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Single Edge Notched Tension (SENT) Testing At Low Temperatures

   
Philippa L Moore and Andreea M Crintea

TWI Ltd, Granta Park Great Abington, Cambridge, CB21 6AL, UK

Paper presented at Proceedings of the 2016 11th International Pipeline Conference IPC2016 September 26-30, 2016, Calgary, Alberta, Canada

Abstract

Much of the established data from SENT tests has been generated on ductile materials in the form of tearing resistance curves (R-curves) in terms of J. The testing of SENT specimens is now standardised in BS 8571 [1] and there is potential to use SENTs for high and low temperature tests, but there is little recently published data showing SENT behaviour at low temperature. This paper presents a comparison of fracture toughness data for equivalent SENT and SENB specimens in three different steels as ductile-to-brittle transition curves over a range of temperatures. SENT specimens in comparison to SENBs show higher fracture toughness on the upper shelf, lower transition temperature, but also a much steeper transition from ductile to brittle behaviour. It is therefore important to characterise SENT behaviour at the lowest anticipated service temperatures to ensure that this sudden change in fracture behaviour will be avoided in service.

This paper also describes methods for carrying out SENT tests at very low temperatures, including the use of threaded ends to allow testing inside a temperature controlled test chamber, while preventing the specimen from yielding at locations away from the intended notch tip.

1. Introduction

Single Edge Notched Tension (SENT) fracture toughness test specimens (Fig. 1) are being used for a wider range of applications. The test specimen became established for use with fitness-for-service assessments of flaws in pipeline girth welds under high strain conditions during installation, as described in DNV RP F108 [2], and were later further established for subsea pipelines in general in DNV OS F101 [3]. The lower constraint of the SENT specimen, compared to the historically more common Single Edge Notched Bend (SENB) specimen, results in higher values of fracture toughness in SENTs on the upper shelf. The crack tip constraint of the SENT specimen can be shown to still be higher than that associated with a flaw in a pipe under uniaxial loading, and the fracture toughness from an SENT test could be conservatively used for the assessment of pipelines. Fracture toughness testing using SENT specimens is now standardised in BS 8571 [1] and it is possible to use SENT specimens for higher and lower temperature tests, for a wider range of material thickness, and for different engineering applications. Provided the structural application to be assessed has lower constraint than the specimen, SENT test results can be considered for use with fitness-for-service assessment, as discussed further in Annex N of BS 7910 [4].

Figure 1 SENT test specimen of square cross section instrumented with a double clip gauge ready for test under clamped loading at ambient temperature

Figure 1 SENT test specimen of square cross section instrumented with a double clip gauge ready for test under clamped loading at ambient temperature

Much of the established data for SENT tests has been generated on ductile materials as tearing resistance curves (R-curves) in terms of J at ambient temperatures. But as SENT specimens become used more widely, lower temperature tests will be performed more often. It is recognised that a change in constraint between SENT and SENB specimens will affect not just the fracture toughness values, but also the ductile-to-brittle transition temperature [5]. This is why characterising the low temperature behaviour of SENT specimens is important. Hauge & Holm [6] published results from a number of data sets in steels tested at temperatures from 0°C to -90°C, in SENB specimens with deep and shallow notches and in SENT specimens, although they do not describe practical details about the testing. Their test results showed that within the test data set, the maximum SENT fracture toughness was higher, and gave a larger range of fracture toughness results for nominally similar specimens, although less difference was seen when all specimens showed predominantly brittle behaviour. This confirms that SENT specimens can give very different results at low temperature compared to SENBs.

2. Nomenclature

a Crack length, mm.
B Specimen thickness, mm.
J J integral fracture toughness, kJ/m2 (or N/mm)
M Mismatch ratio of weld metal to parent metal strength.
T Temperature, °C.
T0 Ductile to brittle transition temperature defined from the mid-point of the tanh curve fit, °C.
W Specimen width, mm.
X, Tanh curve fitting parameter.
Y Tanh curve fitting parameter.
Z Tanh curve fitting parameter.

