Quote:
Originally Posted by Goooh
How was the load speed controlled, or was there a consistent load vs. time throughout all of the tests?
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They make very expensive machines to control load rates in materials testing. Google Instron, for example. These were too expensive for our testing, but it's not to hard with some practice for a skillful person to learn to apply a load in this range with a consistent load rate.
Look at the increase in the load over time shown in Figure 3. Since the loads are being measured as Force vs. time, it is simple to review all the force curves to ensure that the loadings are all occuring over about 1 second. Properties like breaking strength and fracture strength may depend very weakly on the loading rate. Increasing or decreasing the loading rate by a factor of 10 may change the breaking strength by 10-20%. Increasing or decreasing the loading rate by 10% will likely change the breaking strength by less than 1%.
An independent way of knowing that the method is not contributing much to the uncertainty is by observing that the certainties determined from the variations in the five trials with each line is only 1-3% for the lines without any knots. Once the knot is added, uncertainties increase to 1.5-15%. The variations in breaking strength from trial to trial depend much more strongly on whether there is a not than on the small variations from loading with a human pull rather than an expensive machine.