TITLE:

DEATH BY SMALL FORCES: BREAKAGE BY FATIGUE IN WAVE-SWEPT SEAWEEDS

DATE:

Friday, April 16th, 2010

TIME:

3:30 PM

LOCATION:

GMCS 214

SPEAKER:

Katharine J. Mach, Hopkins Marine Station of Stanford University

ABSTRACT:

Repeated force application typifies the lives of many organisms, with sometimes detrimental consequences. In humans, for example, the repeated forces exerted on bones during intensive exercise can cause stress fractures. On wave-swept shores, the study system at the center of my research, waves regularly crash over seaweeds and commonly break them. To date, however, much biomechanical investigation has focused on single applications of force, pulling structures such as algal blades until they rupture. In the case of wave-swept seaweeds, such pull-to-break tests have found that individual waves are often not forceful enough to account for observed rates of breakage. I have investigated an alternative explanation for breakage of seaweed blades: failure may occur by fatigue, with damage accumulating over the course of smaller, repeated loadings. In laboratory tests on the seaweed Mazzaella, I have quantified fatigue by repeatedly loading specimens, mimicking the action of hundreds to millions of waves. I have tracked the entire process of fatigue, from initial formation of small cracks through eventual specimen fracture. Extrapolation of fatigue behavior measured in the laboratory suggests an important role for fatigue failure in breakage observed in the field. I am assessing these predictions for wave-swept seaweeds by comparing models of fatigue failure and intertidal hydrodynamic forces with measured rates of breakage.

HOST:

Jose Castillo

DOWNLOAD:

Techniques for predicting the lifetime of wave-swept macroalgae: a primer on fracture mechanics and crack growth. J EXP BIOL, 210, pp 2213-2230, 2007
Mechanical and biological consequences of repetitive loading: crack initiation and fatigue failure in the red macroalga Mazzaella. J EXP BIOL, 212, pp 961-976, 2007