Generation of Composite Strength Allowables

A- and B-basis allowable strength values are essential for reducing risk in aircraft structural components made from fiber reinforced polymer composite materials. Risk reduction is achieved by lowering the probability of failure of critical aircraft structures through the use of A- and B-basis design values. For A-basis, at least 99% of the population of material strength values is expected to equal or exceed this tolerance bound with 95% confidence. A-basis strength is applied to single members within an assembly whose failure would result in loss of structural integrity. For B-basis, 90% of the population of material strength values is expected to equal or exceed that strength value with 95% confidence. B-basis strength is applied to redundant structures where failure would result in safe load redistribution. Current practices for determining allowables follow procedures recommended by FAA and working draft of the composite materials handbook CMH-17 Rev G (formerly military handbook Mil-HDBK-17-1F). Determining allowables solely by means of testing [1] is costly and time consuming as large number of composite coupons must be tested under various environments: cold, ambient and elevated temperatures (with and without moisture). AlphaSTAR devised technical approach for calculating allowables with reduced testing. The methodology eliminates unnecessary conservatisms that can contribute to increased weight.

Reference
DOT/FAA/AR-03/19, Final Report, “Material Qualification and Equivalency for Polymer Matrix Composite Material System: Updated Procedure” Office of Aviation Research, Washington, D.C. 20591, U.S. Department of Transportation Federal Aviation Administration, September, 2003.

Objective

Provide material and structural engineers with verified capability for rapid and accurate assessment of laminate response under in-service loading.

Approach

Use lamina level testing per CMH-17 guidelines to reverse engineer sources of uncertainties that produce scatter in material response. The variability is generally caused by: (1) scatter in constituent mechanical properties; (2) variability in composite manufacturing parameters; and (3) manufacturing defects such as void, waviness, and gaps. Then use advanced multi-scale multi-physics progressive failure analysis (MS-PFA) coupled with probabilistic analysis and Bayesian statics update to produce scatter at the laminate level. Lamina level scatter can be used to generate laminate level scatter as indicated in [1]. Data from testing of fewer replicates of composite laminates can be used to guide the determination of allowables. AlpahSTAR developed MCQ (Material Characterization and Qualification) software to determine allowables with reduced testing.

Reference
J. Tomblin, and W. Seneviartne, “Laminate Statistical Allowable Generation for Fiber-Reinforced Composite Materials: Lamina Variability Method”. Report number DOT/FAA/AR-06/53, January 2009. US Department of Transportation, Federal Aviation Administration (Office of Aviation Research and Development), Washington, DC.

Tools Used

MCQ is used to determine allowables for laminate level un-notched coupons following ASTM standards. MCQ is a mesh-less unit cell concept that does not use FEA as long as the state of stress is uniform in the coupon. For all other ASTM coupons (open hole, filed hole, etc.) or general structural component design allowables are determined using MS-PFA invoking finite element solution with progressive failure analysis.

Benefits Provided

• Reduce laminate level testing for the first level of FAA’s building block where testing of a large number of coupons becomes inevitable; test reduction can be on the order of a minimum of 50% as compared to current standards.
• Determine A-basis from B-basis.
• Identify root cause for scatter in material strength.
• Provide guidelines for improving material behavior.
• Applicable to determination of B-basis at any level of FAA’s building block (coupon, element, subcomponent, etc.)

Past Experience

Technology was applied successfully to carbon and glass composites (tape and fabric). The approach consistently provided accurate allowable values. Additionally, AlphaSTAR published extensively on the topic with Joint Publications with key Aerospace Industries and University Partners:

Reference
[2] G. Abumeri, M. Garg, F. Abdi, A. McCloskey and R. Bohner, “Validation of a Computational Approach for Composite Material Allowables Using Sealed Envelope Predictions for Reduced Testing,” SAMPE Journal, September/October 2009.