3. Cooling Fracture Toughness Specimens to Low Temperatures

When performing a fracture toughness test at low temperature to test standards such as ISO 12135, ASTM E1820, or BS 7448-1 [7-9] for SENB specimens in parent metals, or BS EN ISO 15653 [10] for SENBs in welds or heat-affected zones, or even BS 8571 [1] for SENTs, it is important to soak the specimen at the test temperature, to enable it to stabilise before performing the test. When the test is performed in a liquid medium (for instance an SENB specimen immersed in cooled methanol), the specimen is soaked for 30s per mm of specimen thickness. For testing in a gaseous medium (such as testing in a thermal chamber cooled with vaporising liquid nitrogen), the specified soak time is 1 minute per mm of specimen thickness.

For SENBs, the ability of the specimen to be immersed in a liquid bath, or for the test to be carried out inside a temperature controlled chamber is well established (Fig. 2). However, the testing of SENT specimens immersed in liquid would generally require the specimen to be loaded horizontally, with the gripped ends of the specimen potentially outside the liquid bath (which would be focused round the notch location); whereas most universal test machines apply loads vertically rather than horizontally. However, immersed SENT tests have been achieved at TWI for specialised testing of SENT specimens in seawater or sour solutions, for example.

Localised cooling can also be applied to an SENT specimen clamped and loaded vertically using a flow of liquid nitrogen vapour within some kind of insulation around the notch location (Fig. 3). This method is effective for modest cooling down to around -60°C, below which it can be difficult to establish a sufficiently constant and stable temperature for the duration of the soak time.

Figure 2 Low temperature SENB specimens tested (a) in a temperature controlled chamber

Figure 2 Low temperature SENB specimens tested (a) in a temperature controlled chamber

Figure 2 Low temperature SENB specimens tested (b) immersed in a cooled liquid bath

Figure 2 Low temperature SENB specimens tested (b) immersed in a cooled liquid bath

Figure 3 SENT specimen tested using localised cooling around the notch, (a) with liquid nitrogen cooling pipes clamped around the notch region

Figure 3 SENT specimen tested using localised cooling around the notch, (a) with liquid nitrogen cooling pipes clamped around the notch region

Figure 3 SENT specimen tested using localised cooling around the notch (b) insulated with wire wool and a double clip gauge added ready for test

Figure 3 SENT specimen tested using localised cooling around the notch (b) insulated with wire wool and a double clip gauge added ready for test

A further limitation of localised cooling around the notch of an SENT specimen is that as a steel becomes colder its yield strength increases. This means that the cooled notch region of the specimen is stronger than the arms of the specimen at ambient temperature. In specimens tested at sufficiently low temperature, the arms of the specimen can yield and deform away from the notch tip location intended by the fracture toughness test. In this situation the test will not be able to characterise the fracture toughness at the notch tip. This issue with yielding in the arms of an SENT specimen can also occur in specimens with moderately shallow notches, and in specimens notched into welds with high strength over-matching to the parent metal. Low localised test temperatures can exacerbate the problem further in these cases.

A potential solution is to adapt the design of a straight-sided clamped SENT specimen to have threaded ends which can then be ‘clamped’ and held in the threaded grips of a universal test machine and tested inside a temperature controlled chamber (Fig. 4) [11]. Having the whole specimen at the same temperature reduces the risk of yielding in the arms. In this configuration the machine grips are still outside the chamber, but apply load to the threaded fitting which protrudes into the chamber and holds the specimen. The specimen is at a more uniform temperature, but the grips themselves do not need to be cooled down (which would affect the hydraulic operation of the machine). This method also allows testing to much colder temperatures (as low as -160°C) with good thermal stability, and the potential to cool and soak subsequent test specimens while the first is being tested, shortening the overall testing time for a set of specimens. However, the method is not without its own challenges, and the optimisation of this specimen design is discussed later in this paper.

An alternative solution to enable SENT specimens to be tested inside a thermal chamber was described by Delliou and Geniaut [12] using a shorter SENT specimen gauge length and bolt holes in a wider clamped area at the end of the specimen to enable it to be gripped inside a thermal chamber, Fig. 5. They carried out SENT tests in this way at temperatures from -40°C to -100°C.

