Development of the AASHTO fracture-toughness requirements for bridge steels

Abstract This paper describes the effects of temperature and strain rate on the fracture-toughness behavior of bridge steels. The test results showed the existence of a fracture-toughness transition that is an inherent material property rather than a behavior caused only by a change in the stress state. The effect of a slow loading rate, compared with impact loading rates, is to shift the fracture-toughness transition to lower temperatures. The magnitude of the temperature shift between stow loading ( ϵ≈ 10 −5 sec −1 )and impact loading ( /.ϵ ≈ 10 sec −1 ) decreased with increased yield strength of the steel. The fracture-toughness behavior of bridge steels under strain rates that are encountered in actual bridge ( /.ϵ t~ 10 −3 sec −1 ) is closer to slow loading than to impact loading. Relationships are presented among fracture-toughness values determined by testing fracture-mechanics-type specimens, Charpy V- notch (CVN) specimens, and nil-ductility-transition (NDT) specimens. Moreover, procedures are presented for using CVN impact-test results to predict K IC values at slow or at moderate loading rates such as occur in actual bridges. The predicted K IC values are shown to be close to those experimentally determined by testing K IC specimens at various strain rates. The test results were used to develop fracture-toughness requirements for bridge steels. These toughness requirements have been approved by the Federal Highway Administration (FHWA) and by the American Association of State Highway and Transportation Officials (AASHTO) and are mandatory requirements on all Federal-aid highway programs in the United States.