Issue 35

Frattura ed Integrità Strutturale, 35 (2016); International Journal of the Italian Group of Fracture

Table of Contents

Y. Matsuda, H. Nishiguchi, T. Fukuda Effects of large amounts of hydrogen on the fatigue crack growth behavior of torsional prestrained carbon steel ……………………………………………………………………………………. 1 W. Ozgowicz, B.Grzegorczyk, A. Pawełek, W. Wajda, W.Skuza. A.Piątkowski, Z. Ranachowski Relation between the plastic instability and fracture of tensile tested Cu-Sn alloys investigated with the application of acoustic emission technique …………………………………………………… 11 A. Pawełek, A. Piątkowski, W. Wajda, W. Skuza, A. Tarasek, W. Ozgowicz, B. Grzegorczyk, S. Kúdela, Jr., S. Kúdela Mechanisms of plastic instability and fracture of compressed and tensile tested Mg-Li alloys investigated using the acoustic emission method …………..................................................................................... 21 R. Konečná, L. Kunz, G. Nicoletto, A. Bača Fatigue crack growth behavior of Inconel 718 produced by selective laser melting ………………….... 31 A. Chahardehi, A. Mehmanparast Fatigue crack growth under remote and local compression – a state-of-the-art review …………..…….. 41 M.V. Bannikov, O. B. Naimark, V.A. Oborin Experimental investigation of crack initiation and propagation in high- and gigacycle fatigue in titanium alloys by study of morphology of fracture ……………………………………………………... 50 A. Vshivkov, A. Iziumova, O. Plekhov, J. Bär Experimental study of heat dissipation at the crack tip during fatigue crack propagation …………..... 57 S. El Kabir, R. Moutou Pitti N. Recho, Y. Lapusta, F. Dubois Numerical study of crack path by MMCG specimen using M integral …………………………… 64 L. S., B. Rui, Z. Ting, F. Binjun Mechanism of crack branching in the fatigue crack growth path of 2324-T39 Aluminium alloy …… 74 S. Morita, S. Fujiwara, T. Hori, N. Hattori, H. Somekawa, T. Mayama Microstructure dependence of fatigue crack propagation behavior in wrought magnesium alloy ………... 82 X. Liu, C. Sun, Y. Hong Crack initiation characteristics and fatigue property of a high-strength steel in VHCF regime under different stress ratios …………………………………………………………………….... 88 P. Bernardi, R. Cerioni, E. Michelini, A. Sirico A non-linear procedure for the numerical analysis of crack development in beams failing in shear ……... 98

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Fracture and Structural Integrity, 35 (2016); ISSN 1971-9883

G. Dhondt Prediction of three-dimensional crack propagation paths taking high cycle fatigue into account ………... 108 V. Shlyannikov, A. Tumanov, A. Zakharov, A. Gerasimenko Surface crack growth subject to bending and biaxial tension-compression ………………………...... 114 A. Mehmanparast, O. Adedipe, F. Brennan, A. Chahardehi Welding sequence effects on residual stress distribution in offshore wind monopile structures ...……….... 125 S. Barter, P. White, M. Burchill Controlling fatigue crack paths for crack surface marking and growth investigations ………………... 132 J. Kramberger, K. Sterkuš, S. Glodež Damage and failure modeling of lotus-type porous material subjected to low-cycle fatigue ……………... 142 S. Glodež, S. Dervarič, J. Kramberger, M. Šraml Fatigue crack initiation and propagation in lotus-type porous material …………………………… 152 E. Dall’Asta, V. Ghizzardi, R. Brighenti, E. Romeo, R. Roncella, A. Spagnoli New experimental techniques for fracture testing of highly deformable materials …………...……….. 161 G. Meneghetti, M. Ricotta Experimental estimation of the heat energy dissipated in a volume surrounding the tip of a fatigue crack 172 J. Albinmousa Multiaxial fatigue crack path prediction using critical plane concept ………………………............. 182 S. Blasón, C. Rodríguez, A. Fernández-Canteli Fatigue characterization of a crankshaft steel: Use and interaction of new models …………………... 187 T. Abe, H. Akebono, M. Kato, A. Sugeta Analytical model of asymmetrical Mixed-Mode Bending test of adhesively bonded GFRP joint …… 196 S. Lesz, A. Januszka, S. Griner, R. Nowosielski Crack initiation and fracture features of Fe–Co–B–Si–Nb bulk metallic glass during compression ….. 206 A. Nikitin, C.Bathias, T.Palin-Luc, A. Shanyavskiy Crack path in aeronautical titanium alloy under ultrasonic torsion loading ………………………... 213 E. Fessler, S. Pierret, E. Andrieu, V. Bonnand Crack growth threshold under hold time conditions in DA Inconel 718 – A transition in the crack growth mechanism ……………………………………………………………………….. 223 C. Gandiolle, S. Fouvry Fretting fatigue crack propagation rate under variable loading conditions ……...………………...… 232 T. Holušová, S. Seitl, H. Cifuentes, A. Fernández-Canteli A numerical study of two different specimen fixtures for the modified compact tension test – their influence on concrete fracture parameters ……………………………………………………………... 242

