Is Scaling Hygrothermal Effects from Coupon to Full-Scale Component Resulting in Over-Conservative Knockdown Factors?
Abstract
Composite airframe assemblies are becoming more common in naval aviation as the knowledgeof how to design, analyze, and manufacture components tailored for specific structural applicationsevolves. If appropriately designed to a specific structural application, composite structures provideweight savings not achievable using traditional isotropic metals, allowing the aircraft to carry morepayload and/or use less fuel. One challenge that remains is assessing the structural effects oftemperature and humidity on full-scale composite airframe components. The current approach isto compare the strain measurements at 150% of design limit load from the full scale test to theresults of environmentally conditioned coupons.Differences in the hygrothermal effects may exist between a coupon fully exposed to theenvironmental conditions and a critical part in a full-scale assembly shielded from the environmentby the surrounding structure. In addition, failure modes between the full-scale article and theenvironmentally conditioned coupon may differ, making the strain comparison method currentlyused less robust. The combination of these two factors could lead to over-conservative estimatesof the hygrothermal effects on the full-scale component, resulting in a larger-than-requiredknockdown factor. These larger knockdown factors translate to reduced flight envelopes and/orfatigue lives and can also result in more frequent inspections, resulting in higher costs and lessmission readiness of the airframe. High knockdown factors may also negate the advantages ofusing composite materials resulting in heavier components, reducing the additional cargo carryingcapability and fuel efficiency.The proposed research will contrast the current approach with results obtained from applying thesame environmental conditions directly to full-scale components during testing. The results ofthis study will provide empirical data to assess the currently employed method of generatingknockdown factors for full-scale components using data generated from environmentallyconditioned coupons. Additional scientific information about environmentally conditionedairframes will also be generated. This study will also provide a greater understanding that apparentfailure modes for composite structure may change, making the strain comparison method lessvalid.This research is relevant to the Navy research presented in ONC Code 33: Structural Materials ~Fracture and fatigue damage of naval structural materials and ONC Code 35: Composite Structures~ Characterization and failure analysis.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 04, 2017
- Source ID
- N000141712030
Entities
People
- Tim Hickey
Organizations
- Office of Naval Research
- United States Navy
- Wichita State University