Continuation of tailored composite structures of ordered staple thermoplastic material

The search for the cost effective composite structure has motivated the investigation of several approaches to develop composite structure from innovative material forms. Among the promising approaches is the conversion of a planar sheet to components of complex curvature through sheet forming or stretch forming. In both cases, the potential for material stretch in the fiber direction appears to offer a clear advantage in formability over continuous fiber systems. A framework was established which allows the simulation of the anisotropic mechanisms of deformation of long discontinuous fiber laminates wherein the matrix phase is a viscous fluid. Predictions for the effective viscosities of a hyper-anisotropic medium consisting of collimated, discontinuous fibers suspended in viscous matrix were extended to capture the characteristics of typical polymers including non-Newtonian behavior and temperature dependence. In addition, the influence of fiber misorientation was also modeled by compliance averaging to determine ensemble properties for a given orientation distribution. A design tool is presented for predicting the effect of material heterogeneity on the performance of curved composite beams such as those used in aircraft fuselage structures. Material heterogeneity can be induced during manufacturing processes such as sheet forming and stretch forming of thermoplastic composites. This heterogeneity can be introduced in the form of fiber realignment and spreading during the manufacturing process causing radial and tangential gradients in material properties. Two analysis procedures are used to solve the beam problems. The first method uses separate two-dimensional elasticity solutions for the stresses in the flange and web sections of the beam. The separate solutions are coupled by requiring that forces and displacements match section boundaries. The second method uses an approximate Rayleigh-Ritz technique to find the solutions for more complex beams. Analyses are performed for curved beams of various cross-sections loaded in pure bending and with a uniform distributed load. Preliminary results show that the geometry of the beam dictates the effect of heterogeneity on performance. The role of heterogeneity is larger in beams with a small average radius-to-depth ration, R/t, where R is the average radius of the beam and t is the difference between the inside and outside radii. Results of the anlysis are in the form of stresses and displacements and are compared to both mechanics of materials and numerical solutions obtained using finite element analysis

The Composite Materials Manufacturaing HUB - Crowd Sourcing as the Norm

The Composites Manufacturing HUB puts compo- sites manufacturing simulations in the hands of those who need them to invent new and innovative ways to capture the extraordinary benefits of these high perfor- mance products at an acceptable manufactured cost. The HUB provides the user simple browser access to power- ful tools that simulate the actual steps and outcome con- ditions of a complex manufacturing process without the need to download and maintain software in the conven- tional manner. Learning use of the manufacturing simu- lation tools will also be accomplished on the HUB in or- der to allow for continuous learning and growth of the human talent required in composites manufacturing

Efficiently Dispersing Carbon Nanotubes in Polyphenylene Sulfide

Thermal plastics are replacing conventional metals in the aerospace, sporting, electronics, and other industries. Thermal plastics are able to withstand relatively high temperatures, have good fatigue properties, and are lighter than metals. Unfortunately, they are not very electrically conductive. However, adding carbon nanotubes to thermal plastics such as polyphenylene sulfide (PPS) can drastically increase the plastic\u27s conductivity at a low weight percent of nanotubes called the percolation threshold. The percolation threshold is the point where adding a little more carbon nanotubes brings together the network of nanotubes and greatly increases the conductivity. We need to learn how to increase the dispersion of nanotubes in PPS to reduce the amount of expensive nanotubes necesarry to reach the percolation threshold. Adding nanotubes to thermal plastics is a difficult procedure. A few different melting and mixing methods have been utilized in previous studies. Initially, we tested how to best disperse the nanotubes using an extruder after physically mixing the two components. We have determined that grinding the PPS pellets to 400 microns and smaller and then coating the PPS powder with the carbon nanotubes in a pulverizer reduces the size and number of carbon nanotube agglomerates in the PPS versus using pellets and mixing by hand. In addition, using moderate screw speeds such as 70 rpm in the extruder helped reduce agglomerates. These results will help us reach the percolation threshold of carbon nanotubes in polyphenylene sulfide while using a smaller amount of the costly nanotubes

