Characterisation of fatigue delamination growth in plain woven hybrid laminated composites subjected to Mode-I loading

Effect of similar and dissimilar crack plane interface configuration on fatigue delamination growth in plain woven hybrid composite laminates under Mode-I has been investigated. Constant displacement amplitude fatigue testing with displacement ratio of 0.1 was carried out on 3 configurations of plain woven glass/carbon epoxy composite laminates. A power law like fit between recorded delamination length and corresponding cycle was used to predict crack length for each of the cycle. Delamination growth rate,da/dN is computed by differentiating the expression of power-law like fit. The obtained crack growth rate for each of the specimens were plotted with respect to two normalised functions, (G) over cap (Imax) = G(Imax)(a)/G(IR)(a) where G(Imax) is maximum mode-I energy release rate and G(IR)(a) is the interlaminar fracture toughness resistance and Delta(G) over cap (Ieff) = (root(G) over cap (Imax) - root(G) over cap (Imin))(2) as crack driving parameters computed on the basis of Modified Beam Theory (MBT) and Valvo's mode partition method (MPV) are used in Paris relation to quantify delamination propagation. It is observed that the exponent values predicted by MBT method for (G) over cap (Imax) is lower as compared to Delta(G) over cap (Ieff). Whereas, exponent values predicted for (G) over cap (Imax) is higher as compared to Delta(G) over cap (Ieff) predicted by MPV method. The higher the exponent value, the higher is the sensitivity of the model leading to uncertainties in the crack growth prediction. Also, it is to be noted that cyclic loading effect is when both GImax and G(Imin) is considered, the use of (G) over cap (Ieff) as crack driving parameter to quantify delamination propagation is justified. Secondly, MBT method does not account for the mode-mixity arising due to hybrid material configuration as in the case of Local Symmetry Fatigue (LSF) and Asymmetry Fatigue (ASF) specimens. Hence, results in higher exponent as compared to MPV method. On the other side, the (G) over cap (Imax) and Delta(G) over cap (Ieff) computed on the basis of MPV method is the pure Mode-I component deduced from the total energy release rate of mode-mixity. The equations of curve fitting is very much the same for Simple Symmetry Fatigue (SSF) specimens indicating that MBT and MPV methods predict pure Mode-I behaviour for symmetric configuration for delamination growth under fatigue Mode-I loading. From the composite laminate configuration point of view, LSF specimens have higher exponents as compared to ASF and SSF specimens indicating, local symmetry configuration laminates are highly sensitive to the small uncertainties and results in unstable crack growth. Comparison of results of all hybrid composite laminates shows that the normalised functions of (G) over cap (Imax) and Delta(G) over cap (Ieff) as crack driving parameters computed on the basis of MPV method is able to capture the effect of interlayers and stacking effect on the delamination growth in hybrid plain woven composites in fatigue loading and MPV method is found to be not sensitive to (G) over cap (Imax) and Delta(G) over cap (Ieff) for displacement ratio of 0.1.