The Department of Defense, specifically the United States Air Force (USAF), is seeking innovative solutions through the Small Business Innovation Research (SBIR) program to develop and verify nondestructive evaluation (NDE) techniques for assessing cold worked fastener holes in titanium and steel aerospace alloys. The primary objective is to create methods that can accurately determine the presence and magnitude of beneficial residual stresses introduced during the cold working process, which is crucial for the life management of metallic structures in USAF aircraft. This initiative is vital for extending inspection intervals and enhancing the longevity of aircraft components, ensuring operational readiness and safety. Interested parties must submit their proposals by November 6, 2024, with further details available on the official SBIR website.

## Description

The intent of this topic is to develop and verify nondestructive evaluation technique(s) to determine if the desired level of beneficial engineered residual stress is present at a cold worked fastener hole. Cold working, also known as cold expansion, is a process that introduces beneficial compressive stresses around fastener holes in metallic aircraft structure. An introduction to the cold expansion process can be found in reference 2. The cold work process is an important life management capability for fastened metallic structures that are found pervasively on US Air Force (USAF) aircraft. The process has been in use for many years, but the verification of the magnitude of residual stress introduced by the process is not possible for previous applications. In addition, current process control methods need confirmation that a satisfactory amount of residual stress is present. As the stress state is not measured, the current approach for life management only takes partial credit for the presence of the desired residual stresses. This is a sub-optimal approach and the need for NDE to determine the state of residual stress has been presented by the Senior Leader for the Aircraft Structural Integrity Program (ASIP) as summarized in reference 3. The pay-off for NDE to measure the amount of residual stress from the cold work process includes the ability to extend inspection intervals by quantifying the impact of the residual stress on the nucleation and growth of fatigue cracks, plus enabling life extension of current components that do not have cold worked fastener holes. Therefore, this capability is relevant for both current and in production USAF aircraft. A recently awarded complementary technical effort is addressing the technical approach using NDE to determine the state of residual stress in the presence of many confounding factors found in typical aircraft structures that will also affect the measurement. It is well known that the changes in ultrasonic velocity, electromagnetic response, and ionizing radiation diffraction are sensitive to the presence of residual stress. However, these changes in the measurements can be exclusively attributed to the presence of residual stress when comparisons are performed on the same test article before and after the residual stress is introduced. Therefore, a technical approach that only performs measurements before and after the application of a stress field is viewed as not addressing this topic.The challenge presented by this topic is to develop an NDE approach to address the challenge of assessing countersunk and/or filled fastener holes and in titanium and steel alloys. The change in behavior of the various NDE modalities in these alloys when compared to aluminum, plus the presence of the countersink and/or fastener, will require addition development of the NDE based methods to assess the presence of the residual stresses induced from the cold working process. Based on this background material and the stated objective of this topic, the technical effort is expected to accomplish the development and validation using the guidance in MIL HDBK 1823A of one or more NDE techniques to determine if the desired amount of residual stress is present after cold working a fastener hole in countersunk fastener holes and/or structures made of titanium or steel alloys. As stated in the objective it is desired that the technical solution uses approved USAF field portable NDE equipment. The technical approach must be applicable to holes of differing diameters with a minimum range being between 0.25 inches to 1.0 inches. The technical approach shall have a demonstrated repeatability to various fastener hole conditions. As a threshold requirement, the technique(s) must be applicable to typical aerospace titanium and steel alloys, e.g. Ti64, Ti 6242, and 4340 steel, and their derivatives, and address typical aerospace thickness values for assembled structures, typically 0.2 inches to 1.0 inches. Capabilities beyond these values is preferred. As an objective, the optimization and production-ready prototype of the NDE technique(s) is desired. This would include required training and field support requirements for the NDE technique(s).In addition to the development of the NDE technique(s), optimization of the sensors, fixtures, and related development systems shall be performed to maximize the sensitivity to the residual stress while minimizing the impact of countersinks and/or installed fasteners. In addition, demonstrations on representative aircraft structures should be accomplished to illustrate how the measurements would be performed within the USAF. The need for specialized fixtures, sensors, and related hardware is recognized, but the desired outcome should minimize their use.The technical effort shall conclude with a detailed technical report of the work accomplished, results of development testing, plus the outcome of the validation study. Any prototype hardware shall be delivered to the USAF accompanied by recommendations for how they need to be specified for acquisition.

