Delamination growth in polymer-matrix fibre composites and the use of fracture mechanics data for material characterisation and life prediction

Abstract

The growth of delaminations in polymer-matrix fibre composites under cyclic-fatigue loading in operational aircraft structures has always been a very important factor which has the potential to significantly affect the service-life of such structures. The recent introduction by the Federal Aviation Administration (FAA) of a 'slow growth' approach to the certification of composites has further focused attention on the experimental data and the analytical tools needed to assess the growth of delaminations under fatigue loads. Specific attention is given to the test and data-reduction procedures required to determine a 'valid' rate of fatigue crack growth (FCG), da/dN, versus the range of the energy release-rate, ΔG, (or the maximum energy release-rate, G max , in a cycle) relationship (a) to characterise and compare different types of composites, and (b) for designing and lifing in-service composite structures. Now, fibre-bridging may occur behind the tip of the advancing delamination and may cause very significant retardation of the FCG rate. Such retardation effects cannot usually be avoided when using the Mode I double-cantilever beam test to ascertain experimentally the fatigue behaviour of composites, so that a means of estimating a valid (i.e. ideally a 'retardation-free' or, at least, a very low-retardation) relationship is needed. The present paper presents a novel methodology, that is based on a variant of the Hartman-Schijve equation, to ascertain a valid, 'retardation-free', upper-bound FCG rate curves.

Fatigue delamination of composite materials – approach to exclude large scale fibre bridging

“…However, if the specimens are no longer available, either because specimens have been taken apart for post-mortem investigations, or because specimens have been thrown away, data might still be used. This is demonstrated recently in [32] using the same dataset from [26] but evaluated with the Hartman-Schijve relationship [33] proposed previously by Jones et al [34][35] [36]. All data obtained on different specimens, tested under different loading conditions, collapse together when evaluated with the Hartman-Schijve relationship, yielding a similar curve as the zero-bridging curve presented here.…”

Interlaminar crack onset in co‐cured and co‐bonded composite joints under mode I cyclic loading

“…The summary of the results is presented on Table . The energy threshold value for cocured joint showed to be close to values of 0.035 N/mm and 0.0298 N/mm reported by Brunner et al and Jones et al, respectively, in similar testing conditions. An extrapolated value of 0.100 N/mm for toughened epoxy system showed to be above the cobonded joint.…”

Crack Growth in a Range of Additively Manufactured Aerospace Structural Materials

“…We also know [19][20][21][22][23] that the Hartman-Schijve equation, i.e., Equation (2) with the term ∆K replaced by ∆ √ G and K max replaced by √ G max where G is the energy release rate (for metals, √ G is linearly proportional to K), also holds for cracking in adhesives and delamination growth in both composites and nano-composites. It is also known [20,21] that the large scatter seen in the delamination growth curves is also captured by allowing for the variability in the term ∆ √ G thr and the effect of the test protocol on the cyclic fracture toughness term A.…”