13th International Conference on Fracture June 16-21, 2013, Beijing, China 4 in Fig. 3: if a Xi=10µm amplitude displacement is required with 300MPa bulk stress, it is only necessary to identify the intersection of the 10µm displacement line (dashed) with the 300MPa line coming from the vertical axis. This point, marked with a cross in the chart, shows where the fretting pads must be placed (around 45mm from the specimen’s bottom – horizontal axis) and the displacement amplitude to be imposed at the specimen’s bottom (X(0)=14µm) – full line that intersects the cross. Figure 3 - Example of an iso-amplitude of displacement abacus. The fretting-apparatus consisted of two 5-mm diameter cylindrical pads made of AFNOR XC48 steel, similar to ASTM510[16], soaked at 830°C for 30 minutes and quenched in water, carefully polished with 1000 CAMI graded sandpapers. Pads were diametrically opposed, pressing specimens with 30N normal load, imposed by two parallel springs, as shown in the schema of the mechanical assembly in Fig. 4. A rigid apparatus supported springs and pads, which were glued with Araldite®. Figure 4 –Schema of mechanical assembly At last, a cooling system using compressed air was necessary to keep specimens at room temperature, since that ultrasonic strain cycling and high fretting frequencies generate high heat rates. 2.1. Specimens The material studied was a low alloy medium strength steel provided by an external supplier. All specimens were sequentially polished with 180, 320, 600, and 1000 CAMI-graded sandpapers in its axial direction. Their dimensions are shown in Fig. 5, and in Table 1.
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