Issue 46

Frattura ed Integrità Strutturale, 46 (2018); International Journal of the Italian Group of Fracture

Table of Contents

B. T. Bounini, B. Bouchouicha, A. Ghazi Simulation of the behavior of aluminum alloys welded in Friction Stir Welding FSW (Case of AA5083 and AA 6082) …………………………………………………………………………... 1 G. B. Manjunatha, K. N. Bharath Investigating the contribution of geometrical parameters and immersion time on fracture toughness of jute fabric composites using statistical techniques ……………………………..…………………….. 14 N. Hiyoshi, M. Yamashita, H. Hokazono Effect of additive elements Bi/Ni/Ge on crack initiation and propagation for low-Ag solders ………... 25 M. El Habiri, M. Benachour, N. Benachour Effect of cold expansion on improving of fatigue initiation life in Aluminium alloy …………………... 34 G. Deng, T. Nakanishi Bending fatigue strength design method for machine components with complex geometries ……………... 45 M. Hack, D. Carrella-Payan, B. Magneville, T. Naito, Y. Urushiyama, W. Yamazaki, T. Yokozeki, W. Van Paepegem A progressive damage fatigue model for unidirectional laminated composites based on finite element analysis: theory and practice ……………………………………………………………….... 54 H. Mekri, B. Bouchouicha, A. Miloudi, H. Christophe, A. Imad Influence of the coupling between the mechanical characteristics and the welding conditions by the FSSW process: case of the bi-material aluminum-steel ………………………………………………… 62 K.I. Tserpes, A. Chanteli, Sp. Pantelakis, E.P. Koumoulos, C.A. Charitidis Mechanical and nanomechanical properties of MWCNT/PP nanocomposite …………………… 73 M. F. M. Yunoh, S. Abdullah, M. H. M. Saad, Z. M. Nopiah, M. Z. Nuawi, S. S. K. Singh Probabilistic-based analysis for damaging features of fatigue strain loadings ……………………… 84 W. Song, X. Liu High cycle fatigue assessment of steel load-carrying cruciform welded joints: an overview of recent results ... 94 S. M. Medjdoub, B. Bachir Bouiadjra, M. Abdelkader Optimization of the geometrical parameters of bonded composite wrap for repairing cracked pipelines …... 102

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Fracture and Structural Integrity, 46 (2018); ISSN 1971-9883

S. Mokadem, B. Mohamed, M. Belaïd, B. B. Bouadjra Effect of the angles of the cracks of corroded plate in bonded composite repair …………………..…… 113 S. Motsa, G. Drosopoulos Failure behaviour of a fire protected steel element ………………………………………………. 124 N. C. M. Ibrahim, S. Boualem, M. Belaïd Analysis of the crack-crack interaction effect initiated in aeronautical structures and repaired by composite patch ……………………...……………………………………………….……………. 140 Z. Lu, B. Feng, C. Loh Fatigue behaviour and mean stress effect of thermoplastic polymers and composites …………………... 150 V. Rizov Analysis of cylindrical delamination cracks in multilayered functionally graded non-linear elastic circular shafts under combined loads ………………………………………………………………... 158 L. Giresini, M. Sassu An on-site teaching laboratory in a village damaged by the 2009 Abruzzo earthquake ……………… 178 M. L. Puppio, S. Novelli, M. Sassu Failure evidences of reduced span bridges in case of extreme rainfalls - The case of Livorno ……………. 190 S. Ivorra, D. Foti, M. Diafero, V. Vacca, D. Bru Resonances detected on a historical tower under bells’ forced vibrations …………………………….. 203 F. Stochino, M. L. Fadda, F. Mistretta Assessment of RC Bridges integrity by means of low-cost investigations ……………………………. 216 L.U. Argiento, A. Maione, C. Casapulla Formulating the in-plane frictional resistances and collapse mechanisms for multi-storey masonry block walls ……………………………………………………………………………………. 226 A. Maione, L.U. Argiento, C. Casapulla, A. Prota Management of multi-source information to identify the typology of the horizontal structures in historical masonry buildings: the case study of the Museum of Capodimonte in Naples (Italy) …………………. 240 A. Baltach, A. Djebli, M. Bendouba, E.H. Besseghier, A. Aid Numerical analysis and optimization of the residual stresses distribution induced by cold expansion technique ……………………………………………………………………………….... 252 A. Akhavan-Safar, A. Queirós Barbosa, L. F. Martins da Silva, M. R. Ayatollahi Micro failure analysis of adhesively bonded joints enhanced with natural cork particles: Impact of overlap length and particles volume fraction ………………………………………………………….. 266 S. Jiang, T. Cai, S. Wu, W. Yao Experimental research on the instantaneous and long-term deflections of pre-cracked concrete beams enhanced by carbon fibre reinforced polymer ……….............................................................................. 275 L. Sorrentino, C. Bellini, W. Polini, S. Turchetta Performance Index of Natural Stones-GFRP Hybrid Structures ……………………………….... 285

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Frattura ed Integrità Strutturale, 46 (2018); International Journal of the Italian Group of Fracture

