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The concept of achieving low cost (≈ $20/m²) and ultra low weight (5 kg/m²) for heliostats is explored theoretically and experimentally. The objective of this work is to significantly improve the cost and performance of the structure under concern, without sacrificing strength and efficiency. The focus is on an innovative design of stretched-membrane heliostats. A reflective membrane of thin film is supported by a taut fishnet structural membrane consisting of graphite fiber-polymer matrix composite. The reflective and structural membranes are attached to a ring frame made of wood. The nonlinear problem of stress-strain analysis is formulated and solved using the finite-element code NASTRAN. The analysis is done for loads which include the initial stretching of the film and structural membrane and the pressure load due to wind. The scope of the present work is limited to analyzing the structural deformation behavior of flat-plate heliostats and partial extension to parabolic and semi-hemispherical dish reflectors.

The dynamic response of subsurface cracks in fiber reinforced composites is analytically studied. The response of layered half-space and three-layered plate with two interface cracks excited by a plane SH-wave and line load respectively are studied by formulating the problem as integral equations in the frequency domain. The governing equations along with boundary, regularity and continuity conditions across the interface are reduced to a coupled set of singular integral equations by using Betti's reciprocal theorem along with the Green's functions. In addition, the transient response of an orthotropic half-space with a subsurface crack subjected to inplane line load at an arbitrary angle is analyzed. Two new Green's functions for the uncracked medium are developed and used along with the representation theorem to derive the scattered field. Satisfaction of the traction free condition at the crack surfaces gives rise to a system of singular integral equations. Singular integrals involved in the analysis are computed numerically by removing the poles. Part of the integrals containing the poles are then obtained analytically by using residue theorem. The solution of singular integral equations are obtained by expanding the unknown crack opening displacements (COD) in terms of a complete set of Chebychev polynomials. The problem is first solved in the frequency domain, the time histories are then obtained numerically by inverting the spectra via Fast Fourier Transform (FFT) routine. Numerical results are presented for isotropic and anisotropic materials for several different crack geometries. The results show significant influence of crack geometries and material properties on the COD and surface response of composites.

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No description available.

No description available.

No description available.

No description available.

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.