For many material applications, the evidence of a visible flaw (for example, a crack) can be regarded as the start of failure. It is important to determine the impact conditions for which incipient crack formation occurs. From the material design standpoint, it is essential to identify the key material properties governing this crack formation process. Therefore, its assessment and characterization become indispensable to polymeric material manufacturers as well as design engineers in material selection.

Typically, a Gardner impact tester has been employed to determine the first crack formed in a material. However, Gardner methods lack the capability of evaluating the crack initiation process as a function of impact speed. The Rheometrics impact tester can be used to determine the crack formation point by adjusting the ram travel distance into a test specimen short of complete penetration. Nevertheless, the large momentum of the impact ram at increasingly high speeds makes its stoppage at a preset travel distance extremely difficult. Hence, this mode of testing is limited to low impact speeds. In order to facilitate crack formation study at high speeds, the authors of this paper have developed a testing technique using a Dynatup impact tester. This technique involves the use of variable-thickness shims to raise the test specimen height from its lowest impact point, where contact between the tup and test specimen is just made. This allows us to control the distance the tup travels into the specimen.

The impact characteristics (that is, impact energy, impact force, and ram displacement) at crack formation are analyzed in connection with material selection. One of these three quantities is suggested for impact toughness evaluation, and its underlying rationale and significance are discussed. Also, the impact characteristics at crack initiation and at complete puncture are compared in relation to material design and selection.