and interface friction. Figure 5.5 shows the model predictions of dynamic strain across the threaded interfaces for varying torque and with different striker velocities. The predictions show a small but observable sensitivity to torque in the stress wave propagation in the incident bar. The sensitivity to striker velocity was much higher as expected, with the peak strain (and stress) varying linearly with the striker velocity. This follows from 1-D wave propagation expression for strain in a Hopkinson bar [16], i.e., εi ¼ σi Ei ¼ ρici Ei vs ¼ 1 ci vs; (5.3) where vs is the striker velocity and ρi, ci, and Ei are the density, wave speed, and elastic modulus of the incident bar, respectively. The local stress state at the interface was also examined in these initial simulations to predict any plastic deformation and also to visualize the stress distribution. Three cases at varying torque levels are shown in Fig. 5.4. No plastic deformation was predicted, although significant local stresses developed at higher torque levels. 5.4 Experiment The experiment test apparatus was set up at the AFRL Shock Dynamics Laboratory. Analog signal conditioning for the strain gages, (i.e., regulated constant-current excitation, analog filtering at 204 kHz, and amplification), is accomplished via a Precision Filter 28000 chassis with 28144A Quad-Channel Wideband Transducer Conditioner with Voltage and Current Table 5.1 The equivalent static loads calculated with the power screw relation Torque Equivalent axial preload Hand tight Negligible 50 ft-lbf 3,075 lbf 100 ft-lbf 6,150 lbf 150 ft-lbf 9,225 lbf a b c Threaded interface Rotation index 14 Strain gages Laser vibrometer Laser extensometer Threaded interface Air gun Fig. 5.3 Pictures of the threaded bar interface experiments showing (a) the details of the interface, (b) the method for applying the torque, and (c) the experiment layout and location of the sensors 40 J.C. Dodson et al.
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