POLYMER COMPOSITES PROGRAM The Polymer Composites Program seeks to facilitate the introduction of lightweight, corrosion-resistant composite materials into commercial applications by expanding the essential science base and generating test methods, reference data, and standard materials. The outstanding properties of composites lead to products that are superior and competitive in international markets. Industries as diverse as transportation, construction, marine, offshore oil, medical devices, and sporting goods have recognized those benefits and are making significant use of these materials. For this to continue, however, two significant barriers must be addressed: the lack of rapid, reliable, cost-effective fabrication methods; and the poor understanding of and predictive capability for long term performance. These barriers were identified in a series of industry workshops, exchange visits, and consultations. In response to these challenges, the composites program initiated two tasks: one on processing science, and the other on interfacial microstructure. The degradation of the interface over time is primarily responsible for the loss of mechanical properties. The automotive industry strongly influences the composites program since many of the processing and durability issues span many automotive applications, and solutions developed at NIST are expected to rapidly propagate throughout the industry. Additionally, the group interacts with companies interested in offshore oil platforms, infrastructure, aerospace, and a variety of other applications. The goal of the Processing Science Task is to develop the technology required to monitor, model, and control the events that occur during composite fabrication. The program focuses on liquid composite molding (LCM) since this fabrication method is of great interest to all industry sectors and is the consensus choice of the automotive industry as the method with the most promise for making structural automotive parts. The approach in this task involves three steps. First, measurement tools are developed and used to characterize the material properties that control processing, for example, permeability. Second, sophisticated process simulation models are formulated to analyze the effects of processing parameters rapidly and inexpensively so they can be optimized. Finally, process monitoring sensors are developed and used to provide feedback for verification and improvement of the simulation models and to help develop the technology for on-line process control. The current activities in this Task involve five projects, including a major industry-university-government program sponsored by the Defense Advanced Research Projects Agency. The work in the Microstructure Task focuses on developing test methods for assessing the resin/fiber interfacial adhesion, and the subsequent degradation of adhesion resulting from fluid attack, particularly moisture. The long term goals are to first develop effective test methods, and then to use those tests to identify the chemical and physical mechanisms of degradation, and finally formulate reliable predictive models. The program focuses on glass fiber materials since they are the primary candidates for automotive applications. In addition, the work is beginning to look at graphite reinforced composites since these systems are important for marine and infrastructure applications. Microscale tests such as the single fiber fragmentation test are currently being analyzed to determine if they can provide realistic estimates of the performance of the resin/fiber interface in composite systems. A variety of interfacial physical and chemical structures are generated during preparation of microscale test specimens by varying the coating chemistry on the fiber, the resin processing speed, and the moisture content of the material. Full scale composite specimens are also produced and tested with identical fiber coatings and processing conditions for comparison with the microscale tests and to provide, in conjunction with the microscale tests, realistic structure-performance relationships. There are currently four specific projects in this Task, including a collaboration with the Automotive Composites Consortium to determine the effects of processing conditions on the interface of polyurethane matrix composites. Significant Accomplishments
Liquid Composite Molding: Development of Permeability Measurement Techniques and Data R.S. Parnas, K.M. Flynn, R. Peterson and M. DalFavero1 1 NIST Standard Reference Data Program Objectives The objectives are to establish a data base of permeability values for use in the design tools used by the composites industry, and to develop new permeability measurement techniques for assessing flow behavior in deformed materials. Technical Description Permeability measurements conducted over several years at NIST have been documented, collected and placed into a Clipper based database. External sources of reliable data have been identified and attempts to document the adequacy of their measurements for inclusion in the database continue. Permeability measurements are continuing with an emphasis on the permeability of fabrics deformed around curves as would be found in molds of complex shape. External Collaborations Industrial: Dr. Henry Friedman, Textile Research Institute, flow behavior in deformed fabrics. Academic: Prof. Raymond Gauvin, University of Montreal, permeability Data.
Planned Outcomes The database released in 1997 is expected to be used by molders to help design their processes and parts. Version 2 of the database is planned for release in 1999 with an expanded data set and enhanced graphics display. Accomplishments
Impact NIST permeability data are being used in flow simulations at several companies, including ATP recipients, in the automotive and aerospace industries. Over the past several years engineers from companies including Ford, Boeing, and Northrup/Grumman, as well as several engineering students, have learned how to make accurate permeability measurements through participation in the NIST permeability measurement project. Outputs Publications R.S. Parnas, Chapter 8. Preform Permeability, in RTM for Aerospace Applications, Chapman & Hall, 1997. H.L. Friedman, R.A. Johnson, B. Miller, D.R. Salem and R.S. Parnas, In-Plane Movement of Liquids Through Curved Fabric Structures, Proc. ASME Symp. on Processing, Design and Performance of Composite Materials, November 1995. R.S. Parnas, K.M. Flynn and M. DalFavero, A Permeability Database for Composites Manufacturing by Liquid Molding, Polymer Composites, in press. Liquid Composite Molding: Development and Verification of Process Simulation Models F. R. Phelan Jr. Objectives The objectives are to develop and apply models, including the effects of preform deformation and heat transfer, that can simulate the events which occur during the LCM process. The model will be developed specifically to simulate injection compression molding for the automotive industry and their suppliers. Technical Description In the past, LCM process optimization has been done with time-consuming and expensive trial and error methods on full scale equipment. Simulation models can greatly reduce the cost and increase the speed of this task. The simulation models developed in this project are based on a finite element / control volume numerical solution procedure to the governing transport equations. For example, the momentum transport equation is expressed by Darcy=s law. In previous work, a Darcy's law simulation, called CRIMSON, for modeling the mold filling phase of LCM was developed. CRIMSON enables modeling of resin injection for either constant flow rate or constant pressure injection conditions, in geometries ranging from 2-D to fully 3-D. In the next phase of this project, CRIMSON is being extended to a second generation LCM process, called