13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- Table 1: Chemical composition of the investigated Al2024 material Material Cu Fe Mg Mn Si Ti Zn Weight-% 4.58 0.21 1.48 0.47 0.073 0.016 0.098 Table 2: Key mechanical properties of Al2024 in the T-351 state Young’s modulus, E [12] Yield strength, Rp0.2 Ultimate tensile strength Elongation at break 73.1 GPa 321.5 MPa 446.5 MPa 18.5 % Prior to testing the chemical composition of the material was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). The results are listed in Table 1. The key mechanical properties are listed in Table 2 (values determined by the supplier). An important aspect of the present work is the influence of the surface state of the material on its VHCF behavior. As shown e.g. in reference [13], scratches significantly reduce the lifetime of clad Al 2024 sheets. To overcome this problem, fatigue samples are usually grinded and polished. This standard process to assure reproducible smooth surfaces is not applicable for clad material, since surface grinding and polishing would remove the thin cladding layer. A detailed analysis of the as-received specimen surface has thus been performed. Scratches on clad samples can be described as small notches with a diameter of a few µm in arbitrary orientation. 3. Experimental setup For fatigue testing an ultrasonic fatigue testing machine, developed at BOKU Vienna, is employed. The testing system is described in references [14] and [15]. Modifications made on the system are described in detail in reference [16]. The sample geometry is presented in Fig. 1(b). Sheet samples are laser cut to their final geometry. The volume (surface) of the gage length is 64 mm3 (105.6 mm2) for 1.6 mm thick sheets and 80 mm3 (112 mm2) for 2 mm thick sheets, respectively. Figure 1. (a) Flat specimen connected to the load frame by an adapter and submerged in its lower part in glycerin. (b) Sample geometry. (c) Typical FFT spectrum of the vibration signal
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