100 J.L. Smith et al. before the actuator starts to move. The data acquisition of the MTS controller starts at the same time as the trigger is sent, and therefore the images taken by both cameras as well as the load cell and LVDT signals of the load frame have a common recorded time base. The infrared camera used in this work can record the temperature data as so-called radiometric temperatures or as nonuniformity corrected photon counts for each pixel. Since the radiometric temperature method assumes that the emissivity of the object is unity, i.e., it is a black body, the temperature readings obtained for stainless steel specimens will be too low because of its lower emissivity. Emissivity values for various substances can be found in the literature, but since the actual emissivity depends on many factors, such as the surface finish of the object, a calibration with real samples is often the only way to obtain reliable temperature values. As stated in the Introduction, in many previous works a coating, such as soot or spray paint, has been applied on the specimen surface to increase its emissivity close to that of a black body. The downside of this approach is that the coating tends to peel off, crack, or thin too much especially in the necking area of the specimen at large strains, which increases the uncertainty of the obtained temperature readings. In this work, calibration measurements with several specimens were conducted in the following manner: thermocouples were attached to the surface of a specimen, which was heated on a hot plate in 10–20 K steps and the true temperature, indicated by the thermocouples, was recorded together with the IR camera reading obtained as an average of an area of reasonable size. To examine the effect of plastic deformation on the emissivity the calibration was done with tested specimens which have areas with different amount of plastic deformation. The necking area is highly deformed and appears a little rougher compared to the surface outside the gage section which is not deformed plastically. In the calibration procedure, IR camera readings at all temperature steps from room temperature up to ca. 350 ıC were recorded from the deformed sections of tested specimens using exposure times of 100 s, 50 s, 20 s, 10 s, 5 s and2 s. From the calibration measurements, polynomial fits were formed separately for each exposure time, which were then used to convert the radiometric temperatures given by the IR camera to true surface temperatures of the specimen. The temperatures were then interpolated at the points where strains were calculated on the surface and using these synchronous measurements the partition of plastic work converted to heat at each point was calculated at each point of strain. 16.3 Experimental Results The measured true strain and temperature at the point of maximum strain at failure in the specimen is displayed in Fig. 16.4. The strain measured by a 4 mm extensometer over the gage length of the specimen is also shown. The two values of strain are comparable until the onset of localization at a strain of approximately 0.3. It is at this point that a large increase of temperature is also observed the peak value of 250 ıC is reached at a local strain of 0.8. Fig. 16.4 History of strain and temperature at the point of maximum strain
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