V. E. Wildemann, T. V. Tretyakova, E. M. Strungar, M. P. Tretyakov Deformation and failure of carbon fiber composite specimens with embedded defects during tension-torsion test ……………………………………………………………...……………………… 295 A. Deliou, B. Bouchouicha Fatigue crack propagation in welded joints X70 ……………………………………..………… 306 C. Bellini, L. Sorrentino Characterization of Isogrid Structure in GFRP ……………………………………………..… 319 A. Kostina, O. Plekhov, S. Aizikovich Numerical simulation of a heat generation in a layered material during ultrasonic wave propagation …... 332 Y. Yanya Blending ratio of recycled aggregate on the performance of pervious concrete …………..……………… 343 J. Liu, J. Xie, Y. Liu, P. Shi Experimental study on concrete damage location by Computerized tomography technology ……………. 352 H. Zhu Stress performance of embedded carbon fiber reinforced plastics plate consolidated reinforced concrete structure ………………………………………………………………………………… 361 I. Čamagić, S. A. Sedmak, A. Sedmak, Z. Burzić, M. Aranđelović 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 ……………………………………...……… 371 I. Shardakov, A. Shestakov, R. Tsvetkov, I. Glot Investigation of the effect of cracks on the vibration processes in reinforced concrete structures ………… 383 W. Hao, X. Xu, Z. Niu Experimental study on the mechanical behavior of RPC filled square steel tube columns subjected to eccentric compression ………………………………………………………….…………… 391 F. Bazzucchi, L. Restuccia, G. A. Ferro Considerations over the Italian road bridge infrastructure safety after the Polcevera viaduct collapse: past errors and future perspectives ………………………………………………………...……… 400

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Fracture and Structural Integrity, 46 (2018); ISSN 1971-9883

Editorial Team

Editor-in-Chief Francesco Iacoviello

(Università di Cassino e del Lazio Meridionale, Italy)

Associate Editors Alfredo Navarro

(Escuela Superior de Ingenieros, Universidad de Sevilla, Spain)

Thierry Palin-Luc

(Arts et Metiers ParisTech, France) (University of Sheffield, UK) (University of Manchester, UK)

Luca Susmel John Yates

Developments in the fracture and fatigue assessment of materials and structures

Guest Editors

Filippo Berto Luca Susmel

(Norwegian University of Science and Technology, Norway)

(University of Sheffield, UK)

Guest Editors

Design of Civil Environmental Engineering

Mauro Sassu

(Università di Cagliari, Italy) (Università di Cagliari, Italy) (Università di Cagliari, Italy)

Fausto Mistretta Flavio Stochino Franco Bontempi

(Università di Roma “La Sapienza”, Italy) (Università di Roma “La Sapienza”, Italy)

Konstantinos Gkoumas

Advisory Editorial Board Harm Askes

(University of Sheffield, Italy) (Tel Aviv University, Israel) (Politecnico di Torino, Italy) (Università di Parma, Italy) (Politecnico di Torino, Italy)

Leslie Banks-Sills Alberto Carpinteri Andrea Carpinteri Emmanuel Gdoutos Youshi Hong M. Neil James Gary Marquis Ashok Saxena Darrell F. Socie Shouwen Yu Ramesh Talreja David Taylor Robert O. Ritchie Cetin Morris Sonsino Donato Firrao

(Democritus University of Thrace, Greece) (Chinese Academy of Sciences, China)

(University of Plymouth, UK)

(Helsinki University of Technology, Finland)

(University of California, USA)

(Galgotias University, Greater Noida, UP, India; University of Arkansas, USA)

(University of Illinois at Urbana-Champaign, USA)

(Tsinghua University, China) (Fraunhofer LBF, Germany) (Texas A&M University, USA) (University of Dublin, Ireland)

Regional Editorial Board Nicola Bonora

(Università di Cassino e del Lazio Meridionale, Italy) (Military University of Technology, Poland) (National Technical University of Athens, Greece)

Dorota Kocańda Stavros Kourkoulis

Carlo Mapelli

(Politecnico di Milano, Italy)

Antonio Martin-Meizoso

(Tecnun Universidad de Navarra, Spain)

Raghu Prakash

(Indian Institute of Technology/Madras in Chennai, India)

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Frattura ed Integrità Strutturale, 46 (2018); International Journal of the Italian Group of Fracture

Luis Reis Elio Sacco

(Instituto Superior Técnico, Portugal) (Università di Napoli "Federico II", Italy) (University of Belgrade, Serbia) (Tel-Aviv University, Tel-Aviv, Israel)

Aleksandar Sedmak

Dov Sherman Karel Slámečka Petro Yasniy

(Brno University of Technology, Brno, Czech Republic) (Ternopil National Ivan Puluj Technical University, Ukraine)

Editorial Board Jafar Albinmousa Nagamani Jaya Balila

(King Fahd University of Petroleum & Minerals, Saudi Arabia)

(Indian Institute of Technology Bombay, India) (Indian Institute of Technology Kanpur, India)

Sumit Basu

Stefano Beretta Filippo Berto K. N. Bharath

(Politecnico di Milano, Italy)

(Norwegian University of Science and Technology, Norway) (GM Institute of Technology, Dept. Of Mechanical Engg., India)

Elisabeth Bowman

(University of Sheffield)

Alfonso Fernández-Canteli

(University of Oviedo, Spain) (Università di Parma, Italy) (Politecnico di Torino, Italy) (University of Porto, Portugal)

Luca Collini

Mauro Corrado

José António Correia Manuel de Freitas Abílio de Jesus Vittorio Di Cocco Milos Djukic Giuseppe Ferro Riccardo Fincato Eugenio Giner Dimitris Karalekas Sergiy Kotrechko

(EDAM MIT, Portugal)

(University of Porto, Portugal)

(Università di Cassino e del Lazio Meridionale, Italy)

(University of Belgrade, Serbia) (Politecnico di Torino, Italy)

(Osaka University, Japan)

(Universitat Politecnica de Valencia, Spain)

(University of Piraeus, Greece)

(G.V. Kurdyumov Institute for Metal Physics, N.A.S. of Ukraine, Ukraine)

