Issue 36

M. Ouarabi et alii, Frattura ed Integrità Strutturale, 36 (2016) 112-118; DOI: 10.3221/IGF-ESIS.36.11 115 If we assume that, the material is isotropic with a linear elastic behavior, the following equation is obtained where E d is the dynamic modulus. 2 2 2 2 ( , ) ( , ) ( , ) 1 0 d U x t U x t U x t dS x S x dx E t           (3) The sinusoidal displacement can be expressed as follow: ( , ) ( ) i t U x t U x e   (4) Finally, the equation for the displacement vibration along the specimen is: 2 ''( ) ( ) '( ) ( ) 0 U x P x U x k U x    (5) where: , 2 f d E k c and c        The geometry of the specimens is designed with a reduced cross section to obtain the maximum stress amplitude in the middle section (Fig. 2a) or/and to obtained the maximum stress intensity factor as shown in Fig. 2b. For having a resonance frequency of 20 kHz, we must calculate the resonance length L1. There are two ways for doing that. The first one is to use the analytic solution of the previous equation.   * 1 2 1 1 arctan (coth( ) ) L L k k     (6) * 2 1 max 0 2 cos( ) sinh( ) L d kL e E U L      (7) where: * 1 2 2 1 ln 2 T L T         (8) The second way is to use the numerical solution computed by Finite Element Analysis (FEA). Ansys software was used in this study. First, the specimen geometry was computed with the analytic solution. Then the resonance frequency was computed by modal analysis. Finally, with an imposed sinusoidal displacement with and amplitude of 1 µm, the stress amplitude in the middle of the specimen was computed by harmonic analysis. The difference between, the numerical and analytical solution was less than 1 %. Crack propagation test The method for calculating the mode I stress intensity factor range is reported in several documents, so that [3, 6]. Usually the Paris’ law is used to determine a crack growth curve, and then it is needed to relate the stress intensity factor range (  K) with the crack growth rate   / da dN . Wu [6] has shown that Eq. 9 can be used for calculating  K during ultrasonic fatigue test since such test is carried out under displacement control. In this equation E d is the dynamic modulus and  is the Poisson ratio of the material, U 0 is the displacement amplitude, a is the crack length, w is the specimen width and f(a/w) is a shape function depending on the specimen geometry. Wu has determined this function for hyperbolic profile crack growth specimen with a thickness of 8 mm (Eq. 10). 0 2 ( ) (1 ) d E a K U f a w      (9)