Fracture Protocol
Mechanical test procedures measuring pure mode II interlaminar fracture toughness utilize three point bending setups on pre-cracked beam specimens. A pre-cut starter notch inserted along the mid-plane guides crack propagation under applied flexural loads, allowing end notched flexure testing to record critical strain energy release rates without axial tension interference. The test fixture supports a specimen across a fixed span while a central loading roller depresses the upper face.
Compliance calibration accounts for machine deflection prior to shear failure.
Shear Energy
Interlaminar shear forces concentrate at the crack tip during bending, driving sliding displacement between adjacent plies.
Interlaminar Delamination
Structural failure in heavy multi-ply paperboard packaging often originates as sub-surface delamination during high speed converting or mechanical folding. End notched flexure testing determines the precise threshold where internal starch interfaces or fiber-to-fiber bonds separate under pure sliding shear forces. Creasing operations and scoring dies induce severe localized shear stresses along the mid-plane of thick containerboard webs, making resistance to mode II crack growth a critical parameter for creasing quality.
Low mode II fracture toughness causes premature wall splitting during box erection on automated packaging lines. Laboratory evaluation requires precise measurement of crack length before loading, using optical microscopy or compliance mapping to track crack front evolution.
