Toughness and Flaw Tolerance by Biologically Inspired Approaches

Nature is a masterful designer of structural composites, put together from mineral and organic constituents in a highly synergistic way that combines stiffness, strength, toughness and fracture resistance. The properties are a result of finely controlled microstructures and carefully balanced interactions. Researchers are attempting to replicate the structural motifs of biological composites in bioinspired materials, envisioning a new class of structural materials to compete with metals and conventional composites. In particular, the nacre found in mollusc shells has captured the imagination of scientists with its brick-and-mortar structure of microscopic mineral platelets combined with a small amount of organic matrix. Among approaches to mimic nacre, self-assembled clay/polymer nanocomposites are attractive due to their simple constituents, bottom-up self-assembly process, low density and high potential performance. Like their natural counterparts, bioinspired materials are often sensitive to humidity. In Publication I, we study the effect of water on our clay/polymer nanocomposite films and find that water affects the glass transition temperature of the composite, which eventually becomes lower than the room temperature, increasing the toughness of the composite. At the same time, stiffness and strength are lowered. We learn that work is needed to ensure the desired properties in different environmental conditions. Defect tolerance is central to the reliability of structural materials. In Publication II, we use a simulation model to investigate the role of dissipativity in interactions between material building blocks in flaw tolerance and notch strength. We find a relationship that has previously been observed in experiments. The work illustrates the mechanism behind the toughening effect of sacrificial bonds, which have been observed in numerous biological materials. Evaporation-induced self-assembly, the method to produce nacre-inspired clay/polymer nanocomposites, can be easily used to form strong films from a dispersion. However, structural applications and fracture mechanical characterization require thicker, bulk scale specimens. In Publication III, we present a lamination method to fuse tens or hundreds of less than 100 µm thick films into a plate of up to centimeter thickness. We test this bulk composite in bending and observe its fracture behavior, and perform the first fracture toughness measurement for nacre-inspired clay/polymer composites. Publication IV continues the fracture mechanical characterization. We adopt laser speckle imaging, previously used to monitor blood flow, to directly image the formation of a fracture process zone in front of a pre-crack in loading. The size of the process zone is important for the reduction of stress concentrations to promote toughness and flaw tolerance, and its shape can give hints on the material properties. In the study, we compare the clay/polymer nanocomposite and red abalone nacre and find differences and similarities.