Figure 4 Machined threaded-end SENT specimen gripped inside a thermal chamber

Figure 4 Machined threaded-end SENT specimen gripped inside a thermal chamber

Figure 5 SENT specimen design with bolt holes (a) which can be gripped inside a thermal chamber

Figure 5 SENT specimen design with bolt holes (a) which can be gripped inside a thermal chamber [12]

Figure 5 SENT specimen design with bolt holes (b) taken from [12]

Figure 5 SENT specimen design with bolt holes (b) taken from [12]

4. Ductile to Brittle Transition Curves in SENT and SENB specimens

4.1 Materials and Test Specimens

In order to investigate the behaviour of SENT and SENB specimens over a range of temperatures, ductile to brittle transition curves were generated in three different steels, selected to show a range of transition temperatures. The full chemical composition of these three steels is given in Table 1. BxB test specimens were used in all cases, notched in the through-thickness direction. Equivalent specimens with the same cross-section size and notch depth were generated as SENBs and SENTs. The three steels were:

Steel M01 was a 26mm thick SA543 Grade B high strength steel plate with 0.2% proof strength of 850MPa and UTS of 910MPa. This steel contained 0.2% carbon, 1.5% chromium and 3% nickel. The fracture toughness specimens were machined with a cross section of 20mm x 20mm and notched to a nominal a/W ratio of 0.5. The actual average a/W ratio for the nine SENT specimens was 0.512 and for the ten SENB specimens was 0.516.

Steel M02 was a 26mm thick SA302 Grade C structural steel plate with 0.2% proof strength of 640MPa and UTS of 760MPa. This steel contained 0.2% carbon and 1.4% manganese. The fracture toughness specimens were machined with a size of 20mm x 20mm and notched to a nominal a/W ratio of 0.5. The actual average a/W ratio for the nine SENT specimens was 0.516 and for the ten SENB specimens was 0.515.

Weld W03 was a girth weld in a 15.9mm thick 22 inch diameter pipeline steel of grade X70, welded using a TIG weld root pass and submerged arc welding for the fill passes. The weld metal had a 0.2% proof strength of 551MPa and UTS of 626MPa, and slightly overmatched the strength of the parent metal. The fracture toughness test specimens were machined to a size of 14mm x 14mm and notched to a nominal a/W ratio of 0.4. The average actual a/W ratio for the twelve SENT specimens was 0.430 and for the thirteen SENB specimens was 0.447.

Charpy transition curves were also generated for each of the three steels so that the transition temperature and transition curve shapes for the fracture toughness specimens could be compared to Charpy test results.

Table 1 Chemical analysis of the three steels tested, with compositions given in weight percent.

Element M01 M02 W03
C 0.17 0.2 0.07
Si 0.38 0.31 0.4
Mn 0.29 1.43 1.3
P 0.007 0.013 0.009
S 0.005 0.004 0.005
Cr 1.46 0.22 0.042
Mo 0.46 0.53 0.19
Ni 2.95 0.59 0.757
Al 0.014 0.012 0.016
Co 0.008 0.006 0.005
Cu 0.018 0.046 0.13
Nb 0.004 0.008 0.005
Ti 0.007 0.002 0.007
V 0.012 0.008 0.003
Ca <0.0003 0.0004 0.0006
Carbon Equivalent 0.80 0.63 0.39

4.2 Fracture Toughness Test Results and Transition Curve Fitting

The results of the fracture toughness tests were plotted as values of fracture toughness in terms of J, against temperature, and a curve fit of the form of a tanh function was fitted to the data as a best fit by eye to define the ductile to brittle transition curve. The S-shaped tanh function is a curve of the form:

J= X + Y tanh ( (T-T0) / Z)                                           (1)

where X, Y, Z and T0 are the curve fitting parameters. The parameter ‘X’ defines the fracture toughness halfway between the upper and lower shelves, and T0 is the temperature of this mid-transition point X. The parameter T0 is therefore also defined as the ductile to brittle transition temperature from the tanh fit. The parameter ‘Y’ defines the magnitude of the difference between upper and lower shelf fracture toughness, where X+Y is the upper shelf fracture toughness and X-Y is the lower shelf fracture toughness. The parameter ‘Z’ is a measure of the steepness of the slope of the transition, and gives the temperature range over which the fracture toughness changes from X-½Y to X+½Y.

The results of the fracture toughness tests in steel M01 are given in Table 2; the SENB and SENT test results are plotted in Figs. 6 and 7 respectively. The test results for steel M02 are given in Table 3 and the SENB and SENT test results are plotted in Figs. 8 and 9 respectively. The test results for weld W03 are given in Table 4 and the SENB and SENT test results are plotted in Figs 10 and 11 respectively. A comparison of all the transition curves is given in Fig. 12 for all three materials and both specimen types. The upper shelf SENT specimens from M01 and M02 (identified by an asterisk in the tables) showed some out-of-plane tearing on their fracture surfaces, with a tearing angle of up to 50° (Fig. 13), therefore the values of J determined from these tests have been corrected by multiplying by the cosine of 50° (0.6428) for the tanh curve fitting [13].

