Observation of intralaminar cracking in the edge crack torsion specimen

• ECT specimens, in their current form, exhibit intralaminar cracking prior to delamination growth.
• ECT specimens, in their current form, are unsuitable for measuring mode III delamination toughness.
• ECT specimens with alternative stacking sequences might constrain intralaminar cracking.

The edge crack torsion (ECT) test is evaluated to determine its suitability for measuring fracture toughness associated with the onset of mode III delamination in laminated composites. ECT specimens with preimplanted midplane inserts of different lengths are tested and examined using nondestructive and destructive techniques. Ultrasonic inspections of all tested specimens reveal that delamination growth occurs one ply interface beneath the midplane. This is confirmed by sectioning and optical microscopy, which also show that macroscopic delamination advance results from a coalescence of angled intralaminar matrix cracks that form in-between and then extend through the midplane plies. The relative orientation of these intralaminar cracks is approximately 45° with respect to the midplane, suggesting that their formation is caused by resolved principal tensile stresses arising due to the global mode III shear loading. Examination of ECT specimens tested to loads below the level corresponding to the onset of delamination growth reveals that initiation of intralaminar cracking occurs prior to or concurrently with the onset of nonlinearity in the specimen’s force–displacement response. The existence of intralaminar cracking prior to delamination growth and the resulting delamination extension at an unintended interface render the ECT test, in its current form, unsuitable for characterizing the onset of mode III delamination growth. The broader implications of the mechanisms observed in this study are also discussed with respect to the current understanding of shear-driven delamination in tape-laminate composites.