Composite hydrophone devices coupling piezoelectricity and tensegrity

The concept of tensegrity as conceived by Buckminster Fuller has been incorporated into a passive hydrophone device. Tensegrity is described as the physical phenomenon that produces a stable geometric structure using solid compressional elements arranged in tandem with flexible tensional cables. In the devices built by the authors, six PZT 5H™ bars acting as compressional elements in the tensegrity structure have been coupled with tensional bands of either polyaramid or carbon fiber. This stable system is then wrapped with an outer layer of either polyaramid or carbon fiber and rubber film to form a sealed device, which is referred to as a piezotensegritive device in this paper. The six bars are arranged in parallel electrical connectivity for all devices described. The resonant frequency for these devices ranged from 19.5 to 20.3 kHz depending on the material used for wrapping the piezoelectric bars. These devices were also tested in a hydrostatic environment to determine the relevant piezoelectric coefficients. For devices wrapped with carbon fiber, d_h peaked at ∼6000 pC/N and g_h at ∼275 mVm/N. For devices wrapped with polyaramid, d_h peaked at ∼2000pC/N and g_h at ∼100mVm/N. Sensitivities from –182–195 db ref. 1V/μPa were calculated for these devices. Received: 7 September 1999 / Reviewed and accepted: 15 September 1999     

Novel piezoelectric ceramics and composites for sensor and actuator applications

 Novel piezoelectric ceramic, and ceramic /polymer composite structures were fabricated by solid freeform fabrication (SFF) for sensor and actuator applications. SFF techniques including: Fused Deposition of Ceramics (FDC), and Sanders Prototyping (SP) were utilized to fabricate a variety of complex structures directly from a computer aided design (CAD) file. Many novel and complex composite structures including volume fraction gradients (VFG), staggered rods, radial and curved composites, and actuator designs such as tubes, spirals and telescoping were made using the flexibility provided by the above processes. Radial composites with various connectivities in the radial direction were made for towed array applications. VFG’s were incorporated into some of these designs, with the ceramic content decreasing from the center towards the edges. Many new designs are also being used to manufacture high authority actuators utilizing the FDC technique. The telescoping actuation of the device is the summation of actuation of all individual tubes making of the actuator, therefore, increasing the number of the tubes which are the driving component of the actuator will further enhance the displacement. The design, fabrication and electromechanical properties of these sensor and actuator structures are discussed in this paper. Received: 2 November 1998 / Reviewed and accepted: 2 November 1998     

New applications of photostrictive ferroics

 Photostriction in ferroelectrics arises from a superposition of photovoltaic and inverse piezoelectric effects. (Pb,La)(Zr,Ti)O₃ ceramics doped with WO₃ exhibit large photostriction under irradiation of near-ultraviolet light, and are applicable to remote control actuators and photoacoustic devices. Using a bimorph configuration, a photo-driven relay and a micro walking device have been developed, which are designed to start moving as a result from the irradiation, having neither electric lead wires nor electric circuits. The mechanical resonance of the bimorph was also induced by an intermittent illumination of purple-color light; this verified the feasibility of applying photostriction for ”photophone” applications. Received: 10 September 1997/Accepted: 9 October 1997     

Influence of fiber length on the mechanical properties of wood-fiber/polypropylene prepreg sheets

Natural fiber based composites have the potential to improve the mechanical properties of plastics while reducing the cost and weight. This study shows a practical method of blending natural-fiber with polypropylene to form a mat and then consolidated into a sheet by hot pressing. The natural fibers assessed were Pinus radiata and Eucalyptus regnan high temperature thermomechanical pulps and sisal (Agave sisalana) fibers. The tensile strength was shown to decrease with an increase in fiber content, while the tensile modulus was shown to increase. Tensile and flexural modulus were positively influenced by fiber length. The water performance tests of the sheets generally showed approximately 20% weight gain and approximately 3% thickness swell at 30% fiber content. The natural fiber surface chemical composition was determined by X-ray photoelectron spectroscopy and shown to be primarily covered with hydrophobic material such as lignin and extractives, while polypropylene was shown to be partially oxidized. Received: 18 September 2000 / Reviewed and accepted: 20 September 2000     

Modeling of 3D transversely piezoelectric and elastic bimaterials using the boundary element method

 This paper presents a numerical model for three-dimensional transversely isotropic bimaterials based on the boundary element formulation. The point force solutions expressed in a united-form for distinct eigenvalues are studied for transversely isotropic piezoelectricity and pure elasticity. A boundary integral formulation is implemented for the modeling of two-phase materials. In this study, the stress distributions are computed for a near interface flaw. The influences of the shape and location of the flaw on the the stress concentration are examined. The accuracy of the numerical procedures is validated through selected example problems and comparison studies. Received 3 October 2001 / Accepted 9 April 2002     