Phase field modeling of crack propagation in double cantilever beam under Mode I

A smeared crack approach using a phase-field approach to fracture with unilateral contact condition was used to study the stress distribution and crack propagation in a double cantilever beam (DCB) specimen. The parameters in the numerical model were informed from atomistic simulations and validated with experimental data for poly(methyl methacrylate) that included data for damage initiation under different levels of volumetric and deviatoric stress components and fracture toughness measurements obtained under Mode I conditions. The phase field model includes two quantities, a length scale that controls the width of the crack and the critical fracture energy density. The study considered a sensitivity analysis of the influence of these two parameters to obtain optimal values. Experiments and simulations of DCB are shown to study the toughness of polymer and polymer composite specimens that include residual stresses developed in the specimen during cure

Prediction of Fiber Orientation in Compression Molded Parts of Short-Fiber-Reinforced Thermoplastics

Short Fiber molded thermoplastics like poly ether ketone ketone (PEKK) have the potential to be used to build reinforcement structures for next generation air crafts. Because of the anisotropic properties of these fibers, it is necessary to know the orientation of fibers in molded parts to determine the strength of parts. The goal of the project is to prepare samples that will be used to determine the spatial distribution of fibers as a function of the parts thickness and for comparison with computer generated models to test for accuracy. Using a full factorial method the effects of four key control parameters, namely compression load, material dimension, incubation time and platen temperature, will be observed on a predetermined mold shape to determine an optimal manufacturing strategy to ensure larger scale production with minimum variability between the parts. Primary investigations have revealed a surprising adverse effect of incubation temperature on the filling of the part. The effect of other parameters has been as expected; increasing pressure and smaller material size promote better parts. However a more detailed study is needed to develop a sound understanding of the role of each parameter in the manufacturing process. The completed work hopes to address these problems and ensure that thermoplastics become a viable alternative for structural reinforcements

On the Off-Axis Strength Test for Anisotropic Materials

ABSTRACT Difficulty in the experimental determination of the interaction component of strength tensor of boron-epoxy composites by off axis tests is discussed. Although off axis data agree well with the prediction of the strength tensor theory, the uniaxial data cannot be used to back calculate the interaction term. The data also showed that responses due to normal and shear stresses can be uncoupled in the linear as well as nonlinear regions

NASA Composite Materials Development: Lessons Learned and Future Challenges

Composite materials have emerged as the materials of choice for increasing the performance and reducing the weight and cost of military, general aviation, and transport aircraft and space launch vehicles. Major advancements have been made in the ability to design, fabricate, and analyze large complex aerospace structures. The recent efforts by Boeing and Airbus to incorporate composite into primary load carrying structures of large commercial transports and to certify the airworthiness of these structures is evidence of the significant advancements made in understanding and use of these materials in real world aircraft. NASA has been engaged in research on composites since the late 1960 s and has worked to address many development issues with these materials in an effort to ensure safety, improve performance, and improve affordability of air travel for the public good. This research has ranged from synthesis of advanced resin chemistries to development of mathematical analyses tools to reliably predict the response of built-up structures under combined load conditions. The lessons learned from this research are highlighted with specific examples to illustrate the problems encountered and solutions to these problems. Examples include specific technologies related to environmental effects, processing science, fabrication technologies, nondestructive inspection, damage tolerance, micromechanics, structural mechanics, and residual life prediction. The current state of the technology is reviewed and key issues requiring additional research identified. Also, grand challenges to be solved for expanded use of composites in aero structures are identified

Reinforcement of dry spun polymeric fibers by cellulose nanocrystal

This study presents the development of composite polymeric fibers using cellulose nanocrystals (CNCs) as reinforcements. CNCs are a class of low cost, renewable and biodegradable materials with high mechanical properties and customizable surfaces. In this study, CNCs were successfully integrated into various polymeric fibers using the method of dry spinning in efforts to improve the fibers’ tensile strength and modulus. The effects of CNCs on two different polymer systems (cellulose acetate and polyvinyl alcohol) were studied. The surface morphologies, mechanical properties, and interactions between the CNCs and the polymer matrix within the fibers were investigated. The results of the characterizations show significant improvement in the tensile strength and modulus of both the cellulose acetate and polyvinyl alcohol fibers with low dosage of CNCs. The presence of CNCs increased the crystallinity of the polymer matrix. The effects of the high shear rates associated with dry spinning on the alignment and dispersion of the nanocrystals in the different systems were also studied. A micromechanical model was developed using data from both systems for the prediction of the fiber mechanical properties as a function of the alignment of the CNC rods