© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

4.3.3.5 Accelerative Onset

Aside from pure acceleration, the rate of accelerative onset -- sometimes called jolt, surge, or jerk -- can produce additional damaging effects on macroscale mechanical and biological structures. For example, the effects of accelerative onset ('a) on the human body were studied by Col. John Paul Stapp¹⁷¹⁴ in a series of aggressive rocket sled experiments conducted during the 1950s.^1714-1716 For human subjects, cardiovascular shock was entirely absent for 'a >~ 500-600 g/sec but began to appear at 'a >~ 1100-1400 g/sec, at 1-40 g accelerations. Human tolerance to linear decelerative force is determined both by rate of onset and peak acceleration; the endurance limit for well-restrained healthy young males has been given as <~500 g/sec and <~50 g for >~0.2 sec durations.¹⁷¹⁷

By 1998, the effects of accelerative onset on nanomechanical structures had not been extensively studied, but similar limits might apply to nanomachines. For example, a diamond rod of length L_rod = 1 micron, cross-sectional area S_rod = 0.01 micron², mass m_rod = 3.5 x 10^-17 kg, and working strength W_rod ~ 5 x 10¹⁰ N/m² (Table 9.3) fails when subjected to a static acceleration a_fail >~ W_rod S_rod / m_rod ~ 10¹² g. But accelerative shocks can produce dynamic amplification in the response spectrum of elastic systems. Dynamic stress rises to a peak of roughly twice the corresponding static value when 'a_peak >~ 4 n_res a_max for a trapezoidal pulse shape, where n_res is the frequency of natural vibration of the system,³⁶³ because the stresses from the free oscillations of the system, induced by the changes in the acceleration of the system, can add to the static stresses from the acceleration, with damage done by the total stress. Taking k_s ~ 1 N/m for the rod described above, then n_res ~ 30 MHz (Section 4.3.3.1) and 'a_peak ~ 10²⁰ g/sec at a_max ~ 10¹² g. For comparison, the teragravity nanocentrifuge (Section 3.2.5) has mass m ~ 10^-17 kg and is spun up to a_max = 5 x 10¹¹ g's in >~10^-6 sec, giving 'a <~ 5 x 10¹⁷ g/sec << 'a_peak.

Last updated on 17 February 2003