Experimental study of the relationship between fracture initiation toughness and brittle crack arrest toughness predicted from small-scale testing

It is vital to prevent brittle cracks in large structures. This is particularly important for a number of industry sectors including offshore wind, Oil & Gas, and shipbuilding where structural failure risks loss of human life and loss of expensive assets. Some modern steels exhibit high Charpy energy – i.e. high initiation fracture toughness, but poor resistance to crack propagation – i.e. low crack arrest toughness. The correlation between initiation and arrest toughness measured through small-scale testing is investigated in five different steels, which include S355 structural steel (with two different thicknesses), X65 pipeline steel, two high strength reactor pressure vessel steels and EH47 shipbuilding steel. Small scale mechanical tests were carried out to characterise the materials' properties and were compared to the materials' microstructures. A wide range of tests were carried out, including instrumented Charpy, drop weight Pellini, fracture toughness, tensile testing, and optical microscopy. Nil ductility transition temperature (NDTT) is used to characterise a material's arrest properties. Initiation fracture toughness correlated with higher upper shelf Charpy energy and smaller average grain sizes, as expected, however none of these correlated well with the arrest toughness measured through NDTT. The NDTT correlated most strongly with the T_27J temperature which indicates the start of lower shelf of the Charpy curve. This correlation held for all materials including those where the NDTT lies on the upper shelf of the Charpy curve. While initiation fracture toughness can be predicted through high Charpy toughness and operation temperatures on the upper shelf, crack arrest behaviour should be predicted from characteristics of the ductile to brittle transition temperature, for example by using the T_4kN from instrumented Charpy tests or T_27J.