13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Materials and experimental procedure 2.1. Materials A 2000 series al alloy (2196) aluminum alloy was investigated with the chemical composition detailed in Table 1. Table 1. Chemical composition of the 2196 aluminum % Cu % Li % Mg % Mn % Ag % Zr % Zn % Si % Fe 2.5-3.3 1.4-2.1 0.25-0.8 < 0.35 0.25-0.6 0.04-0.18 < 0.35 < 1.2 < 0.15 Two different heat treatments were applied: the industrial peak aged on one side called T8, and the experimental treatment at low temperature (120°C during 96 hours) on the other, called UA (Under-Aged). These two ageing treatments involve equivalent elastic properties with an elastic modulus E about 79000 MPa and the Poisson’s coefficient about 0.305, but different monotonic and fatigue properties like the ultimate stress Rm, the yield stress R0.2 and the elongation’s rate A% (Table 2), which imply different ΔKth values. Note that for confidential reasons, all data regarding fatigue properties are not available. Table 2. Mechanical properties of the 2196 aluminum, depending on the heat treatment State R0.2 (MPa) Rm (MPa) A% T8 559 594 5.9 UA 428 520 11.1 For each alloy, small cubic specimen were machined and polished to achieve a 0.2 μm Ra roughness. Samples were adjusted so that the fretting loading was applied along the rolling direction of the alloy (Fig. 1). Figure 1. a) Sample's geometry - b) Microstructure [3] 2.2. Plain Fretting Experiment Plain partial slip fretting tests were performed on a hydraulic set up at the LTDS previously described by Heredia [2]. The normal force (P) is kept constant, while the tangential force (Q) and displacement (δ) amplitudes are recorded. The fretting loop can be plotted and the corresponding amplitude values (respectively Q* and δ*) defined (Fig. 2).
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