Injection/Compression Liquid Composite Molding (I/CLCM). This process has been selected by the automotive companies as the most promising method for fabrication of large structural parts. I/CLCM differs from conventional Ainjection-only@ LCM in that subsequent to preform placement, the tool is only partially closed. An initial charge of resin is then injected, followed by full mold closure. The final closing action of the mold compresses the preform to the desired net shape and volume fraction while distributing the initial shot of resin throughout the part. There are two main I/CLCM process variants. In Aclosed mold@ I/CLCM the tool is closed enough to partially compress the preform. In the Aopen mold@ process a gap exists between the preform and the upper tool surface. The strategy in this case is to try and fill the gap region with fluid first, and then use the compression step to drive the fluid into the preform in the thickness direction. In open mold I/C, there is some penetration of resin into the preform during the injection phase, so that during the compression step there is a combination of in-plane and through-thickness flow. The development of I/CLCM stems from the need to mold high fiber volume fraction components in applications with fast cycle times. In pure-injection LCM, such process constraints can result in excessively high injection pressures that induce undesirable fluid-structure interactions involving preform, foam core, or tool deformation. External Collaborations Industrial: Academic: Planned Outcome Provide The Budd Co. and other interested organizations with NIST simulation tools for design and optimization. Accomplishments This year, work has begun in cooperation with the University of Illinois at Urbana-Champaign (UIUC) to develop a numerical simulation of the open mold I/C process. In the first phase of this collaboration, UIUC has developed a preprocessor which can generate a three-dimensional finite element mesh from a given two-dimensional shell mesh for flow computations. The preprocessor step is an important component of the open mold I/C simulation strategy. In this step, a 3-D flow mesh which functions as input for the CRIMSON program is constructed. Two inputs are needed in the pre-processing step: a 2.5-D mesh of the final molded geometry and all information on the preform Alayup@ in this geometry. This information is normally known by a part designer after a preliminary stress analysis stage has been completed. From this information, a 3-D mesh of the uncompressed preform geometry and Aspines@ are constructed. ASpines@ are lines that run through groups of co-linear nodes from the lower surface to the upper surface of the 3-D FE mesh and are used in the mechanical analysis of the preform as discussed below. Included in the 3-D mesh are elements which account for all the different layers in the preform layup, and the gap spacing present during injection. The development of the preprocessor has been completed at this time and tested for a variety of 2.5-D shell meshes, and preform layups. The second phase of the UIUC collaboration is to develop a module which models the non-linear deformation of the fibrous preform during compression of three-dimensional geometries. This module contains mechanical analysis routines for linking with CRIMSON. This module will enable CRIMSON to compute the volume change that occurs in the elements during compression of the preform which is a necessary calculation in order to track the movement of the flow front during this stage of the fill operation. At the time of this writing, the mechanical deformation module is not fully complete. In the mechanical theory being used to model the preform, local 1-D mechanics with no shear deformation are assumed for the sake of robustness. In response to compression, the preform deforms along the Aspines@ constructed in the preprocessing step. The nodes remain along the spines as the spines rotate in response to the imposed deformation. For each increment of deformation, new nodal locations are computed. The nodal locations satisfy the equilibrium condition of constant stress in all layers of the preform layup. In an effort to link the CRIMSON software with the user community, Structural Dynamics Research Corp. (SDRC) has developed a graphical user interface enabling the interfacing of the NIST flow modeling software CRIMSON with their I-DEAS Master Series mechanical design software. The interface allows the user from within I-DEAS to design a part, specify preform and fluid properties such as permeability and viscosity, enter boundary conditions, and then run the CRIMSON program. Results are automatically read back into I-DEAS when the simulation is finished. Phase II of this effort has now been initiated in which the interface will be modified to include I/C properties. Impact The simulation software has been transfered to the ACC, Budd, Specialty Plastics and Northrup/Grumman, and is regularly used as a design tool. Outputs Publications F.R. Phelan Jr., Simulation of the injection process in resin transfer molding, Polymer Composites, 18, 460 (1997). F.R. Phelan Jr., Analysis of Injection/Compression Liquid Composite Molding, Proceedings of the 5th International Conference on Automated Composites ICAC97, Glasgow Moat House, Glasgow, Scotland, UK, September 1997. F.R. Phelan Jr., Analysis of In-Plane Injection/Compression Liquid Composite Molding, Composites A, submitted. Presentations F.R. Phelan Jr., Simulation of Injection/Compression Liquid Composite Molding, The First Joint Topical Conference on Processing, Structure, and Properties of Polymeric Materials, Chicago, IL, November, 1996. F.R. Phelan Jr., Analysis of Injection/Compression Liquid Composite Molding Process Variants, Symposium on Multi-Disciplinary Issues in Manufacturing of Composites, 1996 ASME International Mechanical Engineering Conference and Exhibition, Atlanta, GA, November 1996. F.R. Phelan Jr., Simulation of Injection/Compression Liquid Composite Molding, Gordon Conference on Computer Aided Engineering (CAE) in Polymer Processing, Harbortown Marina Resort, Ventura, CA, February 1997. F.R. Phelan Jr., Analysis of Injection/Compression Liquid Composite Molding, 5th International Conference on Automated Composites ICAC97, Glasgow Moat House, Glasgow, Scotland, UK, September 1997. Liquid Composite Molding: Development and Verification of Permeability Prediction Models F.R. Phelan Jr., Richard Peterson and Michael A. A. Spaid Objective The objective is to develop theoretical tools to aid industrial designers in predicting the permeability tensor of the fiber reinforcement materials used in Liquid Composite Molding (LCM) from knowledge of their microstructures. Technical Description In previous work, a Lattice Boltzmann (LB) simulation for computing the flow behavior in real preform reinforcement materials was developed. LB methods involve the solution of the discrete Boltzmann equation on a lattice. Traditional flow quantities such as density and velocity are recovered by taking moments of the particle distribution function. The basic LB formulation found throughout the literature has been modified in this study to model flow in heterogeneous media such that the momentum transport is governed by Stokes flow in open media, and by the Brinkman equation in porous media. This enables us to model flow in the heterogeneous preform microstructure and is crucial for modeling void formation. Planned Outcome Simulation tools which can be used to determine the steady and unsteady permeability of fiber preforms used in LCM operations. Accomplishments In the past year, the existing LB formulation was modified to enable modeling of multi-component flow in heterogeneous media in order to study the mechanisms of void formation which occur primarily due to the heterogeneous nature of the preform microstructure in LCM applications.
