Rani W. Sullivan

Guided Wave Behavior Analysis in Multi-Layered Inhomogeneous Anisotropic Plates

homogeneous anisotropic plates. It has been reported before that guided waves in inhomogeneous plates tend to follow preferred directions based on the location of ply-groups as well as the orientation of the fibers [1]. The pattern obtained by imaging the leaked energy into the surrounding fluid (earlier called as Plate Wave Flow Patterns) has been shown to indicate fiber orientations [2]. In this paper, a model based on the Thomson-Haskell transfer matrix is employed to obtain the internal distributions of the energy vector within the inhomogeneous plate. Based on the theoretical results, the plate wave flow patterns can be predicted and compared with the experimental results. The results provide insight into the understanding of the generation mechanism of guided wave mode patterns in inhomogeneous plates. THEORETICAL BACKGROUND Consider a general anisotropic multilayered plate immersed in water. The plate consists of an arbitrary number of general anisotropic layers n, rigidly bonded at their interfaces as shown in Figure 1. Without losing generality, it is assumed that leaky plate waves propagate along the Xl direction. First consider the plane wave propagation in a general anisotropic multi-layered structure as shown in Figure 1. The displacements of the wave are given by

Vibration testing of a carbon composite fuselage

This paper describes the details of an experimental investigation focusing on the vibration characteristics of a composite fuselage structure of an ultralight unmanned aerial vehicle (UAV). The UAV has a total empty weight of 70.3 kg and 6.3 m in length. The fuselage structure consists of the fuselage body with an integrated vertical stabiliser. All structural components are fabricated from oven-cured laminated carbon composite materials using uniaxial and biaxial prepreg fabric. The modal characteristics of the fuselage structure are determined for a free-free configuration which is simulated by suspending the structure from its wing attachment points through the use of springs. A centrally located shaker system is used to induce vertical oscillations in the structure, which is instrumented with nineteen dual axis accelerometers. Dynamic properties such as the frequency, damping and associated mode shapes are obtained for aeroelastic analysis. The design and implementation of the vibration tests along with the experimental results are presented.

Master Creep Compliance Curve for Random Viscoelastic Material Properties

The objective of this study is to apply the time-temperature superposition principle (TTSP) to the viscoelastic material functions that exhibit a large degree of variability to predict the long-term behavior of a vinyl ester polymer (Derakane 441–400). Short-term tensile creep experiments were conducted at three temperatures below the glass transition temperature. Strain measurements in the longitudinal and transverse directions were measured simultaneously using the digital image correlation technique. The creep compliance functions were characterized using the generalized viscoelastic constitutive equation with a Prony series representation. The Weibull probability density functions (PDFs) of the creep compliance functions were obtained for each test configuration and found to be time and temperature dependent. Creep compliance curves at constant probabilities were obtained and used to develop the master curves for a reference temperature of 24 °C using the TTSP.

Fiber Bragg Grating Strains to Obtain Structural Response of a Carbon Composite Wing

The objective of this study was to determine the deflected wing shape and the out-of-plane loads of a large-scale carbon-composite wing of an ultralight aerial vehicle using Fiber Bragg Grating (FBG) technology. The composite wing, subjected to concentrated and distributed loads, was instrumented with an optical fiber on its top and bottom surfaces positioned over the main spar, resulting in approximately 780 strain sensors bonded to the wings. The in-plane strains from the FBG sensors were used to obtain the out-of-plane loads as well as the wing shape at various load levels using NASA-developed real-time load and displacement algorithms. The calculated out-of-plane displacements and loads were generally within 4% of the measured data.Copyright

On the Viability of Energy Harvesting Piezoelectric Devices for Supplemental Power Generation in Uninhabited Aircraft Systems

In this work, a global air vehicle finite element model employing empirically determined cyclic loads is used to assess the viability of energy harvesting piezoelectric devices for supplemental power generation in lightweight uninhabited aircraft systems (UASs) where the “Owl” all-composite ultralight UAS is used as a candidate proof-of-concept platform. The Owl was developed at the Mississippi State University Raspet Flight Research Laboratory as part of the U.S. Army Space Missile Defense Command High Performance Materials/Processes (HIPERMAP) Program. In the HIPERMAP flight test program, straintime histories were recorded at key locations throughout the wing and fuselage. These measured strains were used in the investigation of practical implementation of energy harvesting piezoelectric materials within the Owl UAS with the intent of harvesting electrical power from structural oscillations of the wings during operation. The total power generated was determined from a 80 KIAS 4G pullup maneuver, where piezoelectric patches were simulated in the span-wise direction along the inner surfaces of the upper and lower skins of both wings. The energy harvesting capability of the Owl UAS was insufficient due to the small oscillation frequency and low strain magnitudes of the wing during flight. Nevertheless, such energy harvesting devices remain promising for microscale UASs where power requirements are lower and the oscilation frequency of the wings is higher.

