Tests were carried out to determine the interlaminar fracture toughness of stitch-bonded biaxial polymer matrix carbon nanotube nanocomposites for mode I, II, and I-II including durability effects. Analysis of the test specimens in terms of mode I energy release rates showed good agreement among Modified Beam Theory, Compliance Calibration, and Modified Compliance Calibration methods. End-Notched Flexure (ENF) and four point End-Notched Flexure (4ENF) tests gave very consistent crack initiation and propagation results for mode II fracture. The results show that the critical mode I energy release rate for delamination decreases monotonically with increasing mode II loading. The effects of accelerated aging (60°C and 90% Rh) on fracture properties were studied. Early accelerated aging (0–12 months) has the dominant diminishing effect on energy release rate initiation and propagation in composites and nanocomposites.

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