Journal of Reinforced Plastics and Composites

This paper addresses the flow properties of Resin Transfer Molding (RTM) with primary emphasis on transverse (z-direction) permeability. In the case of RTM, the permeability [K] matrix of the fabric preform is the quantity that governs the accuracy of any numerical flow simulation. Characterization of the in-plane permeabilities (K_xx, K_yy,) is done by simple radial flow experiments. However, the transverse permeability is difficult to measure accurately due to the inability to visualize the flow pattern through the thickness. Also, one dimensional flow experiments, which are ideal, encounter significant racetracking along the edges that inhibits proper monitoring of the true transverse flow. The need for accurate transverse permeabilities stems from use of three dimensional flow simulations to model the manufacture of thick section composite components fabricated by the vacuum assisted resin transfer molding genre of composite processes.

The objective for this work is to develop an improved methodology to determine the transverse permeability of a fabric preform. This will include formation of the experimental apparatus, configuration of visualization tools, analytical solution for that geometry, creation of a numerical model, permeability predictions given flow front location with time, and finally plotting the series of permeabilities as a function of the percentage of fill. The experimental apparatus is based on a vacuum driven operation to simulate the process desired. A modified version of "SMARTweave," which is the enabling technology for insitu flow monitoring, is used for visualization of the fluid phase. Preliminary experimental data is reported and transverse permeabilities are calculated for several preforms. A comparison of the data to established methods is shown.

Resin film infusion (RFI) has been found to be a cost-effective technique for the fabrication of complex shaped composite parts for primary structural applications. Dry textile preforms are infiltrated, consolidated, and cured in a single step, eliminating the labor to lay-up prepreg tape. The large number of processing variables and the complex material behavior during infiltration and cure make experimental optimization of the RFI process extremely inefficient.

The objective of this work was to develop and verify a three-dimensional model to simulate the RFI process. For a specified pressure and temperature cure cycle the code can predict resin pressure, viscosity and degree of cure, flow front progression, and temperature distribution in the preform and tooling components. The model was divided into submodels which describe resin flow, heat transfer, and resin kinetics. A finite element/control volume approach was used to model the flow of the resin through the preform. Boundary conditions include specified pressure, specified flow rate, and vents. A finite element formulation of the transient heat conduction equation was used to model the heat transfer. Thermal boundary conditions include either specified temperature or convection. The code was designed to be modular so the flow problem could be solved alone, or coupled with the thermal problem. The problems are solved sequentially in a quasi-steady state fashion.

Non-isothermal experiments with a reactive resin were conducted to verify the thermal module and the resin model. A two blade stiffened panel, fitted with sensors, was manufactured using the RFI process. Thermocouples were used to measure the temperatures, and FDEM (dielectric) sensors and pressure transducers were used to monitor the flow front progression. Model predictions and experimental results were found to be in close agreement for the temperatures and flow front progression. The predicted and measured infiltration times matched to within 12%, and the temperatures to within 5%.

Existing finite element models can predict the behavior of complex anisotropic structures under combined loading. However, these models typically assume a simplified material model, non-distorted orientation of the fiber materials used in the manufacturing process, and the absence of defects such as voids and dry spots. Such assumptions are particularly questionable in areas of high curvature or significant thickness changes. These areas typically have the double effect of being areas of high stress as well as areas susceptible to manufacturing defects.

This paper will discuss preliminary results from a combined experimental and finite element study that shows the effect of local material variations on actual structural performance. The subject of this study is the Solectria Sunrise composite wheel well. The first part of the paper will present results from testing of an actual wheel well; the second part will briefly discuss our recent efforts at studying one particularly critical component: the tapered sandwich structure and its relationship to resin infusion manufacturing defects and fatigue behavior.

An experimental investigation has been conducted to determine the flexural characteristics of concrete reinforced with chopped graphite/epoxy prepreg fibers. The flexural behavior of concrete beams reinforced with 0.5% and 1.5% fiber volume fractions of 1.0 inch (25.4 mm) fibers, 3.0 inch (76.2 mm) fibers and a mixture of both lengths of fibers were compared with plain concrete beams. A set of thirty-five 30 inch (76.2 cm) long beams with a 6 inch (15.2 cm) square cross section were tested in four-point flexure. The flexural strength, toughness, and stiffness characteristics were evaluated by monitoring load and center deflection. This study provides insight into fiber length and fiber volume fraction that best improve the flexural characteristics of concrete. The flexural toughness of concrete is increased by two and a half orders of magnitude when 1.5% volume fraction of 3.0 inch (76.2 mm) long fibers are included in the mix. Also, when 1.5% volume fraction of 3.0 inch (76.2 mm) long fibers are included in concrete, the deflection prior to failure increases by a factor of six. The maximum load of plain concrete is increased 25% with the addition of 1.5% volume fraction of 1.0 inch (25.4 mm) long fibers and 74% with the addition of 1.5% volume fraction of 3.0 inch (76.2 mm) long fibers.

A Layerwise Trigonometric Shear Deformation Theory (LTSDT) for the analysis of two layered cross-ply laminated beams, taking into account discrete layer transverse shear deformation effects, is presented. The theory is as simple as the first order shear deformation theory and does not require shear correction factor. The sinusoidal function is used in the displacement field in terms of thickness coordinate to represent shear deformation. The most important feature of the theory is that the transverse shear stress can be obtained directly from the use of constitutitve relations, satisfying the stress-free boundary conditions at top and bottom of the beam and satisfying continuity at the interface. Governing differential equations and boundary conditions of the theory are obtained using the principle of virtual work. The system of governing differential equations is reduced to a single sixth order linear differential equation. A two layered asymmetric cross-ply laminated beam is considered for the numerical study to validate the accuracy of the theory.