Mixed & other modes of fracture

There are other modes of fracture that require consideration. Many delaminations in structures will be driven by a combination of mode I and mode II fracture. This is termed mixed mode fracture. While the mode I fracture toughness may give a conservative value to use in a mixed mode case, it is often advisable to measure the actual mixed mode fracture toughness. While many specimen types have been proposed, the mixed mode bending (MMB) test has distinct advantages over most. This specimen allows almost any combination of modes I and II to be evaluated using a specimen with a similar configuration as the DCB or 4ENF can be obtained from one specimen type.

Mode III fracture describes delaminations that are driven in a scissoring shear motion. While this is a less common mode it can occur. The test methods for characterising mode III fracture are less developed. At present the edge crack torsion (ECT) specimen is often specified. This test involves twisting a rectangular plate that has an initial delamination along the edge parallel to the long axis of twist.