Paolo Lonetti Carmine Maletta Liviu Marsavina

(Università della Calabria, Italy) (Università della Calabria, Italy) (University of Timisoara, Romania) (University of Porto, Portugal) (Kyushu University, Japan) (University of Porto, Portugal) (University of Bristol, UK) (Kielce University of Technology, Poland) (National Technical University of Athens) (IMT Institute for Advanced Studies Lucca, Italy) (Università di Parma, Italy) (University of Piraeus, Greece) (Politecnico di Torino, Italy) (Università di Messina, Italy) (Università di Padova, Italy) (Università di Brescia, Italy) (Università di Napoli "Federico II") (Università di Roma "Tor Vergata", Italy) (Università di Cassino e del Lazio Meridionale, Italy)

Sonia Marfia

Lucas Filipe Martins da Silva

Hisao Matsunaga Milos Milosevic Pedro Moreira

(Innovation centre of Faculty of Mechanical Engineering in Belgrade, Serbia)

Mahmoud Mostafavi

Andrzej Neimitz

(Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, Ukraine)

Hryhoriy Nykyforchyn

Pavlos Nomikos

Marco Paggi Oleg Plekhov

(Russian Academy of Sciences, Ural Section, Moscow Russian Federation)

Alessandro Pirondi Dimitris Karalekas Luciana Restuccia Giacomo Risitano Mauro Ricotta Roberto Roberti

Elio Sacco

Pietro Salvini

Andrea Spagnoli

(Università di Parma, Italy)

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Fracture and Structural Integrity, 46 (2018); ISSN 1971-9883

Ilias Stavrakas Dimos Triantis Sabrina Vantadori Natalya D. Vaysfel'd Charles V. White

(University of West Attica, Greece) (University of West Attica, Greece)

(Università di Parma, Italy)

(Odessa National Mechnikov University, Ukraine)

(Kettering University, Michigan,USA)

Shun-Peng Zhu

(University of Electronic Science and Technology of China, China)

Sister Associations help the journal managing: Czech Rep.: Asociace Strojních Inženýrů (Association of Mechanical Engineers) Greece: Greek Society of Experimental Mechanics of Materials - GSEMM India: Indian Structural Integrity Society - InSIS Israel: Israel Structural Integrity Group - ISIG Italy: Associazione Italiana di Metallurgia - AIM Italy: Associazione Italiana di Meccanica Teorica ed Applicata - AIMETA Italy: Società Scientifica Italiana di Progettazione Meccanica e Costruzione di Macchine - AIAS Poland: Group of Fatigue and Fracture Mechanics of Materials and Structures Portugal: Portuguese Structural Integrity Society - APFIE Serbia: Structural Integrity and Life Society "Prof. Stojan Sedmak" - DIVK Spain: Grupo Espanol de Fractura - Sociedad Espanola de Integridad Estructural – GEF Ukraine: Ukrainian Society on Fracture Mechanics of Materials (USFMM)

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Frattura ed Integrità Strutturale, 46 (2018); International Journal of the Italian Group of Fracture

Journal description and aims Frattura ed Integrità Strutturale (Fracture and Structural Integrity) is the official Journal of the Italian Group of Fracture. It is an open-access Journal published on-line every three months (January, April, July, October). Frattura ed Integrità Strutturale encompasses the broad topic of structural integrity, which is based on the mechanics of fatigue and fracture and is concerned with the reliability and effectiveness of structural components. The aim of the Journal is to promote works and researches on fracture phenomena, as well as the development of new materials and new standards for structural integrity assessment. The Journal is interdisciplinary and accepts contributions from engineers, metallurgists, materials scientists, physicists, chemists, and mathematicians. Contributions Frattura ed Integrità Strutturale is a medium for rapid dissemination of original analytical, numerical and experimental contributions on fracture mechanics and structural integrity. Research works which provide improved understanding of the fracture behaviour of conventional and innovative engineering material systems are welcome. Technical notes, letters and review papers may also be accepted depending on their quality. Special issues containing full-length papers presented during selected conferences or symposia are also solicited by the Editorial Board. Manuscript submission Manuscripts have to be written using a standard word file without any specific format and submitted via e-mail to gruppofrattura@gmail.com. Papers should be written in English. A confirmation of reception will be sent within 48 hours. The review and the on-line publication process will be concluded within three months from the date of submission. Peer review process Frattura ed Integrità Strutturale adopts a single blind reviewing procedure. The Editor in Chief receives the manuscript and, considering the paper’s main topics, the paper is remitted to a panel of referees involved in those research areas. They can be either external or members of the Editorial Board. Each paper is reviewed by two referees. After evaluation, the referees produce reports about the paper, by which the paper can be: a) accepted without modifications; the Editor in Chief forwards to the corresponding author the result of the reviewing process and the paper is directly submitted to the publishing procedure; b) accepted with minor modifications or corrections (a second review process of the modified paper is not mandatory); the Editor in Chief returns the manuscript to the corresponding author, together with the referees’ reports and all the suggestions, recommendations and comments therein. c) accepted with major modifications or corrections (a second review process of the modified paper is mandatory); the Editor in Chief returns the manuscript to the corresponding author, together with the referees’ reports and all the suggestions, recommendations and comments therein. d) rejected. The final decision concerning the papers publication belongs to the Editor in Chief and to the Associate Editors. The reviewing process is usually completed within three months. The paper is published in the first issue that is available after the end of the reviewing process.

