Conference Paper

Correlation Between Steel Microstructural Characteristics and the Initiation and Arrest Toughness Determined From Small-Scale Mechanical Testing

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Abstract

Modern high Charpy toughness steels can nonetheless show low crack arrest toughness[1]. In this paper, the relationship between initiation and arrest toughness is investigated in five different carbon steels, including S355 structural steels, X65 pipeline steel, and high strength reactor pressure vessel, RPV, steels. The results from small-scale mechanical tests, including instrumented Charpy, drop weight Pellini, fracture toughness, and tensile testing (including STRA in the through-thickness direction) were used to determine the behaviour of the different steels in terms of initiation fracture toughness and crack arrest toughness parameters. There was no correlation between the upper shelf initiation toughness and the arrest toughness when the results from the five steels were collated. The mechanical test results were then correlated to the steels’ microstructural characteristics, including parent metal microstructure, average grain size and grain aspect ratio to identify the relative roles of microstructure and texture in the fracture initiation and arrest performance of carbon steels.

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... 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]. The results from this study correlate the mechanical properties with the microstructure of five different modern structural steels to evaluate the relevant parameters necessary for fracture prevention. ...
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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.
Article
Brittle crack arrestability of the heavy gauge steel plates for shipbuilding is now an important issue for the recent mega container ships. In the present work, the brittle crack arrestability of the steel plate with different toughness distributions in thickness is examined in ultra-wide duplex ESSO tests. It is examined whether a running long brittle crack arrests or not in flat temperature condition in ultra-wide duplex ESSO test that are harder mechanical conditions similar to an actual ship hull condition. Test temperatures are selected at which arrest toughness, Kca evaluated by temperature gradient type standard ESSO test are the same for two test plates. The steel plate with higher toughness in midthickness (t/2) than that in quarter thickness (t/4) could arrest a running long brittle crack although the plate with lower toughness in midthickness than that in quarter thickness could not arrest it. The typical split nail shape appeared at the arrested crack front in the plate with higher toughness in mid-Thickness than that in quarter thickness. The numerical analyses also demonstrate that the local stress intensity factor at the arrested crack tip is changing sensitively to the crack front shape. It suggests that the higher brittle crack arrestability appears due to the split nail shape of the arrested crack front enhanced by the inhomogeneous toughness in thickness.
Article
Critical crack opening displacement (COD) values have been examined for a range of specimen thicknesses. The COD at the initiation of fracture δ1 is found to be constant, given a plane-strain crack-tip stress-state, whereas the COD at maximum load δmax decreases with increasing thickness. The loads required to produce instability are found to vary with thickness, in a way analogous to behaviour observed under linear elastic conditions. Crack growth under constant load for a range of specimen thicknesses has been examined, and failure has been found to occur at loads below that associated with Δmax,; the minimum load per unit thickness required to cause failure decreasing with specimen thickness.
Article
Recent experimental and computational work by Link and associates has demonstrated that relatively small (W= 150 mm) single edge notched tension specimens (SE(T)) can be used to obtain crack arrest data high in the ductile-to-brittle transition of ferritic structural steel using dynamic computational techniques if a thermal gradient is utilized to aid in the crack arrest. Testing has been reported on two important navy structural steels that clearly defines the relative capability of the two materials to arrest rapidly growing cracks. The HY100 material demonstrated the expected large difference between the initiation and crack arrest toughnesses which has made it impossible in the past to measure crack arrest toughness for this material using the standard ASTM procedure (E1221). The HSLA-100 steel, however demonstrated a much higher crack arrest toughness and a correspondingly smaller drop in toughness below the initiation toughness. This small difference between initiation toughness and arrest toughness suggested that the E1221 procedure, using wedge loaded, compact crack arrest (CCA) specimens would be applicable to this material. Two important issues could then be investigated using this material. First, having completed the expensive and relatively complex testing of the SE(T) specimens using tensile loading and a thermal gradient, a second, quite different geometry could be tested using the E1221 procedure, allowing an important comparison between the crack arrest measurements made using these two distinct geometries. Historically, obtaining crack arrest results using one test configuration has been so difficult, that there have been very few reports of results for the same material using two different test geometries. Transferability of the laboratory results to structural applications has thus been a matter of conjecture. Furthermore, if the E1221 CCA specimens were strain gaged to obtain crack velocity data, and analyzed using the dynamic computational procedure used by Link on the SE(T) specimens, it would be possible to compare the results the E1221 static analysis with the results of the dynamic computation procedure to determine the degree of conservatism present in the E1221 standard procedure. The results of this work have shown that the crack arrest toughness results obtained on these two specimen geometries are similar and hence insensitive to the test geometry and the difference resulting from the application of the complex dynamic computational procedure or the E1221 static analyses is small.
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