Direct measurement of the interlaminar Mode I energy release rate in composite T-joints using digital image correlation

This study proposes a Digital Image Correlation (DIC)-enabled methodology to directly measure the delamination opening and Mode I fracture toughness of a composite T-joint. Opening stresses at the delamination tip were first derived from the opening angles of composite beam specimens made from the same material and layup of the bond line between the flange and the skin of the T-joint, which were subjected to double cantilever beam tests. These experiments produced a representative cohesive zone model (CZM) describing the opening stress-displacement relationship for progressive damage at crack interfaces. Integration of the CZM provided a direct measurement of the Mode I energy release rate, G_I, which was correlated to the G_I calculated using the standard test method, ASTM D5528-13. The CZM was then applied to delamination opening measurements from the composite T-joint to determine the corresponding opening stress. Integrating the opening stress-displacement curve enabled direct measurement of the G_I from the T-joint specimen. Results showed that as the delamination increased in the skin-flange region, the measured G_I decreased, thus suggesting a transition from pure Mode I to mixed-mode, and finally pure Mode II failure. Microscopic observations of fracture surfaces further supported these findings.