Shyh-Shiuh Lih

On the accuracy of approximate plate theories for wave field calculations in composite laminates

Abstract Approximate plate theories have been extremely useful in providing analytical solutions to problems involving static and dynamic loadings of uniform or laminated plates of finite dimensions. The approximate theories can also be used for solving wave propagation problems in laminates of finite thickness and large lateral dimensions. We have recently developed a multiple transform technique coupled with a numerical evaluation scheme to calculate the resulting double integral expressions for the displacements and stresses produced in a composite laminate subjected to dynamic loads. This technique is used to determine the wave field in the laminate due to a concentrated or distributed dynamic surface load with and without the thin plate approximation. The accuracy of the approximate solution is shown to depend upon the pulse width of the forcing function, the plate thickness, as well as the distance between the source and the field points. For a given force time history, the range of validity of the approximate solution is determined through comparison with the exact solution.

Characterization of the Electromechanical Properties of EAP materials

Electroactive polymers (EAP) are an emerging class of actuation materials. Their large electrically induced strains (longitudinal or bending), low density, mechanical flexibility, and ease of processing offer advantages over traditional electroactive materials. However, before the benefits of these materials can be exploited, their electrical and mechanical behavior must be properly quantified. Two general types of EAP can be identified. The first class is ionic EAP, which requires relatively low voltages (<10V) to achieve large bending deflections. This class usually needs to be hydrated and electrochemical reactions may occur. The second class is Electronic-EAP and it involves piezoelectric, electrostrictive and/or Maxwell stresses. These materials can require large electric fields (>100MV/m) to achieve longitudinal deformations at the range from 4 - 360%. Some of the difficulties in characterizing EAP include: nonlinear properties, large compliance (large mismatch with metal electrodes), non-homogeneity (resulting from processing) and hysteresis. To support the need for reliable data, the authors are developing characterization techniques to quantify the electroactive responses and material properties of EAP materials. The emphasis of the current study is on addressing electromechanical issues related to the ion-exchange type EAP also known as IPMC. The analysis, experiments and test results are discussed in this paper.

Response of multilayered composite laminates to dynamic surface loads

A theoretical investigation of the response of multilayered composite laminates to concentrated and distributed dynamic surface loads is carried out. Each layer of the laminate is assumed to be transversely isotropic and dissipative with arbitrarily oriented symmetry axis. The dissipative property of the material is modeled approximately through the introduction of a frequency-dependent damping function. A multiple transform technique is used to calculate the spectra and time histories of the displacements and stresses produced by a variety of dynamic loads, and the quantitative features of the waves produced in the laminate are determined. The methodology developed in this work is expected to be useful in the prediction of the response of composite laminates to impact loads and also in the characterization of acoustic emission (AE) sources in these materials under static and dynamic loads.

Development of Nanolaminate Thin Shell Mirrors

The space science community has identified a need for ultra-light weight, large aperture optical systems that are capable of producing high-resolution images of low contrast. Current mirror technologies are limited due either to not being scalable to larger sizes at reasonable masses, or to lack of surface finish, dimensional stability in a space environment or long fabrication times. This paper will discuss the development of thin-shell, nano-laminate mirror substrates that are capable of being electro-actively figured. This technology has the potential to substantially reduce the cost of space based optics by allowing replication of ultra-lightweight primary mirrors from a master precision tool. Precision master tools have been shown to be used multiple times with repeatable surface quality results with less than one week fabrication times for the primary optical mirror substrate. Current development has developed a series of 0.25 and 0.5 meter spherical nanolaminate mirrors that are less than 0.5 kg/m2 areal density before electroactive components are mounted, and a target of less than 2.0 kg/m with control elements. This paper will provide an overview of nanolaminate materials for optical mirrors, modeling of their behavior under figure control and experiments conducted to validate precision control.

Electroactive polymers (EAP) low-mass muscle actuators

Actuation devices are used for many space applications with an increasing need to reduce their size, mass, and power consumption as well as cut their cost. Existing transducing actuators, such as piezoceramics, induce limited displacement levels. Potentially, electroactive polymers (EAP) have the potential for low-mass, low-power, inexpensive miniature muscle actuators that are superior to the widely used actuators. Under electrical excitation, EAPs contract and thus form a basis for muscle actuators. Efforts are being made to develop EAP materials that provide large displacements, and two EAP categories were identified to produce actuation strain of more than 10%. These categories include: (1) ion-exchange membrane --platinum composite polymer (so-called ionomers); and (2) electrostatically driven polymers. A comparison between EAP and the widely used transducing actuators shows that, while lagging in force delivering capability, these materials are superior in mass, power consumption and displacement levels. This produces an enabling technology of a new class of devices. Several muscle configurations were constructed to demonstrate the capabilities of these EAP actuators. The emphasis of this manuscript is on ionomer actuators.

