Biomechanics

Fracturing seaweed: critical crack length and morphology

By Jessica Bolker, University of New Hampshire


Type of Resource Laboratory activity 
Topic Materials/biomaterials 
Taxa Plant 
Organizational Level Tissue 
Estimated time to do activity 1-3 hours, depending on (1) thoroughness of exploration, (2) ingenuity of students, and (3) desired precision of results. Lab can be done in two sessions if fresh material is available. Or one could do a quick version of one part as an in-class demo. 
Background required/level Some prior discussion of fracture mechanics and (especially) crack propagation. Would fit in nicely with discussions of environmental hazards, in the form of nibbling predators, faced by sessile organisms – who can’t run away, and have therefore had to evolve different sorts of defensive responses, many of which involve the structure and properties of their tissues. 
Role of activity in your course I use this exploration as a somewhat open-ended lab. It’s flexible: one could do a much more structured version (perhaps coupled with quantitative analysis of data), or choose one part for a quick demo during a lecture. (On a nice day with the right tides, it might be possible to pack up equipment and do the whole lab as a field trip to the beach, testing and comparing whatever species were available.) 
What students might learn from this course or activity This series of activities provides clear and sometimes dramatic illustrations of a number of aspects of fracture – and the various ways organisms resist it. Different parts of the lab address the relationship between shape, material, and strength; variation in mechanical properties of tissues in different parts of the same organism; Griffith critical crack length; relationship between stored strain energy, GCCL, and crack propagation; and blunting of crack tip to reduce stress concentration/inhibit crack propagation (students are surprised and impressed that a “mere seaweed” exhibits this rapid, adaptive “behavior”).

Possibilities for interesting comparative observations: Aluminum foil is very sensitive to initial nicks, Saran wrap is nicely resistant; paper is in between - explaining the cutters on rolls for kitchen use. Sheet plastics vary a lot, so they’re interesting, as is Pliofilm (Parafilm) used in the lab.

Skills: depending on how one sets it up, the lab can offer practice in designing experiments, including figuring out exactly what one needs to measure (e.g. to get stress you’ll need to know area as well as force), and devising ways to do so. Trying to get reproducible measurements from a slippery piece of alga may lead to enhanced respect for scientists who actually do this kind of work – and an appreciation of the differences between biomechanics and traditional engineering studies.
 
Special tools, equipment or software needed Biological material: Reasonably fresh, wet Laminaria (or other macroalga). This is easy to come by if one is close to a coast, has access to a beach at low tide, and/or can ask favors of local marine biologists and/or scuba divers. For the landlocked, it is presumably possible to ship wet seaweed – but I don’t know how elapsed time or shipping conditions might affect its mechanical properties.

Possible non-biological comparators: Aluminum foil, Saran wrap, paper, sheet plastics, Pliofilm/Parafilm.

Equipment: digital calipers (nice but not essential: tissue dimensions and crack lengths can also be measured with a ruler, though less precisely), assortment of tools that students can use to devise means to quantitate applied forces (e.g. clamps, weights, spring scales, meter sticks, stepladders), paint or soft-tipped markers for marking prospective crack lengths, razor blades; if possible a dissecting microscope and light, ideally (but not necessarily) with a camera lucida attachment or a digital camera. A mop is likely to be useful.
 
Safety precautions, possible permissions necessary Provide appropriate disposal containers for razor blades. Mop up puddles before someone slips. 
Miscellaneous advice - pitfalls to avoid Three students per group provides enough hands for loading, slicing, and recording observations. If, as I do, you leave it up to students to devise a means of loading the material (putting strain energy into it), provide enough guidance to ensure they do it in a way that will give a reasonably uniform load (not, for instance, by using a wire to hang a weight from the blade). Also remind them about viscoelasticity, and about the difference between force and stress…. I’ve had pairs of students get a rough approximation of load by facing one another and pulling on the Laminaria with one hand, and simultaneously (and in parallel) pulling on a spring scale with the other). If they’ll use a microscope to look at the shape of the crack tip, they should do so right away: it changes surprisingly fast. 
Frequently asked questions by students NA 
Description A slightly quantitative, very accessible, low-tech way to investigate critical crack length, compare material properties of different parts of a single organism, and observe one strategy for reducing stress at crack tip. 
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