The specimen's shape is usually defined by the standard or specification being utilized,
e.g., ASTM E8 or D638. Its shape is important because you want to avoid having a
break or fracture within the area being gripped. So, standards have been developed
to specify the shape of the specimen to ensure the break will occur in the "gage
length" (2 inches are frequently used) by reducing the cross sectional area or diameter
of the specimen throughout the gage length. This has the effect of increasing the
stress in the gage length since stress is inversely proportional to the cross sectional
area under load,
.