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A- & B-Basis AllowablesGeneration of Composite Strength AllowablesA- 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.
ObjectiveProvide the aerospace community with well established methodology for generating strength allowables with minimum testing and without loss in accuracy leading the way for large savings in qualification of composite materials. ApproachUse 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.
Tools UsedMCQ 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
Past ExperienceTechnology 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:
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