Dynamic fracture toughness of X70 pipeline steel and its relationship with arrest toughness and CVN

The dynamic fracture toughness K1d and J1d, arrest toughness K1a and Charpy V-notched impact toughness (CVN) of a pipeline steel, X70, were studied at different temperatures. It was found that fracture toughness was strongly affected by temperature and loading rate. The fracture toughness decreases with decreasing temperature from 213 to193 K and increasing loading rate from K̇=1.0 to 105 MPa12 s−1. At constant temperatures, only increasing loading rate can induce the transition from ductile to brittle. There exists a fracture transition caused by loading rate. Through thermal activation analysis, a quantitative relationship has been derived: J1d=Ja+J0K̇K̇01nexpQfnkT. It can describe the fracture process at different temperatures and loading rates. At a loading rate of K̇=15 MPa m12 s−1, the relationship can predict arrest toughness well. It provides the possibility of measuring arrest toughness with small size specimen. An empirical equation has been derived: CVN=4.84×106T−2.8K1d(K1a), which correlates K1d and K1a with CVN in one equation. This means that we can calculate K1d and K1a when we get CVN.