Damage tolerance requirements for integrally stiffened composite wing skins are typically met using design allowables generated by testing impact-damaged subcomponents, such as three-stringer stiffened panels. To improve these structures, it is necessary to evaluate the critical design parameters associated with three-stringer stiffened-panel compressive behavior. During recent research and development programs, four structural parameters were identified as sources for strength variation: (a) material system, (b) stringer configuration, (c) skin layup, and (d) form of axial reinforcement (tape versus pultruded carbon rods). The relative effects of these parameters on damage resistance and damage tolerance were evaluated numerically and experimentally. Material system and geometric configuration had the largest influence on damage resistance; location and extent of the damage zone influenced the sublaminate buckling behavior, failure initiation site, and compressive ultimate strength. A practical global-local modeling technique captured observed experimental behavior and has the potential to identify critical damage sites and estimate failure loads prior to testing. More careful consideration should be given to accurate simulation of boundary conditions in numerical and experimental studies.
No abstract
The joining of composite materials used in airframe structures has always presented a challenge to the structural engineer. As part of a Survivable Affordable Repairable Airframe Program (SARAP) agreement, research on three advanced joining concepts was conducted to identify and validate designs that would provide improved structural efficiency when compared to conventional joining methods. The first involves using finger joints in thin laminates to produce a joint with high specific strength compared to typical joining methods. The second utilizes a derivative of needling for stabilized dry fabric pre-forms to improve through-the-thickness laminate and joint properties. The third concept focuses on compression preload to improve the performance of a typical lap joint. Within each concept, coupon or element tests were used to validate the performance of these alternative configurations. This paper presents both analytical predictions and test results documenting the effects of these improved joining methods.
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