Reexamination of the edge crack torsion test for determining the mode III delamination toughness of laminated composites

The edge crack torsion (ECT) test is reexamined to address two primary concerns with this method. The first of these relates to inconsistencies in the data reduction procedure that was originally proposed and is most commonly used to compute the mode III interlaminar fracture toughness, G(IIIc), from the experimental data. The second relates to the observation that plies neighboring the preimplanted insert in the standard ECT layup are unable to contain initiation and growth of intralaminar cracks, and that growth of these cracks contributes to errors in the apparent values of G(IIIc). To address these issues, an alternative data reduction technique, which uses a polynomial expression to represent the ECT specimen's compliance versus crack length relation, is proposed. The accuracy and potential drawbacks of this expression are investigated using FE simulations. Next, seven groups of ECT specimens are fabricated and tested. Each of these groups contains specimens with preimplanted inserts that are bounded by plies at different +theta and -theta orientations. The seven groups consist of values of theta that vary from 0 degrees to 90 degrees in 15 degrees increments. Tests on these specimens show that intralaminar cracking occurs in specimens from all batches except those for which theta equals 15 degrees or 30 degrees. However, values of G(IIIc) measured using specimens with these two ply orientations were not equal and, for theta = 30 degrees, the values of G(IIIc) that were obtained were not completely independent of initial starter crack length. Potential issues accounting for these observations and other challenges associated with measuring G(IIIc) using the ECT test are discussed.