The tanh function fitting parameters for each of the transition curves shown in Figs. 6-11 are given in Table 5. Charpy transition curves were generated for the three materials as well, and although not plotted against the same vertical toughness scale, the fitting parameters giving steepness of the transition and the transition temperature (also given in Table 5) can be compared to the SENB and SENT results.

Table 2 Fracture toughness test results for BxB SENT and SENB specimens notched through-thickness in material M01.

Test type Test temperature, °C a0/W ratio J, kJ/m2 Result
SENB -20 0.515 418.74 Jm
-40 0.514 395.22 Jm
-60 0.515 384.36 Jm
-80 0.515 405.58 Jm
-120 0.51 426.79 Jm
-125 0.515 449.37 Jm
-130 0.522 99.87 Jc
-135 0.522 87.34 Jc
-140 0.518 48.95 Jc
-150 0.511 60.94 Jc
SENT -20 0.51 1088.29* Jm
-40 0.512 1024.19* Jm
-60 0.51 949.46* Jm
-80 0.513 1094.02* Jm
-120 0.515 1143.14* Ju
-130 0.514 49.24 Jc
-140 0.51 55.62 Jc
-150 0.513 56.35 Jc
-160 0.507 34.74 Jc
Figure 6 Fracture toughness data and transition curve for steel M01 using SENB specimens

Figure 6 Fracture toughness data and transition curve for steel M01 using SENB specimens

Figure 7 Fracture toughness data and transition curve for steel M01 using SENT specimens

Figure 7 Fracture toughness data and transition curve for steel M01 using SENT specimens

Figure 8 Fracture toughness data and transition curve fit for steel M02 using SENB specimens

Figure 8 Fracture toughness data and transition curve fit for steel M02 using SENB specimens

Figure 9 Fracture toughness data and transition curve for steel M02 using SENT specimens

Figure 9 Fracture toughness data and transition curve for steel M02 using SENT specimens

Table 3 Fracture toughness test results for BxB SENT and SENB specimens notched through-thickness in material M02.

Test type Test temperature, °C a0/W ratio J, kJ/m2 Result

SENB

120 0.511 363.11 Jm
100 0.522 345.64 Jm
80 0.516 368.23 Ju
60 0.515 421.82 Jm
40 0.516 173.77 Jc
20 0.515 84.65 Jc
-20 0.517 64.39 Jc
-40 0.506 172.73 Jc
-60 0.511 16.48 Jc
-80 0.515 9.56 Jc

SENT

60 0.51 830.83* Jm
40 0.522 822.38* Jm
-20 0.517 961.69* Jm
-25 0.52 97.9 Jc
-30 0.517 299.57 Ju
-40 0.511 23.89 Jc
-50 0.511 54.21 Jc
-60 0.518 123.35 Jc
-80 0.517 15.02 Jc

Table 4 Test results for BxB SENT and SENB specimens notched through-thickness in the weld metal of weld W03.

Test type Test temperature, °C a0/W ratio J, kJ/m2  

SENB

-20 0.428 342.38 Ju
-30 0.432 689.06 Jm
-40 0.42 140.55 Jc
-50 0.421 357.09 Jc
-60 0.405 242.65 Jc
-65 0.419 63.13 Jc
-70 0.407 24.73 Jc
-75 0.417 16.76 Jc
-80 0.533 27.56 Jc
-80 0.541 61.43 Jc
-80 0.537 31.64 Jc
-80 0.443 42.32 Jc
-80 0.414 64.09 Jc

SENT

-60 0.43 754.21 Jm
-60 0.427 714.98 Jm
-66 0.427 760.06 Ju
-70 0.423 749.15 Ju
-70 0.44 103.52 Jc
-75 0.439 32.23 Jc
-75 0.421 263.89 Jc
-80 0.429 434.97 Ju
-80 0.437 29.71 Jc
-80 0.435 112.74 Jc
-85 0.437 85.95 Jc
-85 0.418 713.96 Ju
Figure 10 Fracture toughness data and transition curve for steel W03 using SENB specimens

Figure 10 Fracture toughness data and transition curve for steel W03 using SENB specimens

