Abstract
This paper firstly reveals that when assessing if a bonded joint meets the certification requirements inherent in MIL-STD-1530D and the US Joint Services Standard JSSG2006 it is necessary to ensure that: (a) There is no yielding at all in the adhesive layer at 115% of design limit load (DLL), and (b) that the joint must be able to withstand design ultimate load (DUL). Secondly, it is revealed that fatigue crack growth in both nano-reinforced epoxies, and structural adhesives can be captured using the Hartman–Schijve crack growth equation, and that the scatter in crack growth in adhesives can be modelled by allowing for variability in the fatigue threshold. Thirdly, a methodology was established for estimating a valid upper-bound curve, for cohesive failure in the adhesive, which encompasses all the experimental data and provides a conservative fatigue crack growth curve. Finally, it is shown that this upper-bound curve can be used to (a) compare and characterise structural adhesives, (b) determine/assess a “no growth” design (if required), (c) assess if a disbond in an in-service aircraft will grow and (d) to design and life in-service adhesively-bonded joints in accordance with the slow-growth approach contained in the United States Air Force (USAF) certification standard MIL-STD-1530D.
Highlights
The ability of A4EI to accurately predict the quasi-static failure loads for simple joint configurations was first validated as part of the United States Air Force (USAF) Primary Adhesively Bonded Structure
This is an extension of the predictions for quasi-static failure loads in order to try to avoid durability problems. Using this simplistic design approach of applying a “knock-down” factor to the quasi-static strength predictions, no crack growth is permitted in the joint when in-service and it is assumed that durability problems will be avoided. This recommendation is reflected in MIL-STD-1530D Section 5.2.4 [8], which states: “Stress and strength analysis shall be conducted to substantiate that sufficient static strength is provided to react all design loading conditions without yielding, detrimental deformations and detrimental damage at design limit loads and without structural failure at design ultimate loads.”
The adhesive in the joint should not yield at 115% design limit load (DLL); The adhesive in the joint should not failure at design ultimate load (DUL); The joint should have an adequate fatigue life
Summary
The delaminations and disbonds that have been found in US Navy [1] and Royal Australian. The ability of A4EI to accurately predict the quasi-static failure loads for simple joint configurations was first validated as part of the USAF Primary Adhesively Bonded Structure. One of the primary recommendations contained in [4] was that, to avoid durability issues, the adhesive should not be loaded beyond yield This is an extension of the predictions for quasi-static failure loads in order to try to avoid durability problems. Using this simplistic design approach of applying a “knock-down” factor to the quasi-static strength predictions, no crack growth is permitted in the joint when in-service and it is assumed that durability problems will be avoided This recommendation is reflected in MIL-STD-1530D Section 5.2.4 [8], which states:. The adhesive in the joint should not yield at 115% DLL; The adhesive in the joint should not failure at DUL; The joint should have an adequate fatigue life
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