Available online at www.sciencedirect.com Available online at www.sciencedirect.com ScienceDirect ScienceDirect Structural Integrity Procedia 00 (2019) 000–000 Available online at www.sciencedirect.com Structural Integrity Procedia 00 (2019) 000–000 ScienceDirect www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia Procedia Structural Integrity 18 (2019) 205–213 25th International Conference on Fracture and Structural Integrity 25th International Conference on Fracture and Structural Integrity Influence of temperature on fracture toughness values in different Influence of temperature on fracture toughness values in different regions of A-387 Gr. B welded joint regions of A-387 Gr. B welded joint Ivica Čamagićaa, Simon A. Sedmakb,b,*, Aleksandar Sedmakcc, Zijah Burzićdd Ivica Čamagić , Simon A. Sedmak *, Aleksandar Sedmak , Zijah Burzić a Faculty of Technical Sciences, 7 Kneza Miloša Street, K. Mitrovica, Serbia a Faculty Technical Sciences, 7 Kneza Miloša16 Street, K. Mitrovica, Serbia Innovation Center of theofFaculty of Mechanical Engineering, Kraljice Marije Street, Belgrade, Serbia b c Innovation Center of of theMechanical Faculty of Mechanical 16 Kraljice Marije Street, Belgrade, Serbia Faculty Engineering,Engineering, 16 Kraljice Marije Street, Belgrade, Serbia cd Faculty Mechanical Engineering, 16 Kraljice MarijeStreet, Street,Belgrade, Belgrade,Serbia Serbia MilitaryofInstitute of Techniques, 1 Ratka Resanovića d Military Institute of Techniques, 1 Ratka Resanovića Street, Belgrade, Serbia b Abstract Abstract The influence of temperature on fracture toughness values in different regions of a welded joint is analysed low-alloyed Cr-Mo steel The influence of temperature ontemperature fracture toughness valuesHeterogeneity in different regions of a weldedand jointproperties is analysed low-alloyed steel A-387 Gr. B, designed for high applications. of microstructure of welded joint Cr-Mo is evaluated A-387 Gr. standard B, designed high temperature applications. of microstructure and properties of welded joint isweld evaluated by testing 3BPfor specimens with crack tip locatedHeterogeneity at different regions of a joint, including the base metal (BM), metal by testing 3BP specimens with crack tip located at different regions a joint, includingand theatbase metal (BM),temperature, weld metal (WM) and standard heat-affected-zone (HAZ). Experiments were performed both at the of room temperature design working (WM) and heat-affected-zone (HAZ). Experiments were performed both at the room temperature and at design working temperature, 540�C. Based on these results, temperature effect on crack resistance is established for all different regions in a welded joint. 540�C. Based on these results, temperature effect on crack resistance is established for all different regions in a welded joint. © 2019 The Authors. Published by Elsevier B.V. © 2019 Published by Elsevier B.V. B.V. © 2019The TheAuthors. Authors. Published by Peer-review under responsibility of Elsevier the Gruppo Italiano Frattura (IGF) ExCo. Peer-review under responsibility of the Gruppo Italiano Frattura (IGF) ExCo. Peer-review under responsibility of the Gruppo Italiano Frattura (IGF) ExCo. Keywords: : low-alloyed steel; welded joint; crack; plane strain fracture toughness, critical crack length. Keywords: : low-alloyed steel; welded joint; crack; plane strain fracture toughness, critical crack length. 1. Introduction 1. Introduction By analysing the brittle behaviour of bodies with cracks, fracture mechanics provided new possibilities for ensuring By analysing the brittle behaviour bodies withdetermining cracks, fracture mechanics provided new possibilities forcompletes ensuring welded joint safety. Standard ASTM of E399 [1] for of fracture toughness under plain strain, KIc, completes welded joint safety. Standard ASTM E399 [1] for determining of fracture toughness under plain strain, K Ic, the process of linear elastic fracture mechanics application to real structures, made of high strength materials, wherein the process linearresults elasticinfracture mechanics application to real structures, madeofofsuch