Using the LB simulation it is possible to calculate the unsaturated permeability by tracking the location of the fluid front as a function of time. A plot of front position raised to the second power vs. time yields a straight line for simulations at both 39 % and 50 % nominal porosity, indicating a constant permeability. By comparing with previously computed saturated flow results the ratio of the unsaturated and saturated permeability was obtained. These results are given in the Table. The results show that in both cases the unsaturated permeability is lower than the saturated permeability and this effect increases with increasing volume fraction. These results are consistent with experimental observation in which the unsaturated permeability is reduced relative to the saturated permeability.
Outputs Publications M.A.A. Spaid and F.R. Phelan, Jr., Lattice Boltzmann methods for modeling microscale flow in heterogeneous porous media, Phys. Fluids, 9, 2468 (1997). M.A.A. Spaid and F.R. Phelan Jr., Modeling of Void Formation Dynamics in Fibrous Porous Media using the Lattice Boltzmann Method, Proceedings of the 12th Annual Advanced Composites Conference and Exposition ACCE97, 213, April 1997. F.R. Phelan Jr. and M.A.A. Spaid, Modeling of Unsaturated Flow Dynamics in Composites Processing, Proceedings of the 5th International Conference on Automated Composites, Glasgow Moat House ICAC97, Glasgow, Scotland, UK, September 1997. M.A.A. Spaid and F.R. Phelan Jr., Modeling of Void Formation Dynamics in Fibrous Porous Media using the Lattice Boltzmann Method, Composites A, submitted. Presentations M.A.A. Spaid and F.R. Phelan Jr., Lattice Boltzmann Methods for Modeling Microscale Flow in Fibrous Porous Media, Third International Conference on Composites Engineering (ICCE/3), New Orleans, LA, July 1996. F.R. Phelan Jr., Micro-Scale Flow Modeling Issues in Composites Processing, Gordon Conference on Computer Aided Engineering (CAE) in Polymer Processing, Harbortown Marina Resort, Ventura, CA, February 1997. Phelan Jr., F.R., Modeling of Unsaturated Flow Dynamics in Composites Processing, General Electric Corporate Research Center, Schenectady, NY, August 1997. Liquid Composite Molding: Bulk Resin Measurements for Process Monitoring and Control J. P. Dunkers, R. S. Parnas, K. M. Flynn, R. E. Neff and D. D. Sourlas Objective The objective is to develop optical fiber sensors, spectroscopic measurement methods, and control structures for monitoring and controlling chemical and physical processes during composites manufacturing. Technical Description The need to reduce the variation in composite quality has been recognized for many years. Variation in cure between parts and within a part, and flow inconsistencies are major contributors to composite non-uniformity. The cure monitoring work focuses on fluorescence and near infrared spectroscopies, using an optical fiber drawn from commercially available high refractive index glass as the sensing element. Optical systems are designed and built to provide high speed spectral acquisition and chemometric methods are explored to provide equally high speed spectral analysis. The interfacial sensitivity of the evanescent wave sensors is determined with optical theory that quantifies the coupling between the excitation radiation and the fluorescence radiation. The same optical fiber sensors used for cure monitoring are also being explored for flow monitoring by taking advantage of the change in signal strength that occurs as a fiber is covered by fluid when operated in evanescent wave mode. The effects of fiber bending and local environment around the fiber are being explored. In this work, flow simulation is being used to optimize the fiber trajectory in the mold to provide flow data most indicative of proper mold filling. Planned Outcome Demonstrate usefulness of process monitoring to the composites industry and indicate a potential low cost route with optical fiber sensors. External Collaborations Industrial: Carl Johnson, Automotive Composites Consortium and Ford, test sensor system in industrial processing environment. Tom Donnellen, Northrup/Grumman (DARPA), develop control structure for processing ceramic matrix composites. Rob Bannerjee, EDX, develop quality control methods for sandwich panel products. Academic: Dennis Sourlas, University of Missouri, develop process control algorithms. Suresh Advani, University of Delaware, apply sensors to flow monitoring. Accomplishments Optical theory was used to model the sensitivity of the evanescent wave sensor geometry to the region around the optical fiber. An expression was derived for the fluorescence power transmitted by the optical fiber when the same fiber is used to provide the excitation. The model was solved for the fluorescence power in the fiber as a function of radial position around the fiber to determine the radial sensitivity of the evanescent sensing geometry. It was found that the current optical system, which uses all available modes of excitation and fluorescence, collects approximately 2/3 of the fluorescence signal from within 1000 D of the fiber surface at the beginning of resin cure. At the end of resin cure, 2/3 of the signal is collected from a radius of approximately 3000 D. The expansion of the signal collection region is due to the increase of refractive index of the resin, which causes the evanescent field to expand radially around the fiber. These observations are consistent with experiments where no difference was