N221-067 TITLE: Improved Reliability of Composites Pi-Joints for use in Primary Aircraft Structures

OUSD (R&E) MODERNIZATION PRIORITY: General Warfighting Requirements (GWR)

TECHNOLOGY AREA(S): Air Platforms;Materials / Processes

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Improve reliability, reduce uncertainty and scatter in performance and hence enhance the user communities' confidence in the use of pi-preform-based primary bonded composite structures.

DESCRIPTION: While the robustness and load-carrying capability of pi-joint based primary bonded structures have been demonstrated in past development programs [Ref 1], the joining technology is not widely used. Fabricated joints still show variable porosity and resin-pooling [Ref 2]. This in turn provides a large scatter in failure strength. Repeatability of the process remains an issue. This uncertainty hinders characterizing the bondline of as-built geometry and subsequent use of analysis techniques to prognosticate structural performance.

There can be multiple approaches to address this topic:

1. Interface improvement by using film resins that are currently being developed [Ref 2]. Additionally, interface toughening with nano-additives shows promise in providing more control on the interface and reducing the scatter.
2. A method to efficiently create a digital twin of the fabricated joint in the as-built condition using X-Ray/ Ultrasonic (UT)/ or Computed Tomography (CT) can be a solution [Ref 3]. Such digital twin can be used to develop high fidelity models to predict failure. The accumulated digital data can also be used to develop a database of critical joints that can be used with big-data algorithms to extend building block testing.
3. A sensor based Non-destructive Inspection (NDI) system that can monitor cure directly or indirectly. These sensors can also be potentially used during service life of the part for health monitoring system.

The above are suggestions only – any viable method to improve pi-joint reliability will be responsive to the SBIR topic. Additionally, the robustness, manufacturability, maintainability, and affordability of the proposed technology will be important consideration in the selection process.

PHASE I: Focus on establishing feasibility of a proposed concept via a flat panel with a single stringer attached by a pi-joint. The joint should be able to transfer shear, tension, compression, and torsion. Proof of concept testing will be at lab scale to establish joint allowable and associated scatter in data. Additionally the effectiveness of any sensors used for cure monitoring has to be established. Preliminary scale up plans have to be established in this Phase.

PHASE II: Mature and demonstrate the methods developed in Phase I. Further develop and optimize the pi-joint through testing a relevantly-sized, fixed wing or rotorcraft-representative skin-to-frame joint to demonstrate the reliability, durability, inspectability, maintainability, weight efficiency, and affordability of the method. Models to predict performance and inform design choices of the joint shall be developed and verified/validated using the test data. Potential use of any sensors used for health monitoring will need to be planned. A study to assess the maintenance and cost requirements shall be performed in preparation for Phase III.

PHASE III DUAL USE APPLICATIONS: Further mature and commercialize the novel and reliable pi-joint for composite skin-to-frame connection and load transfer. Consideration shall be given to improving manufacturing readiness level and airworthiness qualification through modeling and testing with a vision toward reliable, durable, inspectable, maintainable, lightweight, and affordable joints that will ease their insertion in both manned and autonomous platforms.

Lightweight fastener free joints are as attractive to the commercial sector as it is for the military. This is especially true in the vibrant Urban Air Mobility Sector. Additionally, composites are increasingly used in high end automobiles, especially in electric vehicles.

REFERENCES:

1. Russell, John D. "Composites Affordability Initiative. Transitioning Advanced Aeropace Technologies through Cost and Risk Reduction", AMMTIAC Quarterly, Vol 1, No 3. Pp 3-6, 2006,
2. Ghomi, N. "Secondary Bonded Pi-Joint Out of Autoclave Process." Master’s Thesis, McGill University, Montreal, PQ, 2013. https://escholarship.mcgill.ca/downloads/k930c1599.
3. Seon, G.; Shonkwiler, B. and Makeev, A. "Predicting Defect Formation at Early Stages of Manufacturing Process." American Society for Composites 34th Technical Conference, September 23-25, 2019, Atlanta, Georgia. http://dpi-proceedings.com/index.php/asc34/article/view/31433.
4. Torre-Poza, A. et. al. "Challenges of complex monitoring of the curing parameters in coupons for LRI manufacturing." INCAS Bulletin; Bucharest, Vol 13, Issue 1, 2021, pp. 201-210. https://www.proquest.com/openview/2733066f6b4e14af22cbd529438051e8/1?pq-origsite=gscholar&cbl=2029115.

KEYWORDS: Composites pi-joint; bonded joint; skin-stringer joint; toughened adhesive; in-situ cure monitoring; health monitoring