Adjustable Work Coil Fixture Facilitating the Use of Induction Heating in Mechanical Testing

Induction heating has been used in fatigue testing for over 30 years. Typically, a work coil is used in conjunction with a workstation and a radio frequency power supply to provide localized specimen heating. One disadvantage of the approach is that the localized nature of the heating leads to temperature gradients over the specimen's length. However, experience has shown that the configuration of work coils can be optimized to give relatively uniform temperature profiles over the specimen's gage section. The variables of interest include: coil diameter and overall length, number of turns, spacing of turns, and grouping of turns. Optimization of these variables by trial and error can be a lengthy and frustrating process. This is particularly the case in multiaxial experiments involving large tubular specimens and correspondingly large work coils. The subject work coil fixture was designed and developed to facilitate the optimization process. The approach adopted was to subdivide the work coil into three segments. Each segment has its own positioning mechanism that allows independent adjustment in both the radial and vertical senses. It was shown that use of this fixture allows temperature profiles within ±1% of the mean to be achieved with minimum difficulty. Such levels of precision were found to be necessary in a recent series of thermomechanical experiments conducted on Hastelloy X.