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42. Certification of Composite-Metal Hybrid Structures Using a Single Full-Scale Test Article



The damage-tolerance philosophy is well established for metallic airframes, where proven methods (structural analysis and inspection procedures) and supporting databases exist to detect damage and predict crack growth and residual strength. Damage growth mechanics and load spectra for composite and metal structures have significant differences that make the certification of composite-metal hybrid structures challenging, costly and time consuming. Often composite structural certification uses load-enhancement factors (LEFs) to reduce the test duration. In certain cases, this causes the enhanced load spectrum to have loads beyond the clipping level of metal structural details. Two approaches were proposed for certification of hybrid structures using single full-scale durability test article. First approach called a multi-LEF is introduced so that different LEFs can be used for different parts of the spectrum so that the high loads (that potentially can go beyond clipping level after LEF application) can have a longer test duration and a lower or no LEF. Second concept called deferred severity spectrum (DSS) is introduced so that a single test article can be used to substantiate both metal and composites as a two-phase certification methodology with different LEFs, i.e., first phase with LEF=1 for metal and second phase LEF>1 for composite to reduce the required test duration. The DSS approach provides a mechanism to obtain credit for the loads applied during first phase (focusing metal) so that the composite certification test duration (phase 2) can be reduced. A series of sequencing studies were conducted to understand the implications of spectrum modifications to overall damage growth mechanism of composite structure. The goal of the program is to provide an efficient certification approach that weighs both the economic aspects of certification and the time frame required for certification testing, while ensuring that safety is the key priority.

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