Figure 11 Fracture toughness data and transition curve for steel W03 using SENT specimens

Figure 11 Fracture toughness data and transition curve for steel W03 using SENT specimens

Figure 12 Comparison of SENT and SENB transition curves in three different steels

Figure 12 Comparison of SENT and SENB transition curves in three different steels

Figure 13 Examples of out of plane tearing in part of the crack front in upper-shelf SENT specimens from M01 and M02

Figure 13 Examples of out of plane tearing in part of the crack front in upper-shelf SENT specimens from M01 and M02

Table 5 Tanh function fitting parameters for the ductile to brittle curves in the three materials, for fracture toughness in terms of J (kJ/m2) and Charpy in terms of impact energy (J).

Data set X Y

Z

T0, °C
M01 SENB 236 184

5

-127
M01 SENT 362 331

3

-126
M01 Charpy 73 45

26

-92
 
M02 SENB 206 180

32

+42
M02 SENT 297 272

5

-23
M02 Charpy 62 49

53

+45
 
W03 SENB 142 131

11

-57
W03 SENT 336 289

2

-71
W03 Charpy 95 91

20

-63

4.3 Using Threaded-End SENT Specimens for Low Temperature Tests

Two different production methods were investigated for developing threaded-ends on SENT specimens. The first method was to machine round threaded portions directly onto the ends of the square cross section of the SENT specimen, leaving a specimen length equal to ten times the specimen width (10W) between the threaded portions (Fig. 14). The second method was to use rotational friction welding to weld lengths of round bar onto the ends of a length of square cross section. Threads were then machined onto the round bar (Fig. 15). The length of the square cross section part of the specimen was cut 10mm longer than 10W initially, to allow for some material loss in the flash formed during friction welding.

The machined thread method had the advantage of being quicker and simpler to produce, and in these specimens the strength in the threaded portions was the same as the strength in the main part of the test specimen. The concern in these specimens was that the loss of cross section in the threaded portion might mean that the load bearing capacity of the threads was less than in the notch region.

Figure 14 Machined threaded end SENT specimens, shown with shims attached to the notch mouth ready for the screw attachment of a double clip gauge before testing

Figure 14 Machined threaded end SENT specimens, shown with shims attached to the notch mouth ready for the screw attachment of a double clip gauge before testing

Figure 15 Friction welded threaded end SENT specimens, shown with shims attached to the notch mouth ready for the screw attachment of a double clip gauge before testing

Figure 15 Friction welded threaded end SENT specimens, shown with shims attached to the notch mouth ready for the screw attachment of a double clip gauge before testing

The friction welded method had the advantage that a round bar could be selected with a larger cross section than the square section specimen so that the threaded portion cross section was at least that of the specimen (a round bar with a 30mm diameter was used here). However, the choice of materials for the round bar in this work was limited to commercially available S355 grade structural steel bar (identified here as M04) which had a yield strength of 368MPa, lower than that of the materials being tested, again raising concerns about load bearing capacity of the threaded ends compared to the notch. A hardness traverse across the friction weld (Fig. 16) confirmed that the weld and heat affected regions are stronger than the parent metal in the threaded region, so this parent metal is the limiting yield strength in the joint. The friction welded specimens were more complex to produce and used additional rotary friction welding equipment for their production.

4.4 Strain Capacity Comparison between the Notch and Threads

The feasibility of each of the two threaded-end SENT specimen designs was established by performing a strain capacity analysis comprising both finite element modelling and experimental trials on materials M01 and M02 [14]. The specimen design was considered suitable only if the strain capacity of the plane-sided SENT specimen before the point of fracture instability around the notch (based on the onset of unstable tearing) proved less than the strain capacity of the threaded end due to tensile overload failure. The challenge in using threaded ends for SENT specimens is to ensure the load transmission to the notch location by preventing the failure of the threads, or yielding of the specimen’s arms. The results of the strain capacity analyses are given in Table 7.

The model predictions were supported by experimental tests on specimens with both fatigue pre-cracked and electro-discharge machined (EDM) notches with different notch depths. The strain models predicted that the machined threaded-end SENT specimens would fail in the threads at a lower strain than the notch. Experimental tests on specimens EDM notched with a/W of less than 0.4 confirmed that these would suffer failure in the threads, shown in Fig. 17. However, experimental tests on more deeply notched specimens with fatigue pre-cracked a/W of around 0.5 were successfully used to generate some of the ductile to brittle transition curves without failure in the threads. These were therefore an acceptable design for deeply-notched single point fracture toughness testing in both M01 and M02, despite the conservative model prediction.