A point interpolation mesh free method for static and frequency analysis of two-dimensional piezoelectric structures

 A mesh free method called point interpolation method (PIM) is presented for static and mode-frequency analysis of two-dimensional piezoelectric structures. In the present method, the problem domain and its boundaries are represented by a set of properly scattered nodes. The displacements and the electric potential of a point are interpolated by the values of nodes in its local support domain using shape functions derived based on a point interpolation scheme. Techniques are discussed to surmount the singularity of the moment matrix. Variational principle together with linear constitutive piezoelectric equations is used to establish a set of system equations for arbitrary-shaped piezoelectric structures. These equations are assembled for all quadrature points and solved for displacements and electric potentials. A polynomial PIM program has been developed in MATLAB with matrix triangularization algorithm (MTA), which automatically performs a proper node enclosure and a proper basis selection. Examples are also presented to demonstrate the accuracy and stability of the present method and their results are compared with the conventional FEM results from ABAQUS as well as the analytical or experimental ones. Received: 6 February 2002 / Accepted: 5 August 2002     

Thermal and mechanical properties of porous Y-PSZ/zircon composites

 The thermal and mechanical properties of sintered porous composites of yttria-partially-stabilized-zirconia (Y-PSZ) and zircon (ZrSiO₄) were investigated for a broad range of compositions. Fracture strengths of these composites were significantly improved with the zircon addition (0 to 50wt%). The addition of zircon also improved the thermal shock resistance. Specimens sintered at 1500^oC for 6 h with 15–20% porosity were shown to have superior strength and thermal shock resistance. These findings have been used in the manufacturing of ceramic permanent molds for brass casting. Received: 5 December 1997 / Accepted: 13 December 1997     

Generalized mixed variational methods for Reissner plate and its applications

 Based on the mechanism of shear locking phenomenon and potential functional of Reissner plate bending problem, the generalized mixed variational principle for Reissner plate analysis is presented by parameterized Lagrange multiplier method. The proposed variational functional contains splitting factors which are able to adjust the shear potential energy and shear complementary energy components in it. The generalized mixed finite element formulation of bilinear quardrilateral element for Reissner plate bending analysis is established in terms of the new variational principle. The stiffness of the finite element model can be changed by the alteration of the splitting factors. Thus both the free of shear locking and higher accuracy are obtained by the choice of appropriate splitting factors. The most important is that this paper gives one self-adaptative way to choose the splitting factors for thin and moderately thick plates. This results in the comparative order of magnitude between the bending stiffness and shear stiffness for the arbitrary thickness. In the application of two-by-two exact Gaussian integration scheme to the proposed mixed element model, numerical examples show that free of locking is obtained even in the thin plate limit and high accuracy is given for moderately thick plate. Received: 15 January 2002 / Accepted: 10 September 2002 This work is partially supported by the National Nature Science Fund in China under Award No. 53978376     

Dielectric and magnetoelectric properties of (x)Ni0.8Co0.1Cu0.1Fe2O4/(1 − x)PbZr0.8Ti0.2O3 composites

Ceramic composites of Ni_0.8Co_0.1Cu_0.1Fe₂O₄ and lead–zirconate–titanate (PZT) were prepared using conventional solid state reaction method. The presence of constituent phases in composites was confirmed by X-ray diffraction (XRD). The variation of dielectric constant with frequency (100 Hz–1 MHz) and temperature has been studied. The variation of loss tangent (tan δ) with temperature (at frequency 1 kHz) has also been studied. The magnetoelectric (ME) output was measured as a function of dc magnetic field. The maximum value of ME output (625 mV/cm) was observed for 25% ferrite + 75% ferroelectric phase. The maximum ME response can be explained in terms of the content of ferrite, permittivity of dielectric material and the intensity of magnetic field. The ME response of these composites was observed to be linear within low dc magnetic field. These composites may form the basis for the development of magnetic sensors and transducers for use in solid state microelectronics and microwave devices.     

