On specimen design for size effect evaluation in ultrasonic gigacycle fatigue testing

Literature datasets showed that gigacycle fatigue properties of materials may be affected by the specimen risk-volume, i.e., the part of the specimen subjected to applied stress amplitudes above a prescribed percentage of the maximum applied stress amplitude. The paper proposes a Gaussian specimen shape able to attain large risk-volumes for gigacycle fatigue tests, together with a general procedure for its design: wave propagation equations are analytically solved in order to obtain a specimen shape characterised by a uniform stress distribution on an extended length and, as a consequence, by a larger risk-volume. The uniformity of the stress distribution in the Gaussian specimen is numerically verified through a finite element analysis and experimentally validated by means of strain gauge measurements.