Monotonic Testing and Tension-Tension Fatigue Testing of Free-standing Al Microtensile Beams
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U11.39.1
Monotonic Testing and Tension-Tension Fatigue Testing of Free-standing Al Microtensile Beams Nicholas Barbosa III, Paul El-Deiry, & Richard P. Vinci Lehigh University - Department of Materials Science and Engineering, Bethlehem, PA ABSTRACT Free-standing Al thin films were loaded statically and dynamically through the use of a custom-built microtensile system. The system is capable of performing monotonic loading/unloading up to 50 µm/s (10-1/s) and tension-tension fatigue experiments at 100 Hz on 600 µm long, 100 µm wide, and 1 µm thick free-standing Al microtensile beams. Monotonic loading/unloading and stress relaxation experiments have been performed. The microtensile beams show plasticity as well as a relaxation dependence on strain rate and stress level. Displacement controlled tension-tension fatigue experiments have also been performed. A trend of decreasing cycles to failure with increasing displacement amplitude and increasing mean displacement has been noted but requires further experimental exploration. INTRODUCTION Mechanical properties of small structures may show dramatically different behavior than their bulk counterparts.1,2 Combined with the knowledge that materials loaded under cyclic conditions can be expected to behave in a different manner than those loaded monotonically, an understanding of the behavior of monotonically and cyclically loaded micro- and nano-films is desirable. Strain rate effects on the mechanical behavior of thin films have been observed, but a general understanding over a large range of strain rates is still lacking.3 There have also been various attempts at quantifying the effects of cyclic loading on thin films.4,5 Very few of these studies have investigated the effects of cyclic loading on thin, free-standing films, which in some cases are known to behave differently than thin films attached to substrates. MICROTENSILE APPARATUS A custom built microtensile testing system (Figure 1) was used in both monotonic and fatigue tests performed in this work. The crosshead is piezoelectrically actuated with ±10.0 nm of resolution and 60.0 µm of total displacement. Loads are measured using either a piezoelectric load cell or a conventional load cell with a resolution of ±1.0 mN or ±0.1 mN, respectively. Strain is determined from a capacitance sensor that measures the distance between the top grip and the bottom grip with a resolution of ±10.0 nm. Although this type of strain measurement is not directly measuring sample strain, it improves the accuracy of the strain measurement from previous microtensile systems.6 Temperature fluctuations affect the absolute position between the grips at a rate of 500 nm/oC. In order to maintain accurate control of the grip spacing, a PID controller was added as illustrated in Figure 1.
U11.39.2
1°C change in room temperature changes the grip spacing by 0.5µm, or 0.0008% strain. High voltage amplifier and closed loop feedback to piezotransducer Controls piezotransducer position (input to piezotransducer)
clamp Si frame
COMPUTER: Acquir
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