PSI - Issue 31

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© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. Keywords: Preface, Fracture, Structural Integrity 1. Preface The International Conference on Structural Integrity and Durability, ICSID 2020, with the subtitle “Fatigue and Fracture – Theory and Applications” was organized by the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, and the Croatian National Group of the European Structural Integrity Society (ESIS), in Dubrovnik in Croatia from September 15 to 18, 2020. Due to the Covid-19 pandemic situation, ICSID 2020 was organized as the Hybrid Conference i.e, a combination of on-site and online presentation in order to provide the safety and conveniences for participants. The Conference was held at the Centre for Advanced Academic Studies (CAAS) of the University of Zagreb, in the city of Dubrovnik. The magnificent old building of CAAS is situated in the centre of Dubrovnik on the Croatian Adriatic Coast, in the vicinity of the most prominent historical places of the Old Town (http://icsid2020.fsb.hr/). Online presentations were 4th International Conference on Structural Integrity and Durability, ICSID 2020 Preface Željko Božić a, *, Siegfried Schmauder b , Katarina Monkova c , Aleksandar Sedmak d , Sergio Baragetti e , Fr nces o Iacoviello f a University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Ivana Lu č i ć a 5, 10000 Zagreb, Croatia b University of Stuttgart, Institute for Materials Testing, Materi ls Science and Strength of Materials (IMWF), Pfaffenw ldring 32, Stuttgart, Germany c Technical University of Kosice, Faculty of Manufacturing Technologies, Sturova 31, 080 01 Presov, Slovakia d University of Belgrade, Kraljice Marije 16, 11000 Belgrade, Serbia e University of Bergamo, Department of Management, Information and Production Engineering, Viale Marconi 5, Dalmine 24044, Italy f Università di Cassino e del Lazio Meridionale, via G. DI Biasio 43, 03043, Cassino (FR), Italy © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. Keywords: Preface, Fracture, Structural Integrity 1. Preface The International Conference on Structural Integrity and Durability, ICSID 2020, with the subtitle “Fatigue and Fracture – Theory and Applications” was organized by the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, and the Croatian National Group of the European Structural Integrity Society (ESIS), in Dubrovnik in Croatia from September 15 to 18, 2020. Due to the Covid-19 pandemic situation, ICSID 2020 was organized as the Hybrid Conference i.e, a combination of on-site and online presentation in order to provide the safety and conveniences for participants. The Conference was held at the Centre for Advanced Academic Studies (CAAS) of the University of Zagreb, in the city of Dubrovnik. The magnificent old building of CAAS is situated in the centre of Dubrovnik on the Croatian Adriatic Coast, in the vicinity of the most prominent historical places of the Old Town (http://icsid2020.fsb.hr/). Online presentations were 4th International Conference on Structural Integrity and Durability, ICSID 2020 Preface Željko Božić a, *, Siegfried Schmauder b , Katarina Monkova c , Aleksandar Sedmak d , Sergio Baragetti e , Francesco Iacoviello f a University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Ivana Lu č i ć a 5, 10000 Zagreb, Croatia b University of Stuttgart, Institute for Materials Testing, Materials Science and Strength of Materials (IMWF), Pfaffenwaldring 32, Stuttgart, Germany c Technical University of Kosice, Faculty of Manufacturing Technologies, Sturova 31, 080 01 Presov, Slovakia d University of Belgrade, Kraljice Marije 16, 11000 Belgrade, Serbia e University of Bergamo, Department of Management, Information and Production Engineering, Viale Marconi 5, Dalmine 24044, Italy f Università di Cassino e del Lazio Meridionale, via G. DI Biasio 43, 03043, Cassino (FR), Italy

* Corresponding author. Tel.: +385 1 6168 536; fax: +385 1 6156 940. E-mail address: zeljko.bozic@fsb.hr

2452-3216 © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. 2452-3216 © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. * Corresponding author. Tel.: +385 1 6168 536; fax: +385 1 6156 940. E-mail address: zeljko.bozic@fsb.hr

2452-3216 © 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. 10.1016/j.prostr.2021.03.001

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held using a commercial meeting platform. In total forty on-site and online presentations were given, including three plenary lectures. The objective of the ICSID 2020 Conference was to bring together scientists, researchers and engineers from around the world to discuss how to analyse, predict and assess the fatigue and fracture of structural materials and components. The Conference provided a forum for discussion of contemporary and future trends in experimental, analytical and numerical fracture mechanics, fatigue, failure analysis, structural integrity assessment, and other important issues in the field. Under the Conference subtitle “Fatigue and Fracture – Theory and Applications” a wide range of topics was covered such as: Simulation and testing of crack propagation at all length scales; Models, criteria and methods in fracture mechanics; Finite element methods and their applications; Effect of residual stresses; Fatigue and fracture of weldments, welded components, joints and adhesives; Reliability and life extension of components; Corrosion, environmentally enhanced degradation and cracking, corrosion fatigue; Fracture and damage of cementitious materials; Fatigue and fracture of polymers, composites and biomaterials; and others. Prior to the ICSID 2020 Conference a two-day Summer School with the topic “Fatigue and fracture modelling and analysis” was organized for graduate students, researchers and engineers from industry. Those participants who came to Dubrovnik, besides the excellent technical program, had the opportunity to enjoy their stay in Dubrovnik, one of the most famous Mediterranean cities, world celebrated symbol of historical heritage and beauty, which has found its place in the UNESCO World Heritage List. As the Guest Editors of this Conference Proceedings, we wish to thank all authors for their contributions. Guest Editors of the Procedia Structural Integrity ICSID 2020 Conference Proceedings: Željko Božić, University of Zagreb, Croatia Siegfried Schmauder, University of Stuttgart, Germany Katarina Monkova, Technical University in Kosice, Slovakia Aleksandar Sedmak, University of Belgrade, Serbia Sergio Baragetti, University of Bergamo, Italy Francesco Iacoviello, Università di Cassino e del Lazio Meridionale – DICeM, Italy

