Temperature measurement in the tensile Hopkinson bar test

A thermal scanning camera has been used to measure the temperatures reached 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 mu s, leading to a significant rise in temperature of the specimen. To estimate this rise in temperature from the scanner's signal, account had to be taken 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 the neck and (iii) changing emissivity of the specimen surface as deformation proceeds. Errors in the first two stages are small whereas those in the last stage impose tolerances of approximately +40 degrees C and -30 degrees C on the maximum temperatures. The possibility of scanning 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 present, Together these factors give an overall tolerance of between +100-150 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.