The Mode II fracture toughness is measured using the ENF
Typically, the specimen length is 120 mm, the width B is 20 mm, the thickness is between 3 and 5 mm and the distance between supports, 2L, is 100 mm. The relative crack length a/L is usually 0.5.
When the specimen is loaded, the crack tip is subjected to pure shear (Mode II) and the crack propagates along the mid-plane of the sample.
In the ENF specimen, crack growth is unstable and only one data point per specimen can be obtained.