observed between evanescent wave and bulk resin measurements, indicating that sensing regions of 1000 D and larger do not resolve the interfacial region of amorphous polymer networks. Such a conclusion is also consistent with scaling theory which indicates a length scale of approximately 100 D for the interfacial region of the polymer systems explored here. Thus, the current optical fiber sensor measures bulk cure. The development of an optical fiber sensor to measure the 100 D interphase is described in the next project. To demonstrate the convenience of optical fiber fluorescence spectroscopy, a portable high speed optical fiber fluorometer constructed earlier in this project was shipped to Ford Motor Co. and installed in the structural reaction injection molding development laboratory for testing. The optical fiber sensor was interfaced seamlessly with the Ford molds through existing thermocouple ports by encasing the optical fiber in a steel jacket. The sensor system was able to acquire data in real time and the sensor proved reusable for at least five sequential mold shots. After shipment back to NIST, the fluorescence sensor system was incorporated into an experimental apparatus for evaluating the sensitivity of an evanescent wave optical fiber to fluid flow behavior. Preliminary work indicates that large signal changes occur as the fiber is covered with fluid, and that fiber quality, fiber bending and nonlinear optical effects all play a role in determining signal strength. Outputs Publications R.A. Neff, D.L. Woerdeman and R.S. Parnas, Use of a Charged Coupled Device (CCD) Camera for Evanescent Wave Optical Fiber Cure Monitoring of Liquid Composite Molding Resins, Polymer Composites, 18, 518 (1997). D.L. Woerdeman, J.K. Spoerre, K.M. Flynn and R.S. Parnas, Cure Monitoring of the Liquid Composite Molding Process Using Fiber Optic Sensors, Polymer Composites, 18, 133 (1997). J.P. Dunkers, K.M. Flynn, R.S. Parnas and D.D. Sourlas, The Effect of Noise and Control Parameters on the Efficiency of a Model Assisted Feedback Control Algorithm for Liquid Composite Molding, Proceedings of the 1st Joint Topical Conference on Processing, Structure, and Properties of Polymeric Materials, November 1996. D. Sourlas, S. Maha, G. Peterson and R. Parnas, On-Line Control of Liquid Composite Molding, Proceedings of ISA Control Conference, New Orleans, LA, June 1997. Richard S. Parnas, Joy P. Dunkers, Raymond A. Neff, Optical Fiber Based Sensors for Monitoring and Control of Composites Manufacturing, Proceedings of International Conf. On Automated Composites, Glasgow, Scotland, U.K., September 1997. Presentations J.P. Dunkers, Real-Time Process Control and Mid-Infrared Cure Sensing of Liquid Molding, American Institute of Chemical Engineers, Chicago, IL, November 1996. R.S. Parnas, Measurement and Control of Liquid Composite Molding, American Institute of Chemical Engineers, Chicago, IL, November 1996. R.A. Neff, Real-Time Cure Monitoring of Liquid Molding by Optical Fiber Evanescent Wave Techniques, American Institute of Chemical Engineers, Chicago, IL, November 1996.. Joy P. Dunkers; Kathleen M. Flynn; Mitchell T. Huang, Near Infrared Cure Monitoring and Control of a Resin Transfer Molded Epoxy Composite Using an Evanescent Wave High Index Fiber Optic Sensor, Society of Plastics Engineers Annual Technical Conference, Toronto, Canada, April 1997. Richard S. Parnas, Evanescent Wave Fiber Optic Spectroscopy in Polymer Composites, College de France, Paris, France, September 1997 and Corning, S.A., Fountainbleau, France, September 1997. Liquid Composite Molding: Interphase Sensitive Sensors for Process Monitoring R.S. Parnas, Joseph Lenhart1 and John VanZanten11Department of Chemical Engineering, Johns Hopkins University, Baltimore, MD Objective The objective is to develop sensors with sensitivity to the 100 D region near the fiber surface, a critical region of composite materials that determines many of the bulk mechanical properties. Technical Description One possible route to such a sensor lies through localizing fluorescent dyes at the surface. Previously, a stilbene dye was synthesized with a chemical tail containing a surface reactive silane coupling group. After completing the synthesis, surface grafting was conducted on a flat glass substrate. The fluorescence of the surface bound material was measured in the presence of various solvents and resins to assess the ability of the surface bound fluorophore to respond to the environment. However, reproducibility problems were encountered associated with the limited hydrolytic stability of the Robello siloxane dye. More stable fluorophores are needed to make this approach feasible. Additional analytical methods such as contact angle, atomic force microscopy and IR spectroscopy are used to characterize the surface layer, and calibrate the surface bound fluorophore. Alternative methods of grafting are also explored to determine if low temperature aqueous methods are sufficient to deposit stable surface films. Planned Outcome Develop an integrated optical fiber fluorescence sensor sensitive to the interfacial region of fiber and polymer matrix. External Collaborations Academic: Steve Pollack, Howard University, synthesis of Robello Siloxane compounds. John vanZanten, The Johns Hopkins University, characterization of mixed silane layers. Accomplishments
Microstructure Studies: Performance Relationships D.L. Hunston, C.R. Schulthiesz, W.G. McDonough,
G.A. Holmes, C. L. Schutte, K. Liao1, Z.
