This investigation concerns the frequency dependent heat generation behavior and the heat generation mechanisms for the thermal patterns of delamination in fiber reinforced composites during a vibrothermographic test, which uses real time thermography as a nondestructive evaluation of a structure or a component excited with mechanical vibration. A local resonance model was proposed in the past to describe the frequency dependent heat generation behavior during a vibrothermographic test, and this model was used as a basis for writing software for calculating the natural frequencies of a plate with the size of delamination. Vibrothermographic tests were performed on three glass-epoxy panels that each contained four different sized simulated delaminations. Comparison between the observed vibrothermal peak frequencies and the natural frequencies predicted by the local resonance model, and investigations of the thermoelastic emission field in the delamination region using SPATE, were made to determine the validity of the local resonance model. A significant conclusion of the results is that the local resonance is indeed the mechanics model for the frequency dependent heat generation behavior. A careful measurement of the degree of heating of both sides of [0₅] glass-epoxy panel with delaminations on the 2-3 ply interface, and comparison between the predicted heat patterns generated from a finite difference heat transfer program and observed heat patterns, was made to identify the heat generation mechanism. The results show that the majority of heat generation during vibrothermographic testing results from higher stresses or strains due to local resonance. The heat generation was affected by the combination of the principal strains and shear strain for the lower modes of resonant vibration, and was dominated by the shear strain for the higher modes of resonant vibration. Impact damaged graphite-epoxy panels were also inspected constituting an application of vibrothermography on real damaged components. The degree of heating of the damage were measured through a frequency range, and the damage severity was inspected by ultrasonic C-scan and edge replication. From comparison of two plots of the degree of heating versus exciting frequency, either the area under the curve or the number of vibrothermal peak frequencies, the severity of the damage can be qualitatively identified. / Ph. D.

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of science in Engineering. Johannesburg, 1992. / This project involves the comparison of different failure criteria with experimental results for fibre composite materials, and investigates the usefulness of acoustic emission in composite testing. Three sets of specimens were tested to failure in tension. The specimens had various ply orientations and were tested using acoustic emission to determine ply failures. Carbon and glass fibre reinforced epoxy pre-impregnated specimens were used. The testing machine was an ESH testing machine, and a physical Acoustics corporation computer and data acquisition unit were used to record data from a piezo-electric sensor. Suitable failure criteria should be chosen on the basis of ply orientation and material type (eg fibre stiffness), a combination of criteria being used if necessary. Acoustic emission was successfully used to detect ply failure in multi-layered laminates. / AC2017

A novel approach has been introduced in making flexible armor composites. Armor composites are usually made by reinforcing Kevlar fabric into the mixture of a polymer and nanoscale particles. The current procedure deviates from the traditional shear thickening fluid (STF) route and instead uses silane (amino-propyl-trimethoxy silane) as the base polymer. In addition, a cross-linking fixative such as Glutaraldehyde (Gluta) is added to the polymer to create bridges between distant pairs of amine groups present in Kevlar and silated nanoparticles. Water, silane, nanoparticles and Gluta are mixed using a homogenizer and an ultra-sonochemical technique. Subsequently, the admixture is impregnated with Kevlar - bypassing the heating and evaporating processes involved with STF. The resulting composites have shown remarkable improvement in spike resistance; at least one order higher than that of STF/Kevlar composites. The source of improvement has been traced to the formation of secondary amine C-N stretch due to the presence of Gluta. / by Vincent Lambert. / Thesis (M.S.C.S.)--Florida Atlantic University, 2009. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2009. Mode of access: World Wide Web.

Graduation date: 1993

M. Tech. Mechanical Engineering / The increased use of composite materials in different industries has led to the realization of some of its benefits and disadvantages. One of the major problems, however, is the availability of biaxial test data for different composite materials. This is because structures during application face multi-axial stress states examples of such stress state scenarios include wind turbine blades and pressure vessels. This has also led to diverse range of test methods and material compositions such as combining different fibbers. The material used in this work is polymer matrix honeycomb sandwich panels. Sandwich panels are fabricated by attaching two thin stiff skins of fiber glass or carbon fiber reinforced laminates to a lightweight core. This work addresses some of the major advantages and disadvantages of this testing method. The main objective of this study is to develop a repeatable, cost effective and time efficient method for multi-axial testing of sandwich panels using existing resources.

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A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfiment of the requirements for the degree of Master or Science in Engineering. Johannesburg, 1993. / The stress concentration factors around rectangular holes in carbon-fibre reinforced epoxy plates, subject uniaxial loads, were investigated experimentally and theoretically. To obtain theoretical solutions, two approaches were adopted; the finite element method and the theory of elasticity using the method of complex variable functions. Reflective photoelasticity was used as the experimental method. The determination of the stress concentration factor around a rectangular hole in a glass-fibrereinforced plate was attempted using transmissive photoelasticity, but no meaningful results were obtained. [Abbreviated Abstract. Open document to view full version} / MT2017

The problem examined is an infinite anisotropic layer with a through crack at arbitrary orientation, subjected to uniform in-plane remote loading. The purpose of this study is to gain a better understanding of several theoretical models for predicting the direction of crack propagation and the level of load causing crack extension, and to present a new model for predicting the critical load. The discussed models are particularly examined in detail with regard to the physical parameters affecting the results. Comparison is made with available experimental results. It is shown that the normal stress ratio theory provides good agreement with experimental crack growth direction, independent of physical parameters, and that the newly proposed traction ratio theory predicts well the critical load causing crack extension. / M.S.