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Frattura ed Integrità Strutturale, 35 (2016); International Journal of the Italian Group of Fracture

T. Auger, S. Hémery, M. Bourcier, C. Berdin, M. Martin, I. Robertson Crack path in liquid metal embrittlement: experiments with steels and modeling ……………….…... 250 G. Gobbi, C. Colombo, L. Vergani A cohesive zone model to simulate the hydrogen embrittlement effect on a high-strength steel ………….. 260 S. Tarasovs, J. Krūmiņš, V. Tamužs Modelling of the fracture toughness anisotropy in fiber reinforced concrete ………………………….. 271 N. Oudni, Y. Bouafia Implementation of a damage model in a finite element program for computation of structures under dynamic loading …………….......................................................................................................... 278 E. Giner, J. Díaz-Álvarez, M. Marco, Mª Henar Miguélez Orientation of propagating crack paths emanating from fretting-fatigue contact problems ………….… 285 Y. Besel, M. Besel, U. Alfaro Mercado, T. Kakiuchi, Y. Uematsu Influence of joint line remnant on crack paths under static and fatigue loadings in friction stir welded Al-Mg-Sc alloy ……………………………………………………………………...….. 295 S. Valente, A. Alberto, F. Barpi Sub-critical cohesive crack propagation with hydro-mechanical coupling and friction ……………...… 306 S. Boljanović, S, Maksimović Fatigue failure analysis of pin-loaded lugs ……………………………………....……………. 313 K. Slámečka, P. Skalka, L. Čelko, J. Pokluda, L. Saucedo-Mora, T. J. Marrow, U. Thandavamoorthy Plasma-sprayed thermal barrier coatings: numerical study on damage localization and evolution ……... 322 O. Demir, S. İriç, A. O. Ayhan, H. Lekesiz Investigation of mixed mode - I/II fracture problems - Part 1: computational and experimental analyses 330 O. Demir, A. O. Ayhan, T. Kakiuchi Investigation of mixed mode-I/II fracture problems - Part 2: evaluation and development of mixed mode- I/II fracture criteria …………………………………………………………………….... 340 M. Bozkurt, A. O. Ayhan, M. F. Yaren, S. İriç Finite element modeling and experimental studies on mixed mode-I/III fracture specimens ………….. 350 H. Dündar, A. O. Ayhan Multiple and non-planar crack propagation analyses inthin structures using FCPAS ……………… 360 G. Kullmer, B. Schramm, H. A. Richard Fatigue crack paths under the influence of changes in stiffness ………………………………….... 368 Š. Major, V. Kocour, P.Cyrus Fatigue life prediction of pedicle screw for spinal surgery …………………………………............. 379

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Fracture and Structural Integrity, 35 (2016); ISSN 1971-9883

