Hypodermic needles are usually manufactured out of stainless steel, and the material and design must be tested to evaluate the needle's
maximum tensile strength and
yield strength. The challenge in testing these tubular devices is in gripping the specimen without causing premature failure in the grip faces. We used our
5569 electromechanical testing system with a 5 kN load cell and 5 kN
pneumatic side action grips to perform this test. Pneumatic grips are instrumental in this type of test because they allow you to specify the gripping pressure to an exact value, which may change depending on the material under test. For this sample, a pressure of 85 psi was used. The faces we used in each grip were one 1 in x 1 in rubber-coated face and one 1 in x 1 in metallic face, providing both cushioning and friction to ensure failure in the gauge area.
Before inserting the specimen into the grip faces, it was necessary to plug the inner diameter of the specimen at each end. We inserted the plug to a position that slightly exceeded the length of the grip faces. The purpose of the plug was to prevent collapse of the gripped ends, but not interfere with the gauge area. For these needles, straightened standard staples were used as inserts. The diameter of the staple was small enough to fit into the inner diameter of the needles. With both inserts in place, we loaded the specimen into the grip faces.
Using a test speed of 0.1 in/min, the specimen was tested in tension until failure. For each specimen, failure occurred within the gauge area, ensuring that the test was conducted successfully. We obtained the following results from five specimens: Modulus, 0.2% yield point, maximum load and break load.