Lisa M. Dangora

Shear Characterization of a Thermoplastic Cross-Ply Prepreg

This paper presents the investigation of a thermoplastic cross-ply sheet for use in manufacturing small arms protective helmets. The material system contains four unidirectional layers oriented in a [0°/90°/0°/90°] configuration. This advanced composite is wholly thermoplastic, consisting of ultrahigh molecular weight polyethylene (UHMWPE) fibers within a polyurethane matrix. Due to the polymeric nature of the constituent materials, the mechanical behavior of the composite system will have a dependence on forming temperature. The shear characterization of the prepreg and the investigation of the factors influencing the representative shear stiffness including sample geometry, strain rate, conditioning and temperature are discussed.

A Novel Method for Determining the Stiffness of a Composite Structure Resulting from the Manufacturing Process Using a Discrete Mesoscopic Finite Element Model

This paper presents a methodology for extending the use of the beam-shell forming model to predict the structural properties of the composite part. After the forming simulation has been performed, the material definition will be changed such that the beam elements will represent the fiber reinforcements and the shell elements will represent the resin. The methodology behind the entire approach will be demonstrated using a stitched uniaxial glass fabric. The methodology for characterizing the fabric behavior will be discussed. After the part has been formed, it will be infused with resin. The methodology for characterizing the composite behavior will be introduced. The finite element model will be compared with experimental data to validate the methodology.

Application of a Discrete Mesoscopic Finite Element Approach to Investigate the Bending and Folding of Fiber-Reinforced Composite Materials during the Manufacturing Process

During the manufacturing of fabric-reinforced composite parts using a matched-die compression molding process or liquid composite molding, the fabric may experience local in-plane compressive loads that cause out-of-plane deformations. The waves that result from this outofplane motion can lead to the formation of resin rich pockets (during the infusion stage of a dry fabric) or they may be forced down into a fold by the tooling. Defects such as resin-rich pockets and folds compromise the structural integrity of the formed composite part. Therefore, it is valuable to have a simulation tool that can accurately capture the fabric bending properties and predict the locations where waves or folds are likely to occur as a result of the manufacturing process. The tool can then be used to investigate changes in the forming parameters such that the development of such defects can be mitigated. A hybrid finite element model used with a discrete mesoscopic approach captures the behavior of continuous fiber-reinforced fabrics where the fabric yarn is represented by beam elements and the shear behavior is implemented in shell elements. User-defined material subroutines describe the mechanical behavior of the beams and shells for their respective contributions to the overall fabric behavior. Simulations are used to demonstrate the ability of the modeling approach to predict the amplitude and curvature of out-of-plane waves. The simulation results are compared with experimental data to show the accuracy of the modeling. Additional models are presented to demonstrate the capability of the simulation tool to capture fabric folding.

Investigation of the Mechanical Properties of a Stitched Triaxial Fabric

A traxial fabric was investigated for use in composite forming applications. Three stitched layers of fibers, originally oriented at [-60o/0o/60o], comprise the fabric architecture. The mechanical properties of the material are characterized by testing the tensile, shear, and frictional behavior. Conventional shear frame testing methodology assumes that the yarns are originally oriented perpendicular to one another; however, such an assumption is not valid for this particular fabric geometry and must be adjusted. The material behavior is implemented into a discrete mesoscopic finite element model that can predict the response of the material during deformation. Different element types will be investigated to represent the fabric and used to determine the ideal mesh configuration that best captures the fabric behavior. Different modes of deformation will also be studied, and the observed experimental deformation will be compared to the deformation predicted by the finite element model.

Experiment-Based Assessment of NLBeam for Modeling Geometrically Nonlinear Dynamic Deformations

As the wind energy industry expands, larger wind turbines are being developed to harvest more energy for a given plant site. It has also been noted that gear boxes within the wind turbines are wearing out faster than expected. One possible explanation is that current modeling approaches employed in wind turbine design do not adequately account for nonlinearities associated with the large deformations in wind turbine blades. A finite element code, NLBeam, has been developed to account for nonlinearities using geometrically exact beam theory. To validate the code, experiments were conducted on a surrogate aluminum blade and the results were compared to simulation results from NLBeam. Further development of the NLBeam code will be based off of this research. In the future, NLBeam will be coupled with WindBlade, a computational fluid dynamics based software which has been developed at Los Alamos National Laboratory to study fluid-structure interaction between turbines within wind plants.

Characterization of Cured Composite Materials for Wind Turbine Blades

NCFs (Non-Crimp Fabrics) infused with epoxy resin are popular in the design of wind turbine blades and other complex systems due to their ability to conform to complex shapes. Past work in the development of a combination beam-shell modeling approach to simulate the forming of NCF composites has been demonstrated to capture the change in the orientations of the yarns during a forming process. The structural performance of these manufactured blades is often analyzed using finite element simulations that consider the material properties of the fibers and of the resin based on the rule of mixtures and orthotropic shells where the model is sectioned into zones that account for changes in the material properties due to variations in the orientations of the lamina and number of layers. With the availability of the beam-shell model, the use of zones can be removed if the individual contributions of the yarns (beam elements) and resin (shell elements) can be characterized and the orientations of the yarns resulting from a forming simulation can be used to account for the variations in the material properties of the composite throughout the blade. This research uses a combination of static flexure tests and impact modal tests to ascertain the material properties of the fibers and resin in a unidirectional and biaxial non-crimp fabric laminate plates. The material properties are used in a finite element model of the plate and the model is analyzed in flexure and in a free-free modal configuration to compare to experimental results. Two different approaches are used in the commercially available software Abaqus to model the plate. One approach uses a combination of beam and shell elements to represent the fibers and the resin, respectively. The other approach uses orthotropic shell elements to capture the unbalanced behavior of the fiber/resin composite. The beam/shell modeling approach better represents the overall behavior of a single-layer plate and can be extended to consider multiple plies.