Latent heat effects in inductive heating of shape memory alloy fibers

Motivation of the present work is an inductive heating process, used in manufacturing a new kind of improved ultra-high performance concrete. In this novel material, fibers made out of shape memory alloys are used to increase the maximum possible fiber volume fraction, or to create an internal state of compressive stress. In contrast to other works, the underlying microstructure transformation from martensite to austenite is highlighted based on thermal analysis. Dynamic scanning calorimetry measurements are adapted as basis for development of a phenomenological phase transformation model. It relates local temperature and temperature rate to the rate of change of the phase indicator, modeling the transformation of martensite to austenite. Latent heat is considered by an enthalpy method, the inductive heating process is considered by a phenomenological model. Study results for a purely thermodynamic process of heating a single fiber embedded in concrete are presented. They show that latent heat effects delay phase transformation and the process of fiber activation is very sensitive to the induced heat. Furthermore, it is discovered that latent heat causes a strongly inhomogeneous state of transformation in radial direction of the fiber, which is of great importance for thermomechanical processes and the interpretation of experimental results.