Signatures of the first three modes of stress wave propagation in a Hopkinson bar, given by the Pochhammer-Chree solution a century ago, are derived from a set of recent experiments. The data suggest that, at frequencies at which multiple theoretical modes are possible, the mode with the fastest group velocity dominates the propagation. The Gaussian-windowed Fourier transform technique is used to obtain time-dependent Fourier coefficients from a measured signal. The transformed signal is displayed in a gray Scale plot of the power spectra of the Fourier coefficients as a function of time and frequency. This plot shows clear evidence of the theoretical modes. A new dispersion cuwe for stress wave propagation in bars is derived from the transformed data. The new dispersion curve, used in the DISBAS code (a bar gauge deconvolution technique that uses dispersed basis functions), gives a highly resolved stress wave.
A new method is developed for analysing dispersed stress (or strain) signals from a Hopkinson bar. In this method the incident signal is written as a rectangular-window4 Fourier series. It treats dispersion by amounting for both the time of arrival of the various frequencies and the associated phase shifts in a dispersed signal. An analysis of a shock wave generated by a sphere of high explosives in a water tank shows that this method gives better resolved peaks than the existing FFT method, for signals from blast waves. It alsa gives an indication of when wmponents exist in the signal that are not due to dispersive propagation.
This study investigated the effect of cyclic mechanical stretching on the collagen gene expression and protein synthesis of human patellar tendon fibroblasts (HPTFs). We hypothesized that cyclic mechanical stretching of HPTFs would increase collagen synthesis via transforming growth factor-beta 1 (TGF-β1). To test the hypothesis, the tendon fibroblasts were cultured on microgrooved surfaces of silicone dishes under serum-free conditions. The cells were subjected to cyclic uniaxial stretching with a constant frequency and duration (0.5Hz, 4hr), and one of three stretching magnitudes (no stretch, 4%, and 8%) followed by 4 hours of rest. It was found that the gene and protein expression of both collagen type I and TGF-β1 were significantly increased in a stretching-magnitude dependent manner, whereas collagen type III gene and protein levels were not significantly changed. The exogenous addition of antibody to TGF-β1 eliminated the stretching-induced increase in collagen type I protein synthesis. The results therefore confirmed our working hypothesis and suggest that mechanical stretching of tendon fibroblasts can lead to matrix remodeling by modulating the collagen production of tendon fibroblasts, a process at least particially mediated by TGF-β1.
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