Undergraduate Design and Modification of a Tensile Testing Fixture for Biomaterials

In the freshman materials class of a Mechanical Engineering Technology program, students run tensile tests on a 270 kN universal test machine using standard metal specimens having a crosssectional area of 130 mm. A Biology professor studying the development of crabs asked the MET program for help with measuring mechanical properties of crab shells. This material is a natural, porous composite. Very small test specimens with a cross-sectional area of 5 mm are cut from the crab shells because larger specimens have too much curvature. With expected loads below 75 N, the 270,000 N universal testing machine was not suitable. Instead, a 250 N tabletop tensile tester was purchased. This tester pulls specimens 80 mm long, so it required modification to test crab shell material. The MET students were asked to design clamps to hold the crab shell securely without crushing it, at a predetermined gauge length. In an upcoming semester, a new class of students will customize the software to produce meaningful results. This project was good training for future engineers because it helped students learn to work on an interdisciplinary problem for an external customer (the Biology professor) where the inputs were not all known at the beginning.