Elastic Bands

Objective

We wanted to maximize the elastic band lifespan and performance, so that patients can trust that the device is reliable and a good investment.

This was accomplished by determining the mechanical properties of the elastic bands to help the future optimization of the bands and the overall design.

Experimental Design

Our tensile and cycle fatigue testings were developed from ASTM standards and conducted on an Instron Universal Testing Machine.
- Planned to test 20 samples in total [(5 specimens) x (4 band strengths/colors)] → achieved this amount for the tensile testing, but not the cycle fatigue test.

Testing

Tensile Testing

Rate of 500 mm/min for band extension until breakage.
Values obtained: tensile stress extension, strain, load, maximum load.

The images above show the location of the specimen breaking towards the bottom of the knot. The black knots connect the colored band to the hook. All of the bands that we used during the tensile testing broke near the black knot, with some breaking from the top. This breaking point makes sense because it is consistent with the failure patterns as well as the knowledge of having an existing local stretch at the end knot.

These graphs above show a largely linear relationship between tensile stress and tensile strain, which is expected of elastic materials. The percent elongation for the bands is also very large (1000-1500%), which is good considering that most patients using this device will be stretching the bands quite a bit, seeing as they may be connected from the upper portion of their bodies to as low as their thighs. As shown, the 5 specimens were stretched up until their breakage point, which is denoted by the black triangles at the top of the lines. There was a large variation in breakage values, which we attributed to the slight variation in length of each band in a particular color group.

Cycle Fatigue Testing

Rate of 1450 mm/min for 5000 cycles to determine maximum fatigue.

Values obtained: strain energy, extension ratio, and fatigue life.

The image above show that the bands did not break or undergo significant fatigue, which is good news for the company because it means the bands will have a long life. The first picture shows what it looks like before testing, the second picture shows mid- cycle and in the last picture, the black lines being shifted down after testing shows that the bands were slightly fatigued. Therefore, the bands wear over their used lifetime does not seem to be an issue.

The graph above shows the relationship between load and extension. The thicker lines signify the data collected over 5000 cycles, with earlier cycles shown at the top. The curve shows that the band was not able to withstand larger load quantities over time, which means the bands were getting fatigued. In addition, the differences in the curve lengths are owed to the slight difference in length of specimen.

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