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Correlation between steel initiation toughness and arrest toughness determined from small-scale mechanical testing
Abstract
The ability of a material to arrest a fast-running brittle crack is vital in various industries such as offshore wind, oil and gas, and shipbuilding where cracks can initiate in regions of local stress and put lives at risk. Some modern steels show a high Charpy toughness, but low resistance to crack propagation – i.e. low crack arrest toughness. In this work, the relationship between initiation and arrest toughness is investigated in five different steels, including S355 structural steel, X65 pipeline steel and two high strength reactor pressure vessel (RPV) steels.
Small scale mechanical testing was carried out to determine the material properties, which were correlated against the microstructural characteristics of the materials. The test program included instrumented Charpy, drop weight Pellini, fracture toughess, tensile testing, and microscopy. Nil ductility transition temperature (NDTT) is used as a measure of arrestibility. Initiation toughness showed the expected correlations with upper shelf Charpy and grain size measurements, however these did not correlate with the arrest toughness. The arrest toughness is better correlated against the T27J temperature – i.e. the onset of the lower shelf. This relationship is valid even for steels where the NDTT lies on the upper shelf of the Charpy curve.
... The ductile crack extension length was measured in all thickness directions using the three-point bending test. Under unloading conditions on the surface, the visual method [22,23] was used to estimate the crack length form and compliance [21,[24][25][26][27]. This method directly measured the crack length in the complete unloading condition using beach marks on the crack surface. ...
High-manganese austenitic steel was developed to improve the fracture toughness and safety of steel under cryogenic temperatures, and its austenite structure was formed by increasing the Mn content. The developed high-manganese austenitic steel was alloyed with austenite-stabilizing elements (e.g., C, Mn, and Ni) to increase cryogenic toughness. It was demonstrated that 30 mm thickness high-manganese austenitic steel, as well as joints welded with this steel, had a sufficiently higher fracture toughness than the required toughness values evaluated under the postulated stress conditions. High-manganese austenitic steel can be applied to large offshore and onshore LNG storage and fuel tanks located in areas experiencing cryogenic conditions. Generally, fracture toughness decreases at lower temperatures; therefore, cryogenic steel requires high fracture toughness to prevent unstable fractures. Brittle fracture initiation and arrest tests were performed using 30 mm thickness high-manganese austenitic steel and SAW joints. The ductile fracture resistance of the weld joints (weld metal, fusion line, fusion line + 2 mm) was investigated using the R-curve because a crack in the weld joint tends to deviate into the weld metal in the case of undermatched joints. The developed high-manganese austenitic steel showed little possibility of brittle fracture and a remarkably unstable ductile fracture toughness.
... The shortage of monitoring specimens severely restricts the safety evaluation of RPVs and brings big problems for the life extension of plants. Therefore, small-scale specimen testing [7,8], master curve prediction [9,10] and nondestructive testing (NDT) [11,12] have been widely developed to evaluate the degradation of RPV steels. ...
Reactor pressure vessels (RPVs) are the heart of nuclear power plants. RPV steels suffer from neutron irradiation, which would introduce irradiation damages into materials, such as dislocation loops, tetrahedrons, precipitations, etc. Due to shortage of monitoring specimens, nondestructive testing (NDT) has been developed for the degradation evaluation of RPV steels. In this paper, the ageing behavior of hardness and coercivity of Cu-rich RPV model steels are studied at 643 K, 673 K, 743 K and 773 K for magnetic NDT. It is found that the hardness and coercivity both exhibit ageing peaks during thermal ageing and there is a good correlation between them. Furthermore, the apparent activation energy of Cu-rich precipitations (CRPs) is estimated at around 160 kJ∙mol⁻¹.
... 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. ...
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.
Description
Twenty-two papers appear in this volume. The papers are divided into two areas:
• strength and ductility
• fatigue and fracture
S355 structural steel is commonly used in fabrication of offshore structures including offshore wind turbine monopiles. Knowledge of mechanical and fracture properties in S355 weldments and the level of scatter in these properties is extremely important for ensuring the integrity of such structures through engineering critical assessment. An interlaboratory test programme was created to characterise the mechanical and fracture properties of S355 weldments, including the base metal, heat‐affected zone, and the weld metal, extensively. Charpy impact tests, chemical composition analysis, hardness tests, tensile tests, and fracture toughness tests have been performed on specimens extracted from each of the 3 material microstructures. The experimental test results from this project are presented in this paper, and their importance in structural integrity assessment of offshore wind turbine monopiles has been discussed. The results have shown a decreasing trend in the Charpy impact energy and Jmax values with an increase in yield stress from base metal to heat‐affected zone to weld metal. Moreover, the JIC fracture toughness value in the heat‐affected zone and weld metal is on average around 60% above and 40% below the base metal value, respectively. In addition, the average Charpy impact energy value in the heat‐affected zone and weld metal is around 5% and 30% below the base metal value, respectively. The effects of mechanical and fracture properties on the critical crack size estimates have been investigated, and the results are discussed concerning the impact of material properties on structural design and integrity assessment of monopiles.
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.
The results of a range of different small scale material characterisation tests were correlated with the crack arrest temperature at 2 3 the yield strength of a number of modern ferritic steels and their weldments. The crack arrest temperature was determined experimentally using large scale structurally representative double tension tests. The comparison of small and large scale tests indicates that a safe estimate of the crack arrest temperature for the steels and welds investigated can be obtained using the nil-ductility transition temperature plus 40°C. The 50% fracture appearance transition temperature or the 20% fracture appearance transition temperature plus 20°C obtained from full thickness drop weight tear test are also reliable measures of the lower bound crack arrest temperatures.
A new procedure is described for the engineering design of fracture-sale steel structures representing weldments, forgings, casings, and combinations of these. The procedure is applicable to all steels which have distinct transition temperature features, i.e., excepting the ultrahigh strength types which have poorly defined, low-slope Charpy V transition curves. The new procedure is based on the concept of the fracture analysis diagram, which represents a consolidation of the available knowledge concerning flaw size, stress, and temperature requirements for the initiation and propagation of brittle fractures. The bases for the development of the diagram are explained. Extensive Failure and structural test data are provided as documentation of the validity of the described procedures. The practical engineering use of the fracture analysis diagram is based on the determination of a simple parameter-the NDT temperature of the steel. All other required information involves elements which are normal considerations in design of engineering structures.
Fracture toughness results are often connected with varying thickness effects. Many results are contradictory, with some results indicating no size effect, some indicating increasing toughness with decreasing thickness and others indicating decreasing toughness with decreasing thickness. In this paper the causes for different thickness effects are discussed. It is shown that most of the observed size effects are due to invalid tests. The only test parameters to be regarded as valid are those that correspond to the initiation of crack extension. The theoretical thickness effects for both ductile and brittle fracture initiation are evaluated. It is shown that ductile fracture initiation is thickness independent if B ≥α(J/σy). The thickness effect in brittle fracture is explained by applying a mechanism-based statistical cleavage fracture model. A theoretical thickness correction for cleavage fracture is derived, and its validity is confirmed for a variety of materials.
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