We consider the effects of acoustic pressure on the curing of a two-part epoxy, which can be considered analogous
to the polymer healing process. An epoxy sample is loaded into a tube and monitored throughout the early stages
of curing by measuring its vibrational response upon periodic impulses. By tracing the natural frequencies of the
epoxy-tube system and cross-checking the temperature of the epoxy, the progress of the curing can be quantified. Acoustic stimulation at three different frequencies is investigated and compared to the unstimulated case. We find that external acoustic pressure does seem to affect the curing, though much work remains to be completed.
In this study, we examine the propagation of mechanical waves on thin lightweight
structures, with the aim of developing a method of crack detection in such structures. By comparing
the response of healthy and cracked samples, we are sometimes able to differentiate
between the two. Using a network of sensors it would be possible to determine the presence of
a crack on a structure that is remote. Experimental work has been performed with single-crystal
Silicon thin plates and a thin rectangular sheet of steel. The Silicon plates were tested healthy
and cracked, and the steel was only tested when healthy. Piezo-ceramic stacks were used to
provide actuation and sensing, and wave solutions to the equation of motion are obtained for
the Silicon plate. Calculated and experimental results agree reasonably well.
Space structures would benefit greatly from an ability to tune the dissipation and stiffness of the structural
element. This would provide a compromise between large passive systems, and complex, real-time, active control
implementations. Different elements of a structure could be altered based on the loads that they experience.
This study will focus on thin piezoelectric film strips connected in parallel with an electronic circuit which
provides a "negative capacitance," and an electrical load consisting of a resistor and a capacitor. Due to
the inverse piezoelectric effect, each film forms an electromechanical system in conjunction with the parallel
circuit. The overall impedance of this system can be controlled by correctly varying gain parameters within the
circuit. This work models the PVDF strips of non-vanishing thickness and stretched under a constant, boundary
applied tension. Both flexural stiffness and in-plane tension are accounted for in setting up the partial differential
equation of motion. Harmonic excitation was provided with an acoustic speaker driven by a wave form generator.
Measurements of out-of-plane deflection at a chosen point were taken using an LED/photodiode pair, which was
calibrated experimentally. The voltage developed between the electrodes was also measured. Theoretical and
experimental results are analyzed and compared.
The studies reported on in this paper are of relevance to automated diagnosis of light weight space structures based on membranes. We investigate in-plane vibration response of membranes to in-plane actuation. Identically shaped piezoelectric polymer strips are used both for actuation and sensing. For membrane strips, the frequency response function is obtained using an in-plane vibration model. The model for the intact membrane is then modified to analyze the effect of a transverse crack at mid-span. The theoretical results are compared with experimental measurements. Experiments on a cracked strip show small differences in the frequency response function, which are qualitatively borne out by the theoretical calculations. Some experimental results on membrane sheets are also presented.
This paper presents recent results from an ongoing investigation on the use of non-collocated actuator/receiver
pair to enable dynamic testing of a thin membrane in order to detect cracks. The current focus is on obtaining
the transfer function of a Kapton membrane excited at one end by a polyvinilidine fluoride (PVDF) actuator.
A receiver of identical specifications is located at the other end. The actuator operates in the d31 mode which
under sinusoidal excitation leads to a periodic variation in membrane tension. The paper shows that the resulting
dynamics can be analyzed with the help of the Mathieu equation. As such, the frequency response of this
membrane is complicated. The presence of a crack could in principle be detected by the corresponding decrease
in the output voltage amplitude at the drive frequency in the Fourier transform of the output, but this was found
difficult in practice. Detailed analysis of the parametrically excited dynamics with and without a crack could
lead to precise and reliable signatures for healthy and cracked membranes.