Viscoelastic Creep Compliance Using Prony Series and Spectrum Function Approach

The objective of this study is to compare the viscoelastic material property of a vinyl ester (VE) resin using (1) the generalized 3-D viscoelastic constitutive equation with a Prony series representation and (2) a spectrum function model. The Prony series representation of the Generalized Kelvin model (GKM) is used to determine the Prony series coefficients through the linear least squares (LSQ) method. The Elastic-Viscoelastic Correspondence Principle (EVCP) and the Laplace transform are used in the spectrum function approach, which utilizes a carefully selected distribution function that has the potential to describe a wide range of materials. Short-term unidirectional tensile creep experiments are conducted at two stress levels and at four temperatures below the glass transition temperature of the VE polymer. Experimental strains in both the longitudinal and transverse directions and the applied stress are measured using the digital image correlation (DIC) technique. The measured data is subsequently used to determine the creep compliance function for each test configuration. The potential and limitations of each modeling approach are discussed.

Impact response in polymer composites from embedded optical fibers

This study investigates the feasibility of using embedded optical fibers in polymer matrix composite laminates to characterize delaminations caused by low-velocity impacts with energies between 30 J and 50 J. Impact damage can occur in composite structures during manufacture, in-service, storage and routine maintenance. Because of their small size and light weight, optical fibers can be embedded in composite structures during the manufacture of composite parts, allowing the structure to be monitored for impact-induced delaminations without being removed from service. In this study, optical fibers are embedded in a grid configuration at four selected locations (one-third from impact surface, midplane, two-thirds from impact surface, and farthest ply from impact) in thick autoclave-cured graphite/epoxy laminates. Low-velocity impact testing is performed at four energy levels. Manufacturing procedures for embedding the optical fibers within the composite laminates are investigated. The strain distribution from the optical fibers is correlated with ultrasonic C-scans of the laminates in which they are embedded. X-ray computed tomography scan images are also compared to those from ultrasonic C-scans. Results indicate that embedded optical fibers can provide post-impact strain responses and delamination area from each embedded site within the impacted laminates.

Distributed optical sensing in composite laminates

An optical fiber–based full-field strain measurement technique was used to investigate delamination growth in laminated composites. An experimental setup to load the test samples under idealized modes of delamination was used to investigate the ability to capture the shape and location of the delamination front. It is envisioned that the demonstrated approach has significant field applications in controlled laboratory settings where delaminations have to be located accurately. Furthermore, the ability of this measurement system to provide full-field strain measurements at any given pre-implanted location through the thickness overcomes the surface strain measurements obtained by digital image correlation. In order to demonstrate the technique, distributed fiber optic sensing is used to monitor the propagation of delaminations under pure mode I and II loading. Optical fibers were embedded one ply from the crack plane of both double cantilever beam and end notch flexure specimens. To establish a repeatable fabrication methodology, manufacturing techniques for embedding the optical fibers during the laminate layup process were established. Specimens with and without embedded fibers were tested to verify the fibers did not affect measured fracture toughness values. Crack lengths measured with the optical fibers compared well with true crack lengths, and measured strain distributions compared well with results from finite element analysis.

HIGH PERFORMANCE SCANNING SYSTEM TO EXPLORE ULTRASONIC OBLIQUE WAVE REFLECTION IN COMPOSITE LAMINATES

This paper will provide an overview of the initial reflection spectrum experiments conducted to utilize the unique characteristics of the 12-axis dual transducer ultrasonic workstation, which has been designed and fabricated at MSU [1]. This system was designed as an oblique incidence research scanning tool unlike commercially available multi axis machines which are more suited for contour following and high speed scanning. Through the implementation of DSP processors, it has become possible to process and image the ultrasonic data in near real time, using a personal computer such as a 486 PC.

A Statistical Formulation for Master Modulus Curves of a Vinyl Ester Polymer

ABSTRACT Master modulus curves are developed for a vinyl ester polymer with variability in its material properties. Tensile creep strains were obtained at three temperatures below the Tg through digital image correlation. A spectrum function was used to represent the viscoelastic strain response and modulus. A two-parameter Weibull distribution was used to characterize the probability distribution of the longitudinal modulus. The Weibull probability density functions of the viscoelastic modulus were obtained for each test configuration and found to be time and temperature dependent. Longitudinal modulus curves at constant probabilities were used to develop the master curves using the time–temperature superposition principle. GRAPHICAL ABSTRACT