Publisher Gruppo Italiano Frattura (IGF) http://www.gruppofrattura.it ISSN 1971-8993 Reg. Trib. di Cassino n. 729/07, 30/07/2007

Frattura ed Integrità Strutturale (Fracture and Structural Integrity) is licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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Fracture and Structural Integrity, 46 (2018); ISSN 1971-9883

News from Frattura ed Integrità Strutturale

D

ear friends, Visual Abstracts have been a great success. All the papers in the July issue and in this issue are published also with this fast way to communicate the “core” of the papers and many readers really appreciated this new service. For this reason, Visual Abstracts will not only be offered in the Journal website but they will also be published in the new YouTube channel https://www.youtube.com/channel/UC3Ob2GNW8i0phNiiKjEVv0A … obviously linked to the papers! So, join the channel!  India: Indian Structural Integrity Society - InSIS  Israel: Israel Structural Integrity Group - ISIG  Italy: Associazione Italiana di Metallurgia - AIM  Italy: Associazione Italiana di Meccanica Teorica ed Applicata - AIMETA  Italy: Società Scientifica Italiana di Progettazione Meccanica e Costruzione di Macchine - AIAS  Poland: Group of Fatigue and Fracture Mechanics of Materials and Structures  Portugal: Portuguese Structural Integrity Society - APFIE  Serbia: Structural Integrity and Life Society "Prof. Stojan Sedmak" - DIVK  Spain: Grupo Espanol de Fractura - Sociedad Espanola de Integridad Estructural - GEF  Ukraine: Ukrainian Society on Fracture Mechanics of Materials (USFMM) Obviously, all the associations that wish to help us will be welcome! Finally, some news about the browsable version of the journal. After the last update of the software we use to prepare this version, the browsable version will be completely available also on mobile devices (no matter the OS). Try it and enjoy! The panel of “sister associations” is more and more rich. This is the up to dated list:  Czech Rep.: Asociace Strojních Inženýrů (Association of Mechanical Engineers)  Greece: Greek Society of Experimental Mechanics of Materials - GSEMM

Francesco Iacoviello Frattura ed Integrità Strutturale Editor in Chief

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

Developments in the fracture and fatigue assessment of materials and structures

Simulation of the behavior of aluminum alloys welded in Friction Stir Welding FSW (Case of AA5083 and AA 6082)

Bousekrine Taha Bounini, Benattou Bouchouicha University of Sidi Bel Abbes, Algeria Tahabounini@yahoo.fr, benattou_b@yahoo.fr Abdelkader Ghazi University of Mascara, Algeria ghaziaek@yahoo.fr

A BSTRACT . The friction stir welding FSW is a topical technique to date, either for experimental or numerical research topics. Based on the fact that the majority of the heat generated, comes from the contact between the workpiece and the shoulder, and from the work of friction done by the pin. Thus, it is crucial to approach the reality, by simulating the most of the conditions, related to the FSW process. The development of a numerical model allows us, not only to simulate the FSW welding process, but also to conduct a parametric study and vary the welding conditions as desired. Therefore, a numerical study is conducted on the friction stir welding of AA 5083H111 work piece, in order to perceive the optimal settings. A three dimensional model designed by ANSYS APDL was used. Before proceeding with this, a preliminary experimental study was carried out on FSW welded AA 6082 –T6 specimens and base material, using tensile and fatigue tests. The numerical results obtained, essentially longitudinal residual stresses, are compared to other studies, experimental and numerical. K EYWORDS . FSW; Parametric Study; AA5083H111; AA 6082-T6; Fatigue test.

Citation: Bounini, T., Bouchouicha, B., Ghazi, A., Simulation of the behavior of Aluminium Alloys welded in FSW (Case of AA5083, AA6082), Frattura ed Integrità Strutturale, 46 (2018) 1-13.

Received: 28.03.2018 Accepted: 17.07.2018 Published: 01.10.2018

Copyright: © 2018 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

I NTRODUCTION

riction stir welding (FSW) is a solid-state welding technique that involves the joining of metals without filler materials. A cylindrical rotating tool plunges into a rigidly clamped workpiece and moves along the joint to be welded. As the tool moves along the joint, heat is generated by friction between the tool shoulder and the workpiece. Additional heat is generated by plastic deformation of the workpiece material. The generated heat results in thermal softening of the workpiece material. The translation of the tool causes the softened workpiece material to flow from the F

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

front to the back of the tool where it consolidates. As cooling occurs, a solid continuous joint between the two plates is formed. No melting occurs during the process, and the resulting temperature remains below the solidus temperature of the metals being joined. FSW offers many advantages over conventional welding techniques, and has been successfully applied in the aerospace, automobile, and shipbuilding industries. Thermal and mechanical behaviors are mutually dependent during the FSW process. Because the temperature field affects stress distribution, this example uses a fully thermo-mechanically coupled model. The model consists of a coupled-field solid element with structural and thermal degrees of freedom. The model has two rectangular steel plates and a cylindrical tool. All necessary mechanical and thermal boundary conditions are applied on the model. The simulation occurs over three load steps, representing the plunge, dwell, and traverse phases of the process (Fig. 1) [1].

Figure 1: Schematic drawing of FSW process [2].

Numerical analysis of friction stir welding will allow many different welding processes to be simulated in order to understand the effects of changes in different system parameters before physical testing, which would be time-consuming or prohibitively expensive in practice [2]. This model assumes that two sheets of AA 5083 H111 are welded by a rotating tool. This model takes into account the actual interaction between the tool and the workpiece, and the deformation of the material around the tool. It should be noted that most of the finite element FE models, consider the two sheets as one, or simulate the heat flux without simulating the tool, or a part of the tool. The welding parameters, such as the variation of the welding speed, the rotation speed, and the material of the welded sheets, and their effects on the longitudinal stress, and on the distribution of the temperature flow. An experimental study was carried out, for the purpose of defining the impact of the change of welding parameters on the tenacity of welded joints. The friction stir welding is done using a milling machine. Fatigue and tensile tests were carried out on test tubes of material Alluminium alloy 6082-T6. E XPERIMENTAL STUDY ON AA6082-T6 ur study focuses on the characterization of two materials 6082-T6 (Base material and FSW welded material). The characterization tests are based on two essential tests, tensile test to identify the behavioral laws of the different materials and fatigue test to see the evolution of a crack in the different configurations. The chemical composition of the studied alloy is reported in the table below: O

Al Alloy 6082-T6

Si

Mg 1.2

Fe

Cu Mn Cr

Zn 0.2

Ti

Al

1.3

0.5 0.1 1.0

0.25

0.1

98.3

Table 1 : The chemical composition of AA 6082-T6.

