The most common technique used to report the impact characteristics of a plastic member found in the literature today is the drop-weight-to-fracture test, the ASTM Test for Impact Resistance of Rigid Plastic Sheeting or Parts by Means of a Tup (Falling Weight) (D 3029-82). An impactor or tup of specified mass is dropped from a known height. Through the use of a staircase testing procedure, the probable energy required to crack 50% of the specimens, F₅₀, is obtained. The use of this experimental technique for thermoplastic structural foam members can result in misleading data.

A series of constant velocity instrumented impact tests were conducted on several sets of test plaques. The impact energy correlated with the local density of the member, but relatively large variations did occur between identical positions on “identical” plaques. The experiments clearly indicate that the basic premise that the drop-weight-to-fracture test must be conducted on “identical” sets of specimens from a single population is not valid for thermoplastic structural foam specimens.