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Analysis of Mode I Fatigue Crack Growth in Carbon Fibre Reinforced Polymer Laminates
thesisposted on 07.12.2021, 16:39 authored by Joseph Anthony O'GaraJoseph Anthony O'Gara
With the growing use of composites in aircraft structures to decrease weight, there is a necessity to apply a damage tolerant lifing methodology to increase safety. The strain
energy release rate (SERR) range (Delta sqrt(G)) has been shown to itself not be a similitude metric in literature (like Delta K for metals), however it can be used as the basis for the modified Hartman-Schijve similitude parameter (Delta kappa) which has solid support for being such a parameter. Most literature on damage tolerance in composite materials study delamination growth in double cantilever beam (DCB) specimens which only look at damage growth in a single dimension and therefore does not take into account the growth interactions of intralaminar and interlaminar damage in a multi-directional ply layup as would be used in real aerospace applications. This study examined these interconnected damage growth mechanisms in two multi-directional layups in Compact Tension specimens, with and without pre-existing damage in order to examine SERR. This work has shown that SERR range appeared to have a connection to increasing damage across the lead crack cross-section in both damaged and undamaged specimens. Baseline specimens had increased delamination lengths at higher SERR ranges, and compression damaged specimens had increasing intralaminar crack quantities in plys perpendicular to the lead crack direction. Regardless, large amounts of scatter was seen in the 18 tests carried out, well above that seen in DCB test literature. Analysis through the average SERR per cycle (G*) proved to produce less scatter and more intuitive results, and may work well as a similitude parameter. This study has shown more complex fatigue scenarios in composite materials beyond 1D delamination growth must also be understood for any similitude parameters proposed. This will allow for more well rounded understanding of composite material fatigue performance in aerospace applications.