Browsing by Author "Chaudhry, Zaffir"
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- Design, fabrication, and calibration of an instrumented drop weight impact testerDempsey, Craig Thomas (Virginia Tech, 1994-02-05)In this thesis, the complete design, fabrication, and calibration of an instrumented drop weight impact tester is described. Included in this description are all the sketches and drawings that are needed to duplicate this project, if so desired. This impact tester was built for around $23,000 less than it would have cost to buy and modify a commercial tester for the intended research application. This tester, as designed, was intended to be used in the field of impact location detection using artificial neural networks. Even though this impact tester was built for a specific research application, the design concepts that are presented can easily be adapted to a variety of testing needs. This impact tester was built using an non-working milling machine for a base. This provides a rigid, stable base along with a moveable X-Y table. The tester itself has the capability for drop weights ranging from 3.518 Ib up to 15.408 lb, and impact energy levels ranging from 0.6 ft-lb up to 45.6 ft-lb. Also, it is capable of impacting multiple locations of large plates with variable boundary condition sizes up to 12" x 24". Furthermore, it uses a computer program written using a data acquisition software package to provide output plots for the impact event, including the force and energy applied to the specimen versus time and the force versus displacement. Finally, initial experimental results obtained from this tester agree very well with those obtained from a commercially available tester, allowing it to be used in future tests involving intelligent material systems.
- Electro-dynamic analysis of stack actuators and active members integrated within truss structuresFlint, Eric Michael (Virginia Tech, 1994-08-15)In this thesis, a method of predicting the steady state, dynamic, electromechanical behavior of stack actuators (both electrostrictive and piezoelectric) integrated within complex structures is developed and experimentally verified. This research was motivated by a need to accurately predict transmission force, velocity output, and power consumption for a wide range of applications both terrestrial and space based. The relevant transduction equation / parameters are derived from basic principles. These results are experimentally verified with a PZT stack active member. The derivations are then extended to incorporate the effects of integrating the actuator within a host structure. Specifically, the equations needed to predict actuator output force, resulting velocity and drawn current are derived. To implement and test these results in a structure, the equivalent host structure impedance must be determined. This is done experimentally for a complex truss structure representative of a small satellite. These results are then used to prepare theoretical predictions which compare well with experimentally measured output force. Finally, the derivations are extended to the electrical behavior of active members integrated within truss structures. It is now possible to predict the electrical load imposed by the active member on the power supply system including the effects of coupling with the host structure dynamic boundary conditions. Two implications of this are considered. First, the required power demands directly influence the design and sizing of amplifiers, applied voltage levels and power systems. Second, the dissipative power from actuation losses contributes directly towards raising the internal temperature of an operating stack actuator.
- Experimental/analytical determination of optimal piezoelectric actuator locations on complex structures based on the actuator power factorBhargava, Adesh (Virginia Tech, 1995-02-03)The actuator power factor is defined as the ratio of the total dissipative mechanical power of a PZT actuator to the total supplied electrical power to the PZT actuator. If measured experimentally, it can be used to optimize the actuator location and configuration for complex structures. The concept of actuator power factor is based on the ability of an integrated induced strain actuator such as a PZT actuator to transfer supplied electrical energy into structural mechanical energy. For a given structure such as a beam or a plate, the location and configuration of an actuator will directly influence the authority of the actuator towards driving the structure. Therefore by maximizing the average power factor for a given frequency interval, the actuator driving authority and thus the supply power utilization can be maximized. This thesis describes an experimental technique, based on the actuator power factor, for determining the optimal PZT actuator location(s) on complex structures for actuator power factor maximization and for active structural vibrational and acoustic control. For the actuator power factor analysis, the design of a removable PZT actuator unit is described. The concept of actuator power factor is ini tial1 y evaluated for the simple case of a cantilever beam and there is good agreement between the experimental and theoretical actuator power factor results. The optimization technique is then developed for the case of a complex structure designed to resemble an aircraft panel and shows good prediction for narrow-band as well as broadband power factor maximization.
- Impact failure modes of graphite epoxy composites with embedded superelastic nitinolKiesling, Thomas C. (Virginia Tech, 1995-02-05)Energy absorption during complete penetration of thin graphite composites is experimentally shown to be significantly improved by low volume fractions of embedded superelastic shape memory alloy (SMA) fibers. Graphite/Bismaleimide laminates were embedded with 3% and 6% volume fractions of superelastic nitinol fibers. Quasi-static tests were performed on wide clamped-clamped beams to identify progressive damage mechanisms. Low velocity (13.9 ft/s) impact tests, at an impact energy of 31.5 ft-lbs, resulting in complete penetration were also performed on wide clamped-clamped beams. These tests show that only after peak load is there a contribution made by the SMA to the load deflection behavior of the composite. Owing to the SMA's high strength and high strain to failure it remains undamaged after failure of the base composite. The interaction between the base composite and the SMA creates an increase in absorbed energy over the base composite of as much as 41 % in a Graphite/Bismaleimide laminate embedded with a 6% volume fraction of nitinol fibers. C-scans of the hybrids embedded with bi-directional nitinol fibers show a 22% larger delamination areas compared to plain graphite epoxy. The larger delaminations are a result of the nitinol fibers distributing the impact energy to a larger area of the base composite. This interaction between the nitinol and the graphite is one of the reasons for the increases in absorbed energy. Fiber pull-out and strain energy of the nitinol fibers also adds to the increase in absorbed energy. Although damage initiation and peak loads do not seem to be affected by the embedded nitinol fibers, the energy absorption after peak loads is greatly improved. This improvement is a result of increased energy distribution through the SMA to the graphite. The large improvements in energy absorbing capabilities offered by SMA fibers give SMA hybrid material systems promise in applications where penetration resistance is imperative.
