Tensile
testing involves a sample held in two grips a set distance apart. The
loading arm (attached to the top grip) moves up at a constant speed to
deform the sample, first deforming it elastically then plastically. If
the force required to break the sample is within the limit of the load
cell, fracture will occur. It is a very useful test for
monitoring quality of irregular objects, such as the toughness of pizza
or the texture of fish. However, this test setup can also provide useful
stress-strain data if the sample has a uniform cross-section, providing
accurate measurements are made of the sample’s dimensions. “Dogbone”
shaped specimens are often used in tension, with two wide sections
tapering to a narrower central section.
Although
it has long been known that some materials are harder than others,
indentation tests to find quantitative hardness values only came about
in the 1800s. Once
it was established as a valuable technique, hardness testing machines
started to appear on the market early in the following century. Old
fashioned indentation testing involved the application of a weighted
probe onto a flat sample surface that was left for a set time period.
The hardness of the sample was calculated from the area of the residual
dent left in the sample. “Instrumented” (computer controlled)
indentation testing has now been in use for many years, and involves the
collection of force, displacement and time data, which is why the
Texture Analyser is so well-suited to this type of measurement.