Adhesive elements for stress analysis of bonded patch to curved thin-walled structures

 Bonded composite patching has been recognized as an efficient and economical method to extend the service life of cracked aluminum components. However, current analysis methods and empirical databases for designing composite bonded joint and patch repair are limited to flat plate and/or flat laminate geometries, and the effect of curvature on the performance and durability of composite bonded joints and repairs is not known. This paper presents a novel finite element formulation for developing adhesive elements for conducting quick stress analysis of bonded repairs to curved structures. Illustrative examples are presented to demonstrate the effect of curvature and the effect of patch location, i.e., internal and external patches, patch size and patch thickness on stresses in adhesive layer. Received: 24 April 2002 / Accepted: 10 October 2002     

Combined shape and sizing optimization of truss structures

 In this paper, an evolutionary optimization method is presented for weight minimum problem of a 3-dimensional truss structure in terms of nodal coordinates and element cross-sectional areas. The structure is subject to stress, local buckling and displacement constraints. Two types of design variables with different natures are optimized separately: (1) a fully stressed design (FSD) and scaling techniques are applied to sizing variables and (2) the evolutionary node shift method is applied to shape variables. Alternating procedure is utilized to couple the two types of variables and to combine the results. The optimum solution is achieved gradually from the initial configuration design. Two typical truss structures are examined to illustrate the validity of the method. Received: 22 October 2001 / Accepted: 04 June 2002 This research work is supported by the National Natural Science Foundation of China under the guarantees no. 10072050 and 10172072, respectively.     

A hot-pressed PLZT under DC bias for field-stabilized pyroelectric detector applications

 The temperature dependence of the pyroelectric response of a hot-pressed PLZT (lead lanthanide zirconate titanate) (9.5/65/35) under a modest dc electric bias field has been investigated in an assessment of the performance of this material for applications in small-area pyroelectric devices. A strong pyroelectric response has been induced in the material at and above its transition temperature by the application of an electric field. The electrical resistivity, relative dielectic constant, loss tangent, ferroelectic hysteresis, and pyroelectric coefficients are reported. The material shows a very high figure of merit for a dielectric bolometer application that is competitive with existing relaxor ferroelectric materials such as Pb(Mg_1/3Nb_2/3)O₃ (PMN) and Pb(Sc_1/2Ta_1/2)O₃ (PST). Received: 21 March 1997 / Accepted: 6 May 1997     

Configuration design sensitivity analysis and optimization of beam structures

 A general method for configuration design sensitivity analysis over a three-dimensional beam structure is developed based on a variational formulation of the classical beam in linear elasticity. A sensitivity formula is derived based on a variational equation for the beam structure using the material derivative concept and adjoint variable method. The formulation considers not only the shape variation in a three dimensional direction, which includes translational as well as rotational change of the beam but also the orientation angle variation of the beam's cross section. The sensitivity formula can be evaluated with generality and ease even by employing a piecewise linear design velocity field despite the fact that the bending model is a fourth order differential equation. The design sensitivity analysis is implemented using the post-processing data of a commercial code ANSYS. Several numerical examples are given to show the excellent accuracy of the method. Optimization is carried out for a tilted arch bridge and an archgrid structure to show the method's applicability. Received 29 September 2001 / Accepted 20 March 2002     

Reverse ulta-microtomy: a new method of TEM sample preparation

 Two variations of a new method of TEM metal sample preparation are proposed, hereby labeled RUM1 and RUM2, for ”reverse ultra-microtomy” 1 and 2, respectively. With these techniques, the TEM samples are formed from bulk specimens by successively slicing off layers, with an ultra-microtome, until the remnant is partially thinned to electron transparency. This requires the sample to be embedded with excellent adhesion in a mounting material whose mechanical properties resemble that of the sample. While these methods are by far slower than ordinary microtomy wherein the individual slices are used as samples, preparation times are comparable to those for creating TEM samples by standard techniques. Furthermore, the methods (i) can be used where electropolishing fails, (ii) can be favorably applied to finely layered materials and composites, (iii) permit studies of near-surface structure gradients, and (iv) permit the simultaneous observation of an original surface and the underlying material. The methods have been successfully demonstrated through comparisons between samples created through RUM1 and 2 and standard TEM samples. Artifacts caused by the new method of reverse microtoming are shown to be very much reduced compared to direct microtoming. Received: 26 September 1997 / Accepted: 1 October 1997     

POLYCHAR-8 worldwide forum on polymer applications and theory in 2000

Profile of POLYCHAR conferences is defined. An introduction to the following papers presented at the POLYCHAR-8 conference in January 2000 is provided. Prizes awarded at the POLYCHAR-8 Conference are listed.     