Figure 16 Macrograph showing a cross section through the friction weld attaching the threads (M04) to an SENT specimen (M02). The hardness traverse across this weld is shown superposed. Scale in mm

Figure 16 Macrograph showing a cross section through the friction weld attaching the threads (M04) to an SENT specimen (M02). The hardness traverse across this weld is shown superposed. Scale in mm

Table 7 Strain capacity of the plane-sided SENT specimen for different notch depths determined from numerical models [14].

Material Location Strain capacity, [%]
High strength
M01
Notch 1.81
High strength
M01
Machined thread 1.77
Medium strength M02 Notch 1.72
Medium strength M02 Machined thread 0.97
Round bar
M04
Friction welded thread 1.94
Figure 17 Examples of failure in machined threaded ends in SENT specimens in M01, with (a) yielding in a specimen with a notch depth of a/W of 0.33

Figure 17 Examples of failure in machined threaded ends in SENT specimens in M01, with (a) yielding in a specimen with a notch depth of a/W of 0.33

Figure 17 Examples of failure in machined threaded ends in SENT specimens in M01, with (b) a thread fracture from a notch depth of a/W of 0.23

Figure 17 Examples of failure in machined threaded ends in SENT specimens in M01, with (b) a thread fracture from a notch depth of a/W of 0.23

Where the model predicted a strain capacity of 1.8% around the notch (in M01 and M02) but only up to 1% in the thread (in M02), in practice such tests actually failed at the notch rather than the thread, demonstrating the conservatism in the analysis approach. The friction welded specimens manufactured with threads of 30mm diameter and commercially available structural steel M04 proved feasible for all notch depths in the range of 0.2 to 0.5, having greater strain capacity in the threads than around the notch. They were shown through both model and experiment to be suitable for a wide range of SENT testing.

For relatively shallow notched specimens, where the notch is in weld metal which has a much higher yield strength than the parent material there can be a risk of yielding in the arms, even for SENT tests performed at ambient temperature. For overmatched welds the mismatch ratio, M, is defined as the yield strength of the weld metal divided by the yield strength of the parent metal, and it is recommended in BS 8571 [1] to restrict the notch depth so that:

a/W ≥ -0.107M2 + 0.536M - 0.261                               (2)

This means that for mismatch of M=1.1 the a/W ratio must exceed 0.228 and for M=2.5 a/W must exceed 0.410. This risk of yielding in the arms is exacerbated for a specimen with machined threaded ends. A comparison was made between the strain capacity results from models of the threaded ends with lower tensile properties than around the notch in M01 for different levels of mismatch, M, between 1.1 and 1.5 [14], summarised in Table 8. At higher levels of mismatch the strain capacity of the threads reduced below 1% for mismatch greater than 1.1.

The friction welded design had already been shown to be feasible with material around the notch of 850MPa yield and threaded ends from steels with 368MPa yield. Based on the predictions for the parent metal specimens, it can be concluded that this design can be used for any weld strength mismatch, based on this validation using weld strength up to 850MPa, provided the conditions in Equation (2) also hold. Additionally, attention must be paid to the diameter of the round bar that is friction welded to the SENT blank, to ensure it has a cross section larger than the specimen gauge.

Table 8 Strain capacity of the SENT specimens’ threaded ends for different levels of weld strength overmatching in relation to a weld metal with strength equal to M01.

Material / Weld Location Strain capacity [%]
Postulated ‘weld metal’ M01 Notch 1.81
Even-matching M01 Machined thread 1.77
Mismatch ratio,
M =1.1
Machined thread 1.11
Mismatch ratio,
M =1.2
Machined thread 0.77
Mismatch ratio,
M =1.3
Machined thread 0.63
Mismatch ratio,
M =1.4
Machined thread 0.60
Mismatch ratio,
M =1.5
Machined thread 0.59
Round bar M04 Friction welded thread 1.94

5. Discussion

5.1 Transition Curve Behaviour with SENT Specimens

A wide range of ductile to brittle transition temperatures were shown by the three steels that were characterised in this work. The transition curves were fitted to quite scattered data in some instances, particularly for weld W03, which may be due to greater inhomogeneity in the weld metal sampled between specimens. An average-fit transition curve was used for this work to compare the different materials. Had a lower-bound transition curve been the intention, then the curve fit parameters might have been slightly different to those given in Table 5. The values of transition temperature, T0, from the SENB specimens ranged from +42°C for M02, to -127°C for M01. Yet for these three very different types of steel, there were some common features between the SENT transition curves for all of them.

