The test specimen design is based on a standard tensile specimen. It must be proportional (see Connect Article No. 69) in order that results can be compared and ideally should be machined to tighter tolerances than a standard tensile test piece. In particular the straightness of the specimen should be controlled to within some ½% of the diameter. A slightly bent specimen will introduce bending stresses that will seriously affect the results. The surface finish is also important - the specimen should be smooth, scratch free and not cold worked by the machining operation. The extensometer should be fitted on the gauge length and not to any of the other load carrying parts as it is difficult to separate any extension of these parts from that in the specimen.