B. Lian, A. Ueno, T. Iwashita Development of in-situ fatigue crack observing system for rotating bending fatigue testing machine ……... 389 A. Tzamtzis, A. T. Kermanidis Fatigue crack growth prediction in 2xxx AA with friction stir weld HAZ properties …….................. 396 R.A. Cardoso, J.A. Araújo, J.L.A. Ferreira, F.C. Castro Crack path simulation for cylindrical contact under fretting conditions ….............................................. 405 O. Plekhov, O. Naimark, M. Narykova, A. Kadomtsev, V. Betechtin The study of a defect evolution in iron under fatigue loading in gigacyclic fatigue regime ………………. 414 T. Abe, H. Akebono, M. Kato, A. Sugeta Fatigue properties and fracture mechanism of load carrying type fillet joints with one-sided welding ….… 424 W. Ozgowicz, E. Kalinowska-Ozgowicz, K. Lenik, A. Duda Analysis of cracking of low-alloy copper stretched at elevated temperature ………………………… 434 S. Xinhong, Z. Jianyu, X. Qingshan, L. Tianqi Crack initiation and early propagation plane orientation of 2A12-T4 aluminum alloy under tension- torsion fatigue loading including mean tensile stress ………………………………………….. 441 A. Kowalski, W. Ozgowicz, M. Kurek, A. Kurek, T. Łagoda Fatigue cracking of aluminium alloy AlZn6Mg0.8Zr subjected to thermomechanical treatment ……... 449 L. C. H. Ricardo, C. A. J. Miranda Crack simulation models in variable amplitude loading - a review ……………………………..… 456 T. Haiyan Damage of bamboo and wooden materials based on linear elastic fracture mechanics in garden design … 472 W. Xu, L. Ronggui Effect of steel reinforcement with different degree of corrosion on degeneration of mechanical performance of reinforced concrete frame joints …………………………………………………………… 481 T. Sadowski, P. Golewski Cracks path growth in turbine blades with TBC under thermo – mechanical cyclic loadings ………….. 492 L. Chunjiang, Z. Huabo, L. Ninghua Influence of different sizes of concrete and roller compacted concrete on double-K fracture parameters …... 500 X.C. Arnoult, M. Růžičková, K. Kunzová, A. Materna Short review: Potential impact of delamination cracks on fracture toughness of structural materials …… 509 R. Citarella Residual strength evaluation by DBEM for a cracked lap joint …………...…………………….. 523 R. Sepe, E. Armentani, F. Caputo Static and fatigue experimental tests on a full scale fuselage panel and FEM analyses ………. 534

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Frattura ed Integrità Strutturale, 35 (2016); International Journal of the Italian Group of Fracture

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) (Ecole Nationale Supérieure d'Arts et Métiers, Paris, France)

Thierry Palin-Luc

Luca Susmel John Yates

(University of Sheffield, UK) (University of Manchester, UK)

Guest Editors A. Carpinteri

(University of Parma, Italy)

Les P. Pook

(21 Woodside Road, Sevenoaks TN13 3HF, UK)

L. Susmel R. Tovo

(University of Sheffield, UK) (University of Ferrara, Italy)

Advisory Editorial Board Harm Askes

(University of Sheffield, Italy) (Politecnico di Torino, Italy) (Università di Parma, Italy) (Politecnico di Torino, Italy) (University of Plymouth, UK)

Alberto Carpinteri Andrea Carpinteri Donato Firrao M. Neil James Gary Marquis Ashok Saxena Darrell F. Socie Shouwen Yu Ramesh Talreja David Taylor Robert O. Ritchie Cetin Morris Sonsino Elisabeth Bowman Roberto Citarella Claudio Dalle Donne Manuel de Freitas Vittorio Di Cocco Giuseppe Ferro Eugenio Giner Tommaso Ghidini Daniele Dini Editorial Board Stefano Beretta Nicola Bonora

(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)

(Politecnico di Milano, Italy)

(Università di Cassino e del Lazio Meridionale, Italy)

(University of Sheffield) (Università di Salerno, Italy) (EADS, Munich, Germany) (EDAM MIT, Portugal)

(Università di Cassino e del Lazio Meridionale, Italy)

(Imperial College, UK)

(Politecnico di Torino, Italy)

(Universitat Politecnica de Valencia, Spain) (European Space Agency - ESA-ESRIN)

Paolo Leonetti Carmine Maletta Liviu Marsavina

(Università della Calabria, Italy) (Università della Calabria, Italy) (University of Timisoara, Romania) (University of Porto, Portugal)