## Objective

The objective of this topic is to develop and verify a nondestructive evaluation (NDE) technique or techniques as a quality assurance and surveillance capacity of engineered residual stresses generated by cold working of countersunk and/or filled (i.e. fasteners in place) fastener holes in metal structure. The NDE technique(s) can be based on electromagnetic or mechanical wave propagation and must address how the technique can address titanium, steel, and other metal alloys besides aluminum to assess the presence of beneficial residual stresses. The objective of the NDE measurements include the ability to detect if beneficial residual stresses are present after an applied expansion of three percent, for titanium (threshold) and steel (objective) alloys with fastener hole diameters between 0.25 inches and 1.0 inches with multiple thickness values. The measurement method shall determine if the residual stress is present after the cold working process is completed for quality assurance/verification and after the system has been in use for extended periods of time, i.e. 10+ years, for surveillance to ensure the residual stresses is still present. The technical solution must be applicable to both manufacturing and Depot/field maintenance environments, minimizing the use beyond US Air Force approved portable NDE systems. The proposed solution should minimize the need for any surface preparation and provide meaningful results considering possible confounding factors, such as paints, localized hole conditions, material variability, and any plasticity associated with the cold working process. A comprehensive validation study that follows the guidance of MIL HDBK 1823A (reference 1) is expected to provide a statistical metric of performance for all relevant metal alloys, to include aluminum in addition to the ones previously mentioned. Optimization of the technique is expected combined with steps to develop a production ready prototype system for manufacturing, Depot, and field applications.

## Phase I

As this is a Direct-to-Phase-II (D2P2) topic, no Phase I awards will be made for this topic. To qualify for this D2P2 topic, the Government expects the applicant(s) to demonstrate feasibility by means of a prior &ldquo;Phase I-type&rdquo; effort that does not constitute work undertaken as part of a prior SBIR/STTR funding agreement. This includes determining the scientific and technical merit and feasibility of ideas appearing to have commercial potential. This includes test results that demonstrate the ability to nondestructively measure beneficial residual stresses and address the impact of countersinks, installed fasteners, and titanium plus steel alloys. Preliminary demonstrations relevant for typical USAF metallic structures is desired. Applicant(s) must have validated the product-market fit between the proposed solution and potential AF stakeholders. The applicant(s) should have defined a clear, immediately actionable plan with the proposed solution and the AF customer. The feasibility study should have:- Identified the prime potential AF end user(s) for the commercial offering to solve the AF needs,- Described integration cost and feasibility with current mission-specific products,- Described how the capability can be used by other DoD or Governmental customers.

## Phase II

Under the Phase II effort, the offeror shall sufficiently develop the technical approach and process to conduct relevant demonstrations and a statistically significant validation study following the guidance of MIL HDBK 1823A that includes countersunk and/or installed fasteners, plus titanium and steel alloys that could affect the measurement process. The technical effort should document any specific challenges in the manufacturing and production of the desired NDE-based measurement system to ensure the technical approach is sustainable for the life cycle of weapon systems currently in acquisition as well as be applicable to weapon systems in current USAF inventory.

## Phase III: Dual Use Applications

The awardee(s) will pursue commercialization of the residual stress measurement technologies developed in Phase II for transitioning expanded mission capability to a broad range of potential government and civilian aviation-based users, plus industries outside of aviation that use the cold working process to enhance life management. Direct access to USAF end users and government customers will be provided for opportunities to develop and receive Phase III awards to tailor the solution to the specific customer needs and provide additional development, if needed, or procurement of products developed as part of the tailoring process for the specific customer needs.

Nondestructive Evaluation for Assessing Cold Worked Countersunk and/or Filled Fastener Holes in Titanium and Steel Aerospace Alloys

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