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4th International Conference on Structural Integrity and Durability, ICSID 2020 Analysis of SA 387 Gr. 91 welded joints crack resistance under static and impact load Milivoje Jovanović a , Ivica Čamagić a , Aleksandar Sedmak b , Zijah Burzić c Simon Sedmak d *, Predrag Živković a

a Faculty of Technical Sciences, 7 Kneza Miloša Street, K. Mitrovica, Serbia b Faculty of Mechanical Engineering, 16 Kraljice Marije Street, Belgrade, Serbia c Military Institute of Techniques, 1 Ratka Resanovi ć a Street, Belgrade, Serbia d Innovation Centre of Faculty of Mechanical Engineering, Serbia

© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. Abstract This paper presents the results of experimental testing of crack resistance of specimens taken from a welded plate made of steel SA-387 Gr. 91, while taking into account different filler materials and welding procedures that were used. This type of steels is typically used in pressure vessels, pipelines and gas installations in the chemical and petrochemical industry, as well as in thermal installations. Since it is operating under extreme conditions, which include elevated temperatures and/or corrosion, the mechanical properties of SA-387 Gr. 91 will deteriorate over time, especially its welded joints. For this reason, it is very important to thoroughly analyse the behaviour of welded joints, taking into account the possibility of cracks initiating in any of the three welded joint regions, the parent material (PM), the weld metal (WM) and the heat affected zone (HAZ). This can be achieved by determining the total impact energy of Charpy specimens with V-2 notches in each of the three regions, along with their components, crack initiation and crack propagation energy, as well as by measuring the fracture toughness. © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. Keywords: SA 387 Gr. 1; welded joint; crack initiation energy; crack propagation energy; fracture toughness

* Corresponding author. Tel.: +381 62 295 496; E-mail address: simon.sedmak@yahoo.com

2452-3216 © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers.

2452-3216 © 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. 10.1016/j.prostr.2021.03.008

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1. Introduction Steel SA 387 Gr. 91 is used for high temperature welded component due to its high creep resistance properties in all zones of its welded joints, Milovic et al (2008). Properties of parent material and welded joint significantly differ when subjected to impact load, compared to static load. In both loading cases, crack resistance is of utmost importance, so impact testing on instrumented Charpy pendulum and fracture toughness testing, under plane strain conditions, is crucial to determine welded joint behaviour, as the most critical region of a steel structure, including SA 387 Gr. 91 steel. Such tests are also often used to determine the quality level and homogeneity of the material. Additionally, impact tests can help determine the tendency of materials towards brittle fracture during exploitation (aging), especially in the case of creep resistant steels. Research presented in this paper was focused on determining of crack initiation and propagation, as well as fracture toughness for each individual welded joint region (parent metal - PM, weld metal - WM and heat affected zone - HAZ). All impact tests were performed using an instrumented Charpy pendulum, Čamagić, Jović et al. (2016), in accordance with relevant standards. Fracture toughness, K Ic , was determined via J Ic , also using the standard procedure, as described in Čamagić, Sedmak S. et al. (2019) for similar material, A 387 Gr. B. This approach could prove applicable to a number of other real problems which involve crack initiation, including steels and Al alloys with different applications, SRPS EN ISO 6947:2020 (2020), Milovanović et al. (2019), Baragetti, Borzini et al. (2020). Additionally, other types of failure mechanisms can be investigated using the method presented here, including fatigue, among others, as shown in the works of Baragetti, Božić, Arcieri (2020), Solob et al. (2020) and Pastorčić et al. (2019). 2. Material and methods Steel SA-387 Gr. 91, with yield stress of 450 MPa and minimum impact energy 41 J at room temperature. Material used in this research was manufactured by “Železarna ACRONI”, Jesenice, with chemical composition given in table 1. Welding of the plates needed for testing involved two different procedures and filler materials, Grbović et al. (2020), SRPS EN ISO 21952:2013 (2013), SRPS EN ISO 9692-1:2014 (2014), SRPS EN ISO 3580:2017 (2017): • Root weld – 4 passes – BOEHLER C9 MV-IG filler metal, gas tungsten arc welding (TIG). • Filler welds – 10 passes – BOEHLER FOX C9 MV, Ø2.5 mm and Ø3.25 mm electrodes, manual metal arc welding. To avoid issues with defining notch position in the HAZ, symmetric K-groove was selected for the butt weld joint. Chemical composition of filler metals is shown in table 2 for both welding procedures. Finally, mechanical properties of both filler materials are given in table 3, including both electrode diameter used in the manual arc welding.

Table 1. Base metal chemical composition, steel SA-387 Gr. 91

Chemical composition, weight %

C

Si

Mn

P

S

Cr

Mo

Ni

V

Nb

Cu

0.129

0.277

0.443

0.001

0.001

8.25

0.874

0.01

0.198

0.056

0.068

Table 2. Filler metal chemical composition (%) Filler metal C Si

Mn 0.5

P

S

Cr

Mo

Ni

V

Nb

Cu

C9 MV-IG

0.11 0.09 0.11

0.23 0.19 0.26

0.006

0.003 0.006 0.005

9.0 8.5 8.5

0.93

0.5 0.5 0.5

0.19 0.19 0.20

0.07 0.04 0.06

0.0.