Lucas Filipe Martins da Silva

Hisao Matsunaga

(Kyushu University, Japan) (University of Sheffield, UK)

Mahmoud Mostafavi

Marco Paggi

(IMT Institute for Advanced Studies Lucca, Italy)

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Fracture and Structural Integrity, 35 (2016); ISSN 1971-9883

Oleg Plekhov

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

Alessandro Pirondi

(Università di Parma, Italy)

Luis Reis

(Instituto Superior Técnico, Portugal)

Giacomo Risitano Roberto Roberti

(Università di Messina, Italy) (Università di Brescia, Italy) (Università di Bologna, Italy) (Università di Parma, Italy)

Marco Savoia

Andrea Spagnoli Charles V. White

(Kettering University, Michigan,USA)

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Frattura ed Integrità Strutturale, 35 (2016); 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 (July, October, January, April). 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 iacoviello@unicas.it. 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 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, 35 (2016); ISSN 1971-9883

…. ECF21 is more and more near!!

D

ear friends, just an update about the most important event that IGF is organizing: the 21 st European Conference on Fracture, under the auspices of the European Structural Integrity Society. This event will be held in Catania, Italy (June 20- 24, 2015) and, according to the ECFs events tradition, will allows to hundred of researchers to meet in the wonderful land of Sicily. I don’t want to bother you describing the sea, the culture, the food, the people of Sicily … join us and you will see by your own!!! In these lines I wish only to underline few details: - Abstract summission deadline has been postponed: 15.01.2016; - Proceedings will be published in Procedia Structural Integrity (Elsevier); Templates are available in the ECF21 website; - Special issues will be published in Engineering Fracture Mechanics, Engineering Failure Analysis, International Journal of Fatigue and Theoretical and Applied Fracture Mechanics; - A Summer School titled “Understanding and modelling fracture and fatigue of materials and structures” will be organized before the ECF event (June 18-19, 2015) You can find all the details in the ECF21 website www.ecf21.eu …. Looking forward to meeting you in Catania!!!

Francesco Iacoviello F&IS Chief Editor

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

Focussed on Crack Paths

Effects of large amounts of hydrogen on the fatigue crack growth behavior of torsional prestrained carbon steel

Yuta Matsuda Advanced Engineering Course, National Institute of Technology, Sasebo College, Japan me1419@st.sasebo.ac.jp Hiroshi Nishiguchi, Takayuki Fukuda Department of Mechanical Engineering, National Institute of Technology, Sasebo College, Japan hiroshin@sasebo.ac.jp

A BSTRACT . The effects of large amounts of hydrogen on the fatigue crack growth properties of torsional prestrained ferritic–pearlitic low-carbon steel were investigated. Hydrogen-precharged specimens were produced by conducting cathodic charge to the virgin material and to torsional prestrained JIS-S10C and JIS- S25C steels (hereafter S10C and S25C steels). Rotating bending fatigue tests were conducted in air at room temperature. Hydrogen content, C H , increased with torsional prestrain for both S10C and S25C steels; the C H of the torsional prestrained S25C steel precharged with hydrogen was lower than that of S10C at the same torsional prestrain. No clear difference between the maximum C H values of the torsional fractured S10C and S25C hydrogen-precharged steel specimens. With respect to crack initiation, there was no obvious difference between the uncharged and precharged specimens in spite of the large amount of C H induced by torsional prestrain. The acceleration of fatigue crack growth by hydrogen was the main cause of the decreased fatigue life. For the virgin material, hydrogen had no obvious effect on the fatigue crack growth rate. In contrast, for the torsional prestrained materials, the acceleration ratios, {(d a /d N ) H /(d a /d N ) U }, increased with the torsional prestrain and C H . However, {(d a /d N ) H /(d a /d N ) U } did not exceed the value of about 30, even when a large amount of hydrogen was charged (10.0 ≤ C H ≤ 30.3 mass ppm). A hydrogen content threshold was found; hydrogen content above this limit enhances the growth of the non-propagated crack, even for metals with lower hardness (HV < 200). K EYWORDS . Fatigue crack growth property; Non-propagating crack; Torsional prestrain; Large amounts of hydrogen; Carbon steel.

