In any item of structural or pressure equipment there is a need to avoid the occurrence of brittle fracture. As we saw in API 571, brittle fracture is a catastrophic failure mechanism caused by the combination of low temperature and a material that has a low resistance to crack propagation at these temperatures. Under these conditions a material is described as having low toughness (or impact strength) — i.e. it is brittle. Impact strength is measured using a Charpy or Izod test in which a machined specimen is impacted by a swinging hammer.