- Modeling of induced strain actuation of shell structuresChaudhry, Zaffir; Lalande, Frederic; Rogers, Craig A. (Acoustical Society of America, 1995-05-01)Based on the thin-shell Donnell theory, a model to represent the action of discrete induced strain actuator patches symmetrically bonded to the surface of a circular cylindrical shell has been developed. The model provides estimates of the bending curvatures due to the out-of-phase actuation and the in-plane strains due to the in-phase actuation of the bonded actuator patches. The magnitudes of the induced curvature and the in-plane strain are found to be identical to those of plates; however, due to the strain-displacement relations in cylindrical coordinates, the in-plane and out-of-plane displacements are coupled. Expressions for the equivalent forces and moments that represent the action of the actuator patches have been developed. Due to the curvature of the shell, the representation of the in-phase actuation with an equivalent in-plane line force applied along the edge of the actuator results in the application of erroneous rigid-body transverse forces. To avoid these rigid body forces, a method to represent the in-phase actuation with a system of self-equilibrating forces is proposed. The action of the actuator is then represented by an equivalent in-plane force and a transverse distributed pressure applied in the region of the actuator patch. Finite element verification of the proposed model is presented. The displacements due to the actual actuator actuation are compared with the proposed model, and very good agreement is found.
- Modeling of the induced strain actuation of shell structuresLalande, Frederic (Virginia Tech, 1995-04-05)When discrete piezoelectric actuator patches bonded on structures are used for active shape, vibration, and acoustic control, the desired deformation field in the structure is obtained through the application of localized line forces and moments generated by expanding or contracting bonded piezoelectric actuators. An impedance-based model to predict the dynamic response of cylindrical shells subjected to excitation from surface-bonded induced strain actuators is presented. The essence of the impedance approach is to include the actuator/structure impedance ratio in the calculations of the actuator forces applied to the structure, which will retain the dynamic characteristics of the actuators. The appropriate representation of the loading due to in-phase and out-of-phase actuation is discussed. Due to the curvature of the shell, the representation of the in-phase actuation with an equivalent in-plane line force applied along the edge of the actuator results in the application of erroneous rigid-body transverse force. To avoid these rigid body forces, the action of the actuator needs to be represented by am equivalent in-plane force and a transverse distributed pressure applied in the region of the actuator patch to maintain the structure self-equilibrium. A full derivation of the impedance model is included, taking great care in the structural and actuator impedance definition. It is found that the actuator's output dynamic force in the axial and tangential direction are not equal. Various case studies of a cylindrical thin shell are performed to illustrate the capabilities of the developed impedance model. The in-phase and out-of-phase actuation authority of induced strain actuators bonded to the surface of a shell is compared. It is shown that out-of-phase actuation has better authority in exciting the lower order bending modes, while in-phase actuation has better authority in exciting the higher order circumferential modes. Dynamic finite clement analysis has been performed using piezoelectric elements available in ANSYS 5.0. The good correlation between the finite clement results and the impedance model confirms the analytical solution. Experimental data of a circular ring actuated in-phase and out-of-phase by a piazoelectric material (PZT) were also compared to the derived impedance model.
- Torsional and flexural control of sandwich composite beams with piezoelectric actuatorsKoike, Ayako (Virginia Tech, 1994-09-15)A mathematically one-dimensional model was developed to predict the static response of composite sandwich beams subjected to loads induced by piezoelectric (PZT) actuators. The model was derived using Reddy's (1984) displacement field for a laminated plate which consists of cubic variation of the in-plane displacement through the thickness. In this model, beam deformations include extension, bending, transverse shear, St. Venant torsion, and torsion due to warping of the cross section out of its plane. The PZT actuators can be configurated to induce a bimoment, resulting in twist cf the beam through the warping of the cross section. Hence directionally attached PZT (DAP) actuator elements, which cause twist by inducing tensile and compressive strains at 45° to the longitudinal axis of the beams, are not necessary to actuate twist. For an aluminum beam example, it is shown that the PZT bimomet control produced about 2.7 times more twist than the conventional DAP control.