I NTRODUCTION

ecently, hydrogen energy systems have been proposed as an alternative energy source to solve environmental and energy problems related to the depletion of fossil fuel, global warming, and the nuclear reactor accident caused by the eastern Japan great earthquake of 2011. However, it has been reported that the strength properties of R

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

materials that are directly exposed to high-pressure hydrogen gas are degraded. This problem is known as hydrogen embrittlement. The effects of hydrogen on fatigue crack growth properties within the low and middle stress intensity factor ranges have been investigated. [1, 2] The findings of these studies have provided important insights that can be used to inform the design of hydrogen-related devices for a future hydrogen economy. Hydrogen pre-charging was reported to increase the fatigue crack growth acceleration rate of 0.08 mass%C ferritic–pearlitic carbon steel JIS-SGP, compared with the uncharged specimen [1]. The effect of hydrogen on fatigue crack growth rate was related to the test frequency; however, the enhancement in rate acceleration due to hydrogen reached an approximately 10 times rate, which should be an upper bound on the acceleration of fatigue crack growth by hydrogen. Another study indicated that the upper bound of fatigue crack growth acceleration in JIS-SM490B under a high-pressure hydrogen gas atmosphere is 30 times for hydrogen gas pressures less than 45 MPa [2]. These upper bounds have been used to predict fatigue life for the design of hydrogen-related devices. In addition, hydrogen was reported to have little effect on the fatigue limits of lower- hardness materials (HV ≤ 200), such as carbon steel, whereas for materials with higher hardness (HV ≥ 200), hydrogen decreased the fatigue limit by 25 % [3]. Previous review articles on the effects of internal hydrogen on fatigue properties mainly discussed low hydrogen contents. However, the fatigue data for high hydrogen contents are important for the design of hydrogen-related devices. The effects of larger amounts of hydrogen on the crack growth rate and fatigue limit have not been studied because the hydrogen contents of BCC materials precharged with hydrogen are small. According to recent studies, hydrogen diffuses to and concentrates at fatigue crack tips. Therefore, it is important to clarify the effects of larger amounts of hydrogen during the design stage of a hydrogen-related device. According to a previous study by the authors using prestrained carbon steels, hydrogen content increases with the prestrain [4]. Furthermore, for torsional prestrained JIS-S25C steel (hereafter S25C steel) specimens, it is possible to introduce a larger amount of hydrogen into carbon steel (up to 35 mass ppm) than that into the tensile prestrained material. Therefore, in this study, the effects of large amounts of hydrogen on fatigue crack growth and fatigue limit were investigated using two carbon steels (JIS-S10C and JIS-S25C steels, hereafter S10C and S25C steels). he materials used in this study were ferritic–pearlitic carbon steels S10C and S25C. Tab. 1 shows the chemical compositions and Vickers hardness values (indentation force, 9.8 kN; holding time, 30 s; number of measured points, 20) of the steels. Heat treatment was conducted for 1 h at 920 °C (S10C steel) or 900 °C (S25C steel). The Vickers hardness was HV = 97.8 for S10C steel and HV = 129 for S25C steel. Fig. 1 shows the shapes and dimensions of the rotating bending fatigue test and hydrogen measurement test specimens. Fig. 2 shows the results of torsional prestrain testing. After polishing with #2000 emery paper, all specimen surfaces were finished by buffing. The specimens were then subjected to torsional prestrain testing to yield torsional prestrained specimens. The specific angles of twist (  pre ) ranged from 0 to 45.0 deg/mm for S10C steel and from 0 to 20.0 deg/mm for S25C steel. After testing, the prestrained specimens were finished by rebuffing, and small holes with diameters of 500  m and depths of 500  m were introduced into the rotating bending fatigue test specimens. T M ATERIALS AND EXPERIMENTAL METHODS

C

Si

Mn

P

S

HV 97.8

0.10

0.17

0.36

0.09

0.17

(a) S10C steel

C

Si

Mn

P

S

HV 125

0.22

0.20

0.39

0.010

0.018

(b) S25C steel Table 1 : Chemical compositions (mass %) and Vickers hardness values of the experimental steels.

