13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- hence tensile force in the specimen, is controlled by the magnet strength and the air gap between the rotor and magnet. The air gap varies with the rotation of the eccentric rotor; therefore, the force varies in a cyclic manner. A mean tensile stress is achieved by use of a weight applied through a cable and pulley attached to the sliding grip/magnet assembly. To control the cyclic loading frequency, the eccentric steel rotor is driven by variable speed DC motor. For the test apparatus shown, the DC motor had a power rating of ½ HP, and a maximum rotational speed of 34 RPM. Figure 1. Schematic illustration of the fatigue testing apparatus. For a given magnet strength and rotor speed, the waveform of the applied axial load is determined by the geometry of the rotor (shape and number of lobes). For the current test configuration, the motor speed was adjusted to provide a loading frequency of 2 Hz. Although a simple eccentric rotor was used in the present investigation, a multi-lobed rotor can also be used to increase the fatigue loading frequency or provide different peak and mean load levels within a given cycle. The shape of the rotor lobe (s) can also be changed to produce other load versus time waveforms. Alternatively, the shape or number of magnets that interact with the rotor can be altered to achieve a desired waveform. Signal monitoring. An impulse counter was used to record the number of fatigue cycles. The load cell and impulse counter signals were monitored by a data acquisition board and DASYLAB software.1 By monitoring the load cell signal, the DA board also enabled stopping the motor when the specimen failed. Figure 2 shows a close-up view of the actual fatigue testing apparatus with a 30 µm thick specimen installed. 3. Experimental Procedure 3.1. Material and Specimen Preparation The material used for this study, commercially available annealed Ti foil (99.6% purity), was obtained as 30 �m thick sheets.2 General information regarding the composition and mechanical 1Measurement Computing Corporation, Norton, MA USA. 2Goodfellow Corp., 125 Hookstown Grade Road, Coraopolis, PA 15108-9302 USA.
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