New Method Measures Localized Stiffness of Thin Paper Materials with a Single Test
Researchers at the Forest Service's Forest Products Laboratory in Madison, Wisc., developed a method to measure localized stiffness of thin paper materials with a single test. Central to this work is the development of a novel load fixture which is capable of exciting all stiffness components simultaneously without wrinkling. The load fixture displaces a portion of the paperboard sheet at specific locations along the specimen's circular boundary. Displacements through the entire sheet are captured by a camera and used to separate the stiffness components. Critical to the analysis is the extension of the Virtual Fields Method (VFM) analysis procedure to paperboard, which made it possible to determine localized stiffness at millimeter resolution within the paperboard samples. Since product failures initiate in regions of low stiffness, the ability to determine the variation in mechanical properties within a sheet of paper is critical to improving paperboard performance. It is estimated that as much as 20 percent of the pulp fiber used in paperboard is needed to make up for the non-uniformity in stiffness distributed throughout the sheet. This new method provides a way to make progress on paperboard formation problems that lead to this wasteful use of wood pulp. Although developed for use on problems in the paper industry, this equipment and analysis are equally useful for several other material systems: in aircraft, where many wing assemblies are designed with specific stiffness behaviors; in printing, where mis-registration can be caused by unintentional stiffness interactions; and in the textile industry, where �stretchiness' and tactility have important stiffness components.
Forest Service Partners