           ANCA STANCIU, DIANA COTOROS, MIHAELA BARITZ, LILIANA ROGOZEA Mechanical Engineering Faculty University Transilvania of Brasov 500036 Bra)ov, 29 EROILOR St. Romania http://www.unitbv.ro , ancastanciu77@yahoo.com , dcotoros@yahoo.com , mbaritz@unitbv.ro   These strain gauges have the largest use in measuring strains due to their simplicity and easiness to be applied on the measured part, also due to their low cost, high accuracy and various possibilities offered for performing measurements in complicated and difficult working and loading conditions. The obtained results using strain gauges help us to determine some optimization possibilities for the studied part.   strain gauges, strains, composite materials, unit loading, Young’s modulus   Electric strain measuring, whose fundamental concepts were established approx. six decades ago, had a fast and wide development due to its important benefits: the method is non;destructive so the shape and size of the measured part or structure are not changed; allows performing of measurements in real working conditions; assures a much higher sensitivity and accuracy than mechanical, optical, pneumatic or acoustical methods; also the use of electronic devices, practically lacking inertia, allows measuring and recording of fast variable phenomena.    The strain gauge, attached to the studied part so that it thoroughly follows the deformations, presents a variation of the electric resistance during deformation. It has been found that the specific variation of the transducer resistance is proportional to its specific deformation occurred at the same time with the deformation of the studied part on which it is attached. This phenomenon of electric resistance variation of a conductor due to its mechanical deformation is the basis of using electric strain measuring. It is well;known that the electric resistance of a wire with constant cross;section is    ρ = (1) where ρ is the wire material resistivity in  ⋅  ;  wire length, m;  wire cross;section, m 2 . Using logarithms and differentials we get        − + =  ρ ρ (2) which, for a finite variation becomes        −  +  =  ρ ρ (3) In this expression, ε =    , and ε 2 − =    where  is Poisson’s coefficient. In order to asses the term ρ ρ  we currently apply Bridgeman’s law     =  ρ ρ (4) where  is a material constant (called Bridgeman’s constant) whose value is experimentally determined by wire stretching tests. Considering that ( )          − =  +  =   2 1 , the expression (4) becomes (V is the wire volume) ( )     − =   ρ ρ 2 1 (5) The relation (5) is approximate because ρ odes not depend only on the volume but also on the material crystals orientation; actually the value of  is different according to the crystallization directions. A more accurate reasoning is based on Ohm’s law applied to a cubic crystal subjected to a certain stress, with small deformations, expressed by ( )          π δ ρ + = (6) where   is the electric field,   current density,  ρ resistivity,  δ fundamental metric tensor (Kronecker’s RECENT ADVANCES in SIGNAL PROCESSING, ROBOTICS and AUTOMATION ISSN: 1790-5117 254 ISBN: 978-960-474-157-1