Andrew Croll (North Dakota State U.) - Adhesion and the Strength of Crumpled Films

A thin sheet can easily be bent along one axis, however, bending orthogonally to this leads to stress focusing and the creation of one or more developable cones (d-cones).  Additional folding leads to the creation of more complex objects, such as 'ridges' and 'folds'.  A randomly crumpled sheet is an excellent example of the complex structures possible with a thin sheet, though origami and other organized systems are made from the same building blocks (bending, d-cones, ridges, folds).  While the basic mechanics of the 'building blocks' have been studied in certain limits, it is still unclear if a crumpled film of realistic physical dimensions can be understood from such pieces.  In this talk, I will present our experimental studies of crumpled polymer sheets and 'building blocks'.  Precisely, we will examine the force response of crumpled polymer films, and attempt to supply enough understanding that crumpled structures could fill a role in modern engineering.  I will show that proper attention to materials is needed (though the details are often lost in macroscopic measurement) ,that adhesion plays a substantial role in the structure's resistance to compression, and that a simple empirical model allows quantitative prediction.  While we have no theoretical justification of the model, we will finish with some hints from the 'building blocks'.