hightests strength materials, wherein presence of of cracks plane strain state. The main condition for the validity is that the plastic strain presence of cracks results in plane strain state. The main condition for the validity of such tests is that the plastic is only present in a negligible area around the crack tip, prior to crack propagation and fracture. Since for strain most is only present in a negligible area around the crack tip, prior to crack propagation and fracture. Since for most * Corresponding author. * Corresponding E-mail address:author. simon.sedmak@yahoo.com E-mail address: simon.sedmak@yahoo.com 2452-3216 © 2019 The Authors. Published by Elsevier B.V. 2452-3216 2019responsibility The Authors. of Published by Elsevier Peer-review©under the Gruppo Italiano B.V. Frattura (IGF) ExCo. Peer-review under responsibility of the Gruppo Italiano Frattura (IGF) ExCo. 2452-3216  2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Gruppo Italiano Frattura (IGF) ExCo. 10.1016/j.prostr.2019.08.155 Ivica Čamagić et al. / Procedia Structural Integrity 18 (2019) 205–213 Sergio Cicero et al./ Structural Integrity Procedia 00 (2019) 000–000 206 2 structural materials and welded joints, the plastic strain area around the crack tip is large, direct determining of the KIc parameter is not possible, and its application to real conditions is limited. Parent material (PM) testing, as well as testing of welded joint components (weld metal – WM and heat affected zone - HAZ) of a low-alloyed steel involved the determining of fracture mechanics parameters of the PM and welded joint components, at room temperature and at an elevated temperature of 540°C, [2]. 2. Materials for testing The parent material was steel A-387 Gr. B with thickness of 102 mm. Chemical composition and mechanical properties of the PM are given in tables 1 and 2, [2, 3]. Table 1. Chemical composition of PM specimens Specimen mark N % mas. C Si Mn P S Cr Mo Cu 0,13 0,23 0,46 0,009 0,006 0,85 0,51 0,035 Table 2. Mechanical properties of PM specimens Specimen mark N Yield stress, Tensile strength, Elongation, Rp0,2, MPa Rm, MPa A, % 325 495 35,0 Impact energy, J 165 Welding of steel sheets made of this parent material was performed in two stages, according to the requirements given in the welding procedure provided by a welding specialist, and these stages include:  Root weld by E procedure, using a coated LINCOLN S1 19G electrode (AWS: E8018-B2), and  Filling by submerged arc welding (SAW), wherein wire denoted as LINCOLN LNS 150 and powder denoted as LINCOLN P230 were used as additional materials. Chemical composition of the coated electrode LINCOLN S1 19G, and the wire LINCOLN LNS 150 is given in tab. 3, whereas their mechanical properties are given in tab. 4, [2, 3]. Table 3. Chemical composition of additional welding materials % mas. Filler material C Si Mn P S Cr LINCOLN Sl 19G 0,07 0,31 0,62 0,009 0,010 1,17 Mo 0,54 LINCOLN LNS 150 0,10 0,14 0,71 0,010 0,010 1,12 0,48 Table 4. Mechanical properties of additional materials Additional material Yield stress, Tensile strength, Elongation, Rp0,2, MPa Rm, MPa A, % Impact energy, J at 20C LINCOLN Sl 19G 515 610 20 > 60 LINCOLN LNS 150 495 605 21 > 80 Butt welded joint was made with a U-weld. The shape of the groove for welding preparation was chosen based on thickness, in accordance with appropriate standards SRPS EN ISO 9692-1:2012, [4], and SRPS EN ISO 9692-2:2008, [5]. 3. Determination of plane strain fracture toughness, KIc The influence of temperature on the parent material and welded joint components tendency towards brittle fracture was assessed by determining fracture toughness in plain strain conditions, i.e. by determining the critical value of Ivica Čamagić et al. / Procedia Structural Integrity 18 (2019) 205–213 Author name / Structural Integrity Procedia 00 (2019) 000–000 207 3 stress intensity factor, KIc. Tests were performed at room temperature of 20°C, as well as at the elevated temperature of 540°C. For the purpose of determining KIc, three point bending specimens (SEB) were used for room temperature testing, and their geometry was defined in