Issue 49

M. Bannikov et alii, Frattura ed Integrità Strutturale, 49 (2019) 383-395; DOI: 10.3221/IGF-ESIS.49.38 385 Figure1: Geometry of the samples (scales in mm) The fatigue loading was carried out on an ultrasonic Shimadzu USF-2000 resonant-type testing machine (Fig. 2) at stress levels of 90-162 MPa and a symmetrical cycle R = -1. The principle of operation testing machine is based on formation of standing wave in the generator-horn-sample system [2]. In this case, the maximum amplitude of displacements occurs at the ends of the sample, and the maximum amplitude of stresses at its center. On the side of the free end of the specimen, an inductive sensor is fixed, which makes possible to determine the displacement of the end with an accuracy of tenths of a micron and a frequency of up to 1 MHz. The machine allows testing materials on the basis of 10 9 -10 10 cycles with amplitude from the 1st to several tens of microns with a frequency of 20 kHz, which reduces the test time to several days, in contrast to the classical fatigue installations, where this number of cycles is achieved during the years of testing. Figure 2: The experimental setup: 1 - Horn, 2 - sample, 3 - displacement sensor, 4 - cooling system. The formation of cracks in cylindrical samples undergoing high cycle loading in the range 10 6 -10 7 cycles begins from the surface. For the dynamic preloading samples AlMg6 subject to cyclic loads in the range exceeding 10 8 cycles the crack formation is started in the bulk of the sample. The fracture surface in this case exhibits a fatigue zone known as a “fish- eye” which is a distinguishing feature of such fatigue regimes. The central part of this region comprises a fracture nucleus surrounded by the region of refined (submicrocrystalline) structure so-called FGA [13] (Fig. 3a).