Understanding of Tensile Test Results on Small Size Specimens of Certified Reference Material BCR-661

Abstract

Abstract BCR-661 is a Nimonic 75 Certified Reference Material for tensile testing, available from the European Institute for Reference Materials and Measurements. The certification is based on specimens with a diameter of 10 mm. Since for several purposes small specimens are desirable, an attempt was made to qualify a specimen with a diameter of 2.4 mm. It was found that for cylindrical specimens made by turning neither the 0.2 % proof stress, nor the tensile strength values corresponded with the certified values. Rectangular section specimens made by electro-discharge machining gave correct stress values but too high elongations. The reduction of area values were for all specimens satisfactory. It was shown that the stress deviations found in the turned specimens are due to the presence of a work hardened layer caused by machining. Annealing of turned specimens resulted in correct strength values. With regard to the elongation values the influence of the specimen shape and possible specimen head deformation seem to be important factors.