accordance with standards ASTM E399 [1] and ASTM E1820, [6]. For determining KIc at the temperature of 540°C, modified CT specimens, whose geometry was defined in accordance with standard BS 7448 Part 1 [7], were used. Fracture toughness, KIc, determined directly using critical J-integral, JIc, by using elastic-plastic fracture mechanics (EPFM), as defined by standards ASTM E813 [8], ASTM E 1737 [9], ASTM E1820 [6] and BS 7448 Parts 1 and 2, [7,10], i.e. by monitoring crack propagation under plastic conditions. American Society for Testing and Materials (ASTM) defined a standard procedure for obtaining of resistance curves for metallic materials according to crack propagation [8]. The European Structural Integrity Society (ESIS) then worked on improving of this standard [11]. Some of the solutions suggested by this standard were accepted, and in this paper they are related to determining of a fitted regression line. Standards [1,6,8,9,12-14] are updated regularly and thus it is important to use only the most recent versions. Experiments were performed by testing a single specimen via successive partial unloading, i.e. by single specimen yield method, as defined by standard E813 [8]. The goal of the yield method is to register the magnitude of crack propagation, Δa, which occurred during the test, after unloading. The testing itself was performed with specimens which had fatigue cracks in PM, WM and HAZ, at room temperature of 20°C and the elevated temperature of 540°C, using an electric mechanical tensile test machine. In the case of room temperature testing, the specimens was equipped with a COD extensometer for the purpose of measuring crack tip opening. This was not the case when testing was performed at elevated temperatures. Namely, due to the lack of extensometer that can work at these temperatures, crack tip opening was registered using an inductive sensor, with previously registered calibration curve, showing the ratio between values obtained using the extensometer and those obtained from the sensor. Bending or tensile load (depending on the type of specimen being tested) was applied at a slow rate, 1 mm/min. The load was applied with periodic unloading, up to a point where considerable plastic strain started occuring or the specimen fractured, i.e. once the extensometer /inductive sensor range was exceeded. During this time, an A/D converter was used Based on the yield, which represents the ratio between force increment and crack tip opening increment on the unloading line, it is possible to determine crack length using the following expression:  b   C  Ci 1   ai  ai 1   i 1    i  i 1   Ci 1  (1) J (i )  J el  J pl (2) where: ai-1 – previous crack length; Ci = tgi – slope of the observed unload line; Ci-1 = tgi-1 – slope of the previous unload line; i-1 = 2 –SEB specimen coefficient; i-1 = 2 + 0,522 bi/W – CG specimen coefficient; W – specimen width and bi-1 – previous ligament length. J-integral is equal to the sum of its elastic and plastic components [15]: For SEB and CT specimens, the elastic J-integral component, i.e. the elastic energy component, is calculated based on the expression [15]: J el (i )   K i2  1   2 E  where: Ki – stress intensity factor, defined according to standard ASTM E 399; (3) Ivica Čamagić et al. / Procedia Structural Integrity 18 (2019) 205–213 Sergio Cicero et al./ Structural Integrity Procedia 00 (2019) 000–000 208 4 ν - Poisson’s ratio and E – Elasticity module. Stress intensity factor Ki for SEB specimens is calculated using the following expression: Ki  Fi  S B  BN 1 2  W 3 2  f a0 W  (4) whereas for CT specimens: Ki  Fi B  BN  W 1 / 2 (5)  f ai W  The geometry term f(a0/W), in the case of SEB specimens, is determined as: 1,99  a 0 W 1  a 0 W    12 3a 0 W   2      2 , 15 3 , 93 a W 2 , 7 a W     0 0  f a 0 W   32 21  2 a 0 W 1  a 0 W    (6) and for CT specimens: f a i W   2  a i 0,866  4,64a i W   13,32a i W 2   W   3 4  14,72a i / W   5,6a i / W   1  a i / W 3 / 2 (7) Plastic component of the J-integral is calculated based on the following expression [15]:     Apl (i )  A pl ( i 1)   ai  ai 1  (8) J pl (i )   J pl (i 1)   i    1   i  b