In order to do a notched Charpy test, a precise notch must be cut into the specimens. The purpose of the notch is to serve as a stress concentrator. The notch is probably the most critical part of specimen preparation and there is a tight tolerances defined by ASTM E23 on the type of the notch, the depth of the notch (actually the material remaining under the radius of the notch), the angle of cut and the radius at the base (or apex) of the notch. Research has shown that the notch in the specimen is perhaps the greatest source of variability of test data.

Specimens can be notched using a milling machine or a commercially available, specifically designed, notching machine. ASTM E23 does not define the specific operation of the notching machine, but most involve adjusting the cutter height on the machine so that it cuts a notch that leaves 8 mm of material remaining under the apex of the notch and a radius of curvature at the apex of 0.25 mm.

While the notch depth, the included angle of the notch and the radius of curvature are critical, only the measurement of the notch depth is relatively simple and can be easily done on specimens prior to testing. The angle of the notch and the radius of the angle at the base of the notch are more difficult to measure. Therefore, specimens must be periodically sampled for inspection under magnification.

A method of identifying the individual specimens within the sample should be devised. Measure and record the width of each individual specimen in the area of the notch using a micrometer or similar instrument. A notch depth verification device, which consists of an analog dial indicator positioned above an anvil, is commonly used to measure the depth of the notch. A digital indicator may also be used and is particularly useful if the actual depth of the notch is to be determined or if it is necessary to switch between measuring in traditional US units and metric units. However, for those laboratories interested in doing a go/no-go type determination, analog dials are very convenient because they can be equipped with follower arms that serve to mark the upper and lower limits of the depth tolerance.