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Cold Expansion Tests for Plates Containing Elongated Holes

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Cold-expansion testing of D6ac plates containing an elongated non-circular hole, of 21 aspect ratio, has been undertaken using an AMRL designed interference fit plugsleeve arrangement The aim has been to determine the practical viability of the process as an option for addressing the cracking problem at the non-circular fuel flow vent hole number 13 in the wing pivot fitting of the F-111C aircraft in service with the RAAF. The key issue being that in the open literature, only cold expansion to circular holes has been reported, and application to non-circular holes would represent a significant new development. In the testing, three nominally identical plate specimens were cold expanded with sleeves of two different material types i.e., high strength tool steel D2 and stainless steel being assessed. Strain gauge readings around the elongated hole boundary were recorded at various stages of the cold-expansion process, as well as some typical full-field qualitative photoelastic strain distributions. In all cases nominal expansion levels greater than 2.5 were achieved, along with maximum peak strains of more than 10,000 micro-strain and maximum residual strains greater than 1600 micro-strain. These results indicate that highly effective cold expansion has been achieved. It was also demonstrated that subsequent to cold expansion, effective interference fitting could be achieved, without the need for post cold-expansion machining. The particular advantages available by using stainless steel sleeves have been identified, and an improved surface finish at the holes in the plug is also recommended. Fatigue testing of the existing design, with minor amendments is now proposed to confirm its anticipated suitability as an effective fatigue life extension option.

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  • Mechanics

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