Tensile Test For the different parts of the mechanical tests allowing the characterization of the materials, we carried out simple tensile tests on "dumbbell" specimens machined in aluminum sheets (longitudinal). The geometry of the specimens is described in Fig. 2.

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

The following figure shows the true stress-true strain curves that characterize an elasto-plastic behavior of the studied materials.

Figure 2: Details of the dumbbell specimen.

700

600

500

400

300

 v (MPa)

200

100

6082 6082 FSW

0

0,00

0,02

0,04

 v (%)

Figure 3: Tensile behavior of 6082 T6 aluminum alloy with and without welding.

The usual mechanical characteristics of the two materials are given in Tab. 2 below:

E (MPa)

σ e (MPa)

σ max (MPa)

ε y (%)

ε failure (%) 0.029

K (MPa)

Material

n

Al 6082-T6

74000

314

487

0.0046

1351

0.21

Al 6082-T6 FSW

69000

260

391

0.0038

0.019

848

0.19

Table 2: Tensile Mechanical characteristics.

The conventional yield stress in the base material (314MPa) is well above the yield strength in FSW welded specimens (260MPa). Fatigue Test The cracking tests were carried out on CT50 test specimens of thickness 06 mm, in accordance with ASTM-E-647 norm, the dimensions of which are given in Fig. 4. The cracking tests were carried out at the University of Sidi Belabbes, on an INSTRON electrohydraulic machine of capacity 100 KN in static and 50 KN in dynamics.

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

The crack length was measured using a magnifying optical binocular bezel and a stroboscope mounted on a device attached to the machine.

Figure 4: Dimensions of the CT50 specimen used for cracking tests.

Propagation rates The fatigue crack propagation tests were carried out for both materials (Fig. 5), in the range of speeds between 4.10 -5 and 2.10 -2 mm/cycle at ambient air and with a load ratio (R = 0.1). The values of  K vary from: - 10 to 47 in aluminum 6082-T6 - 10 to 45 in FSW welded aluminum 6082-T6. The results obtained for the different specimens are represented by Fig. 5:

1

6082 welded 6082

0,1

0,01

1E-3

da/dN (mm/cycle)

1E-4

10

100

 K (MPa m 1/2 )

Figure 5: Evolution of the cracking speed as a function of  K for the two materials.

In order to compare more clearly and more systematically the evolution of the cracking rate in the two grades, we chose to represent them by their respective linear regression lines obtained from the experimental points (seven point method) on the linear parts of the curves. Indeed, we can notice that the curves present an almost rectilinear look on a large part of the domain explored, being able to be presented by PARIS’ law of the form:

da C K dN  

  m

(2.1)

The results of the cracking tests obtained in the two materials respectively recorded in the Tab. (3).

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01



Alloy

Paris’ law

 K

6082-T6

da/dN=7.41E -8  K 3.02 da/dN= 3.32E -14  K 7.32

10 to 47 10 to 45

mMPa

6082-T6 FSW

m MPa

Table 3: Equations characterizing the Paris’ law in the different zones studied.

The cracking rate da/dN, for low values of (ΔK <10) is decreasing respectively in both cases (6082-T6 and 6082-T6 FSW), however, this gap decreases as increases, which means that: microstructure in the two shades possesses almost the same appearance and the same characteristics. In this area (typical of Paris), the cracking rate is almost similar in the two materials.

 M ATERIAL AND NUMERICAL STUDY ON AA5083-H111 Introduction

T

he development of a numerical model allows us not only to simulate the FSW welding process, but also to make a parametric study and vary the welding conditions as desired. Song and Kovacevic have modeled the heat transfer in FSW using the finite difference method [4, 5]. Few papers have dealt directly with the modeling of thermo- mechanical stresses in FSW. Chao [6] proposed a model to predict the thermal history and the subsequent thermal stress and distortion of the workpiece without involving the the tool mechanical effect. Dong [7] developed several models to deal separately with the subproblems of heat transfer, material flow, and plastic flow. From the point of physics for the FSW process, the tool mechanical effect needs to be included into the thermo-mechanical model. C.M. Chen, R. Kovacevic has considered the the tool mechanical effect (only the shoulder was included) [8]. The model used is based on the model developed by ANSYS, which took as a reference the the Zhu and Chao model. The approach presented in this article is more realistic, because it takes into consideration the whole tool: the shoulder and the pin, what it approaches us to reality. The main parameters affecting the mechanical behaviour are multiple: the rotational and advancing speed, the tool size and the tool penetration. However, the parameters taken under consideration are the rotation speed, the welding speed and material change. We observed the residual stress and the heat distribution. Experimental Reference Study on AA 5083 H111 To be able to compare and validate the numerical study we chose the experimental study on the material AA 5083H111. The results obtained by GHAZI [3], on another material AA 5083, by varying the welding parameters, rotational and welding speed, led to identify the optimal welding parameters. The alloy studied is Al 5083H111, which is an Al-Mg (Tab.4) in the H111 state (deformed by 25% by rolling). This strain hardening alloy will serve us to understand the different phenomena occurring during welding without involving precipitation. The alloy 5083H111 was chosen because it has magnesium and manganese levels fairly close to those of the alloy 2024 used in aeronautical structures for the fuselage. The chemical composition of Aluminum Al 5083H111 is given by the Tab. 4. The mechanical properties of the aluminum alloy were determined along the longitudinal directions L. The following table shows the material properties:

Al

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

0.15

0.40

0.40

0.10

0.10

4.90

0.25

0.25

0.15

E [MPa]

YS[MPa]

UTS[MPa]

A[%]

Hv

K[J/cm 2 ]

71008

155

236

16.5

88

45

Table 4: Plates material characteristics: Aluminum Alloy 5083H111.

