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

• Cohesive zone model was measured directly from double cantilever beam specimen using digital image correlation. • Measured CZM gave good representation of the GI measured using ASTM D5528 . • Reverse calculations allow for estimation of defect width at the delamination tip. 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, GI, which was correlated with the same parameter 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 GI from the T-joint specimen. Results showed that as the delamination increased in the skin-flange region, the measured GI 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.