Jean-Christophe Thomas

Deflections of inflatable fabric panels at high pressure

Inflatable structures made of modem textile materials with important mechanical characteristics can be inflated at high pressure (up to a several hundreds of kPa). They can be used as strong building elements thanks to their mechanical strength. The aim of the paper is to present experimental and analytical studies on the behaviour of inflated fabric panels at high pressure and submitted to bending loads. It is shown that inflatable structures cannot be viewed as ordinary plates or beams, because their deformation pattern is quite different. Experiments show that their behaviour depends on the applied load, the inflation pressure, and the constitutive law of the fabrics. Equilibrium equations are written in the deformed state to take into account the influence of geometrical stiffness and the following forces. A Timoshenkos beam theory must be used because sections of the panels do not satisfy the usual Bernoullis beam theory. A new inflatable beam theory is developed. Wrinkling loads are derived from equilibrium equations. Deflections satisfy the fact that the compliance of the inflatable panel is the sum of the beam compliance and of the yarn compliance. Comparisons between the results of our modelling and experimental results are shown and prove the accuracy of this theory on the mechanical strength of inflatable structures at high pressure.

Continuous and Finite Element Methods for the Vibrations of Inflatable Beams

Inflatable structures are under increasing development in various domains. Their study is often carried out by using 3D membrane finite elements and for static loads. There is a lack of knowledge in dynamic conditions, especially for simple and accurate solutions for inflatable beams. This paper deals with the research on the natural frequencies of inflatable Timoshenko beams by an exact method: The continuous element method (CEM), and by the classical finite element method (FEM). The dynamic stiffness matrix D(ω) is here established for an inflatable beam; it depends on the natural frequency and also on the inflation pressure. The stiffness and mass matrixes used in the FEM are deduced from D(ω). Natural frequencies and natural modes of a simply supported beam are computed, and the accuracy of the CEM is checked by comparisons with the finite element method and also with experimental results.

Strength of inflatable fabric beams at high pressure

Inflatable structures made of modern textile materials can be inflated at high pressure in order to be used as strong building elements. The aim of the paper is to present results from research on the mechanics of highly inflated structures. Experimental, analytical and numerical studies on the behavior of inflatable fabric beams are displayed. We will first describe experimental studies on two kinds of inflatable prototypes: flat panels and tubes. Experiments show that their behavior is a linear combination of yarns and beams shapes. The usual theory of collapse analysis is then applied to the computation of wrinkling loads of these fabric beams. The second section of the paper is devoted to build a new inflatable beam theory and to show that the compliance of the inflatable beams is the sum of the beam compliance and of the yarn compliance. A new inflatable beam finite element is developed in the third section and used to compute deflections of hyperstatic beams. Our first results on the buckling of inflatable panels are displayed in the last section. Comparisons between experimental and analytical results are shown and show that this new theory on the mechanical strength of inflatable structures at high pressure is satisfactory.

Transverse piezoelectric coefficient measurement of flexible lead zirconate titanate thin films

Highly flexible lead zirconate titanate, Pb(Zr,Ti)O3 (PZT), thin films have been realized by modified sol-gel process. The transverse piezoelectric coefficient d31 was determined from the tip displacement of bending-mode actuators made of PZT cantilever deposited onto bare or RuO2 coated aluminium substrate (16 μm thick). The influence of the thickness of ruthenium dioxide RuO2 and PZT layers was investigated for Pb(Zr0.57Ti0.43)O3. The modification of Zr/Ti ratio from 40/60 to 60/40 was done for 3 μm thick PZT thin films onto aluminium (Al) and Al/RuO2 substrates. A laser vibrometer was used to measure the beam displacement under controlled electric field. The experimental results were fitted in order to find the piezoelectric coefficient. Very large tip deflections of about 1 mm under low voltage (∼8 V) were measured for every cantilevers at the resonance frequency (∼180 Hz). For a given Zr/Ti ratio of 58/42, it was found that the addition of a 40 nm thick RuO2 interfacial layer between the aluminium substrate and the PZT layer induces a remarkable increase of the d31 coefficient by a factor of 2.7, thus corresponding to a maximal d31 value of 33 pC/N. These results make the recently developed PZT/Al thin films very attractive for both low frequency bending mode actuating applications and vibrating energy harvesting.

New process for transferring PZT thin film onto polymer substrate

This paper reports a simple, quick and cheap method for transferring Pb(Zr,Ti)O3 (PZT) thin film onto polymer substrate. To achieve a low-cost transfer process, piezoelectric thin film are deposited onto a sacrificial commercial aluminium substrate to target polymer substrate by a wet etching technique. This novel flexible piezoelectric structure was characterized to evaluate its crystal structure and its morphologic properties. This new process should allow to develop novel applications for low frequency vibrations energy harvesting.

Reliability of inflatable structures: challenge and first results

Inflatable structures are part of the family of tensioned textile membrane structures. The operating principle is common for all these type of structures: the stiffness of the structure relies on t...

Flexible PZT/aluminium thin films characterizations for energy harvesting at very low frequencies (∼ 1 Hz)

With the objective to harvest the airflow energy, flexible PbZrTiO3(PZT) thin films have been deposited onto ultra thin aluminium (Al) foil (16 μm).The piezoelectric properties of a PZT/aluminium cantilever have been investigated, and the measurements show d31 values around 30 pm/V. Due to use of a very lightweight substrate, a very large deflection (in the millimeter range) of the tip of the beam have been observed, which is very promising for actuators applications. In a next step, PZT/aluminium films have been encapsulated with laminated polyethylene terephthalate (PET) in order to increase the flexibility of the device. The obtained structures were subjected to sinusoidal stress at very low frequencies (0.3 to 8 Hz) and the output voltages across a resistive load have been measured and compared to the theory with a good agreement. In addition, an intermediate ruthenium dioxide (RuO2) layer has been used at the PZT/aluminium interface and its influence on the piezoelectric properties and the harvested power has been demonstrated.