Measurements and macro models of ionomeric polymer-metal composites (IPMC)

Ionomeric Polymer-Metal Composites (IPMC) are attractive type of electroactive polymer actuation materials because of their characteristics of large electrically induced bending, mechanical flexibility, low excitation voltage, low density, and ease of fabrication. The diffusion of ions between the electrodes causes the material to bend. The unique features of the IPMC materials and their need for special operating environment require new approaches to measuring their characteristics. A macro model that relates the electric input and mechanical output is required for the material characterization and application. This paper addresses the macro models for the electric inputs and electromechanical actuation of IPMC. A distributed RC line model is developed to describe the varying capacitance of the electric input behavior and a four-parameter model to express the relaxation phenomena. The power capacities of the IPMCs are estimated according to the established models and the measured results. Results for several types of IPMCs, which present different behaviors, are presented.

Wave Attenuation in Fiber-Reinforced Composites

Attenuation of waves in graphite/epoxy composite laminates is studied through an ultrasonic experiment and theoretical analysis of the recorded waveforms. The specimens are immersed in water, insonified by a beam of acoustic waves from a broadband transducer, and the reflected signals are recorded by a second transducer in a pitch-catch arrangement. The received signals are analyzed by means of a theoretical model in which the composite is assumed to be a transversely isotropic and dissipative medium. A simple model of dissipation is proposed and calculations based on wave propagation in the laminate are carried out. The values of the damping parameters are determined through comparison between the measured and calculated waveforms of the reflected signal. Results are presented for four unidirectional specimens of different thicknesses. The assumed model of attenuation is shown to yield excellent agreement between measured and calculated waveforms in all four cases.

New technologies for the actuation and control of large aperture lightweight optical quality mirrors

This paper presents a set of candidate components: MEMS based large stroke (>100 microns) ultra lightweight (~0.01 gm) discrete inch worm actuator technology, and a distributed actuator technology, in the context of a novel lightweight active flexure-hinged substrate concept that uses the nanolaminate face sheet.

In-service monitoring of steam pipe systems at high temperatures

An effective in-service health monitoring system is needed for steam pipes to track through their wall the condensation of water in real-time at high temperatures. The system is required to measure the height of the condensed water inside the pipe while operating at temperatures that are as high as 250°C. The system needs to be able to make time measurements while accounting for the effects of water flow and cavitation. For this purpose, ultrasonic waves were used to perform data acquisition of reflected signals in pulse-echo and via autocorrelation the data was processed to determine the water height. Transmitting and receiving the waves is done by piezoelectric transducers. There are transducers with Curie temperatures that are significantly higher than the required for this task offering the potential to sustain the conditions of the pipe over extended operation periods. This paper reports the progress of the current feasibility study that is intended to establish the foundations for such health monitoring systems.

Rapid Characterization of the Degradation of Composites Using Plate Waves Dispersion Data

NDE methods are needed to determine the structure integrity, stiffness and durability (residual life) of structures and they can be extremely useful in assuring the performance of structures using smaller safety factors. While the integrity and stiffness can be extracted directly from NDE measurements, strength and durability can not be associated with physical parameters and therefore, cannot be measured by NDE methods. Specifically, NDE methods are developed to detect and characterize flaws and to determine the material properties of test specimens. For many years, composites as multi-layered anisotropic media, have posed a challenge to the NDE research community. Pulse-echo and through-transmission are the leading methods that are used in practice to evaluate the quality of composites. However, these methods provide limited and mostly qualitative information about the material properties and many defects. Following the discovery of the LLW and the Polar Backscattering phenomena in composites [1,2], numerous experimental and analytical studies have taken place using obliquely insonified ultrasonic waves [3–5]. These studies led to the development of effective quantitative NDE capabilities to determine the elastic properties, to accurately characterize defects and even to evaluate the quality of adhesively bonded joints [6, 7]. In spite of the progress that was made both theoretically and experimentally, oblique insonification techniques are still academic tools and have not yet become standard industrial test methods for NDE of composite materials. The authors investigated the issues that are hampering the transition of these methods to the practical world of NDE and are involved with extensive studies to address these issues. This paper covers the progress that was made by the investigators in tackling the theoretical and experimental issues to solidify the foundation of the techniques and their transition to practical NDE tools.