Miyagi2, D. Raghavan3 and J. He3 Objectives The objectives are to develop and apply measurement tools that establish relationships between the microstructural features generated during processing and the performance properties of polymer composite systems. The focus is durability and the role of the polymer-polymer and the polymer-fiber interfaces in composite performance. Technical Description The work in this project involves three areas: resins, interfaces, and full composites. With resins, the focus is toughened thermosets. In these two phase material systems, the fracture resistance or toughness depends on the morphology of second phase (the toughener). Since the morphology in most such systems develops by phase separation during cure, the microstructure is difficult or impossible to control and vary systematically. The work here uses materials made from suspensions of preformed toughener particles to minimize morphological change during cure. Consequently, features like particle size, size distribution, and concentration can be systematically varied by changing the starting suspension and the dilution prior to cure. The work on interfaces uses simplified geometries that isolate the interface behavior so it can be studied independently. This information can then be combined with results on full composite to determine the role of the interface in the total performance. Two such studies are underway. One is developing an ultrasonic technique to characterize non-destructively the interface between a flat surface and a polymer. Once developed, the technique will be used to monitor the interface region during polymer cure or attack of the cured polymer by water. The second interface study uses the single fiber fragmentation test to obtain information about the fiber-matrix interface itself and how that interface is affected by water. The interface region is varied through the use of coupling agents and surface treatments. For some systems, full composites are fabricated and tested to examine the relationship between the fragmentation results and composite behavior. The final study in this project examines the fatigue behavior of pultruded composites to develop test methods for durability and generate data to improve the understanding of degradation mechanisms. External Collaboration Industry: Dr. David Dwight, Owens Corning, interface studies examining the relationship between fragmentation test results and the behavior of full composites. Mr. Glenn Barefoot and Dr. Daniel Witcher, Strongwell (formerly Morrison Molded Fiber Glass), durability of studies on pultruded composites used in infrastructure applications. Academic: Prof. D. Raghavan, Howard University, structure-property relationships in rubber-toughened epoxy systems. Planned Outcome The work on toughening will generate structure-property relationships that provide generic guidelines to help industry develop more fracture resistant materials. The ultrasonic studies will produce a fully automated instrument for characterizing and monitoring changes in the shear mechanical properties of thin films. The durability studies will provide guidelines and methodologies for testing the resistance of composite materials to attack by long term exposure to moisture. Accomplishments The work on toughened resins has provided the first definitive experiments that show the effects of changing toughener concentration at a fixed morphology (size and size distribution of toughener particles in the matrix resin). The results shown in the figuristance goes through a maximum at 15 phr (parts toughener per hundred parts epoxy by mass). This contradicts previous ideas which asserted that the decrease in toughness seen at high concentrations was the result of changes in the morphology from a dispersed particle-matrix microstructure at higher concentrations. Work on the ultrasonic test for characterizing the dynamic shear storage and loss moduli of coatings and interface regions has now succeeded in totally automating the measurement and data analysis processes. A new temperature chamber was also built which will make it possible to use the technique to study moisture attack on the interface between quartz and a polymer resin.
Studies of environmental fatigue for pultruded composites were conducted by testing samples in water and in air. The results show that water had a significant detrimental effect on fatigue life only at loads below 45% of the static strength. Consequently, using higher loads to accelerate studies on moisture effects will be very misleading. Pre-exposure to water before testing in water had no effect so both water and load are needed for reductions in fatigue life. Pre-exposure to water at elevated temperatures did increase degradation rates but there were indications that the mechanisms changed so temperature is also a poor way to accelerate this type of environmental testing. Outputs Publications K. Liao, R. Altkorn, S.M. Milkovich, J. Gomez, C.R. Schultheisz, L.C. Brinson, J.M. Fildes, and D.L. Hunston, Long-Term Durability of Glass Fiber Reinforced Composites in Infrastructure Applications, Journal of Advanced Materials, 28, 54 (1997). D.L. Hunston, Liquid Composite Molding--Processing Methods for the Future, Composite Fabrication, in press. K. Liao, C.R. Schultheisz, D.L. Hunston and L.C. Brinson, Environmental Fatigue of Pultruded Glass Fiber-Reinforced Composites, ASTM Seventh Symposium on Composites: Fatigue and Fracture, in press. K. Liao, C.R. Schultheisz, D.L. Hunston, R. Altkorn, S.M. Milkovich, J.M. Fildes and J. Gomez, Long-Term Durability of Composites in Infrastructure Applications, Proceedings, The National Seminar on Advanced, Composite Material Bridges, in press. Presentations D.L. Hunston, K.S. Macturk, C.R. Schultheisz, G. Holmes, W.G. McDonough, and C.L. Schutte, The Role of Silane Surface Treatments in Strength and Durability of Fiber-Matrix Bonding in Composites, EURAD=96, European Adhesion Conf., Cambridge, England, September 1996. D.L. Hunston, Failure Behavior of Polymers Used in Structural Applications like Adhesives and Composites, Chemistry Department Seminar, Howard University, Washington, DC, October, 4, 1996. D.L. Hunston, Toughening of Thermoset Resins, Symposium on Polymers for Electronics, Photonics, and Communications: Science & Technology for New Leading Industries, Tokyo, Japan, January 8, 1997. D.L. Hunston, Fiber-Matrix Interface Bonding in Composites, Symposium on Polymers for Electronics, Photonics, and Communications: Science & Technology for New Leading Industries, Tokyo, Japan, January 