Rotating bending fatigue tests were conducted at room temperature in air. Cracks were observed using the replica method, and the crack lengths (2 a ) were measured. Hydrogen was charged into the virgin and torsional prestrained specimens of S10C and S25C steels using the cathodic charge method with a platinum electrode at a current density of 100 A/m 2 in an aqueous solution of H 2 SO 4 (pH 2.0) at 313 K for 24 h. Hydrogen content was previously confirmed to be saturated in

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

each specimen. Hydrogen content was measured using gas chromatography thermal desorption analysis (TDA). The rate of temperature increase was 0.028 °C/s. During hydrogen charging, the areas of fatigue cracking and small artificial holes were locally protected to prevent corrosion. After removing the protection, rotating bending fatigue tests were conducted for both specimens (S10C and S25C steels) at test frequencies of f = 1.6–3.2 Hz, corresponding to stress amplitudes of   a = 250 MPa. The fatigue tests at fatigue limit were conducted at f = 30 Hz and  a = 150 MPa for S10C torsional prestrained (  pre = 45.0 deg/mm) steel and at f = 30 Hz and  a = 135 MPa for S25C torsional prestrained (  pre = 20.0 deg/mm) steel. The stress intensity factor was calculated according to the handbook [5].

(a) Rotating bending fatigue test specimen

(b) Hydrogen measurement specimen Figure 1 : The shapes and dimensions of specimens (mm).

Figure 2 : Relationship between torque and specific angle of twist.

R ESULTS AND DISCUSSION

Effect of torsional prestrain on hydrogen entry properties ig. 3 shows the TDA hydrogen release profiles during continuous heating of hydrogen-precharged S10C and S25C specimens. As shown in Fig. 3(a), the hydrogen desorption peaks were located at around 100–150 °C for virgin and prestrained specimens for both S10C and S25C steels. The height of the peak increases with the prestrain due to the increasing density of dislocations, which act as hydrogen trap sites. Hydrogen released from around 20–200 °C is classified as diffusible hydrogen and causes hydrogen embrittlement because hydrogen can diffuse to crack tips or regions of stress concentration [6]. Small peaks corresponding to the so-called non-diffusible hydrogen were observed at temperatures close to 300 °C. F

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

) and equivalent prestrain at the specimen surface (  eq,s

Fig. 4 shows the relation between hydrogen content ( C H

) along with

the specific angle of twist (  pre ). Previous results [4] for S25C steel are also included in Fig. 4. Hydrogen content increased of S10C steel is smaller at the same torsional prestrain due to the difference in the fraction of pearlite. As shown in Fig. 4, S10C steel has a higher ductility than S25C steel; thus, higher prestrain can be introduced into S10C steel. However, no clear difference in the maximum hydrogen contents in torsional fracture specimens between S10C and S25C steels exists. In S25C steel, for  pre ≥ 25.0 deg/mm, some cracks are initiated during the introduction of torsional prestrain. In contrast, for the prestrained S10C steel specimen, few cracks are initiated, even when  pre = 45.0 deg/mm. Therefore, a fatigue test with a hydrogen-precharged specimen containing a large hydrogen content ( C H = 36.7 mass ppm) could be conducted using torsional prestrained (  pre = 45.0 deg/mm) S10C steel. with the prestrain. Compared with that of S25C steel, C H

(a) S10C steel (b) S25C steel [4] Figure 3 : TDA hydrogen release profiles during continuous heating of hydrogen-precharged S10C and S25C specimens.  pre : Specific angle of twist.

Figure 4 : Relationship between hydrogen content and  eq,s .

Effects of a large amount of hydrogen on the fatigue property of carbon steel Fig. 5 shows the relations between the number of cycles to failure ( N f

) and equivalent strain at the specimen surface (  eq,s ).

