CompositesWorld
Issue link: https://cw.epubxp.com/i/830100
NEWS CompositesWorld.com 27 BBA vs. VA Testing firm e-Xstream engineering (Newport Beach, CA, US), put forward the critics' case in his presentation, "Virtual allowables approach to accelerate continuous fiber-reinforced polymers develop- ment and insertion." Lockheed Martin Aeronautics' (Fort Worth, TX, US) senior staff engineer and stress analyst Dr. Carl Rousseau defended conventional building block practice as the author of "Why virtual allowables are not cost-effective." It's important to note at the outset that Assaker and Rousseau agree physical testing will never be completely eliminated, but they diverge when discussing how many physical tests are required and at what level in the BBA pyramid computer-aided virtual simulation could and should be used to reduce that number. Rousseau believes physical testing is the best method for quantifying variability in constituent fibers, resins and manu- factured laminates. Assaker asserts variability is easily simulated by using the deviation in fiber and resin properties per manu- facturers' data, while laminate defects from processing can be included in the models and explored via PDA simulations. Each has respect for the other's position, and though they see value in comparing the time and cost of the two approaches, each contends that his position in such comparisons is often misrepre- sented with incorrect supporting numerical data that needs to be clarified. ere is no disagreement, however, about the priority of aircraft safety, and that current levels of protection must be main- tained no matter what methods are adopted to facilitate insertion of new materials into an aircraft program. CW invited these two experts to this forum, not to emphasize the divide, but instead to examine each side's assertions, looking for common ground. Counting the cost of coupons Speaking first, in defense of BBA practices, Dr. Rousseau contends: "is whole issue of an excessive amount of test coupons required has been misrepresented," adding that the norm is 1,500-3,000 coupons per material form, and this can be reduced by up to 75% by using extensive data pooling (see "Statistical data pooling for design allowables" online | short.compositesworld.com/DataPool ). Rousseau points out that coupon testing consumes a relatively small fraction of the overall cost (Fig. 1). "Half of the cost of certi- fication testing is in the full-scale test articles," he explains, noting that most programs complete two full-scale airframe tests — one static, one fatigue — and that these account for half the BBA program's cost. "Details and subcomponents are two-thirds of the remainder, so coupon testing may total US$500,000 per material. To us, that's just cheap insurance." In other words, unlike metal, a composite's properties vary based on laminate stacking sequence, lot/batch properties of fiber and resin, laminate manufacturing/ Much of coupon allowables development requires testing — effects of defects, variability, environment, process stability, NDI Validate Strength & Deformation Capabilities Validate/Calibrate External Loads Distributions & Magnitudes Validate Systems Ops & ECD Analytical Method Validation Validate Durability Requirements are Achieved Flight Test Full Scale (Static & Fatigue) Component (Static & Fatigue) Analysis & Modeling Validation & Correlation Complex Loading Scale-up Effects Failure Criteria Validation Inspection Criteria & Scale-up Load Distribution Validation Subcomponents (Static & Fatigue) Analysis & Modeling Validation & Tuning Combined Load Effects Certification Approach Validation Scale-up Effects Failure Modes & Criteria Correlation Inspection Criteria & Scale-up Damage Tolerance Environment Knockdowns Joint Strength Allowables Combined Load Effects Process Spec Compliance Joint Failure Modes Inspection Criteria Development Damage Tolerance Coupons Elastic Properties Material Strength & Variability Environment Knockdowns Process Stability & Qualification Failure Modes Inspection Technique Selection & Development Effects of Defects Potential PDA Opportunities: Areas where reduction in testing is occurring over time Element Tests Validate Internal Loads Distribution FIG. 2 Simulation already used in BBA Rousseau notes that simulation is already used in analysis and modeling valida- tion at the subcomponent, component and full-scale levels. Source | Lockheed Martin