TEMPERATURE-MEASUREMENT IN THE TENSILE HOPKINSON BAR TEST

A thermal scanning camera has been used to measure the temperatures re ached in a small tensile specimen of ductile Remco iron deformed at a strain rate of about 1600 s-1 in a split Hopkinson bar apparatus. True strains as high as 3.3 were reached in times of about 200 mus, leadin g to a significant rise in temperature of the specimen. To estimate th is rise in temperature from the scanner's signal, account had to be ta ken of (i) movement of the specimen with respect to the camera during the scanning period, (ii) changing orientation of the specimen surface with respect to the infrared detector due to strain localization in t he neck and (iii) changing emissivity of the specimen surface as defor mation proceeds. Errors in the first two stages are small whereas thos e in the last stage impose tolerances of approximately + 40-degrees-C and -30-degrees-C on the maximum temperatures. The possibility of scan ning away from the axis of the specimen increases the upper tolerance by as much as 25-degrees-C. Finally, the detector's rise time may have prevented the scanner resolving the steep temperature gradients prese nt. Together these factors give an overall tolerance of between +100-1 50-degrees-C to -30-degrees-C on an estimated temperature of about 300 -degrees-C. It is clear, therefore, that despite this large margin of error there is a significant rise in temperature within the specimen, sufficient to affect the observed mechanical response.