The uncharged virgin specimen fractured after 3.67 × 10 4 cycles, whereas the fatigue lives of both S10C and S25C steels increased with the torsional prestrain due to work hardening. However, in specimens precharged with hydrogen, fatigue life decreased with increasing prestrain. Fig. 6 shows the relations between crack length (2 a ) and number of cycles ( N ) for S10C and S25C steels. With respect to crack initiation, there is no obvious difference between the uncharged and

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

hydrogen-precharged specimens in spite of the large amount of hydrogen introduced by torsional prestrain. For the virgin specimen, hydrogen does not affect crack growth or initiation. However, for all torsional prestrained specimens, the presence of hydrogen accelerated crack growth; thus, the N f values of torsional prestrained specimens precharged with hydrogen were clearly decreased.

Figure 5 : Relations between number of cycles to failure and  eq,s .

(a) S10C steel

(b) S25C steel

Figure 6 : Relations between crack length and number of cycles.  pre

: Specific angle of twist.

Effects of a large amount of hydrogen on the fatigue crack growth acceleration rate of carbon steel Fig. 7 shows the relation between fatigue crack growth rate (d a /d N ) and stress intensity factor range (  K ) for S10C steel. For the virgin material, no obvious effect of hydrogen on the fatigue crack growth rate was observed. For torsional prestrained materials, the acceleration ratios {(d a /d N ) H /(d a /d N ) U } were estimated, where (d a /d N ) H represents the value of d a /d N for the hydrogen-precharged specimen, and (d a /d N ) U is the d a /d N for the uncharged specimen. For instance, when  pre = 15.0 deg/mm, {(d a /d N ) H /(d a /d N ) U } = 4 at  K = 15 MPa √ m, indicating that the fatigue crack growth rate of the hydrogen-precharged specimen is 4 times higher than that of the uncharged specimen. For  pre = 30.0 deg/mm, {(d a /d N ) H /(d a /d N ) U } = 13 at  K = 15 MPa √ m. These results show that the fatigue crack growth rate of hydrogen- precharged materials increases with the hydrogen content ( C H ). For  pre = 45.0 deg/mm the results were drown by dot lines at positions apart far from other prestrained specimens. {(d a /d N ) H /(d a /d N ) U } for  pre = 45.0 deg/mm was about 16 at  K = 15 MPa √ m, which is almost the same as that obtained for  pre = 30.0 deg/mm. Fig. 8 shows the relation

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

between fatigue crack growth rate (d a /d N ) and stress intensity factor range (  K ) for S25C steel. S25C steel exhibits similar tendencies in the acceleration and saturation of fatigue crack growth rate in the  pre range of 0–20.0 deg/mm. Based on these results, Fig. 9 shows the relation between {(d a /d N ) H /(d a /d N ) U } and C H . The fatigue crack growth acceleration rates of S10C and S25C steels do not increase, even when C H exceeds 10 mass ppm. This indicates that {(d a /d N ) H /(d a /d N ) U } has an upper bound estimated to be in the range of about 30 for both S10C and S25C steels. At this point, the effects of hydrogen precharging on fatigue crack growth rate have been reported in the previous studies. In case of 0.08 mass%C ferritic–pearlitic carbon steel ( C H = 1.0 mass ppm), for f ≤ 0.01 Hz, the fatigue crack growth acceleration rate {(d a /d N ) H /(d a /d N ) U } = 10 [1]. Tanaka et al. [7] reported a {(d a /d N ) H /(d a /d N ) U } upper bound of 30 for Cr–Mo steel, JIS-SCM435, (HV = 330; C H = 0.24–0.59 mass ppm). Previous studies indicate that the maximum upper bound of {(d a /d N ) H /(d a /d N ) U } is about 30 for small hydrogen contents. In this study, even for large hydrogen contents, the maximum upper bound remained at around 30. The upper bound value found in this study can be used as one parameter in the design of hydrogen-related devices. Further investigation is important to consider {(d a /d N ) H /(d a /d N ) U } at lower frequencies.

(a ) Virgin materials

* The dotted line shows 30 times acceleration of fatigue crack growth rate for hydrogen-precharged specimen than the uncharged specimen.

(b) Torsional prestrained materials Figure 7 : Relations between fatigue crack growth rate and stress intensity factor range in S10C steel.  pre

: Specific angle of twist.

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

(a) Virgin materials

* The dotted line shows 30 times acceleration of fatigue crack growth rate for hydrogen-precharged specimen than the uncharged specimen.

