Plastic deformation mechanisms in polyimide resins and their semi-interpenetrating networks

High-performance thermoset resins and composites are critical to the future growth of space, aircraft, and defense industries in the USA. However, the processing-structure-property relationships in these materials remain poorly understood. In the present ASEE/NASA Summer Research Program, the plastic deformation modes and toughening mechanisms in single-phase and multiphase thermoset resins were investigated. Both thermoplastic and thermoset polyimide resins and their interpenetrating networks (IPNs and semi-IPNs) were included. The fundamental tendency to undergo strain localization (crazing and shear banding) as opposed to a more diffuse (or homogeneous) deformation in these polymers were evaluated. Other possible toughening mechanisms in multiphase thermoset resins were also examined. The topological features of network chain configuration/conformation and the multiplicity of phase morphology in INPs and semi-IPNs provide unprecedented opportunities for studying the toughening mechanisms in multiphase thermoset polymers and their fiber composites

Space environmental effects on polymers and composites

The response of polymers and polymer-based composites to the space environment is being investigated. A wide range of materials are covered in this study, including elastometer seals for Space Station Freedom, polymer films for thermal control, and composites for space structural elements. Space environmental agents of concern include atomic oxygen, thermal cycling, space debris impacts, UV, charged particles and other forms of high-energy radiation. This ambitious project is potentially a multi-year research effort and the success of such a project could be expected to have a profound impact on the design of future space-based structures. The research goal of this first Summer is to identify the priority areas of research and to carry out the initial phase task so that a collaborative research can proceed smoothly and fruitfully in the near future

Composite Layer Manufacturing with Fewer Interruptions

An improved version of composite layer manufacturing (CLM) has been invented. CLM is a type of solid freeform fabrication (SFF) -- an automated process in which a three-dimensional object is built up, point-by-point, through extrusion of a matrix/fiber composite-material precursor. In CLM, the translation and the extrusion operation are such that the final size and shape of the fabricated object are as specified by a computer-aided design (CAD). Sometimes, in order to achieve the desired final shape

Solid Freeform Fabrication of Composite-Material Objects

Composite solid freeform fabrication (C-SFF) or composite layer manufacturing (CLM) is an automated process in which an advanced composite material (a matrix reinforced with continuous fibers) is formed into a freestanding, possibly complex, three-dimensional object. In CLM, there is no need for molds, dies, or other expensive tooling, and there is usually no need for machining to ensure that the object is formed to the desired net size and shape. CLM is a variant of extrusion-type rapid prototyping, in which a model or prototype of a solid object is built up by controlled extrusion of a polymeric or other material through an orifice that is translated to form patterned layers. The second layer is deposited on top of the first layer, the third layer is deposited on top of the second layer, and so forth, until the stack of layers reaches the desired final thickness and shape. The elements of CLM include (1) preparing a matrix resin in a form in which it will solidify subsequently, (2) mixing the fibers and matrix material to form a continuous pre-impregnated tow (also called "towpreg"), and (3) dispensing the pre-impregnated tow from a nozzle onto a base while moving the nozzle to form the dispensed material into a patterned layer of controlled thickness. When the material deposited into a given layer has solidified, the material for the next layer is deposited and patterned similarly, and so forth, until the desired overall object has been built up as a stack of patterned layers. Preferably, the deposition apparatus is controlled by a computer-aided design (CAD) system. The basic CLM concept can be adapted to the fabrication of parts from a variety of matrix materials. It is conceivable that a CLM apparatus could be placed at a remote location on Earth or in outer space where (1) spare parts are expected to be needed but (2) it would be uneconomical or impractical to store a full inventory of spare parts. A wide variety of towpregs could be prepared and stored on spools until needed. Long-shelf-life towpreg materials suitable for such use could include thermoplastic-coated carbon fibers and metal-coated SiC fibers. When a spare part was needed, the part could be fabricated by CLM under control by a CAD data file; thus, the part could be built automatically, at the scene, within hours or minutes

Apparatus and process for freeform fabrication of composite reinforcement preforms

A solid freeform fabrication process and apparatus for making a three-dimensional reinforcement shape. The process comprises the steps of (1) operating a multiple-channel material deposition device for dispensing a liquid adhesive composition and selected reinforcement materials at predetermined proportions onto a work surface; (2) during the material deposition process, moving the deposition device and the work surface relative to each other in an X-Y plane defined by first and second directions and in a Z direction orthogonal to the X-Y plane so that the materials are deposited to form a first layer of the shape; (3) repeating these steps to deposit multiple layers for forming a three-dimensional preform shape; and (4) periodically hardening the adhesive to rigidize individual layers of the preform. These steps are preferably executed under the control of a computer system by taking additional steps of (5) creating a geometry of the shape on the computer with the geometry including a plurality of segments defining the preform shape and each segment being preferably coded with a reinforcement composition defining a specific proportion of different reinforcement materials; (6) generating programmed signals corresponding to each of the segments in a predetermined sequence; and (7) moving the deposition device and the work surface relative to each other in response to these programmed signals. Preferably, the system is also operated to generate a support structure for any un-supported feature of the 3-D preform shape

Space environmental effects on polymer composites: Research needs and opportunities

The long-term performance of polymer-based composites in the space environment is discussed. Both thermoset and thermoplastic matrix composites are included in this discussion. Previous efforts on the space environmental effects on composites are briefly reviewed. Focus of this review is placed on the effects of hygrothermal stresses, atomic oxygen, ultraviolet (UV), and space debris/micrometeoroid impacts along with the potential synergism. Potential approaches to estimating the residual strength of polymer composites after exposure to atomic oxygen erosion or space debris/micrometeoroid impact are evaluated. New ground-based data are then utilized to illustrate the effects of atomic oxygen and thermal cycling on the failure behavior of polymer composites. Finally, research needs, challenges, and opportunities in the field of space environmental effects on composite materials are highlighted

CHARACTERIZATION OF TISSUE SCAFFOLDS FABRICATED BY RAPID PROTOTYPING TECHNIQUES AIDED BY FINITE ELEMENT ANALYSIS

ABSTRACT Numerous techniques for fabricating tissue engineering scaffolds have been proposed by researchers covering many disciplines. While literature regarding properties and efficacy of scaffolds having a single set of design parameters is abundant, characterization studies of scaffold structures encompassing a wide range of design parameters are limited. A Precision Extrusion Deposition (PED) system was developed for fabricating poly-ε-caprolactone (PCL) tissue scaffolds having interconnected pores suitable for cartilage regeneration. Scaffold structures fabricated with threedimensional printing methods are periodic and are readily modeled using Computer Aided Design (CAD) software. Design parameters of periodic scaffold architectures were identified and incorporated into CAD models with design parameters over the practical processing range represented. Solid models were imported into a finite element model simulating compression loading. Model deformation results were used to identify apparent modulus of elasticity of the structure. PCL scaffold specimens with design parameters within the modeled range were fabricated and subjected to compression testing to physically characterize scaffold modulus. Results of physical testing and finite element models were compared to determine effectiveness of the method