The welding parameters are shown in the Tab. 6 .

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

Density (Kg/m 3 )

2660

Thermal conductivity( w/m.K)

110 -140

Thermal expansion coefficient K -1

24.2 10 -6

Specific heat (J/Kg°C)

900

Table 5: Plates material other mechanical and thermal characteristics.

Speed of rotation ω [rpm]

710

1000

1400

2000

40 80

40 80

40 80

40 80

100 200 400

100 200 400

100 200 400

100 200 400

Welding speed V [mm/min]

Table 6: Welding parameter (speed of rotation - welding speed).

(a)

(b)

as a function of the rotational speed . b) Variation of the elastic limit R e

as a function of the

Figure 6: a) Variation of the elastic limit R e

welding speed.

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

The analysis of the tensile curves, clearly shows the variation of the elastic limit Re as a function of the variation of the rotational speed and the welding speed (Fig. 6). Tensile tests have shown that these parameters give a good quality of welding: - [710 rpm, 80 mm/min]. - [1000 rpm, 100 mm/min]; [1000 rpm, 200 mm/min]. - [1400 rpm, 100 mm/min]. - [2000 rpm, 80 mm/min]; [2000 rpm, 100 mm/min] Modeling and Meshing For reducing the calculation time, dimensions are reduced. The reduced model is constituted by two rectangular shaped plates 1 x 0.7 x 0.1 mm. The tool shoulder diameter is 0.2 mm. the pin has a height of 0.3 mm. A direct coupled-field analysis is performed on a reduced-scale version of the Zhu and Chao model [6]. Also, rather than using a moving heat source as in the reference model, a rotating and moving tool is used for a more realistic simulation. The simulation welds two Alu alloy 5083 H111 plates (workpiece) with a cylindrical shape tool, as shown in the Fig. 7. The FSW process was simulated taking into account a fully three-dimensional model, a thermo-mechanical analysis was developed with ANSYS software. Both the workpiece and the tool are modeled using coupled-field element SOLID226. The contact pairs has (contact analysis) was identified by the two elements CONTA174 and TARGE170 for contact and target surfaces (Fig. 8).

Figure 7: Geometry of the workpiece and tool by ANSYS.

Figure 8: Finite Element mesh of FSW.

Mechanical Boundary Conditions The workpiece is fixed by clamping each plate [1]. The clamped portions of the plates are constrained in all directions. To simulate support at the bottom of the plates, all bottom nodes of the workpiece are constrained in the perpendicular direction (z direction). Thermal Boundary Conditions T he frictional and plastic heat generated during the FSW process propagates rapidly into remote regions of the plates. On the top and side surfaces of the workpiece, convection and radiation account for heat loss to the ambient. Conduction losses also occur from the bottom surface of the workpiece to the backing plate. Mechanical and physical properties Accurate temperature calculation is critical to the FSW process because the stresses and strains developed in the weld are temperature-dependent. Thermal properties of the Alluminium Alloy 5083 H111 plates such as thermal conductivity, specific heat, and density are temperature-dependent. Mechanical properties of the plates such as Young’s modulus and the coefficient of thermal expansion are considered to be constant due to the limitations of data available in the literature. It is assumed that the plastic deformation of the material uses the von Mises yield criterion, as well as the associated flow rule and the work-hardening rule [6 ) . Therefore, a bilinear isotropic hardening model is selected [1].

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

Heat generation The friction between the shoulder and workpiece results in the biggest component of heating. From the heating aspect, the relative size of pin and shoulder is important, and the other design features are not critical. The shoulder also provides confinement for the heated volume of material. The tool plunges into the workpiece, then rotates to generate heat. The depth and rotating speeds are critical parameters for the weld temperatures. The tool travels from one end of the welding line to the other [9]. Loading The FSW process consists of three primary phases:  Plunge -- The tool plunges slowly into the workpiece  Dwell -- Friction between the rotating tool and workpiece generates heat at the initial tool position until the workpiece temperature reaches the value required for the welding.  Traverse (or Traveling) -- The rotating tool moves along the weld line. During the traverse phase, the temperature at the weld line region rises, but the maximum temperature values do not surpass the melting temperature of the workpiece material. As the temperature drops, a solid continuous joint appears between the two plates. N UMERICAL RESULTS AND DISCUSSION he mechanical properties studied are the longitudinal residual stress and temperature distribution. The analysis takes different translation speeds (40, 80, 100 and 200 mm/ min) and two rotational speed (710 rpm and 1400 rpm).

T

(a)

(b)

(c) (d) Figure 9: Heat distribution on cross-section of the workpiece: a) Welding speed of 40mm/min, b) Welding speed of 80mm/min, c) Welding speed of 100mm/min, d) Welding speed of 200 mm/min.

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

Influence of Welding Speed on Heat generation From the Fig. 9, it is remarkable that decreasing the welding speed produces a hotter weld.

Heat generation during friction stir welding arises from two main sources: friction at the surface of the tool and the deformation of the material around the tool. Increasing the welding speed produces less work of deformation and in result, less heat generated. The temperature distribution shows an asymmetric profile due to the stirring effect of the pin. Influence of welding Speed on Longitudinal Residual Stress It should be observed that residual stresses in FSW processes are due both to thermal and mechanical causes. The former are due to the friction forces work and to the deformation work both decaying into heat. The latter are strictly connected to the strong gradients of strain and strain rates occurring due to the tool pin action on the material to be welded and which determine relevant microstructure changes in the parent material but low influence in the residual stresses [10].

