chapter  10
24 Pages

Impact Testing

The fracture behavior of a material is different when it is tested at low strain rates (i.e., the rate at which the specimen is strained, dε/dt where ε is the strain and t is the time) or high strain rates. The strain rate is low during conventional tensile testing and is typically about 10−4-10−2 s−1. A material may exhibit ductile fracture under these conditions. However, if the same material is subjected to a sudden, intense blow, that is, when loading occurs at high strain rates, it may behave in a much more brittle manner than under low strain rates. Such a behavior can be seen very clearly in the case of plastics. Let us take a plastic such as polyethylene or Silly Putty and subject it to stretching slowly. The polymer molecules will have time to disentangle or for the chains to slide past each other and cause large amounts of elongation. On the other hand, if the loading is done suddenly, that is, at high strain rates, the molecules will not have suf“cient time to disentangle and therefore the specimen will experience brittle fracture, that is, the specimen will break suddenly. Thus, the rate of loading plays an important role in determining the fracture behavior of a material. Accordingly, an impact test is frequently used to ascertain the brittleness of a material. During impact loading, the strain rates are typically high, about 103 s−1, that is, 5-7 orders of magnitude faster than during a conventional tensile test.