FOX C9 MV Ø2.5 mm FOX C9 MV Ø3.25 mm

0.55 0.66

0.01

1.0

0.1 0.1

0.008

0.94

Table 3. Mechanical properties of filler metals Filler metal

Yield stress, MPa

Tensile strength, MPa

Elongation, %

Impact energy, J

C9 MV-IG

≥ 530 ≥ 550 ≥ 550

≥ 620 ≥ 680 ≥ 680

≥ 17 ≥ 17 ≥ 17

≥ 50 ≥ 47 ≥ 47

FOX C9 MV Ø2.5 mm FOX C9 MV Ø3.25 mm

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3. Impact test procedure and results Impact tests were performed using an instrumented Charpy pendulum, enabling separation of the total impact energy into crack initiation and crack propagation energy. The test procedure is defined by standard SRSP EN ISO 9016:2013. Three specimens with V-2 notches in every welded joint region were made for testing, for a total of 9 specimens, in order to obtain more accurate results, Sedmak S. et al. (2020). Testing also included the drawing of force-time and impact energy-time diagrams. Specimen geometry, with a detailed view of the notch position and dimensions, as well as the specimen cutting plan, which includes specimens taken from each individual welded joint region, are shown in figure 1, Čamagić, Jović et al. (2016), SRPS EN ISO 9016:2013 (2013). Results of impact testing of all three groups of V-2 notch specimens are shown in tables 4-6 and corresponding examples of diagrams in figures 2-4, while results for J Ic testing are shown in tables 7-9, and corresponding diagrammes in figures 5-7.

I

II

III

Fig. 1. Specimen geometry (left) and cutting plan (right)

Table 4. Impact test results for specimens with a V-2 notch in the PM

Impact total energy, AT, J

Crack initiation energy, AI, J

Crack propagation energy, AP, J

Specimen mark

PM - 1 PM - 2 PM - 3

251 268 275

58 60 55

193 208 220

Table 5. Impact test results for specimens with V-2 notch in WM

Impact total energy, AT, J

Crack initiation energy, AI, J

Crack propagation energy, AP, J

Specimen mark

WM - 1 WM - 2 WM - 3

144 168 157

52 55 54

92

113 103

Fig. 2. Impact test diagrams, for specimen PM-1

Fig. 3. Impact test diagrams for specimen WM-1

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Table 6. Impact test results for specimens with V-2 notch in HAZ

Impact total energy, AT, J

Crack initiation energy, AI, J

Crack propagation energy, AP, J

Specimen mark

HAZ - 1 HAZ - 2 HAZ - 3

248 246 249

70 59 70

178 187 179

Fig. 4. Impact test diagrams, for specimen HAZ-1

Table 7. Calculated values of K Ic for specimens notched in BM Specimen mark Testing temperature,  C

Critical J-integral, J Ic , kJ/m 2

Critical stress intensity factor, K Ic , MPa m 1/2

PM-1K PM-2K PM-3K

131.1 144.2 124.0

173.9 182.4 169.2

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Table 8. Calculated values of K Ic for specimens notched in WM Specimen mark Testing temperature,  C

Critical J-integral, J Ic , kJ/m2

Critical stress intensity factor, K Ic , MPa m1/2

WM-1K WM-2K WM-3K

71.6 64.8 69.2

128.5 122.3 126.4

20

Figure 5. Diagrammes F-  (a) and J-  a (b) for the specimen PM – 1K

Milivoje Jovanović et al. / Procedia Structural Integrity 31 (2021) 38 –44 M. Jovanovi ć et. al. / Structural Integrity Procedia 00 (2019) 000–000

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Table 9. Calculated values of K Ic for specimens notched in HAZ Specimen mark Testing temperature,  C

Critical J-integral, J Ic , kJ/m2

Critical stress intensity factor, K Ic , MPa m1/2

HAZ-1K HAZ-2K HAZ-3K

97.6 88.9 92.1

150.1 143.2 145.8

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Figure 6. Diagrammes F-  (a) and J-  a (b) for the specimen WM-1K

Figure 7. Diagrammes F-  (a) and J-  a (b) for the specimen HAZ-1K

4. Discussion and conclusions The results of impact testing confirmed that the location of the notch in the V-2 specimens has a noticeable effect on its impact properties. Highest impact energy, 265 J, was measured BM, with significantly larger crack propagation energy than crack initiation energy. This ratio, which is also the most favourable from structural integrity point of view, was 3.6:1. Heat affected zone specimens has slightly lower values of total impact energy, 248 J, with similar distribution of crack initiation and crack propagation energies, while WM has the lowest impact energy values, 156 J, as expected. Crack propagation to crack initiation energy ratios were 1.9:1 for WM and 2.8:1 HAZ. This suggests that BM has the best ductility (high total energy and much higher crack propagation energy), and that the weld metal has lowest ductility. Additionally, the values for all three specimens in each group had small differences, suggesting good homogeneity of each welded joint region’s structures. The detailed approach to experimental testing