40 mm/min 80 mm/min 100 mm/min 200 mm/min

80 mm/min 100 mm/min 200 mm/min

1,2

3,2

1,0

2,4

0,8

1,6

0,6

0,4

0,8

0,2

Longitudinal Residual Stress (MPa)

Longitudinal Residual Stress (MPa)

0,0

0,0

-8,0E-04

-4,0E-04

0,0E+00

4,0E-04

8,0E-04

8,0E-04

4,0E-04

0,0E+00

+4,0E-04

+8,0E-04

Distance from the weld center (mm)

Distance from weld center (mm)

(a) (b) Figure 10: a) longitudinal stress profiles in a cross-section of the workpiece (Rotational speed: 710 rpm). b) Only 80, 100 and 200 mm/min welding speeds are represented.

40mm/min 80mm/min 100mm/min 200mm/min

40mm/min 80mm/min 200mm/min

1,2

10

1,0

8

0,8

6

0,6

4

0,4

2

0,2

Longitudinal Residual Stress (MPa)

Longitudinal Residual Stress (MPa)

0

0,0

-8,0E-04

-4,0E-04

0,0E+00

4,0E-04

8,0E-04

-8,0E-04

-4,0E-04

0,0E+00

4,0E-04

8,0E-04

Distance from the weld center (mm)

Distance from welding center(mm)

(a) (b) Figure 11: a) Longitudinal stress profiles in a cross-section of the work piece (Rotational speed: 1400 rpm). b) only 40, 80 and 200 mm/min welding speeds are represented.

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

The graphs show the influence of the welding speed, on the longitudinal residual stresses LRS. We noticed that the following parameters are optimal: (1400 rpm-100 mm/min), (1400 rpm-80 mm/min), (710 rpm-100 mm/min) (710 rpm -80 mm/min), which is consistent with the study of GHAZI already quoted [3]. The studies show that residual stress has three regions. There are the nugget region, the thermo-mechanically affected zone TMAZ and the heat affected zone HAZ. Each region has qualitatively or quantitively distinct values of microstructure, hardness and residual stress [11]. Influence of rotational speed on heat generation Higher tool rotational speed resulted in a higher temperature and slower cooling rate in the friction stir processed FSP zone after welding. Fig. 12 shows that the temperature is higher with the rotational speed 1400 rpm. In the retreating side, the temperature distribution is lower than the advancing side. The temperature is decreasing, when the distance from weld center is increasing.

(a) (b) Figure 12: Heat distribution on cross-section of the workpiece : a) rotational speed of 1400 rpm, b) rotational speed of 710 rpm. Influence of Rotational Speed on Longitudinal Residual Stress Rotational speed appears to be the most significant process variable since it also tends to influence the translational velocity. Very high rotational speeds (>10,000 rpm) could raise strain rate, and there by influence the recrystallisation process; which in turn could influence the FSW process [12]. At a constant welding speed of 80 mm/min, residual stresses increased as rotational speed augmented from 710 to 1400 rpm. A higher rotational speed causes excessive release of stirred materials to the upper surface, which resultantly left voids in the FSP zone. Lower heat input condition due to lower rotational speed resulted in lack of stirring [13].

25

710 rpm 1400 rpm

AA 5052 AA 5083

1,2

1,2

20

1,0

0,9

0,8

15

0,6

0,6

10

0,4

0,3

5

0,2

longitudinal residual stress (MPa)

longitudinal residual stress (MPa)

Longitudinal Residual Stress (MPa)

0

0,0

0,0

-8,0E-04

-4,0E-04

0,0E+00

4,0E-04

8,0E-04

-8,0E-04

-4,0E-04

0,0E+00

4,0E-04

8,0E-04

distance from the center (mm)

Distance from weld center (mm)

Figure 13: Influence of rotational speed on longitudinal residual stress.

Figure 14: Influence of material properties on residual longitudinal stress.

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

The heat input (generated by the friction between tool shoulder and base metal) increases with increase in tool rotational speed and decrease in welding speed. Low heat input causes the intermittent metal flow and improper stirring action around the tool pin due to insufficient plasticization of the base metal under the tool shoulder. On the other side, high heat input causes turbulent metal flow around the tool pin due to excess plasticization of base metal under the tool shoulder. Both these welding conditions produced defective welds. The base metal with low yield strength, low hardness and higher ductility can be plastically deformed very easily and the flow of plasticized metal around the non-consumable tool will also be uniform compared to the base metal with high strength, high hardness and low ductility. [14] Influence of Material Properties in the Longitudinal Residual Stress Fig. 14 shows that the material AA 5052 has undergone higher stresses at the nugget, TMAZ and less at TAZ, compared to AA 5083 alloy, which means higher deformation.

80 mm/min 100 mm/min 200 mm/min

1,2

1,0

0,8

0,6

0,4

0,2

Longitudinal Residual Stress (MPa)

0,0

8,0E-04

4,0E-04

0,0E+00

+4,0E-04

+8,0E-04

Distance from weld center (mm)

(a) (b) Figure 15: Comparison between: a) Influence of welding speed on LRS (710 rpm), b) Influence of welding speed on Micro-hardness [3].

