The geometry and the dimensions of specimens are given on the figure 1 FIGURE 1: SPECIMEN SHAPE The specimens were subjected to cyclic loading at room temperature with a sinusoidal waveform of a frequency of 30 Hz. For all tests the stress ratio (Smin/Smax) is the same and taken at the value 0.57. In this condition then crack is always opened. EXPERIMENTAL RESULTS From the curves crack length versus number of cycles fig(2) we can determine the time life and the minimal crack propagation rate versus hole radius and versus cold expansion degrees table 2. For each case the time life versus stress amplitude can be put in the form of power law in the same way that the well known Wöhler curves. The fatigue limit corresponding to a time life of 2*106 cycles can be expressed from the hole radius by a law N = N0* d where N0 corresponds with = 1. FIGURE 2: CRACK LENGTH VERSUS NUMBER OF CYCLES The figure 2 shows clearly the crack arrest and the great time life gain we can get by modifying the notch sharpness and the cold expansion. 10 15 20 25 30 35 40 0,00E+00 4,00E+05 8,00E+05 1,20E+06 1,60E+06 Number of cycles Crack Length mm Expanded (5,75+6mm) Expanded (5,8+6mm) Expanded (5,9+6mm) Diameter 6 mm Diameter 5 mm Diameter 2 mm Cold expansion effect Initial crack Notch sharpness effect Crack initiation Delay
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