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 joints 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.
Benefit: The primary beneficiary of the proposed solution are the designers of composite aircraft structures. A digital twin model which can be used to efficiently predict the strength of both pristine and flawed pi joints would significantly improve the confidence in the design of these structures. Previous methods for evaluating composite pi joint designs involve performing several thousand tests for a variety of configurations and load cases. This make-and-break method is employed mostly to compensate for the current inadequacies associated with evaluating these critical structures. A secondary, but also significant benefit of a digital twin model is to provide an uncertainty quantification tool to evaluate the effect that geometric and fabrication flaws have on effective strengths. The commercialization strategy for this program will be primarily based on an ongoing relationship with Northrop Grumman. Northrop Grumman is an industry leader in the design manufacture and integration of high-strength, lightweight composite material for the aerospace and defense industries. From wing products to fuselage parts to engine components, NGC components are currently flying on numerous commercial and military aircraft. MR&Ds plans to develop a digital twin of a polymer matrix composite pi joint has the potential to help NGC engineers better understand what causes the scatter in pi joint strength measurements while also providing input into the uncertainty quantification of certain design variables. MR&D will take a building-block approach for this problem, first starting with a unit-cell analysis of the different fiber architectures and building to a full 3-D model that will be capable of evaluating failure both within the laminate and at the bondline.
Keywords: progressive damage, progressive damage, Composite Pi Joints, Digital Twin Model, Finite Element Analysis, polymer matrix composites, airframe, Vacuum Assisted Resin Transfer Molding
