Macroalgal materials: Foiling fracture and fatigue from fluid forces

The persistence of qualitatively weak and flimsy algae on wave-exposed shores is generally attributed to their structural flexibility, which is presumed to reduce hydrodynamic forces. In this dissertation I describe the material properties of over 30 algal materials and investigate the relationship between material properties and fracture in macroalgae using fatigue fracture mechanics, computer modeling, and a novel, virtual-reality based, dynamic testing system.; Among biomaterials, non-coralline algae have generally low strengths (0.20 to 8.08 MN/m2) and high breaking strains (0.14 to 0.79). The absence of upright, non-extensible algal structures supports the idea that extensibility is critical to the persistence of algae on wave-swept shores.; As algae are subjected to roughly 8,000 waves each day, I examine the resistance to cyclic crack propagation in algal blades using protocols and analyses gleaned from the rubber technology industry. The algal species, M. flaccida and P. occidentalis, from wave-exposed sites have higher critical energy release rates than U. expansa blades collected from protected sites. However, for blades with large cracks already present, U. expansa blades are predicted to break at higher stresses than blades of the two wave-exposed species.; While the flexibility of algae generally results in reduced drag forces, the potential for large inertial forces concomitantly increases with freedom of motion. I use a computer model of the bull kelp N. leutkeana to investigate the effects of material properties on the loading imposed on the kelp stipe under ocean waves. Using the cyclic breaking values of stress and strain for N. leutkeana stipe, the safety factor under large waves is maximized when stipe stiffness is 20 MN/m 2.; I incorporated the N. leutkeana computer model into the testing apparatus to create a closed-loop system that reproduces natural loading patterns in the lab. Under large simulated waves, N. leutkeana stipes exhibit an initial stiffness (40 MN/m2) near the predicted optimum of 20 MN/m2. The resilience of N. leutkeana stipes under natural loading patterns is remarkably low (60%). Comparison to a standard linear solid model suggests that stipe material properties are well tuned to the kelp's natural period of oscillation such that resilience is minimized.