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Innovative Methodology for Composite Structure Allowables and Analytical Validation


OBJECTIVE: Develop a methodology for composite structure design allowables leveraging analytical techniques and existing test databases to validate methods of prediction for composite material properties, allowing reduction of future design allowable matrices. DESCRIPTION: Composite materials are widely used in DoD programs; however each new material system, form (tape grade, fabric weave, resin content, etc), and layup process (e.g., tape hand layup, tow placement, automated tape layup) requires a large, costly, time-consuming test program for allowables, design values, structural properties, and subtle property variations. This often inhibits the use of new material systems. Nevertheless, surprises uncovered during weapon system development and test often necessitate the introduction of new material/process solutions without the time and budget luxury afforded with traditional test programs. Large test databases are now available for a significant number of carbon fiber reinforced plastic (CFRP) materials, both from DoD programs and industry shared databases, such as NCAMP. 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, since many of these trends are subtle, complicated, and confounded by material and testing variations, very large datasets are necessary for development of accurate property models. Data for large numbers of batches (tens to hundreds) and identical data-points (hundreds to thousands) are needed to evaluate subtle effects and overcome apparent discrepancies seen in small 3 to 5 batch datasets. The database size issue can now be addressed as sufficient material systems have been used for a number of years on production programs, providing not only large initial allowables and qualification databases but also on-going production batch acceptance data. Further, test method bias and error is the source of many apparent differences within and between databases. The property correlation effort must isolate these issues and problems with a combination of test method subject-matter-expert judgment and statistical analysis in order to minimize test method influence on property estimation. Analytical methods incorporating advanced material failure and damage models have shown promise for use in composite material property prediction. There is a significant potential and benefit for reducing composite material test programs by leveraging existing material property data combined with analytical methods for filling out and extending the property trends. Well documented structural level property trends and factors linked to fundamental material properties will provide a robust database for validation of current and future analytical methods; successful validation of improved analytical methods has the potential for further reduction in material and structural test programs, and corresponding acceleration of insertion of new material systems. In Phase I, it is recommended that proposers should seek to leverage as much data as is currently available through interaction with Prime contractors, material suppliers, and weapon system program offices. A commercialization plan should be included as part of the Phase I proposal and refined in preparation for Phase II. The work should also describe how the empirical data correlations 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. As a deliverable, several copies of the software with instructions/user manuals should be delivered for test and validation by the government. PHASE I: Obtain/construct database of DoD/industry CFRP test data at coupon/element levels. Develop initial cross-material correlation & factors linking to 1 related mech property (e.g. compression, tension, etc). Remove test method bias effects. Evaluate trends of different forms of the same material. Assess analytical methods for property prediction & demonstrate correlation to empirical data trends. PHASE II: Expand database to glass and quartz. Develop cross-material correlations for properties. Avoid bias from test differences/small databases. Develop software with property correlation, trends, and factors. Generate full range of design values with input of key test data. Document key tests required to characterize a new material system and demonstrate by correlation. Validate analytical predictions on a range of property trends. Write instructions for the test procedures and manuals for software. PHASE III: Upon meeting the customer's validation requirements, the composite material allowables program approach and associated analytical methods will be incorporated into the airframe development process. The computer code supporting the methodology will be available as a commercial product. REFERENCES: 1. Composite Materials Handbook, CMH-17. 2. JSSG 2006. 3. 4. H Bau and D M Hoyt,"Global Approach to Characterizing Composites Strength with Empirical, Analytical and Progressive Damage Methods", STTR AF96T009 Phase II, SVELT Workshop, AF Materials Directorate, 2000. 5. H Bau, Damage and Failure Mechanisms in Composite Bolted Joints, ASTM Symposium on Joining and Repair of Composites Structures, Kansas City, 2003.
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