All three materials showed upper shelf fracture toughness values in the SENT specimens of at least 150kJ/m2 greater than the SENB values. Care must be taken in the interpretation of upper shelf SENT results where there is out of plane tearing [4], the data presented here was corrected for out-of-plane tearing in some specimens in M01 and M02 based on the tearing angle [13]. The higher fracture toughness in SENT specimens is the well-known consequence of the difference in constraint between the SENB and the SENT specimen, and the reason why SENT specimens are so widely preferred, particularly for pipeline and high strain applications. The difference in lower shelf fracture toughness in terms of J was less significant, the SENT being within 40 kJ/m2 of the SENB value in all cases, and it was less than the SENB lower shelf for M01. These results agree with similar findings by Hauge and Holm [6] who also found little difference in lower shelf behaviour, but significant difference in transition and upper shelf between SENB and SENT.

For two of the materials, M02 and W03, there was a difference between the transition temperature of the SENB specimens and the SENTs, with the SENT specimens giving a lower ductile to brittle transition than the SENBs, but for material M01 the transition temperature was almost the same for both SENT and SENB specimens. The transition temperature determined from the Charpy transition curves was closer to the SENB transition temperature for M02 and W03, but not very close to either for M01. The difference in T0 between the SENT and SENB specimens, and the SENT and Charpy specimens, plotted against the SENT transition temperature, T0, is shown in Fig. 18. This trend suggests that the relative transition temperatures between SENT and SENB specimens is due to factors other than simply the difference in constraint of the specimens, such as those affecting the fracture toughness of the steel itself.

For all three materials, the slope of the ductile to brittle transition (characterised by parameter ‘Z’ in the tanh fit) for the SENT specimens was not more than 5, and lower than the equivalent SENB and Charpy curves in all three cases. This means that the SENT transition curve for all the materials gives a very steep and sudden transition from ductile to brittle behaviour. For the SENB specimens the steepness of the transition was greater for the materials with lower transition temperature, so there was not a common relationship between SENB and SENT transition slope. However, the experimental work presented here suggests that SENT specimens show a very sudden change from ductile to brittle behaviour which must be recognised and accounted for when SENT specimens are being used to assess structures at low temperature. The steepness of the transition could be much sharper than would be predicted from either SENB or Charpy specimens.

Figure 18 The difference in transition temperature, T0 between SENT and SENB specimens, plotted against the T0 for SENT specimens

Figure 18 The difference in transition temperature, T0 between SENT and SENB specimens, plotted against the T0 for SENT specimens

5.2 Choosing a Suitable SENT Specimen Design

The model predicted that the machined threads would never exceed the strain capacity of the notch, and could not be used. However, the experimental SENT tests on specimens fatigue pre-cracked with a/W of around 0.5 showed that these were an acceptable design for single point fracture toughness tests in M01 and M02. Therefore a predicted strain capacity of 1.8% around the notch and 1% in the thread nonetheless still failed at the notch rather than the thread. The discrepancy between model and experimental results can be attributed mainly to the conservatism in the numerical modelling approach, which is described more fully in [14]. This means that the other predictions from the model can be applied with confidence in their conservatism. The case where the model predicted thread strain capacity of at least 1% in relation to notch strain capacity of 1.8% was experimentally acceptable. Therefore machined end specimens with sufficiently deep notches, say greater than a/W of 0.45 were considered to be acceptable based on the experimental and model results. A similar level of conservatism was also the case for the model of mismatch ratio up to M = 1.1, predicting a thread strain capacity above 1%, and therefore this scenario could be considered equivalent to the deeply notched specimen and likely to be an acceptable application for machined threads.

The friction welded threaded-end SENT specimens allow for the attachment of lower strength threads, but greater diameter than the width of the specimen, such that it can be used for a broader range of notch depths and weld strength mismatch levels. The disadvantage lies in the fact that the manufacturing procedure involved additional rotational friction welding equipment, which can be expensive and may add to the specimen preparation time. However, other welding procedures may be used as long as the quality of the weld is sufficient to avoid yielding of the specimen at the weld location.