Characterization of Pt/AlN/Pt-based structures for high temperature, microwave electroacoustic devices applications

Highly c-axis oriented AlN films, 3.15 μm thick, were grown by rf reactive sputtering technique at 200 °C on bare and Pt-covered Si(100) substrates previously oxidized to a thickness of about 2 μm in wet oxygen atmosphere. A Pt film, 2200 Å thick, was then sputtered on the free surface of the AlN/Pt/SiO₂/Si multilayer at 200 °C without breaking the vacuum in order to avoid any oxidation effects of the layers. The multilayers were then annealed in air at 900 °C for different time lengths up to 32 h in order to test the materials' resistivity to harsh environment. The influence of this high temperature annealing (HTA) on the thin films' crystallinity, as well as on the c-AlN piezoelectricity and Pt sheet resistivity was investigated at room temperature before and after each annealing. X ray diffraction investigations revealed that the films' crystallinity was improved by the HTA: the full width of half maximum of the AlN(002) and Pt(111) peaks decreases from 0.39° to 0.24°, and from 0.42° to 0.28° after 32-hours-HTA. Scanning electron microscopy, four points probe and piezoelectricity tests revealed that the morphology and the sheet resistivity (in the range from 0.6 to 0.5 Ω/sq) of the outer Pt film, as well as the AlN piezoelectric constants d₃₃ (in the range from 6.2 to 7.4⋅10⁻¹² C/N) was quite unaffected by the HTA even after 32 h of annealing.     

A new model for the analysis of composite steel – concrete slab and beam structures with deformable connection

 In this paper a solution to the bending problem of reinforced concrete slabs stiffened by steel beams with deformable connection including creep and shrinkage effect is presented. The adopted model takes into account the resulting inplane forces and deformations of the plate as well as the axial forces and deformations of the beam, due to combined response of the system. The analysis consists in isolating the beams from the plate by sections parallel to the lower outer surface of the plate. The forces at the interface producing lateral deflection and inplane deformation to the plate and lateral deflection and axial deformation to the beams are related with the interface slip through the shear connector stiffness. Any distribution of connectors along the interface and any linear or non-linear load–slip relationship or partial shear connection for the shear connectors can be handled. The creep and shrinkage effect relative with the time of the casting and the time of the loading of the plate is taken into account. The solution of the arising plate and beam problems, which are nonlinearly coupled, is achieved using the analog equation method (AEM). The adopted model compared with those ignoring the inplane forces and deformations, approaches more reliable the actual response of the plate–beams system. Moreover, it permits the evaluation of the shear forces at the interfaces, the knowledge of which is very important in the design of composite steel–concrete structures. Received: 21 June 2002 / Accepted: 5 March 2003     

CNT composites for aerospace applications

Carbon nanotubes were synthesized by thermal arc plasma process after optimization of the synthesis parameters. These samples were then analysed by scanning and transmission electron microscopes (SEM and TEM), in order to establish the morphology of the nanostructures. Atomic force microscopy (AFM) and electron diffraction studies were also carried out before using the sample for the composite material preparation. Composites of epoxy resin with curing agent as well as a mixture of graphite and carbon nanotubes were prepared with varying proportions of the mixture. The electrical resistivity of the material was studied under varying pressure and voltage conditions. Preliminary results of these studies present interesting features which are reported here.     

Inorganically filled starch based fiber reinforced composite foam materials for food packaging

 A process for the manufacture of highly inorganically filled starch based fiber reinforced foam composites for food-packaging use is described. Starch, obtained from potato, corn, or other sources, is used as binder to form net shape foam composite substrates under hydrothermal conditions. The starch content is 25 to 60 wt. % of the product. Ground calcium carbonate is the principal inorganic material, being 25 to 60 wt. % of the product. The composite substrate is foamed by steam at temperatures between 160° and 220°C in a single 15 to 200 seconds step within heated molds. The organic-inorganic composite is reinforced with 5 to 20 wt. % wood or plant fiber. The substrate is rendered functional with food and beverages by thin overlay coatings that provide moisture barrier properties and additional mechanical flexibility. The product is used to package dry and wet foods and meets required food packaging standards. The substrate biodegrades rapidly in the presence of sufficient moisture or microbes. An environmental life-cycle model and an economic model are described as the additional forces to a materials model for research, development and commercialization. The primary goal of this development is to provide disposable packaging materials from renewable resources that are low in cost, low in environmental impact, and meet commercial performance requirements. Received: 22 October 1998 / Reviewed and accepted: 23 October 1998     

Shape optimization of periodic structures

This paper describes a numerical approach to the optimization of effective properties of periodic perforations in an infinite body, in the frameworks of heat conduction and of linear elasticity. We implement a special finite element mesh in order to deal with the periodic nature of the problem. We compute the gradient of the functional to be minimized. We describe the process of mesh deformation and mesh regeneration. We give several numerical examples, some of them having practical relevance.