Critical phase-transition temperature for freezing stress in thermo-sensitive photopolymers used for visualizing stress fields in solids

Abstract

Quantitative visualization of the hidden full-field stresses in three-dimensional (3D) solids is crucial for solving various engineering problems; however, it is challenging to achieve using the conventional experimental techniques. Frozen stress technique is an effective method to characterize internal stress fields. However, difficulties in fabricating the complex 3D models impede the extension of this method. The method combining additive manufacturing or 3D printing, printable transparent photopolymers, and frozen stress techniques provides a new approach to overcome these challenges. However, the effects of continuous temperature rise and loading during the frozen stress tests on the stress birefringence of the thermo–sensitive photopolymers have not been studied thoroughly. In this study, the stress birefringence characteristics of the photopolymers were examined using 3D printed transparent disk models and frozen stress tests under various temperatures and radial loads. A reflection polariscope system incorporated with a high-temperature loading chamber was designed to freeze and capture the stress fringes in the printed model. The thermo–optic curve, stability of full-field fringes, and elasticity and plasticity of the photopolymer under high temperatures and remaining loads were investigated. The effects of high temperatures and remaining loads on the stability of the fringe orders were analyzed. The linear relationship between the fringes and stresses in the material was verified using the fringe orders in the central points of the disk models. The critical temperatures that discriminate between the three different thermo–stress states, i.e., glassy state, transition state, and freezing state, were discussed. Our results indicate that the lowest critical temperature for freezing stresses in the tested photopolymer is 80 °C.