11, 1997. D.L. Hunston, Comments on the Role of Viscoelasticity in Adhesion, Plenary Lecture at Society of Rheology Meeting, Hilton Head, SC, February 24, 1997. D.L. Hunston, J. He, D. Raghavan, and D. Hoffman, The Effects of Particle Morphology on Toughness for a Rubber Modified Epoxy, Polymer Stabilizers and Modifiers >97, Conference and Exhibition, Hilton Head, SC, March 5, 1997. D.L. Hunston, and C.L. Schutte, Opportunities and Research Challenges for Commercial Applications of Composites, CMMC Technical Forum, Composites: New challenges - New Opportunities, Center for Micro- and Molecular Composites, Akron, OH, April 18, 1997. K. Liao, C.R. Schultheisz, D.L. Hunston, R. Altkorn, S.M. Milkovich, J.M. Fildes, and J. Gomez, Long-Term Durability of Composites in Infrastructure Applications, The National Seminar on Advanced, Composite Material Bridges, Arlington, VA, May 1997. K. Liao, C.R. Schultheisz, D.L. Hunston, and L.C. Brinson, Environmental Fatigue of Pultruded Glass-Fiber Reinforced Composites, ASTM Seventh Symposium on Composites: Fatigue and Fracture, St. Louis, MO, May 1997. D.L. Hunston, Performance Relationships, Industry Workshop on Micro-Mechanics Measurement Technologies, NIST, Gaithersburg, MD, May 29, 1997. Gale A. Holmes, Donald L. Hunston, Richard C. Peterson, Walter G. McDonough, and Carol L. Schutte, Influence of Model Interfaces on Composite Durability, Symposium on Application of Fiber Composites in Offshore and Marine Technologies, 1997 International Mechanical Engineering Conference and Exposition, ASME, Dallas, TX, November 16, 1997. Microstructure Studies: Improve the Utility of the Single Fiber Fragmentation Test G.A. Holmes, R.C. Peterson, W.G. McDonough,
D.L. Hunston, R.S. Parnas and J.F. Cheng1 Objectives The objectives in this project are to investigate the effects of resin viscoelasticity and assess the influence of moisture absorption on the interpretation of single fiber fragmentation test data. Technical Description The interfacial shear strength (or shear stress transfer coefficient) is typically calculated from experimental test data using models that assume the matrix material is linear elastic or elastic-perfectly plastic. Because the matrix material must have a high extension to failure relative to the failure strain of the embedded fiber, the linear elastic and elastic-perfectly plastic matrix condition is rarely met in experimental conditions where the matrix material is polymeric. This project seeks to quantify the impact of non-ideal matrix behavior on the interfacial shear strength determined by the single fiber fragmentation test. This will be accomplished by: (1) monitoring the change in load in single fiber fragmentation test (SFFT) specimens with increasing strain, (2) monitoring the evolution of the fragmentation process with increasing strain, (3) investigating the impact of loading rate on the matrix modulus and fragmentation of the embedded fiber, (4) development of shear stress transfer models to account for the impact of matrix property changes with increasing strain and strain rate, and (5) comparing results from the new models with values predicted by traditional linear elastic and elastic-plastic based shear stress transfer models. Research designed to obtain reliable values of the strength of the fiber-matrix interface has been pursued in an effort to assess the durability of E-glass coatings made from silane coupling agents. These coupling agents are designed to promote adhesion of the matrix to the embedded fiber and provide a protective barrier against moisture absorption. Because moisture absorption alters the properties of the matrix material, a significant portion of the change in interfacial shear strength determined from moisture exposed SFFT specimens is due to stiffness changes in the matrix. Hence, assessing the durability of an E-glass coating requires the decoupling of changes in interfacial shear strength resulting from changes in matrix properties from changes in interfacial shear strength due to degradation of the chemical bonds formed between the silane coupling agent and the E-glass fiber. To accomplish this aspect of the project, model systems will be prepared in which the impact of resin stiffness on the calculated interfacial shear strength can be determined independently of fiber-matrix interface degradation and fiber strength degradation. The matrix behavior from these model systems will be compared with resin systems plasticized by moisture to assess the applicability of this approach. Analytical models will be developed to predict theoretically the change in interfacial shear strength arising from changes in matrix properties. These values will be compared with values derived from experimental data to determine the durability of E-glass coatings. External Collaborations Industry: David Dwight, Owens Corning, develop model composite systems. Eric Pohl, OSI, durability of industry coatings. Academia: John Nairn, University of Utah, dynamics of debond region formation. Larry Drzal, Michigan State University, data analysis procedures. International: Graham Sims, Versailles Advanced Materials and Standards Program (VAMAS), establish standardized test for interfacial shear strength. Planned Outcome Establish international protocol for single fiber fragmentation test. Accomplishments
During the recent Workshop on Micro-Mechanical Measurement Technologies for Fiber-Matrix Interfaces, NIST announced the official initiation of an international round robin testing program for interfacial shear strength measurements. The effort is co-organized by Michigan State University and is under the auspices of the Composites Technical Working Group of VAMAS, the Versailles Project on Advanced Materials and Standards. This is an international organization that promotes pre-standards research. Currently, 17 laboratories representing 8 countries have agreed to participate. The program has three tasks. The first is to develop a consensus procedure for preparing the samples and conducting the tests. The second is to complete a round robin using the recommended test procedure and samples prepared in a single batch. The final task is to use the program as a forum for identifying critical research issues, exchanging results, and encouraging cooperation among researchers active in the area. The initial focus is the fragmentation test, but the work will expand to other methods if successful. In addition to officially initiating the program during the past year, the first draft of a test procedure for fragmentation was developed.
