In Mode II delamination, failure occurs due to excessive shear stress. The End Notched Flexure (ENF) test determines the elastic energy threshold required for the growth of a predefined delamination for this mode of failure.

Such a test should be performed to the standard proposal EN 6034 [11]. Figure 5.14: Schematic of the ENF Configuration Used to Determine the Interlaminar Fracture Energy G_IIC shows a schematic of the experimental set-up and geometry of the sample. The sample, containing a predefined crack is loaded in a test machine in a three point bending configuration. Displacement of the loading device and applied load is recorded continuously during the tests.

Figure 5.14: Schematic of the ENF Configuration Used to Determine the Interlaminar Fracture Energy G_IIC

The predefined crack grows further as a result of the Mode II loading: specimen bending and the resultant shear forces at the crack tip. The total fracture toughness energy G_IIC is calculated from the initial crack length and from the critical load P to start the crack, and, the displacement of the test machine d at onset of crack extension [11]. All remaining coefficients are sample and test rig dimensions. Further detailed information can be found in [2].

(5–3)

The interlaminar fracture toughness energy is the energy per unit plate width that is necessary to grow an interlaminar crack. A typical force displacement curve is presented in Figure 5.15: Typical Load Displacement Curve for Determination of the Mode II Interlaminar Fracture Energy and shows an interruption of the initial slope in the force displacement curve characterizing the critical load at the onset of crack propagation in the specimen. Afterwards, the reduced bending stiffness influences the force displacement curve.

Figure 5.15: Typical Load Displacement Curve for Determination of the Mode II Interlaminar Fracture Energy