B b i N i       where: B – specimen thickness; a0 – initial fatigue crack length; Apl – plastic energy component; S – span between the supports; BN – specimen net width; i = 2 – SEB specimen coefficient; i = 2 + 0,522 bi/W – CT specimen coefficient; i = 1- for SEB specimens and i = 1+ 0,76 bi/W – for CT specimens. Based on the obtained data, a J-Δa curve is drawn, and the regression line is then constructed on it, according to standard ASTM E1152 [13]. Critical value of J-integral, JIc, is then obtained from this regression line. By knowing the values of JIc, it is possible to calculate the critical stress intensity factor (fracture toughness), KIc, for plane strain, using the following relation: K Ic  J Ic  E 1  2 (9) Calculated values of critical stress intensity factor are given in table 5, for specimens with the notch in the PM, tested at room temperature and at the elevated temperature of 540°C, [15]. It is important to note that the calculation of fracture toughness for plane strain used different elasticity module values for room temperature (210 GPa) and for elevated temperature (cca 160 GPa for 540°C). Typical F-δ and J-Δa diagrams for specimens taken out of the parent material, tested at room temperature and the elevated temperature of 540°C are not shown here, due to the scope of this paper [15]. The influence of test temperature Ivica Čamagić et al. / Procedia Structural Integrity 18 (2019) 205–213 Author name / Structural Integrity Procedia 00 (2019) 000–000 209 5 on critical stress intensity factor values, for PM specimens is graphically represented in fig. 1, whereas its influence on critical crack length, ac, is shown in fig. 2, [15]. Table 5. Values of KIc notched specimens in PM Critical J-integral, Critical crack length, ac, mm JIc, kJ/m2 Critical stress intensity factor, KIc, MPa m1/2 60,1 117,8 38,5 63,9 121,4 40,8 PM-1-3n 58,6 116,3 37,5 PM-2-1n 43,2 87,2 40,0 44,7 88,7 41,4 45,3 89,2 41,9 Specimen mark Testing temperature, C PM-1-1n PM-1-2n PM-2-2n 20 540 PM-2-3n Figure 1. Change in value of KIc, depending on the testing temperature for PM specimens Figure 2. Change in value of ac, depending on the testing temperature for PM specimens Calculated value of critical stress intensity factor, KIc, and critical crack length, ac, are given in table 6, for specimens with the notch in the WM, tested at both room and elevated temperatures [15]. Table 6. Values of, KIc notched specimens at WM Critical J-integral, JIc, kJ/m2 Critical stress intensity factor, KIc, MPa m1/2 Critical crack length, ac, mm 72,8 129,6 20,2 74,3 130,9 20,7 WM-1-3 71,1 128,1 19,8 WM-2-1 50,2 93,9 17,4 52,6 96,2 18,2 48,4 92,2 16,8 Specimen mark Testing temperature, C WM-1-1 WM-1-2 WM-2-2 WM-2-3 20 540 F-δ and J-Δa diagrams for the specimen with a notch in the WM are shown in fig. 3, for specimen denoted as WM1-1, tested at room temperature, and in fig. 4, for specimen denoted as WM-2-1, tested at 540°C. The effect of test temperature on critical stress intensity factor values for specimens with the notch in the WM is shown graphically in fig. 5, whereas the effect of test temperature on critical crack length values, ac, is shown in fig. 6, [15]. Calculated value of critical stress intensity factor, KIc, and critical crack length, ac, are given in table 7, for specimens with the notch in the HAZ, tested at both room and elevated temperatures [15]. 210 6 Ivica Čamagić et al. / Procedia Structural Integrity 18 (2019) 205–213 Sergio Cicero et al./ Structural Integrity Procedia 00 (2019) 000–000 Figure 3. F-δ (left) and J-Δa (right) diagrams for specimen WM-1-1 Figure 4. F-δ (left) and J-Δa (right) diagrams for specimen WM-2-1 Figure 5. Change in value of KIc, depending on the testing temperature for WM specimens Figure 6. Change in value of ac, depending on the testing temperature for WM specimens Ivica Čamagić et al. / Procedia Structural Integrity 18 (2019) 205–213 Author name / Structural Integrity Procedia 00 (2019) 000–000 211 7 Typical F- and J-a diagrams for specimens taken from the HAZ, tested at room and elevated temperatures, are not shown here, due to the scope of this paper [15]. The