Adaptive Changes in the Morphology of Natural Materials

Natural materials have shown their ability to adapt their geometry in response to mechanical stress. In examining their shape and structure, it becomes apparent that the major strategic function of structural materials and systems involves mechanical support under changing load. This comparison study examines how the morphology of four biological materials (bone, wood, plant stems, and blood vessels) is influenced by environmental loading conditions. As these materials remodel their shape through adaptive growth, they seek to maintain an optimal structure that is able to sustain a particular mechanical support requirement. Bone and wood adapt to increased mechanical load by increasing their density while plant stems and blood vessels develop thicker vessel walls. For bone, trabecular bone density and cortical bone thickness are increased with load in order to maintain a roughly constant peak functional strain; wood and blood vessels remodel their shape under elevated levels of stress in order to maintain a constant surface stress and constant circumferential stress, respectively. In addition, plant stems and blood vessels which develop thicker vessel walls due to mechanical stressing show a tendency to reduce their elastic modulus so that they could overcome their stiffness and become more flexible, preventing rupture. The examination of these materials suggests that natural materials remodel functions to maintain a constant stress or strain, giving a constant factor of safety. Thesis Supervisor: Lorna J. Gibson Title: Professor of Civil and Environmental Engineering