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presented in this paper confirmed the importance of impact tests in determining the predicting the behaviour of welded joint regions in the presence of a crack, depending on its location within the welded joint itself. Regarding fracture toughness, the highest values are obtained for BM than for HAZ and the lowest for WM, similar with the impact load and corresponding energies, with one significant difference: values for HAZ are now much closer to WM, indicating higher sensitivity of HAZ to static than to impact loading. If compared with separated energies, situation is somewhat different, since HAZ has somewhat better resistance to crack initiation than BM and WM but all values are in the narrow range, whereas BM and HAZ has significantly higher resistance to crack propagation. Therefore, similarity with fracture toughness is clear for crack initiation energy, but for crack propagation energy WM is significantly reduced, making it more sensitive to impact load than the other two zones. Good agreement between different zones for crack initiation energies and fracture toughness can be interpreted as a counteraction of two effects – crack vs notch and crack initiation vs. propagation, as also noticed in previous research with similar steel A 387 Gr. B, Čamagić, Sedmak A. et al. (2019), Čamagić, Marsenić et al. (2018). Finally, one should keep in mind that in all presented cases, both impact and fracture toughness values indicate high resistance to crack propagation, i.e. brittle fracture. In the future work temperature effects will be analysed, as well as comparison with similar steel A 387 Gr. B, previously tested in the same way, with results presented in Čamagić, Vasić et al. (2106), Čamagić (2017), Čamagić, Arnđelović et al. (2018), Jovanović et al. (2020). References Baragetti, S., Borzini, E., Božić, Ž., Arcieri, E.V., 2019. On the fatigue strength of uncoated and DLC coated 7075-T6 aluminum alloy. Engineering Failure Analysis 102, 219-225 Baragetti, S., Božić, Ž., Arcieri, E.V., 2020. Stress and fracture surface analysis of uncoated and coated 7075-T6 specimens under the rotating bending fatigue loading. Engineering Failure Analysis 112, 104512. Čamagić I., Jović S., Radojković, M., Sedmak S., Sedmak A., Burzić Z., Delamarian C., 2016. Influence of Temperature and Exploitation Period on the Behaviour of a Welded Joint Subjected to Impact Loading. Structural integrity and life, 16(3), 179-185 Camagic, I., Vasic, N., Cirkovic, B., Burzic, Z., Sedmak, A., Radovic, A., 2016. Influence of temperature and exploitation period on fatigue crack growth parameters in different regions of welded joints, Frattura ed Integrita Strutturale, ISSN 1971-8993, 36(10), 1-7, DOI: 10.3221/IGF ESIS.36.01, Čamagić I., Sedmak A., Sedmak S., Burzić Z., 2019. Relation between impact and fracture toughness of A-387 Gr. B welded joint. 25th International Conference on Fracture and Structural Integrity, Procedia Structural Integrity, ISSN 2452-3216, 18, 903-907 Čamagić, I., Sedmak, S., Sedmak, A., Burzić, Z., Marsenić, M., 2018. Effect of temperature and exploitation time on tensile properties and plain strain fracture toughness, K Ic , in a welded joint, IGF Workshop “Fracture and Structural Integrity”. Procedia Structural Integrity, ISSN 2452 3216, 9, 279-286, https://doi.org/10.1016/j.prostr.2018.06.034 Čamagić, I., Sedmak, S., Sedmak, A., Burzić, Z., 2019. Influence of temperature on fracture toughness values in different regions of A-387 Gr. B welded joint. Procedia Structural Integrity, 18, 205-213 Camagic, I., Sedmak, S., Sedmak, A., Burzic, Z., Arandjelovic, M., 2018. The impact of the temperature and exploitation time on the tensile properties and plain strain fracture toughness, K Ic in characteristic areas of welded joint. Frattura ed Integrita Strutturale, ISSN 1971-8993, 46(12), 371-382, DOI: 10.3221/IGF-ESIS.46.34. Čamagić, I., Sedmak, S., Sedmak, A., Burzić, Z., Todić, A., 2017. Impact of Temperature and Exploitation Time on Plane Strain Fracture Toughness, K Ic , in a Welded Joint. Structural Integrity and Life, 17(3), 239–244 Grbović, A., Sedmak, A., Kastratović, G., Petrašinović, D., Vidanović, N., Sghayer, A., 2020. Effect of laser beam welded reinforcement on integral skin panel fatigue life. Engineering Failure Analysis 101, 383-393 Jovanović, M., Čamagić, I., Sedmak, S., Živković, P., Sedmak A., 2020. Crack initiation and propagation resistance of HSLA steel welded joint constituents. Structural Integrity and Life, 20(1), 11–14 Milovanović, N., Sedmak, A., Arsić, M., Sedmak, S.A., Božić, Ž., 2020. Structural integrity and life assessment of rotating equipment. Engineering Failure Analysis 113, 104561. Milovic, L., Vuherer, T., Zrilic, M., Sedmak, A., Putic, S., 2008. Study of the simulated heat affected zone of creep resistant 9-12% advanced chromium steel. Materials and Manufacturing Processes, 23(6), 597-602 Pastorcic, D., Vukelic, G., Bozic, Z., 2019. Coil spring failure and fatigue analysis. Engineering Failure Analysis 99, 310–318. Sedmak, S., Aranđelović, M., Jovičić, R., Radu, D., Čamagić, I., 2020. Influence of Cooling Time t8/5 on Impact Toughness of P460NL1 Steel Welded Joints. Advanced Materials Research, ISSN: 1662-8985, 1157, 154-160, doi:10.4028/www.scientific.net/AMR.1157.154, Trans Tech Publications Ltd, Switzerland. Solob, A., Grbović, A., Božić, Ž., Sedmak, S.A. 2020. XFEM based analysis of fatigue crack growth in damaged wing-fuselage attachment lug. Engineering Failure Analysis 112, 104516. SRPS EN ISO 6947:2020 Welding and allied processes - Welding positions (ISO 6947:2019), 2020.

