NONLINEAR DYNAMICS OF SHELLS SUBJECTED TO RANDOM PARAMETRIC FORCING

This study explores the dynamic response of a polyethylene terephthalate (P.E.T) cylindrical shell subjected to strong random excitation. The shell is clamped at its base and connected at the top to a heavy diaphragm, allowing significant vertical vibrations. It is mounted on a 40kN shaking table integrated with a climate chamber, enabling precise temperature control from -70°C to 180°C to assess thermal effects on vibrational behavior. A standard modal analysis is conducted to determine the shell’s natural frequencies, mode shapes, and damping ratios across temperatures. To investigate nonlinear dynamics, we apply high-level, band-limited random excitation targeting the first axisymmetric mode, which maximizes energy absorption. This results in extreme vertical accelerations up to 100g. Under these conditions, we observe a novel nonlinear phenomenon-Ghost hammering. Beyond a critical excitation threshold, large, irregular spikes appear in the shell’s lateral (radial) response, absent in vertical signals and the input excitation. Detected via Laser Doppler vibrometry and accelerometers, these spikes suggest a purely internal dynamic effect linked to parametric excitation. Their emergence depends on both excitation level and temperature, offering insight into complex vibrational behavior in nonlinear, thermally sensitive structures.