A Digital Twin Model for Robust 3D Woven PMC Pi Joints

There is an increasing use of organic matrix composites in the airframes of many modern aircraft, including the F-15, F-16 and F-22.  These materials are being integrated to achieve weight savings as well as additional fatigue resistance and strength. Recent advancements in Vacuum Assisted Resin Transfer Molding (VARTM) have further expanded the benefits offered by these materials by providing a lower cost process that cures at low temperature. This has enabled manufacturers to fabricate larger components, thus reducing part counts, processing costs and reduced assembly complexity by limiting the number of fasteners required. Regardless, these structures still require mechanical joints to provide local reinforcement, facilitate the assembly of intersecting hardware and provide load transfer. Three-dimensional (3D) woven preform-reinforcing joints shaped in the form of the Greek letter p (hereafter referred to as a “pi” joint) have been used to fuse the web and flanges of webbed support structures.  While this application has been successful in maintaining the integrity of the airframe, use of the technology has required extensive testing to certify a joint’s use in the airframe.  In most cases, several thousand tests may be necessary to achieve flight qualification, a tedious and cost intensive process. These test series must include multiple repetitions of specimens under a variety of loading conditions. Some tests also require intentional flaws in order to quantify design ‘allowables’.  Furthermore, any prospective change in the pi design, such as to the fiber architecture, thickness of geometry, will require an entirely new round of tests. An alternative to the classical build-and-break approach, is the use of finite element analysis. However, an accurate and robust analytical tool that can predict the performance of a joint system at a conceptual level has thus far eluded the designers of 3D woven pi joints for airframes.  Therefore, there exists a need for an innovative tool to improve the reliability and robustness of polymer matrix composite pi joints. The proposed innovation for this Phase I effort will explore the use of computed tomography (CT) images to provide the input for the creation of a digital twin of as-fabricated pi-joints. As part of this effort, Materials Research & Design will work with Northrop Grumman Corporation (NGC) and NTS Chesapeake. NGC will provide MR&D with a recently fabricated PMC pi-joint structure, with a composite base plate and composite web attached, for NDE inspection at NTS. Using the CT output, MR&D will work to create a digital-twin of the as-fabricated pi joint for analysis. Exact details of the fiber architecture, resin pockets and pores will all be captured in both the scan and finite element model. Using this information, MR&D will survey a suite of analytical tools to create a digital twin of the pi joint that will enable the designer to analyze.