Accurate mechanical testing requires not only familiarity with measurement systems, but also some understanding of the planning, execution, and evaluation of experiments. Much experimental equipment is often “homemade,” especially in smaller companies where the high cost of specialized instruments cannot always be justified. If the designer of the “homemade” equipment does not carefully consider how the design functions under test conditions, then the stress vs. strain diagram may be in error.
ACCURACY, REPEATABILITY, RESOLUTION
Three basic properties determine how well a testing machine can measure stress and strain: accuracy, repeatability (precision), and resolution. To understand the meaning of each, consider the positioning of the crosshead on an electromechanical testing machine.
• Accuracy is the ability to tell the true position of the crosshead. Accuracy is the maximum error between any two crosshead positions.
• Repeatability (precision) is the ability of the crosshead to return to the same position over and over again. Repeatability error is the difference between the positions after several successive attempts to move the crosshead to the same position.
• Resolution is the larger of the smallest programmable steps in crosshead position or the smallest mechanical step the crosshead can make.
Although these definitions seem straightforward, how measurements should best be made to determine them is often a hotly debated topic. The biggest concern is the certainty of the measurements themselves.