This specimen design proved feasible for the entire range of a/W between 0.2 and 0.5. Overmatching levels of up to 1.5 were also acceptable. Based on the results presented here, and other R-curve results discussed further in [14], a decision flowchart summarizing the selection of SENT threaded-end design is given in Fig. 19.

6. Conclusions

  1. SENT specimens can exhibit a much steeper transition between ductile and brittle behaviour than would be observed from SENB or Charpy specimens, and sometimes a lower transition temperature.
  2. SENT specimens with threaded ends can be tested in an environmental chamber to very low temperatures using established tensile test grips.
  3. Threads can be simply machined onto square section SENTs when testing single point toughness in deeply notched specimens without significant weld strength overmatching.
  4. For specimens with shallower notches and weld strength mismatch, round bar can be welded to the ends of the specimen for threads to be machined.
  5. It is therefore feasible to perform fracture toughness testing using SENT specimens to characterize material at temperatures as low as -160°C, for the assessment of structures operating in very low temperature environments.
Figure 19 Decision flowchart to aid selection of the appropriate SENT specimen design for low temperature tests

Figure 19 Decision flowchart to aid selection of the appropriate SENT specimen design for low temperature tests

7. Acknowledgments

Thanks are owed to Phillip Cossey and Jerry Godden for carrying out, and providing advice on, the experimental SENT tests presented in this paper. Part of the work presented here was carried out as part of a joint-industry project funded by Subsea7, Saipem, BP Ltd and TWI Ltd. Thanks to the sponsors of this work for their support throughout the project, and their permission to publish. Part of this work was carried out with funding provided by the Industrial Members of TWI as part of the Core Research Programme, and through efforts of the NSIRC and Brunel University MSc in Structural Integrity.

8. References

  1. BS 8571, 2014, “Method of test for determination of fracture toughness in metallic materials using single edge notch tension (SENT) specimens”, British Standards Institution.
  2. DNV-RP-F108, 2006, “Fracture control for pipeline installation methods introducing plastic strain”, Det Norske Veritas Recommended Practice.
  3. DNV-OS-F101, 2013, “Submarine Pipeline Systems”, Det Norske Veritas.
  4. BS 7910:2013+A1:2015, “Guide to methods for assessing the acceptability of flaws in metallic structures”, British Standards Institution.
  5. Pisarski, H. and Wignall, C.; 2002, “Fracture toughness estimation for pipeline girth welds”, Proceedings 4th International Pipeline Conference IPC2002, Calgary, Alberta, Canada, September 2002.
  6. Hauge, M. and Holm, H., 2011, “Statistical interpretation of fracture toughness test data for qualification of weldability and integrity assessment of Arctic structures”, Proceedings, 21st International Offshore and Polar Engineering Conference, ISOPE2011, Maui, Hawaii, USA.
  7. ISO 12135, 2002, “Metallic materials - Unified method of test for the determination of quasistatic fracture toughness” International Standards Organisation.
  8. ASTM E1820, 2015, “Standard Test Method for Measurement of Fracture Toughness”, ASTM.
  9. BS 7448-1, 1991, “Fracture mechanics toughness tests - Part I: Method for the determination of KIC, critical CTOD and critical J values of metallic materials”, British Standards Institution.
  10. BS EN ISO 15653, 2010, “Metallic materials - Method of test for the determination of quasistatic fracture toughness of welds”, British Standards Institution.
  11. Moore P, 2015, “Optimisation of SENT specimen design”, Proceedings, EUROJOIN9, Bergen, Norway, June, 2015.
  12. Delliou, P. and Geniaut, S., 2014, “Tests on SENT specimens to study geometrical effects in the ductile to brittle transition”, Proceedings, ASME 2014 Pressure Vessels & Piping Conference, PVP2014, Anaheim, California, USA.
  13. Hutchison E, Moore, P. &. Bath, W., 2015, “SENT Stable Tearing Crack Path Deviation and its Influence on J,” Proceedings, ASME 2015 Pressure Vessels & Piping Conference PVP2015, Boston, USA.
  14. Crintea, A. and Moore, P., 2016 “Ensuring the Strain Capacity in Threaded-End Single Edge Notched Tension (SENT) Specimens,” Proceedings, The 26th International Ocean and Polar Engineering Conference, ISOPE2016, Rhodes, Greece, June 2016.

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