10

8

6

4

2

Longitudinal Residual Stress(MPA)

0

-8,0E-04

-4,0E-04

0,0E+00

4,0E-04

8,0E-04

Distance from weld center(mm)

(a) (b) Figure 16: a) Longitudinal Residual stress with (1400 rpm, 100 mm/min), b) Experimental and numerical longitudinal stress profiles in a cross-section of the weld piece [10]. Validation of Numerical Results Comparison With The Validation Study: Stress – Microhardness Relationship The relationship between stress and strain (deformation) and microhardness is a factor to check the numerical model. Strain or reduced deformation is a term that expresses the trend of the deformation change among the material field. Strain is the

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T. Bounini et alii, Frattura ed Integrità Strutturale, 46 (2018) 1-13; DOI: 10.3221/IGF-ESIS.46.01

deformation per unit length [15]. Applying a stress to material results in a strain. On the other side, hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. From these definitions, it is deductible that the relationship between hardness and strain is an inverse relationship, to the opposite of the positive relationship between stress and strain. From this perspective, the longitudinal stress results in the numerical study shown in the Figs. (10, 11, 12, 13), are comparable to the micro-hardness results, obtained by GHAZI [3]. The inverse relationship is shown in Fig. 16. By Longitudinal stress Profile in a cross-section of the weld piece: Fig. (16-a) shows longitudinal stress profile, achieved with ANSYS APDL for a rotation speed equal to 1400 rpm, and a welding speed equal to 100 mm/min. Fig. (16-b) shows the residual stress profiles used as a typical stress profile for comparaison. It can be seen that the residual stresses are compressive in the weld with external regions subjected to compressive stresses [10]. The comparison between Figs. (16-a) and (16-b), shows a similarity. Experimental - The conventional yield stress in the base material AA 6082-T6 (314 MPa) is well above the yield strength in FSW welded specimens AA 6082 – T6 FSW (260 MPa). - The cracking rate da/dN, for low values of (∆K < 10 ) is almost similar in the two materials. Numerical - Decreasing the welding speed produces a hotter weld. - The optimal parameters are these combinations (1400 rpm-100 mm/min), (1400 rpm – 80 mm/min), (710 rpm – 100 mm/min) and (710 rpm – 80 mm/min). - Higher temperature is generated with the rotational speed 1400 rpm, then with 710 rpm, for a constant welding speed 80 mm/min. - Residual stress increased as rotational speed augmented from 710 to 1400 rpm. - Comparing AA 5083 with AA 5052, the latter has undergone higher stresses. This is translated into deformation, which means less weldabilty. Validation - The longitudinal stress results in the numerical study shown in Figs. (10, 11, 12 and 13) are comparable to the micro- hardness results obtained by GHAZI [3]. The inverse relationship is shown in the Fig. 17. - The longitudinal residual stresses LRS profile resulted in the numerical study, shows similarity with the typical LRS profile [10]. C ONCLUSIONS [1] ANSYS 14.5 Help// Technology Demonstration Guide // 30. Friction Stir Welding (FSW) Simulation. [2] He, X., Gu, F. and Ball, A. (2014). A Review Of Numerical Analysis Of Friction Stir Welding, Progress In Materials Science 65, pp. 1-66. [3] Ghazi, A. (2013). Caractérisation Mécanique des Assemblages Soudés par Friction Malaxage (Etude Numérique et Expérimentale). [4] Song, M. and Kovacevic, R. (2002). Numerical Simulation And Experimental Analysis Of Heat Transfer Process In Friction Stir Welding Process, Proceeding Of Institution Of Mechanical Engineers, Part B, Journal Of Engineering Manufacture 216 (12), pp. 73–85. [5] Song, M. and Kovacevic, R. (2003). Thermal Modeling Of Friction Stir Welding In A Moving Coordinate And Its Validation, International Journal Of Machine Tool And Manufacturing 43 (6), pp. 605–615. [6] Zhu, X. K. and Chao, Y. J. (2004). Numerical Simulation of Transient Temperature and Residual Stresses in Friction Stir Welding of 304L Stainless Steel. Journal of Materials Processing Technology 146(2), pp. 263-272. [7] Dong, P., Lu, F., Hong, J.K. and Cao, Z. (2001). Coupled Thermomechanical Analysis Of Friction Stir Welding Process Using Simplified Models, Science And Technology Of Welding And Joining 6(5), pp. 281-287. R EFERENCES

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[8] Chen, C.M. and Kovacevic, R. (2003). Finite Element Modeling Of Friction Stir Welding—Thermal And Thermomechanical Analysis. Science Direct, International Journal Of Machine Tools & Manufacture 43, pp. 1319– 1326. [9] Mishra, R.S. and Ma, Z.Y. (2005). Friction stir welding and processing, Materials Science and Engineering R, 50(1-2), pp. 1-78. [10] Buffa, G., Fratini, L. and Pasta, S. (2009). Residual Stresses In Friction Stir Welding: Numerical Simulation And Experimental Verification, JCPDS-International Centre for Diffraction Data, pp. 444-453 [11] Kambouz, Y., Benguediab, M. and Bouchouicha, B. (2017). Numerical Study of the Mechanical Behavior and Fatigue in a Weld Bead by Friction Stir for a 6082–T6 Aluminum Alloy. Mechanics and Mechanical Engineering, 21(1), pp. 67- 83. [12] Elangovan, K. and Balasubramanian, V. (2007). Influences of pin profile and rotational speed of the tool on the formation of friction stir processing zone in AA2219 aluminium alloy, Material Science and Engineering: A, 459 (1-2), pp. 7-18. [13] Elangovan, K., Balasubramanian, V. and Valliapan, M. (2008). Effect of Tool Pin Profile and Tool Rotational Speed on Mechanical Properties of Friction Stir Welded AA6061 Aluminium Alloy Materials and Manufacturing Processes, 23(3), pp. 251-260. [14] Fernandez, G.J., and Murr, L.E. (2004). Characterization of tool wear and weld optimization in the friction-stir welding of cast aluminum 359+20% SiC metal-matrix composite. Materials Characteization 52(1), pp. 65-75. [15] Beer, F. P., Russell Johnston Jr., E., DeWolf, J. T. and Mazurek, D.F. (2009). Mechanics of materials (5th ed). Mc Graw Hill, p.49.

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