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SRPS EN ISO 21952:2013 Welding consumables - Wire electrodes, wires, rods and deposits for gas shielded arc welding of creep-resisting steels - Classification (ISO 21952:2012), 2013. SRPS EN ISO 9692-1:2014 Welding and allied processes - Types of joint preparation - Part 1: Manual metal arc welding, gas-shielded metal arc welding, gas welding, TIG welding and beam welding of steels (ISO 9692-1:2013), 2014. SRPS EN ISO 3580:2017 Welding consumables - Covered electrodes for manual metal arc welding of creep-resisting steels - Classification (ISO 3580:2017), 2017. SRPS EN ISO 9016:2013 Destructive tests on welds in metallic materials - Impact tests - Test specimen location, notch orientation and examination (ISO 9016:2012), 2013.

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© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. Abstract In this work, Design of Experiments is applied to identify which factors mostly influence the residual stress distribution in an hourglass specimen subject to the impact of a foreign object. The specimen is made of 7075-T6, that is one of the most popular alloys in the aeronautical sector. After the impact, the specimen is assumed to be tested with an axial or bending fatigue load. For this reason, the study is conducted on the axial stresses induced by the impact of the object on the specimen, assessed by means of finite element analysis. Only the stresses in the region x=[-2 mm; 2 mm], with x=0 corresponding to the position of the minimum cross section of the specimen before the impact, were considered. In this region, stress concentrations are probable during the fatigue test. The results showed that high impact forces induce high tensile stresses, unfavorable from a fatigue point of view. Therefore, it is preferable that the impacting object has a low weight and reduced speed. © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. Keywords: FOD, Design of Experiments, Taguchi method, residual stresses, 7075-T6 1. Introduction Lightweight alloys are attractive to the aeronautical sector, because they allow the fabrication of components with high specific strength. The higher the specific strength, the higher the performance, as the reduced mass of the components means low inertia and consequently low fuel consumption. For this reason, the Authors’ research activity 4th International Conference on Structural Integrity and Durability, ICSID 2020 Application of Design of Experiments to Foreign Object Damage on 7075-T6 Emanuele Vincenzo Arcieri a , Sergio Baragetti a *, Željko Božić b a Department of Management, Information and Production Engineering, University of Bergamo, Viale Marconi 5, Dalmine 24044, Italy b Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, I. Lu č i ć a 5, Zagreb 10000, Croatia

* Corresponding author. Tel.: +39-035-205-2382; fax: +39-035-205-2221. E-mail address: sergio.baragetti@unibg.it

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is focused on the characterization of these materials. The studies on titanium alloys are presented in Baragetti (2013), Baragetti et al. (2019a), and Baragetti et al. (2019b), the behavior of aluminum alloys is described in Baragetti et al. (2019c). The components of the aeronautical industry must have a high stiffness and a high static and fatigue strength. Strength must be guaranteed in the presence of defects, which are always existent. It is essential to consider flaws in the design phase and monitor them in terms of shape and propagation speed. Defects are indeed possible nucleation sites for fatigue cracks that propagate and cause failures according to the mechanism reported in Božić et al. (2014), Mlikota et al. (2017), Babić et al. (2018), Mlikota et al. (2018), Božić et al. (2018) and Babić et al. (2019). This mechanism can be summarized as follows: (i) slip bands are formed as a consequence of the sliding on continuous crystallographic planes; (ii) pores nucleate and form micro cracks; (iii) micro cracks join to form macro cracks and (iv) cracks propagate until the failure. Fatigue can drastically reduce the life of a component as reported for instance in Papadopoulou et al. (2019). It is therefore mandatory to evaluate the fatigue life of the components and to propose methodologies to assess it. For this purpose, simulation is gradually assuming an important role because it allows to save time and setup cost. Some examples are reported in Pastorcic et al. (2019), Babić et al. (2020), Cazin et al. (2020), Solob et al. (2020) and Rølvåg et al. (2020). As known, residual stresses affect the fatigue strength of the components. Residual stresses can be induced by the deposition of a coating as described in Baragetti et al. (2005) and Baragetti et al. (2020), by the implantation of chemical species as reported in Voorwald et al. (2019) and by the impact of objects. In this regard, it is mandatory to mention shot peening and Foreign Object Damage (FOD). Shot peening refers to a global effect, provided by the impact of a large number of objects. According to Nicholas (2006), FOD is a typical expression in the aerospace and aviation industry, indicating the damage of engine components due to the impact of alien objects ingested. The effect of the FOD is local. This paper investigates the residual stress distribution induced by the FOD on an hourglass specimen made of 7075-T6 aluminum alloy, which is one of the most widespread alloys in the aeronautical industry. The stresses are assessed by Finite Element (FE) modelling and Design of Experiments (DoE) is applied to the results to identify the best levels of the most important input data to minimize the (tensile) residual stress quickly and cost effectively.

