (Fig. 4a). The calculated heat source power data were verified by analyzing the data recorded by the Seebeck effect-based heat flux sensor. The heat source obtained by both methods do not coincide completely, which can be explained by different sensitivities of the devices, and errors of numerical processing of the infrared thermography data. Nevertheless, these results suggest the possibility of using non-contact measurements to estimate the heat source distribution on the specimen surface. One part of the irreversible plastic work contributes to heat generation, and another is stored as the energy of crystal defects accompanying plastic deformation, known as the stored energy of cold work. For flat specimens, the plastic work spent on deformation of the specimen can be defined as a function of strain rate V and loading force   F t :      p W t F t V (5) The stored energy is determined as a difference between the plastic work spent on deformation and the integral heat dissipation. The time dependences of these values are presented in Fig. 4b. Analysis of the data in Fig. 4b suggests that, when the material approaches fracture, the value of stored energy in the material reaches a critical value, and the rate of stored energy tends towards zero. In work [10], the algorithm allowing estimation of the heat transferred by convection, conduction and radiation:                                                    2 4 4 cv ir ij ij cv n ir p V V S S V T d dV T dV T T dS T T dS c E dV t (6) where  is the thermal conductivity of the material,  is the heat transfer coefficient by convection,  is the surface emissivity,  is the density, c is the specific heat,  n is the Stephan–Boltzmann constant equal to 5.67·10 8 W/(m 2 K 4 ),  p E is the rate of variation of the stored energy,  T is the room temperature, ( ) T x, y,z,t is the time-dependent temperature field, and cv S , cd S , ir S are the three parts of the external surface of the control volume V through which the heat Q is transferred by convection, conduction and radiation, respectively.