Guidelines for Carbon Fiber Tape Prepreg Specifictions

1. Guidelines for Carbon Fiber Tape Prepreg Specifictions W.T.McCarvill S.Ward August 6, 2002

2. Guidelines Were Limited to Hot Melt Uni-tape • Easiest product form to tackle first. • Familiar industry standard for manufacture, design, and use. • Addresses common themes such as resin mixing, combining resin with fiber, and testing. • Once the philosophy behind the guidelines is accepted, it can applied to other product forms and processes. • The material supplier will benefit by: • Not conducting multiple qualifications of the same material to various user specifications. • Manufacturing fewer materials gaining better efficiency. • The user will benefit by: • Simpler, easier, cheaper qualifications and certifications. • Better understood, more consistent materials.

3. Let the Material Drive the Specification #1 • Historically, users developed a specification and material suppliers strove to provide materials that met them. The same material would be qualified to many different specifications. • A limited number of batches qualified a material to the user specification. The qual tests were rarely run again. • The approach led to: • A ‘test to pass’ minimum specification values mentality. • A lack of knowing what the true material variability was. • Barn door manufacturing process limits to ensure the supplier had plenty of room to make material that passed. • Preaching, but not practicing, SPC. • A failure to continually add to the qualification database and keep test methods current. • Scary requalifications.

4. Let the Material Drive the Specification #2 • Now the material supplier will establish the database and define the specification. • Preliminary targets and ranges for material characteristics and properties will be established in a limited ‘qualification’. • Acceptance testing for certification and purchase • Minimum testing for a database. • Additional testing to show a material is acceptable for the intended use. • This database will be augmented on an ongoing basis. • Establish true material variability. • Allow for SPC monitoring and control. • No shocks or surprises when a change happens.

5. The Use of Industry Standards for Definitions of Terms, Test Methods, and Change Level is Encouraged • Test Methods can come from SACMA and ASTM. • Definitions can come from MIL-HDBK-17. • Methods for calculating specification limits are found in DOT/FAA AR/00-47. • Allowables can be calculated from procedures found in MIL-HDBK-17. • Five levels of change are recommended that will guide the material supplier on what degree of testing will be required to show that the prepreg remains true to the original database and allowables.

6. Establishing and Following a Process Control Document is Critical to The New Philosophy • The PCD should document all aspects of the material fabrication. • Raw materials. • Key process parameters. • SPC procedures and requirements. • Process parameter targets and ranges should be determined by engineering trials and should be re-evaluated on an ongoing basis like the material property database. • The PCD is maintained by the material supplier and should be made available for review by the end-users and certification agencies.

7. The Guidelines Recommend Minimum Testing Requirements so the PCD is Data Driven • It is expected that behind the process limits and in process tests are a body of data and an understanding of the material and the effects of the process steps upon the material. • The selection of appropriate test methods and generation of data will result in a database that can ensure the sale of consistent material and a reservoir of knowledge to aid problem solving and identify change. • It is recommended that tolerances be stacked and the resulting material evaluated to demonstrate that its cure characteristics and mechanical properties are still within statistical limits. • The effects of stacked tolerances on handling characteristics such as tack and drape, and cured composite quality, should not be neglected.

8. SPC Methods are Key to Reducing Variability • Set targets and ranges for process parameters and test results. • Control limits will be set based on natural variability of prodcut and processes. • Data should be collected, plotted, analyzed and acted upon. • Action should be taken based on standard criteria for non-random variability-and before data shows product is still within the control limits. • High levels of SPC control and capability may allow reduced testing.

9. A Clear Protocol for Dealing With Change Will Material Equivalency to be Determined • 0- change in ingredient name, correction of typos. • No notification of end users or FAA is required. • 1- change in release paper, changes to packaging materials. • Current end users are notified, but approval by end user and FAA not required. • 2- change in resin ingredient supplier, change in resin ingredient precursor, modifications of process equipment, addition of new similar equipment. • Notify current end users and obtain FAA approval. • 3- change in fiber manufacturing process, change in resin mixing, filming and prepregging equipment, change in resin or fiber manufacturing site. • Full equivalency testing and FAA approval • 4-change in fiber type, change in resin formulation • New product specification

10. Ongoing testing Will Keep the Original Database Up to Date • After the initial databsae is established periodic testing will be conducted. • At the completion of periodic testing, the material batch acceptance limits and material equivalency will be recalculated. • The periodic testing will include: • Minimum set of cured laminate mechanical properties. • Some subset of the additional cured laminate mechanical properties. • Some subset of the optional cured laminate mechanical properties. • Since the database and test methodology will be kept current, a drift in properties can be detected early and corrective action taken. • A current database will also make requalification and material equivalency demonstration easier.

11. What do We Expect to Get From the Next Two Days? • Comments, criticisms, and contributions from you. • I am sure you have closely studied the documents. • Keep in mind the goal is to reduce cost to suppliers and users. • This way of writing material specifications is what the rest of the world does. • These will be reviewed, discussed, and incorporated into the final document. • Thanks in advance for your help.