Requirements and Variability Affecting the Durability of Bonded Joints

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.

Fatigue crack growth in epoxy polymer nanocomposites

“…D , n , A and ΔGthr, deduced from the Hartman–Schijve relationships may then be used to compute the respective d a/ d N versus ΔG curves. In the present study, as in previous work [26,27], there is invariably a good agreement between the computed and experimentally measured FCG rate curves. These parameters have also been employed to further understand the form of such FCG relationships between d a/ d N versus ΔG, with respect to the structures of the epoxy nanocomposite polymers.…”

“…The microcomposite polymers are commercially available structural adhesives, namely ‘FM73’ and ‘FM300’ which are both from Cytec, USA. Both of these epoxy microcomposite polymers are rubber-toughened structural adhesives containing a dispersed phase of spherical rubber particles of a few micrometres in diameter [26,27,47]. Furthermore, the epoxy polymeric matrices for these two structural adhesives have very different chemistries to each other, and also when compared to the epoxy nanocomposite polymers studied in the present paper.…”

“…Note that the linear relationship for [22,23] encompasses all the various types of CNF and GNP epoxy nanocomposite polymers which have identical values of slope, n , and intercept, D , see tables 2 and 3. The Hartman–Schijve plots for the results [26,27,47] from two rubber-toughened epoxy microcomposites are also shown for comparative purposes. (The references are given for the sources of the original fatigue results).…”

“…Recently, the present authors have developed an approach for analysing the cyclic-fatigue behaviour of polymer-matrix continuous-fibre composites [24,25] and structural adhesives [26,27] using a fracture mechanics approach based upon the Hartman–Schijve equation [28]. The advantages of using this approach are that (i) a unique, ‘master’, linear representation may be obtained which yields the threshold value of the energy release-rate, G , below which no significant fatigue crack growth (FCG) occurs, (ii) having ascertained the values of the constants in the Hartman–Schijve equation the complete curve for the experimentally measured results may be computed with a relatively high degree of accuracy, and (iii) an ‘upper-bound’, i.e.…”

Effect of Bond-Line Thickness on Fatigue Crack Growth of Structural Acrylic Adhesive Joints

“…In the case of fatigue crack resistance, there are no standards available. However, fatigue crack growth (FCG) of composites and adhesives is one of the key issues in the aviation industry; therefore, many studies on FCG have been conducted mainly based on empirical correlations in experiments [ 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ], and some used the Cohesive Zone Model (CZM) for numerical analysis [ 23 , 24 , 25 , 26 ]. In the fatigue tests, FCG behavior is examined by relating crack initiation per cycle with a fracture parameter.…”