Nomenclature D

diameter of the ball Young’s modulus empty parameter 1 empty parameter 2

E

E1 E2

i run number MSD mean square deviation, measure of data dispersion n number of tests per run R s yield stress S11 stress in axial direction S/N average of S/N in which factor x is at level y S/N’

signal to noise ratio, measure of both the location and dispersion of the measured effect

V X

impact speed

position of the maximum axial stress in x direction, in the region x=[-2 mm, 2 mm] Y’ average of σ in which factor x is at level y α impact angle measured on xz plane β impact angle measured on yz plane Δ S/N’ modulus of the difference between S/N’ at level 1 and S/N’ at level 2 for the considered parameter μ coefficient of friction ν Poisson’s ratio ρ density σ maximum axial stress in the specimen, in the region x=[-2 mm,2 mm]

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3

Fig. 1. FE model, adapted from Arcieri et al. (2021).

2. Materials and methods DoE was applied to assess the most important factors and choose the preferred levels to minimize the residual stresses in the sample, which were evaluated with the FE models of Fig. 1. These models are similar to those of Arcieri et al. (2021) and they are described in the following lines. The Taguchi L 8 (2 7 ) array filled with the factors shown in Table 1 was employed. This array allows to optimize seven factors with only eight runs. The empty columns E1 and E2 allow to understand whether some parameter was not taken into account. A full description of the DoE methodology adopted is reported in Condra (1993), Baragetti (1997) and Baragetti and Terranova (2000).

D ( mm ) α ( ° ) β ( ° ) E1 5 0 0 1

Table 1. Taguchi L 8 (2

7 ) array: factors and respective levels.

Run

V (m/s)

Material of the ball

E2

1 2 3 4 5 6 7 8

80 80 80 80

steel steel

1 2 2 1 2 1 1 2

5 7 7 7 7 5 5

20

20

2 2 1 1 2 2 1

ceramic ceramic

0

0

20

20 20

120 120 120 120

steel steel

0

20

0

ceramic ceramic

0

20

20

0

The impacts were studied by FE analysis in Abaqus Explicit 2019. Indeed, impact problems are usually solved using explicit integration schemes, as reported in Arcieri et al. (2018), Baragetti and Arcieri (2019), Baragetti and Arcieri (2020a) and Baragetti and Arcieri (2020b). In the models, the 7075-T6 specimen and the steel ball were positioned as shown in Fig. 1. The sample was modelled with 92160 linear hexahedral elements, C3D8R according to the Abaqus terminology. The global mesh size in the impact area was 0.25 mm. It was assumed that the behaviour of the material was elastic perfectly plastic, with the properties reported in the upper left corner of the figure. The ball was considered stiff, whatever the material. It was modelled with linear quadrilateral rigid elements, R3D4, with a global mesh size of 0.25 mm. The mass and moments of inertia were then assigned to the Reference Point (RP) of the ball, which was positioned at its centre. For the calculation of the inertial properties, it was assumed =7860 kg/m 3 for steel and =2300 kg/m 3 for ceramic. The ball was assumed to be fired close to the external surface of the specimen, with the RP lying in the same plane as the specimen's minimum cross-section, x=0, whatever the direction of impact. The minimum gap between the external surface of the sample and the external surface of the ball was 0.1 mm. The models did not consider the contribution of air to the movement of the ball. Due to the size of the ball and the specimen, the contribution of weight to the dynamics of the studied system and to the stress distribution was considered negligible. Therefore, gravity load was not implemented. The specimen was assumed to be located in the support shown in the lower left corner of Fig. 1, with its two cylindrical parts housed in the through holes and locked with two set screws. For this reason, the nodes on the external surface of the sample corresponding to the holes (yellow areas in Fig. 1) were locked. The symmetry conditions were implemented on the sample and on the RP of the ball

Emanuele Vincenzo Arcieri et al. / Procedia Structural Integrity 31 (2021) 22–27 Emanuele Vincenzo Arcieri et al./ Structural Integrity Procedia 00 (2019) 000–000 with respect to the planes of symmetry SP1 and SP2 of Fig. 1 whenever they were consistent with the impact direction analysed. A coefficient of friction =0.6 was set between the ball and the sample in all the eight runs. A dynamic step of 7 ms was adopted. At this time, the stresses in the sample can be considered stabilized. 3. Results and discussion The specimen was assumed to be tested with an axial or bending fatigue load after impact. Both loading conditions provide an axial stress distribution and for this reason the residual stresses induced by the impact in the axial direction are considered in this paper. Tensile stresses are unfavorable from a fatigue point of view and for this reason the Taguchi method was used to identify the preferable levels of the most important parameters to minimize the tensile residual stresses. To do this, the maximum axial stress σ in the region x=[-2 mm, 2 mm], shown in Fig.1, was assessed for each run. Not the whole specimen, but only this region was considered because it is reasonable to assume that the maximum stresses will be reached here in the fatigue test. In this region, indeed, the cross-sectional areas are the smallest in the specimen and the notch induced by the impact provides stress concentrations. Table 2 is called ‘Response table’ and it was built with the results shown in Fig.2. The table reports the results regarding the effect ‘maximum axial stress’ according to the Taguchi method. The maximum axial residual stresses for each run in x=[-2 mm, 2 mm] region are reported in the second column of the table. For each stress, it is indicated its position in x direction, X. As expected, X=0 mm for run 1, 3 and 7, where the x-component of the impact speed was zero. For Run 5, the maximum stress, 465 MPa, was at X=±3.5 mm, outside the considered region; considering the region x=[-2 mm, 2 mm], the maximum stress, 448 MPa, was at X=0 mm according to the symmetry of the problem. As the goal was to minimize the stresses, the results were analyzed according to the modality ‘smaller is better’ and MSD was calculated as MSD= ∑ σ i 2 /n, assuming n=1 since one (deterministic) simulation per run was carried out. The calculation of S/N=-10log(MSD) and of Y’ and S/N’ for each level of each parameter allowed to identify V, Material and D as the most important factors, since Δ S/N’ for these parameters is much greater than Δ S/N’ for the others. For this reason, V, Material and D are in bold in Table 2. The preferred levels of the important parameters correspond to the highest values on S/N’ and are underlined in Table 2: V=80 m/s, Material=ceramic and D=5 mm. The impact direction does not seem to be one of the most important factors for the residual stresses in x=[-2 mm, 2 mm]. Probably α and β are more effective once the post-impact stress concentrations associated to the notch created and the total stresses in the specimen subject to fatigue are assessed. The analysis of the empty columns confirmed that all the factors were taken into consideration. The F-test performed for the analysis of variance revealed that V, Material and D are significant with a confidence greater than 95%. 25 4

