Heat Source Reconstruction and Its Relationship with Functional Fatigue of Pseudoelastic NiTi Ribbons

Abstract

In this study, the local and global effects of functional fatigue of pseudoelastic NiTi ribbons are revisited through the integrated use of digital image correlation and infrared thermography. Firstly, a detailed phenomenological description regarding the global effects of functional fatigue, with special emphasis on Lüders-type deformation stages is presented. The advantages of using in situ full-field techniques become evident here, since they allow us to thoroughly characterize strain field inhomogeneities, recoverable strains, and fronts geometry. Secondly, from appropriately processed Lagrangian temperature maps, a heat source reconstruction procedure was performed. A strong linear correlation between the calculated heat source and measured local strain rate during the inhomogeneous deformation stages of the first cycles was found. As the cycling progresses, this correlation is lost due to the appearance of a spatial or temporal lag, which is detected by cross-correlation analysis. The proportionality factor between both variables enabled us to perform energy balances and determine latent heats using an alternative procedure different from the usual calorimetric techniques. The experimental data obtained for ribbon geometry were also used to validate a previously developed thermomechanical model for pseudoelastic NiTi wires, which includes thermal effects and nucleation or collapse of transforming domains on the overall mechanical response.