Proceedings of COBEM 2005 18th International Congress of Mechanical Engineering Copyright © 2005 by ABCM November 6-11, 2005, Ouro Preto, MG THE THERMOELASTIC TECHNIQUES FOR THE MEASUREMENT OF STRESSES DISTRIBUTION ON COMPONENT OF STRUCTURES OR MACHINES Roberto Marsili Università degli Studi di Perugia misure@unipg.it Gianluca Rossi Università degli Studi di Perugia gianluca@unipg.it Abstract. Because of the continuous evolution of the market in terms of quality and performance, the mechanical production industry is subjected to more and more pressing technological challenges. In this frame the use of advanced measurement technique as the thermoelasticity, allows the engineers to have a fast and reliable tool of experimental investigation, optimization and validation of the FEM models of those critical parts as for example parts of car frames. In this work it is shown how the thermoelastic measurement technique can be used to optimize mechanical components, as method of experimental investigation and as technique of validation of numerical models. The measurement technique developed for this purpose is described together with the calibration method used in the test benches normally used for fatigue testing and qualification of these mechanical components. The results obtained show a very good deal with FEM models and also the possibility to experimentally identify the concentration levels of stress in critical parts with a very high spatial resolution and testing the effective geometry and structure material. Keywords: measurement of stress pattern, thermoelastic stress analysis, car frames analysis, stress pattern on differential gears, FEM model validation. 1. Introduction The design and development of a new component of structures or machines, when reliability, safety or costs due to component failure are relevant normally requires both an experimental and a theoretical approach (Tomlinson John R. Yates, 2001). For the second finite element models (FEM) are of common use, both for static and dynamic loading conditions (Ju S.H. Lesniak and J.R. Sandor, 1997). Experimental validation of FEM models is normally performed by strain gauge for the deformation measurements on the component prototype or on a model of it, under operating conditions or standard loadings in laboratory test benches. Strain gauges, anyway, allow only measuring stresses in few points of the component and have a limitation in spatial resolution due to sensor grid dimensions. To overcome these limitations other non contact and full field techniques have been developed: photoelasticity, for example that requires a model of the components made of a particular material or photoelastic coatings on the components surface. Other no contact full field techniques, able to measure dynamic component deformations but only in dynamic conditions, are based on laser scanning vibrometers. Also another optical technique, like holography, Moiré and speckle, for example, has been used for this purpose. In this paper thermoelasticity is considered and tested in order to have an experimental experience about its possibilities for FEM models validation of mechanical components. Two applicative examples are illustrated in order to demonstrate the validity of this technique: the measurement of stress pattern on high performance car frames and the measurement of stress pattern on differential gears. In all the activities we have designed and realized test benches able to load the components as similar as possible to their working conditions. 2. Thermoelastic’s theory The phenomenon of material changing temperature when it is stretched was first noted by Ghough in 1805 who performed some simple experiments using strand of rubber, but the first observation in metals of what is now known as the thermoelastic effect was made by Weber in 1830: he noted that a sudden change in tension applied to a vibrating wire did not cause the fundamental frequency of the wire to change as suddenly as he expected, but the change took place in a more gradual fashion (Rocca R., and Bever M. B., 1950). He reasoned that this transitory effect was due to a temporary change in temperature of the wire as the higher stress was applied. In 1974, the Admiralty Research Establishment approached Sira Ltd to determine relationship between stress and the temperature changes that may be produced by an applied load. Sira confirmed feasibility and over the next four years, with fundings from English Ministry of Defence, developed a laboratory prototype called Spate (Stress Pattern Analysis by measurement of Thermal Emissions) for an application research.