Abstract
The USAF requirements for the durability and damage tolerance certification for additively manufactured (AM) aircraft structural parts, which are detailed in Structures Bulletin EZ-19-01, raise a number of new and, as yet, unanswered questions. The present paper attempts to address three questions: How to perform a fracture mechanics-based analysis of crack growth in an AM part so as to account for the residual stresses, how to perform a fracture mechanics-based durability analysis of a cold spray repair so as to account for both the induced residual stresses and the presence of multiple co-located cracks, and how to perform a fracture mechanics-based durability analysis of an AM part so as to account for the presence of multiple collocated surface braking cracks. In this context, the present paper reveals the potential of the Hartman–Schijve variant of the NASGRO crack growth equation to accurately predict the growth of each of the individual (collocated) cracks that arose in a cold spray-repaired specimen and in a specimen from a crack that nucleated and grew from a rough surface.
Highlights
The March 2019 memo by the Under Secretary, Acquisition and Sustainment [1] enunciated that, as of March 21st 2019, the US Department of Defense (DoD) will use additive manufacturing (AM) to “enable the transformation of maintenance operations and supply chains, increase logistics resiliency, and improve self-sustainment and readiness for DoD forces.” This memo further stated that:“additively manufactured (AM) parts or AM repair processes can be used in both critical and non-critical applications
World of Science (WOS), the seven books/book chapters referenced are all listed in SCOPUS, one report is on the North American Space Administration (NASA) Technical Report Server, two references are contained in the proceedings of 13th International Conference on Materials (ICM13) with the ISBN
Number is given, two references are available on the US Department of Defense DTIC website, one is on the NATO-RTO web site, one is on the US Federal Aviation Administration (FAA) website, and another is on the US Navy navy.mil website
Summary
Aerospace-related examples that illustrate the ability of the Hartman–Schijve equation to accurately compute the crack growth histories that are obtained in conventionally manufactured metallic structures tested under an operational flight load spectra include: The growth of small, naturally occurring cracks in the 1969 F-111 D6ac wing test [6,32]. (This finding is relevant to the durability/economic life assessment of an AM replacement part where the equivalent initial size (EIDS) required is small (e.g., sub mm) [4].) It has been shown [46] that for cold spray repair, which is sometimes referred to as supersonic particle deposition (SPD) [47] which is being used to repair damaged aircraft structures [46,60,61,62,63,64], the Hartman–Shijve equation can be used to compute the fastest growing crack in the substrate. These findings are significant given the central role that analysis has in the certification of AM and AM repairs and that, as noted, the above role of testing is to validate or correct analysis methods
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