Mode II fracture

There are several different test configurations for measuring mode II fracture toughness. The specimen configuration is largely the same, it is the method of applying load to promote a mode II fracture that varies. The configuration is a parallel, unidirectional beam as in the DCB. The three main method of loading the specimen are:

End loaded shear (ELS)- specimen is loaded as a cantilever in specialised fixturing
End notched flexure (ENF) - specimen is loaded in 3 point bending resulting in unstable delamination growth
4 end-notched flexure (4ENF) - as for the ENF except the specimen is loading in 4 point bending resulting in stable delamination growth.

The 4ENF has advantages over the other tests and this section will focus on this test method. The 4-point bending loading fixture uses rollers to support the specimen and to allow it to rotate freely. The upper loading rollers are allowed to rotate about the vertical centre line of the fixture to account for the asymmetric bending of the specimen caused by one half of the beam containing a delamination. The specimens is loaded in displacement control and generally stable delamination results. Fibre bridging does not always occur for delaminations grown in mode II, however, delamination values at delamination initiation are generally used to quote a mode II toughness.

A compliance calibration technique has been developed for the 4ENF specimen where the mode II fracture toughness is determined from