Delamination fatigue growth in polymer-matrix fibre composites: A methodology for determining the design and lifing allowables

Abstract

The introduction, originally in 2009, by the FAA of a 'slow growth' approach to the certification of polymer-matrix fibre composites has focused attention on the experimental data and the analytical tools needed to assess the growth of delaminations under cyclic-fatigue loads. Of direct relevance is the fact that fatigue tests on aircraft composite components and structures reveal that no, or only little, retardation of the fatigue crack growth (FCG) rate occurs as delamination/impact damage grows. Therefore, of course, the FCG data that are ascertained in laboratory tests, and then employed as a material-allowable property to design and life the structure, as well as for the development, characterisation and comparison of composite materials, must also exhibit no, or only minimal, retardation. Now, in laboratory tests the double-cantilever beam (DCB) test, using a typical carbon-fibre reinforced-plastic (CFRP) aerospace composite, is usually employed to obtain fracture-mechanics data under cyclic-fatigue Mode I loading. However, it is extremely difficult to perform such DCB fatigue tests without extensive fibre-bridging developing across the crack faces. This fibre-bridging leads to significant retardation of the FCG rate. Such fibre-bridging, and hence retardation of the FCG, is seen to arise even for the smallest values of the pre-crack extension length, a p-a 0 , that are typically employed. The results from the DCB tests also invariably exhibit a relatively large degree of inherent scatter. Thus, a methodology is proposed for predicting an 'upper-bound' FCG curve from the laboratory test data which is representative of a composite laminate exhibiting no, or only very little, retardation of the FCG rate under fatigue loading and which takes into account the inherent scatter. To achieve this we have employed a novel methodology, based on using a variant of the Hartman-Schijve equation, to access this 'upperbound' FCG rate curve, which may be thought of as a material-allowable property and which is obtained using an 'A basis' statistical approach. Therefore, a conservative 'upper-bound' FCG curve may now be calculated from the DCB laboratory test data for material development, characterision and comparative studies, and for design and lifing studies.

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

“…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. The position and slope of the curve in [32] seems to agree rather well with the curves presented here, which indicates there are more ways to verify the validity of delamination resistance curves.…”

“…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.…”

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.…”

Fibre-bridged fatigue delamination in multidirectional composite laminates

“…Fig. 13 provides a comparison between the upper-bound given in [30] As expected, the upper-bound FDG curve slightly locates on the left side of these experimental data, indicating a faster crack growth. The results given in Fig.…”

“…It is worth highlighting that the use of the modified Paris relation in fatigue data interpretation indeed determines FDG behaviors free of fibre bridging. In a recent study [30], a novel methodology based on statistical analysis has been proposed to determine an "upper-bound" FDG curve, which represents fatigue delamination with no or very little bridging retardation and takes inherent data scatter into account. Fig.…”