Table 2. Response table. Run σ ( MPa )

MSD ( MPa 2 ) S/N

Parameters

Levels 80 m/s

Y' ( MPa )

S/N'

Δ S/N’ 2.723

X ( mm )

V

1 2 3 4 5 6 7 8

198 254 215 229 448 378 219 234

0.0 1.1 0.0 1.0 0.0 2.0 0.0 0.8

3.920E+04 6.452E+04 4.623E+04 5.244E+04 2.007E+05 1.429E+05 4.796E+04 5.476E+04 -------------- 6.487E+05

-45.933 -48.097 -46.649 -47.197 -53.026 -51.550 -46.809 -47.384

224.000

-46.969 -49.692 -49.651 -47.010 -47.056 -49.605 -48.104 -48.557 -47.879 -48.782 -48.385 -48.276 -47.872 -48.789

120 m/s 319.750

Material

steel

319.500

2.642

ceramic 224.250

D

5 mm 7 mm

226.250 317.500 270.000 273.750 256.250 287.500 277.250 266.500 256.000 287.750

2.549

α β

0°

0.453

20°

0°

0.903

20°

E1

1 2 1 2

0.109

E2

0.917

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Fig. 2. Residual stresses (GPa) in axial direction: (a) Run 1; (b) Run 2; (c) Run 3; (d) Run 4; (e) Run 5; (f) Run 6; (g) Run 7; (h) Run 8.

The results therefore show that small impact forces are preferable to reduce the axial residual stresses in the sample in the region x=[-2 mm; 2 mm]. Material and D affect the mass of the ball and the product of the mass and V is its initial momentum. The greater the momentum, the greater the impact force and the greater the stresses, which are associated to a greater deformation. A final consideration should be made on the values of stresses reached in the runs: Run 5 gives very high residual stresses that can be responsible for an early failure of the sample when the fatigue stresses are superposed. The high stresses reached can be due to the size of the ball, comparable to that of the sample, and the high impact speed. 4. Conclusions In this work, DoE was applied to FE analyses to identify which factors mostly affect the residual stress distribution in a 7075-T6 hourglass specimen subject to the impact of a ball. Assuming an axial or bending fatigue stress

