Computing the growth of naturally-occurring disbonds in adhesively-bonded patches to metallic structures

Abstract

In 2009 the US Federal Aviation Administration (FAA) introduced a slow growth approach for certifying composite and adhesively-bonded structures. This approach requires that delamination or disbond is slow, stable and predictable under cyclic-fatigue loads. The present paper addresses the challenge of developing a methodology capable of enabling this approach to certification to be implemented. To this end we have examined the growth of disbonds from small naturally-occurring material discontinuities in adhesively-bonded structures. It is shown that, for the examples studied, the disbond growth histories can be accurately computed using a form of the Hartman and Schijve variant of the NASGRO crackgrowth equation. It is also shown that the scatter in the disbond growth histories can be captured by allowing for small changes in the fracture-mechanics threshold term. These findings suggest that the Hartman-Schijve crack-growth equation has the potential to address the 'slow growth' approach to certifying composite/bonded structures and bonded repairs outlined in the US FAA Airworthiness Advisory Circular No: 20-107B.

Cyclic-fatigue crack growth in composite and adhesively-bonded structures: The FAA slow crack growth approach to certification and the problem of similitude

“…Now, there are several reasons why the SERR, G, approach, and not the stress-intensity factor approach, has invariably been used when employing a fracture-mechanics approach to investigate the failure of polymeric-matrix fibre-composite materials and adhesively-bonded joints, see for example [5,7,8,[22][23][24][25][26][27][28]. There is the difficulty of calculating the stressintensity factor around the crack tip in anisotropic and inhomogeneous materials and bonded joints.…”

“…where D, n and A are constants and, as for metals, the value of A is generally chosen to so as to capture the entire da/dN history [1,11,24,25]. The term ∆� ℎ represents the fatigue threshold.…”

“…This approach has the advantage that it can represent the growth of both long and small, naturally-occurring, cracks and can capture the variability seen in crack-growth histories as observed in both constant amplitude and operational flight-load spectra [24,32] as well as the coalescence of small cracks and their subsequent growth.…”

“…For composites and structural adhesives, it is often found from experimental tests that a clearly defined threshold value exists, below which little fatigue crack growth occurs [11,26]. In this case the value of the threshold, ∆� ℎ , is taken to be the experimentallydetermined value, see [11,24]. If this is not the case, then the concepts described in the ASTM Standard [45], which are widely used by the metals community, may be employed.…”

“…The Hartman-Schijve equation used in [1,8,11,20,24,25,32,36,[39][40][41][42][43][44] is basically an empirical equation that for metals takes the form:…”