Development of a modified edge crack torsion test for mode III fracture toughness of laminated composites

Modifications to the edge crack torsion (ECT) test are investigated to address two primary concerns. The first of these relates to errors in the data reduction procedure that is typically used to compute the mode III interlaminar fracture toughness, GIIIc, from ECT test data. The second relates to the observation that plies neighboring the preimplanted insert in standard ECT layups are unable to contain the growth of intralaminar cracks, and that the initiation and growth of these cracks contributes to errors in the apparent values of GIIIc that are obtained. To address the first concern, an alternative data reduction technique, which uses a polynomial form to represent the ECT specimen's compliance versus crack length relation, is proposed and its accuracy demonstrated by finite element simulations. Next, seven batches of ECT specimens are fabricated and tested. Each of these batches contain specimens with a preimplanted midplane edge delamination which is bounded by plies at + and - orientations. The seven batches consist of values of that vary from 0 to 90 in 15 increments. Tests on these specimens show that intralaminar cracking occurs in specimens from all batches except those for which = 15 or 30. However, values of GIIIc measured using specimens with these two ply orientations were not equal and, for = 30, the values of GIIIc were not clearly independent of insert length.