Plate impact experiments have been carried out to examine the influence of grain boundary characteristics on the dynamic tensile response of Cu samples with grain sizes of 30, 60, 100, and 200 lm. The peak compressive stress is $1.50 GPa for all experiments, low enough to cause an early stage of incipient spall damage that is correlated to the surrounding microstructure in metallographic analysis. The experimental configuration used in this work permits real-time measurements of the sample free surface velocity histories, soft-recovery, and postimpact examination of the damaged microstructure. The resulting tensile damage in the recovered samples is examined using optical and electron microscopy along with micro x-ray tomography. The free surface velocity measurements are used to calculate spall strength values and show no significant effect of the grain size. However, differences are observed in the free surface velocity behavior after the pull-back minima, when reacceleration occurs. The magnitude of the spall peak and its acceleration rate are dependent upon the grain size. The quantitative, postimpact, metallographic analyses of recovered samples show that for the materials with grain sizes larger than 30 lm, the void volume fraction and the average void size increase with increasing grain size. In the 30 and 200 lm samples, void coalescence is observed to dominate the void growth behavior, whereas in 60 and 100 lm samples, void growth is dominated by the growth of isolated voids. Electron backscatter diffraction (EBSD) observations show that voids preferentially nucleate and grow at grain boundaries with high angle misorientation. However, special boundaries corresponding to Rl (low angle, < 5) and R3 ($60 <111> misorientation) types are more resistant to void formation. Finally, micro x-ray tomography results show three dimensional (3D) views of the damage fields consistent with the two dimensional (2D) surface observations. Based on these findings, mechanisms for the void growth and coalescence are proposed. V

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This paper presents the design, fabrication, and testing of a miniature latching accelerometer that does not require electrical power. Latching is attained by using a bistable compliant mechanism that switches from one mechanical position to another when the force on the accelerometer exceeds a threshold value. Accelerometers were fabricated by laser cutting the compliant mechanism switch out of both ABS and Delrin plastic sheets. Packaging consisted of gluing the single compliant layer to a supporting substrate. The switching thresholds of the accelerometers were varied from 10g to 800g by varying the surface area of the free moving section between 100 and 500 mm2.

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