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Analytical Tool for Assessing Short Fiber Composite Structural Behavior

Description:

OBJECTIVE: Develop/demonstrate a methodology for assessing the structural behavior of short fiber carbon composites that leverages existing analytical techniques and test databases. Develop/validate methods for the prediction of short fiber composite material properties allowing reduction of future design allowable matrices. 

DESCRIPTION: Short fiber composites, such as chopped carbon fiber sheet molding compound (SMC), bulk molding compound (BMC), and other emerging product forms, are used in commercial programs for compression molding fabrication of small, yet complicated, composite parts. Some examples include access covers with cut-outs and fastener holes and integrally rib-stiffened panels. A few of the advantages of the short fiber based compression molding materials and processes over their aluminum or continuous carbon fiber reinforced counterparts are the following: part complexity, dimensional stability (drop-in replacement of matching metal part outer moldline geometry), light-weighting (lower specific density materials), the ability to add integrated inserts and rib stiffeners, dimensional control, corrosion resistance, high volume throughput, and lower recurring finished part cost (reduced fabrication, preparation, cutting, and kitting costs). Note that the material systems of interest should be competitive from a mechanical property perspective with continuous carbon fiber systems in the 50-60% fiber volume fraction range. SMC/BMC materials that utilize fiber volume fractions less than this and materials that use "nano" reinforcements are not the materials of interest for this topic. However, much like continuous carbon fiber prepreg materials for structural applications, each new short fiber material system requires a large, costly, time-consuming testing program to generate allowables, design values, and structural properties as well as statistically account for subtle property variations. In addition, it is unclear if the various property relationship factors that exist and have been used for multiple continuous fiber materials are appropriate for use with short fiber materials. As a result, short fiber materials have been limited to tertiary structure in DoD applications. What is needed is an innovative computational tool that can leverage continuous fiber knowledge to reduce the non-recurring experimentally intensive test program size, predict SMC/BMC batch acceptance ranges, or assist in estimating allowables based on its ability to correlate trends among mechanical properties and accurately predict short fiber material properties. Large carbon fiber reinforced plastic (CFRP) experimental databases are now available from DoD programs and industry shared databases. Initial evaluations of data from multiple material systems indicate that general property trends, such as ratios of notched to unnotched strength, and bolted joint geometry factors, are consistent across material systems. However, the availability of large compiled databases are somewhat limited for short fiber composite systems that could compete with the continuous materials. By leveraging any existing short fiber composite material property data combined with analytical methods for filling out and extending the property trends among molding compound materials and continuous CFRP materials, there is a significant potential and benefit for reducing material test programs and acceleration of insertion of new material systems in both DoD and commercial applications. In Phase I, it is recommended that proposers seek to leverage as much experimental data as currently available through interaction with Prime contractors, material suppliers, the Federal Aviation Administration (FAA)-funded National Center for Advanced Materials Performance (NCAMP) and Advanced General Aviation Transport Experiment (AGATE) databases, and weapon system program offices. The work should also describe how the empirical methodology can validate the analytical method and how future test programs could be reduced in scope as a result. During Phase II, the database correlation should be tested by using data from a material not found in the database or with data developed during Phase II. The property correlation effort must isolate issues, such as test method bias and error, with a combination of test method subject-matter-expertise and statistical analysis in order to minimize test method influence on SMC/BMC property estimation. As a deliverable, the software with instructions/user manuals shall be delivered for test and validation by government and other industry users. 

PHASE I: Obtain/construct a limited database of DoD/industry short fiber composites test data. Develop initial cross-material correlations and factors linking to one mechanical property (e.g. compression, tension, etc.). Develop a methodology for assessing the structural behavior of short fiber carbon based composite systems. Evaluate trends among the molding compound materials and how they compare to continuous CFRP empirical data trends. 

PHASE II: Expand database to more short fiber materials and develop cross-material correlations for multiple properties. Remove bias effects from test method differences and small datasets. Develop software with property correlation, trends, and factors. Generate a range of design values with input of key test data. Document key tests required to characterize a new short fiber composite. Validate analytical predictions on a range of property trends. Refine software based on user feedback. Document user manual guidance, best practices, and case studies to facilitate transition. 

PHASE III: The commercialization plan shall be in place for maintenance, support, and dissemination of the computer code supporting the methodology. Upon meeting the customer's validation requirements, the short fiber composite design data methodology and associated analytical methods shall be incorporated into the airframe development process. 

REFERENCES: 

1: H. Bau and D.M. Hoyt, "Characterizing Notched Composites Strength with Empirical and Analytical Methods", AFRL-ML-WP-TR-2000-4109, AF Materials and Manufacturing Directorate, 2000.

2:  Composite Materials Handbook, CMH-17. Rev. G. SAE International, 2012.

3:  G.P. Thomas, "Compression Molded Composites: Processes, Benefits and Applications – An Interview with TenCate", AZoNetwork, 10 February 2014, http://www.azom.com/article.aspx?ArticleID=10665, January 2017.

KEYWORDS: Short Fiber Composites, Bulk Molding Compound, Sheet Molding Compound, Modeling, Composite Structural Properties, Structural Testing 

CONTACT(S): 

Kara Storage (AFRL/RXSA) 

(937) 255-4784 

kara.storage@us.af.mil 

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