Determining the fracture resistance of thin sheet fiber composites – Paper as a model material

In nonlinear fracture mechanics testing of thin-sheet short-fiber composites, special problems occur that do not appear in other engineering materials, such as steels. The most important problem is the formation of a long process zone, where fiber pull-out, realignment and breakage occur, making an optical crack length measurement impossible. This impedes the determination of a reproducible value of the fracture toughness and the construction of a crack growth resistance curve. Two new approaches are presented to overcome this problem. In the first one, a procedure is presented to determine experimentally the cohesive zone relation on deeply-notched double-edge notch tension specimens. The cohesive zone relation enables us, together with the mechanical properties, to simulate numerically a fracture mechanics test on an arbitrary geometry and to determine a crack growth resistance curve. In the second approach, the displacements and strains around the process zone are measured during in situ experiments under an optical microscope using digital image analysis. With this local deformation analysis, a critical local strain is determined where the load bearing capacity of the material decreases to zero. The knowledge of this critical strain is used to find the location of the crack tip and to determine a crack growth resistance curve. The application of the two approaches is demonstrated on commercial printing paper as model material. It is shown that reproducible fracture toughness parameters can be determined with both procedures.