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distribution after impact, only residual stresses in axial direction were considered. Only the stresses in the region x=[- 2 mm; 2 mm] were considered because stress concentrations are expected here during the fatigue test. The results showed that small impact forces are preferable, that can be reached with a low impact speed and low ball mass. References Arcieri, E.V., Baragetti, S., Fustinoni, M., Lanzini, S., Papalia, R., 2018. Study and modelling of the passenger safety devices of an electric vehicle by finite elements. Procedia Structural Integrity 8, 212-219. Arcieri, E.V., Baragetti, S., Lavella, M., 2021. Effects of FOD on fatigue strength of 7075-T6 hourglass specimens. IOP Conference Series: Materials Science and Engineering (submitted). Babić, M., Verić, O., Božić, Ž., Sušić, A., 2018. Reverse engineering based integrity assessment of a total hip prosthesis. Procedia Structural Integrity 13, 438-443. Babić, M., Verić, O., Božić, Ž., Sušić, A., 2019. Fracture analysis of a total hip prosthesis based on reverse engineering. Engineering Fracture Mechanics 215, 261-271. Babić, M., Verić, O., Božić, Ž., Sušić, A., 2020. Finite element modelling and fatigue life assessment of a cemented total hip prosthesis based on 3D scanning, Engineering Failure Analysis, 113, 104536. Baragetti, S., 1997. Shot peening optimisation by means of 'DoE': Numerical simulation and choice of treatment parameters. International Journal of Materials and Product Technology 12, 83-109. Baragetti, S., 2013. Corrosion fatigue behaviour of Ti-6Al-4V in methanol environment. Surface and Interface Analysis 45, 1654-1658. Baragetti, S., Arcieri, E.V., 2019. Study on a new mobile anti-terror barrier. Procedia Structural Integrity 24, 91-100. Baragetti, S., Arcieri, E.V., 2020. A new mobile anti-ramming system. ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 14, 15960. Baragetti, S., Arcieri, E.V., 2020. Study of impact phenomena for the design of a mobile anti-terror barrier: Experiments and finite element analyses. Engineering Failure Analysis 113, 104564. Baragetti, S., Borzini, E., Arcieri, E.V., 2019. Quasi-static crack propagation in Ti-6Al-4V in inert and aggressive media. Corrosion Reviews 37, 533-538. Baragetti, S., Borzini, E., Božić, Ž., Arcieri, E.V., 2019. Fracture surfaces of Ti-6Al-4V specimens under quasi-static loading in inert and aggressive environments. Engineering Failure Analysis 103, 132-143. Baragetti, S., Borzini, E., Božić, Ž., Arcieri, E.V., 2019. On the fatigue strength of uncoated and DLC coated 7075-T6 aluminum alloy. Engineering Failure Analysis 102, 219-225. Baragetti, S., Božić, Ž., Arcieri, E.V., 2020. Stress and fracture surface analysis of uncoated and coated 7075-T6 specimens under the rotating bending fatigue loading. Engineering Failure Analysis 112, 104512. Baragetti, S., Gelfi, M., La Vecchia, G.M., Lecis, N., 2005. Fatigue resistance of CrN thin films deposited by arc evaporation process on H11 tool steel and 2205 duplex stainless steel. Fatigue & Fracture Engineering Materials & Structures 28, 615-621. Baragetti, S., Terranova, A., 2000. Non-dimensional analysis of shot peening by means of DoE. International Journal of Materials and Product Technology 15, 131-141. Božić, Ž., Schmauder, S., Mlikota, M., Hummel, M., 2014. Multiscale fatigue crack growth modelling for welded stiffened panels. Fatigue & Fracture Engineering Materials & Structures 37, 1043-1054. Božić, Ž., Schmauder, S., Wolf, H., 2018. The effect of residual stresses on fatigue crack propagation in welded stiffened panels, Engineering Failure Analysis 84, 346-357. Cazin, D., Braut, S., Božić Ž., Žigulić R., 2020. Low cycle fatigue life prediction of the demining tiller tool. Engineering Failure Analysis 111, 104457. Condra, L.W., 1993. Reliability improvement with Design of Experiments. Marcel Dekker Inc., New York. Mlikota M., Schmauder S., Božić Ž., Hummel M., 2017. Modelling of overload effects on fatigue crack initiation in case of carbon steel. Fatigue and Fracture of Engineering Materials and Structures 40(8), 1182–1190. Mlikota, M., Schmauder, S., Božić, Ž., 2018. Calculation of the Wöhler (S-N) curve using a two-scale model. International Journal of Fatigue, 114, 289-297. Nicholas, T., 2006. Foreign Object Damage in High Cycle Fatigue, Elsevier Science Ltd (Oxford). Papadopoulou, S., Pressas, I., Vazdirvanidis, A., Pantazopoulos, G., 2019. Fatigue failure analysis of roll steel pins from a chain assembly: Fracture mechanism and numerical modeling. Engineering Failure Analysis 101, 320-328. Pastorcic, D., Vukelic, G., Bozic, Z., 2019. Coil spring failure and fatigue analysis. Engineering Failure Analysis 99, 310-318. Rølvåg, T., Haugen, B., Bella, M., Berto, F., 2020. Fatigue analysis of high performance race engines. Engineering Failure Analysis 112, 104514. Solob, A., Grbović, A., Božić, Ž., Sedmak, S.A., 2020. XFEM based analysis of fatigue crack growth in damaged wing-fuselage attachment lug. Engineering Failure Analysis 112, 104516. Voorwald, H.J.C., Bonora, R.G., Oliveira, V.M.C.A., Cioffi, M.O.H., 2019. Increasing fatigue resistance of AISI 4340 steel by nitrogen plasma ion-implantation. Engineering Failure Analysis 104, 490-499.

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Procedia Structural Integrity 31 (2021) 33–37

© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. Abstract The blades of axial turbomachines (steam and gas turbines, axial compressors) are highly stressed by centrifugal force, temperature, and alternating fluid forces, which leads to vibrations and fatigue cracking. This requires the studious engagement of designers and technologists. In this paper, a procedure for accelerated fatigue testing of a gas turbine compressor blade based on the modified Locati method is presented. The tests were performed on a vibration shaker with the frequency that equal the blade’s first natural frequency. S-N curves were estimated according to two approximate representation namely one-slope and two-slope models. Then, the blades were tested at different stress levels starting at a level below foreseen fatigue strength at the specified fatigue life. In each loading block, a blade was subjected to 2ˑ10 7 cycles. After the first block was completed, the stress level in the second loading block was increased by 20 MPa and a new 2ˑ10 7 cycles were applied. A procedure was repeated until the blade failure occurred. It was considered that a blade has failed when its natural frequency drops by more than 2%. After the failure, the fatigue strength was calculated using Palmgren -Miner rule. © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. Keywords: Modified Locati method, accelerated fatigue test, compressor blade r c 4th International Conference on Structural Integrity and Durability, ICSID 2020 Application of modified Locati method in fatigue strength testing of a turbo compressor blade Sanjin Braut a *, Marina Tevčić b , Mirko Butković a,b , Željko Božić c , Roberto Žigulić a a University of Rijeka, Faculty of Engineering, Vukovarska 58, 51000 Rijeka, Croatia b University of Applied Sciences, Mechanical Engineering Department, I. Meštrovi ć a 10, Karlovac, Croatia c University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture., I. Lu č i ć a 5, 10000 Zagreb, Croatia

* Corresponding author. Tel.: +385 51 651 502 E-mail address: sbraut@riteh.hr

2452-3216 © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers.

2452-3216 © 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ICSID 2020 Organizers. 10.1016/j.prostr.2021.03.007