Model composites with and without a single bare E-glass fiber embedded in a diglycidyl ether of bisphenol-A / m-phenylene diamine (DGEBA/m-PDA) epoxy resin were prepared. Fragmentation of the bare E-glass fibers was found to initiate in the stress-strain region where the DGEBA/m-PDA epoxy matrix exhibits nonlinear viscoelastic behavior. To account for this nonlinear viscoelastic behavior in the analysis procedure, the Cox model was extended to the nonlinear viscoelastic regime by replacing the linear elastic modulus with a strain-dependent secant modulus. Interfacial shear strength calculations using the experimental data from one particular sample and the assumption that the strength in the fiber at the critical length is approximately 2.5 GPa are shown in the Table below. These results show that assuming linear elastic or elastic perfectly plastic behavior for the DGEBA/m-PDA epoxy resin stress-strain behavior yields drastically different answers for the calculated interfacial shear strength. Calculations using the strain-dependent secant modulus gave relative results approximately 15 % lower than the linear elastic modulus results. These results show clearly that the calculated interfacial shear strength is dependent on the matrix properties. Tabulated values of interfacial shear strength using different models for data analysis.
The hydrophilic nature of model interfaces were characterized by coating fibers and glass plates with various molar compositions of g-aminopropyl triethoxysilane (g-APTES) and propyl trimethoxysilane (PTMS). An aqueous coating procedure, analogous to the procedure used in industry, was developed. Dynamic contact angle data from the model interfaces obtained by using equal chain length bonding (g-APTES) and non-bonding (PTMS) silane coupling agents indicate that significant damage to the fibers can occur during their separation from the fiber tow. In addition, data on the E-glass plate specimens indicate that the hydrophilic character of a surface coated with PTMS is sensitive to the pH of the depositing solution. In addition, the hydrophilic character did not increase monotonically with increasing g-APTES proportion of total silane concentration. This suggests that at these higher proportions appreciable amounts of the amino head group on the g-APTES may be oriented towards the glass surface, resulting in a more hydrophobic surface than expected based on the hydrophilic character at lower proportions. More research is being conducted to confirm these results. To resolve the issue of fiber damage during sample preparation a single fiber coater was designed and built. Fibers coated by this method should minimize damage during the sample preparation procedure.
Automated the single fiber fragmentation test by designing and building a specialized instrument that collects high resolution images under precisely controlled loading conditions. This will permit the development of standardized test procedures for assessing interfacial properties of polymer composites. Outputs Publications G.A. Holmes, R.S. Parnas and D.L. Hunston, Report on the Industry Workshop for On-line Composite Process Monitoring, NISTIR 6044, (1997). Presentations Gale A. Holmes, Richard C. Peterson, Donald L. Hunston, Walter G. McDonough, Refinement of the Single Fiber Fragmentation Procedure, 24th Annual Conference of the National Organization for the Professional Advancement of Black Chemist and Chemical Engineers, Orlando, FL, March 24-29, 1997. Gale A. Holmes, Richard C. Peterson, Donald L. Hunston, Walter G. McDonough, Impact of Nonlinear Viscoelasticity on Single Fiber Fragmentation Measurements, General Electric Corporate Research Center, Schenectady, NY, August 1997. Optical Coherence Tomography of Polymer Composites J.P. Dunkers, F.R. Phalen Jr., C.G. Zimba, R.S. Parnas, B. Bouma1 and J. Fujimoto1 1Electrical Engineering Department, Massachusetts Institute of Technology, Cambridge, MA Objective The objective is to evaluate nondestructively glass reinforced polymer composites using optical coherence tomography (OCT) and optical coherence microscopy (OCM). Of particular interest is the determination of residual porosity and the three-dimensional structure of the glass preform within the polymer matrix. Technical Description The development of technologies, such as ultrasound, computed x-ray tomography and magnetic resonance imaging has had an impact in a wide range of industries, including steel, ceramics, composites, polymers, and semiconductors, as well as medicine. The recent invention of OCT at MIT offers a new, cost-effective NDE tool. Preliminary work indicates that OCT can likely provide information on the microstructural properties of polymeric materials leading to improved designs and manufacturing techniques. In the context of polymer composites, rapid evaluation of fiber orientation and voids is a pressing need to support process and product design. Most of the techniques now used in industry involve labor-intensive and time-consuming evaluation of the material after the processing is complete. The OCT technology is nondestructive, can be used to probe structure well beneath the surface, and can be used in real-time. OCT is essentially a light scattering technique that rejects multiple light scattering events by interferometrically combining a probe beam with a reference beam. Currently, a 25 fs, diode laser pumped, optical fiber cavity pulse laser is used. The laser can generate radiation at either 800 nm or 1300 nm wavelengths, and both wavelengths have been used successfully. The instrument focuses the light into the sample such that an interior focal plane is imaged at up to 5 :m resolution, thereby providing cross-sectional images of the sample. A computer controlled, piezoelectric actuated, optical fiber interferometer provides precise control of the focal plane, allowing cross-sectional images to be collected at spatial increments as small as 10 nm and at speeds as high as 8 frames/s. Limitations in image collection are primarily determined by optical density of the sample. Either highly absorbing or highly scattering material is difficult to image since single light scattering events do not occur deeply in such materials. Planned Outcome To provide the composites community a tool for predicting permeability in complex materials and in complex geometries not readily accessible with standard permeability measurement techniques. External Collaborations Academic: James Fujimoto, MIT, OCT imaging. Accomplishments A high degree of correlation between the residual porosity observed in OCT images and that observed by optical microscopy has been demonstrated in unidirectional glass / vinyl ester composites. The OCT images were obtained from within a block of material with dimensions of several millimeters on a side. Each image represents a transverse slice of the sample in a plane perpendicular to the upper surface. The presence of the fiberglass bundles, both tows and crossing threads, as well as voids, are clearly indicated in the OCT images. Optical microscopy of cross-sections of the same sample confirmed the presence of these features, albeit with considerable more effort required to prepare the sample. The quality of OCT imaging is shown in the Figure for a sample in which the refractive indices of the unidirectional glass fibers and an epoxy polymer matrix are well matched. In this low magnification image, two separate images taken from opposite sides of the sample were combined to show the entire 4 mm x 6 mm sample. The variety of pore shapes clearly demonstrates the difference between model structures and this real structure. In higher magnification images, individual fibers within each bundle can clearly be observed. The clarity of this image when combined with others obtained at different imaging planes provides a
