Abstract
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 T27J 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 T4kN from instrumented Charpy tests or T27J.
Highlights
Crack initiation and propagation is often experienced by engineering components and structures subjected to operational loading conditions
The purpose of this study is to develop a better understanding of the crack arrest behaviour in a range of modern steels by performing mechanical testing and metallurgical analysis of the materials’ microstructures, which have been suggested to influence crack arrest properties [9,26,27,28,29]
As seen in this figure, there is a weak linear correlation between upper shelf Charpy energy and CTOD δm fracture toughness, excluding M03, which has the highest Charpy energy, but the median fracture toughness compared to the other materials
Summary
Crack initiation and propagation is often experienced by engineering components and structures subjected to operational loading conditions. An important issue that needs to be understood for design and life assessment of such structures is the ability of a material to arrest a fastrunning brittle crack, for structures where fracture may initiate in areas of high local stress or low toughness, for example in the welds [1] This is 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. Structures in offshore environments are exposed to very harsh loading conditions, with both wind and wave loading in addition to the operating loads i.e. its own mass and moving components [2,3] In such structures a crack may initiate around a weld region, which is the part of a structure most susceptible to cracking due to material mismatch, heat input which changes material properties, locked-in residual stresses and increased chance of impurity inclusions during the welding process [4]. Smaller components can be heat-treated after welding, to reduce the damaging effects of welding residual stresses and reset the materials microstructure and larger structures can receive local heat treatment
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