It seems so simple: Place specimen in grips, perform tensile test, and achieve consistent results, right? Not really. Unfortunately, gremlins lurk, all conspiring to introduce errors. The “fishbone diagram” illustrates how errors and variability can occur. Each “rib” of the fishbone is a source group of contributing factors. In particular, we will focus on alignment—why it is important and what to do about it.
In an ideal world, the force applied to the specimen is exactly aligned with the axis of a perfectly straight specimen. So, it is true that any deviation from perfect uniformity of stress in the specimen is a potential cause of variability. (This is particularly true with stiffer, stronger materials and components—where slight misalignments can cause dramatic variations in cross-sectional stress.) Knowing this, the operator must assess the specimen for various factors: Is the tensile specimen axially perfect? Is it uniform—free of curvature in the gage area and cross-section? Was it machined properly—including nicks, scratches and machining marks? Is the “tab” (where the specimen is gripped) flat and parallel to the gage section to ensure that the force is properly concentrated in the desired failure zone? Once the specimen passes inspection, the grips are next on the examination table.