Journal of Aerospace Engineering

The internal cracking patterns of a series of small mortar specimens were observed using a technique called X-ray microtomography. The experiments were arranged such that three-dimensional images of internal cracks could be made at several different times during the load history of the sample. This arrangement allowed progressive crack growth to be observed at micron-scale resolution in a single specimen. The specimens were loaded in compression. Image processing routines were used to make quantitative measurements of internal crack growth and damage progression. Preliminary measurements indicate that the maximum increase in crack area occurs around the peak load. Three-dimensional renderings qualitatively illustrate features of internal cracking such as the effects of different microstructural features.

An experimental investigation was conducted to determine the flexural characteristics of advanced composite grid reinforced-concrete structures. Concrete beams reinforced with orthogonal grids composed of corrosion-resistant fiber-reinforced polymer composites (carbon/vinyl-ester and fiberglass/vinyl-ester) were compared to standard steel-reinforced beams. Flexure tests were performed on 26 0.762 mm (30 in.) long beams with a 0.1524 mm (6 in.) square cross section under four-point bending. The various test configurations incorporated #3 steel-reinforcing bars and/or composite grids with 0.0508 or 0.1016 mm (2 or 4 in.) spacing. The flexural strength, stiffness, and failure characteristics were evaluated by monitoring the load, deflection, and strain coupled with visual observation. The present study provides insight into the relevant load-transfer mechanisms, geometric issues, and dependence on material properties. The results show that the behavior of composite grid reinforced beams can be predicted with the same accuracy as traditional steel reinforcement enabling a simple, straightforward extension of existing design criteria. Composite grids designed and fabricated to have the same axial rigidity as steel rebar result in equal deflections for equal loads and significantly higher ultimate strengths, with reduced toughness due to the linear elastic characteristics of the reinforcement materials.

In this paper we present a methodology and a rigorous solution formulation for multiple crack problems in nonhomogeneous plates using the dislocation complex potential technique. The fundamental perturbation problem is investigated by establishing a system of singular integral equations, whose solution gives mixed-mode stress intensity factors [(SIFs), k] and strain energy release rate [(SERR), G]. The numerical integration of the singular integral equations is carried out by using a Lobatto type collection technique. Finite-element analysis (FEA) of the representative problem is provided for accuracy comparison. Finally, studies for two cracks located on the opposite side of the interface are presented.

The elasticity solutions for composite laminates in bending originally developed by Pagano are extended for Hertzian contact-type loading in this paper. Two solutions are obtained, corresponding to cylindrical bending and three-dimensional bending of a rectangular plate. Loads simulating contact with a spherical indentor are obtained by superposition of the elasticity solutions for individual terms of truncated Fourier sine series representations of Hertzian contact stresses. In this way, a benchmark for models of impact loaded plates is provided in which the loading is more representative of contact than the solutions often used for that purpose. The solutions provide insight into the interpretation of experimental data from low-velocity impact tests and are used to judge the performance of finite-element–based low-velocity impact algorithms in predicting stress under the impactor.

The paper presents a methodology that can be employed in numerical and analytical analyses of composite plates subjected to the action of a laser beam. The problem is related both to protection of composite components of aerospace structures from hostile high-energy laser weapons as well as to drilling holes in composites using a laser. In both cases, a three-dimensional state of stresses in the vicinity of the focal spot (the contact zone where the structure-laser beam interaction takes place) has to be evaluated. In addition, if a hostile laser locks on a structure, it is necessary to estimate changes in the structural response during and after the interaction. The paper outlines the solution of the problem of local three-dimensional stresses as well as an approach to the global response analysis. Finally, a practical problem where local damage exists in the structure as a result of a limited exposure to a laser beam is discussed. Numerical examples illustrate that even a small indentation produced by a laser can noticeably affect the natural frequencies of a composite beam.

Previous experimental work has shown a reduction in the effective elastic response of [±45°]_s SiC/Ti composite tubes under axial and torsional loading. There is considerable evidence of fiber/matrix debonding in off-axis plies and the presence of preexisting fiber cracks, extending radially from the carbon core of the SiC fiber, is also well documented. In this work, the Generalized Method of Cells (GMC) micromechanical model is used to calculate a constitutive equation for the composite material in each ply, in the presence of fiber cracks or fiber/matrix debonds. These constitutive equations are used in the exact analytical solution for the problem of an arbitrarily laminated tube under axisymmetric or torsional loading. By employing GMC in the solution to the tube problem, predictions are made as to the degree of degradation of the elastic response of these tubes due to partial or full debonding of the fiber/matrix interface or preexisting fiber cracks or a combination of both. These theoretical predictions are compared to experimental results.