Comparison and analysis of in-plane compression and bending failure in paperboard

In-plane compression of paperboard, using long (LCT) and short compression test (SCT), and bending, using the L&W creasability tester, have been analyzed using three paperboards with similar strength properties but different ZD profiles. The failure loads from the methods have been compared and the failure mechanisms analyzed. It was shown that at maximum bending moment from bending of samples using L=10 mm and L=50 mm, the SCT value and the maximum bending force from LCT all varied linearly with each other, indicating that same damage mechanism is activated. It was suggested that delamination cracks were initiated during SCT and LCT as well as during bending when plastic deformation had been initiated in an outer ply subjected to compressive stress. The plastic deformation would be initiated when the yield stress in the ply was reached, determined by an in-plane tensile test. When plastic deformation takes place, it will generate shear induced delamination cracks in locations with low shear strengths, e.g. in the interfaces or within the middle ply. The location depends on the material design strategy used in manufacturing the paper quality. To control the in-plane compression properties in paperboard one should control the yield stress (or possibly the failure stress) of the outer plies. Increased stress gives higher in-plane compression strength. Moreover, the interfaces strength is important, since increased interface strength (or lack of interfaces) prevents delamination.