effect of test temperature on critical stress intensity factor values for specimens with the notch in the HAZ is shown graphically in fig. 7, whereas the effect of test temperature on critical crack length values, ac, is shown in fig. 8, [15]. Table 7. Values of, KIc notched specimens at HAZ Specimen mark Testing temperature, C HAZ-1-1n HAZ-1-2n 20 HAZ-1-3n HAZ-2-1n HAZ-2-2n HAZ-2-3n 540 Critical J-integral, JIc, kJ/m2 Critical stress intensity factor, KIc, MPa m1/2 Critical crack length, ac, mm 53,6 111,2 34,3 51,7 109,2 33,0 49,8 107,2 31,8 33,6 76,9 31,1 34,2 77,5 31,6 36,1 79,7 33,4 Figure 7. Change in value of KIc, depending on the testing temperature for HAZ specimens Figure 8. Change in value of ac, depending on the testing temperature for HAZ specimens 4. Discussion Based on the results obtained by testing of specimens taken out of the PM, it can be seen that the increase in test temperature leads to decreased critical J-integral values, hence fracture toughness, KIc also decreases. Fracture toughness values for PM specimens, shown in tab. 5, range from 118 MPa m1/2 at room temperature to 88 MPa m1/2 at 540°C [15]. Obtained critical crack length, ac, values in the case of the PM did not show noticeable changes with temperature. This was expected since critical crack length was calculated using real yield stress values, obtained from tensile tests [16]. Based on the results obtained by testing of specimens with the notch in the WM, it can be seen that the increase in test temperature leads to decreased critical J-integral values, hence fracture toughness, KIc also decreases. Fracture toughness values for these specimens, given in tab. 6, range from 130 MPa m1/2 at room temperature, to 94 MPa m1/2 at 540°C [15]. Obtained critical crack length, ac, values, are significantly lower relative to yield stress level, ranging from 20.2 mm at room temperature to 17.5 mm at 540°C. However, if critical crack length values are calculated based on the PM yield stress, they are considerably higher, suggesting high brittle fracture resistance of the WM [16]. Based on the results obtained by testing of specimens taken out of the HAZ, it can be seen that the increase in test temperature leads to decreased critical J-integral values, hence fracture toughness, KIc also decreases. The same can be concluded for critical crack length, ac. 212 8 Ivica Čamagić et al. / Procedia Structural Integrity 18 (2019) 205–213 Sergio Cicero et al./ Structural Integrity Procedia 00 (2019) 000–000 Fracture toughness values for HAZ specimens, shown in tab. 7, range from 109 MPa m1/2 at room temperature, to 78 MPa m1/2 at 540°C, [15]. Obtained critical crack length values do not change significantly with the temperature, for HAZ specimens [15]. 5. Conclusion Weakest resistance towards crack growth under static force, i.e. lowest KIc values were measured for specimens with the notch in the HAZ, whereas highest resistance was measured in the case of specimens with the notch in the WM. The change in the slope of the curves is caused by changes in test temperature, notch location and exploitation duration. By analysing the obtained curves, it can be seen that the individual curves in each groups exhibit almost identical dependence from the above mentioned factors, with differences in maximum force values, Fmax, which is directly related to fatigue crack length [15]. Results obtained from fracture mechanics parameters (KIc, JIc, and ac) indicated that the tendency toward brittle fracture under static loading is lowest in the case of specimens with cracks in the WM and the PM, whereas it was highest for specimens with a crack in the HAZ, i.e. HAZ specimens had lowest resistance toward brittle fracture in this case. 6. Acknowledgements Parts of this research were supported by the Ministry of Sciences and Technology of Republic of Serbia through Mathematical Institute SANU Belgrade Grant OI 174001 Dynamics of hybrid systems with complex structures. Mechanics of materials and Faculty of Technical Sciences University of Pristina, residing in Kosovska Mitrovica. 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