Transient temperature measurement using embedded thermocouples

The response time of thermocouples is generally considered to be a limiting factor when transient temperature changes need to be assessed in solids. As an example, transient temperature changes which develop during dynamic straining of materials, adiabatic shear band formation, dynamic fracture and related fields are often investigated using sophisticated noncontact measurement techniques such as infrared detectors. In these phenomena, the time scale is of the order of the microsecond. In this paper, the authors revisit the application of thermocouples to such measurements using small embedded thermocouples (ETC). Experiments with dynamically loaded polymeric disks (characteristic strain rate of 10³ s^–1) show that the thermocouples record transient temperatures with a short typical rise time of 10 s as a result of the conversion of plastic deformation into heat. This observation is corroborated by the solution of the temperature distribution in a sphere subject to constant surface temperature which predicts the same fast reaction. Specifically, considering a sphere which is representative of the sensing bead, the average temperature is shown to rise in a few microseconds. These theoretical results can be used to deconvolve the experimental results with respect to a calculated impulse response of the sensor to recover the actual temperature variations. The results show that small thermocouples can be embedded to yield useful information about the transient temperature evolution in a solid. This technique is easy to use and provides an important complement to other noncontact techniques.