volumetric representation of the sample that can be used for the predictive modeling of permeability. Combination of the individual OCT images to form a three-dimensional digital representation of the sample is currently underway. Outputs B. Bouma, J. Dunkers, R. Parnas, J. Fujimoto, Non-destructive Imaging of Biological and Synthetic Materials with Optical Coherence Tomography, presented at the Workshop on Intelligent Processing and Inspection of Composite Materials, Baltimore, MD, August 1997. Environmental Durability Studies: Development of Processing Methods to Fabricate Urethane Samples W.G. McDonough, Y.-H. Kim, K.M. Flynn and R.S. Parnas Objective The objective is to develop new processing procedures that will enable the preparation of urethane test specimens that can be used in the microstructure program and which are equivalent to materials made in industry by structural reaction injection molding (SRIM). Technical Description Single fiber specimens for the single fiber fragmentation test (SFFT) are typically prepared by pouring premixed resin into an open rubber mold, and then curing the resin in an autoclave. This method cannot work with rapidly curing resins of interest to the auto industry. Consequently, an injection molding procedure is being developed that will closely mimic the processing speed, temperature and pressure observed in the SRIM process used with the resins of interest. The dog bone samples thus prepared will be tested by SFFT to determine if the interface strength is degraded under rapid processing conditions. Another goal of the program is to use the technology described in the Liquid Composite Molding: Bulk Resin Measurements for Process Monitoring and Control project to monitor a fast curing polyurethane system. Whereas resin transfer molded epoxy resins may take hours to cure, the rapid reaction of polyurethanes can result in gel formation in less than a minute. The Fourier Transform Infrared Sensor shall be used to monitor and control the curing of the much faster reacting polyurethane resin. An extension of this part of the project will be to assess the effect of processing changes on the interfacial shear properties. External Collaborations Industrial: Thomas Dearlove, Automotive Composites Consortium, processing effects on the interface. James Entringer, The Dow Chemical Co., tailored resin supply. Charles Seagrave, Bayer Corporation, tailored resin supply. Accomplishments An existing mold was modified by adding an insert that contains several dog bone shaped cavities. When the resin is injected, the cavities produce multiple fragmentation samples. A specially designed injection system was made to simulate the SRIM process. The isocyanate resin is put into one chamber and the polyol mixture is put into another chamber, and during processing, the liquids are combined together in a static mixer and injected into the mold. Preliminary trials with the Dow system and the Bayer system were very encouraging. Flow visualization experiments have been carried out with non-reacting fluids to simulate the hydrodynamic loads experienced by the single fiber in each mold cavity. The fibers survived high speed injections, indicating that dog bone samples can be prepared in a rapid injection and cure process. The Fourier Transform Infrared sensor has been used successfully to follow the cure of the polyisocyanurate system from Dow Chemical. Impact The ACC makes void free polyurethane composites in an SRIM process using a NIST developed procedure involving the controlled application of back pressure to the mold during and after injection. Outputs Workshop on Micro-Mechanics Measurement Technologies for Fiber-Polymer Interfaces, NIST, Gaithersburg, MD, May 28-30, 1997. Publications: R. Clough and W. McDonough, The Measurement of Fiber Strength Parameters in Fragmentation Tests by Using Acoustic Emission, Composite Science and Technology, 56, May 2, 1996. |
The Figure shows the
filling patterns obtained from a numerical simulation
with a constant pressure injection for flow over a single
column of circular tows, where the nominal porosity, or
the percentage of the unit cell unoccupied by the porous
tows, is 39 %. The simulation predicts that the flow
front advances more rapidly in the gaps between fiber
tows, resulting in air entrapment within the fiber tow.
The trapped air is initially elliptical in shape with the
major axis perpendicular to the flow direction, while at
later times surface tension forces cause the voids to
assumes a more circular shape. The simulation predicts
that the trapped air volume shrinks as a function of time
and eventually dissipates. That is the reason why in the
Figure there is no air trapped in the first two tows.
This is not due to compressibility effects, but due to
trapped air being slowly carried away and escaping with
the bulk flow. This effect is currently being
investigated experimentally and preliminary results
indicate the simulation is correctly modeling the physics
of the process.
Interface studies
have compared results from the single fiber fragmentation
test with data from experiments on full composites where
both sample types are immersed in water for times up to
4500 hours. All samples were prepared at NIST to minimize
variations. The moisture-induced fiber degradation
patterns seen in the fragmentation tests are quite
similar to those seen in the composite data although
quantitative prediction of one result from the other is
not presently possible. Moisture-induced degradation of
the interface measured in the fragmentation tests is also
reflected in the composite results, but the comparison is
complicated by the fact that the composite tests are
dependent on a number of factors in addition to the
interface behavior. The conclusion was that qualitative
correlations can be made but to go beyond that will
require advances in both the tests methods and our
ability to analyze the results.