(b) Torsional prestrained materials Figure 8 : Relations between fatigue crack growth rate and stress intensity factor range in S25C steel.  pre

: Specific angle of twist.

Figure 9 : Relation between fatigue crack growth acceleration rate in the presence of hydrogen and hydrogen content.  pre : Specific angle of twist.

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

d a /d N defined as the boundary between non- propagating and propagating.

Figure 10 : Relation between fatigue crack growth rate and number of cycles ( f = 30 Hz).  pre

: Specific angle of twist.

Figure 11 : Relation between residual hydrogen content in the specimen and time after the end of hydrogen charge for the torsional prestrained S10C and S25C steel.  pre : Specific angle of twist. Effects of a large amount of hydrogen on the fatigue limit of carbon steel Fig. 10 shows the relation between d a /d N and N , while Fig. 11 shows the relation between the residual hydrogen content (diffusible hydrogen content) in the specimen and time, following the first hydrogen charging of the torsional prestrained (  pre = 45.0 deg/mm) S10C steel and (  pre = 20.0 deg/mm) S25C steel. The results shown in Fig. 11 were obtained by measuring the hydrogen content of the hydrogen measurement specimen (Fig. 1(b)) used for TDA at 30  C and heated to 520  C after 48 h. The hydrogen content values during the fatigue test shown in Fig. 10 were calculated using the results given in Fig. 11. The fatigue test of the torsional prestrained (  pre = 20.0 deg/mm) uncharged S25C specimen was first conducted at  a = 135 MPa. Under this condition, a non-propagating crack was observed at the edge of the artificial small hole, and the fatigue crack growth rates were d a /d N = 2.0 × 10 −11 m/cycle at N = 8–9 × 10 6 cycles and d a /d N = 4.1 × 10 −11 m/cycle at N = 9–10 × 10 6 cycles. Hence, d a /d N = 2.0 × 10 −11 to 4.1 × 10 −11 m/cycle was defined as the threshold d a /d N value between non-propagation and propagation in this paper. After hydrogen charging, the observed d a /d N was greater than 10 −10 m/cycle, indicating crack propagation. Following this propagation, d a /d N then decreased to 3.8 × 10 −11 m/cycle, while the estimated hydrogen content decreased to 7.1  4.2 mass ppm. Then, the same specimen was hydrogen

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

charged for a second time with almost the same amount of hydrogen as in the first charging, as was previously confirmed. At the beginning of the repeat fatigue test, d a /d N was 5.2 × 10 −11 m/cycle, which is slightly higher than the 4.1 × 10 −11 m/cycle threshold value, indicating crack propagation. However d a /d N then decreased to 1.0 × 10 −11 m/cycle and crack propagation was not clearly observed; this was accompanied by a decrease in hydrogen content to less than 4.8 mass ppm. Hence, it was presumed that hydrogen enhances the growth of a non-propagating crack. However, in the case involving hydrogen content of less than 4.8 mass ppm, the non-propagating crack was not caused to propagate by the hydrogen. The explanation for this behavior is that, as the hydrogen was released into the atmosphere during the test, there was insufficient contained hydrogen for enhancement of the fatigue crack growth. At the end of the S25C steel fatigue test, crack propagation occurred at a lower hydrogen content. It is possible that small cracks initiated by the introduction of the torsional prestrain grew in locations other than the main crack site, and all of these cracks then coalesced. For a hydrogen content range of 4.8 to 1.4, the main crack did not propagate at the surface, however it was presumed that the small cracks propagated and coalesced with the main crack at a subsurface. Then, the main crack finally exhibited propagation at a lower hydrogen content, due to coalescence between the small cracks and the main crack. Thus, in the case involving hydrogen content of less than 4.8 mass ppm, the hydrogen itself did not enhance the main crack growth. A similar fatigue test of torsional prestrained (  pre = 45.0 deg/mm) S10C steel was conducted at  a = 150 MPa. A non- propagating crack also occurred at the edge of the artificial small hole in the absence of hydrogen. The fatigue crack growth rates were d a /d N = 4.5 × 10 −11 m/cycle at N = 8–9 × 10 6 cycles and d a /d N = 2.3 × 10 −11 m/cycle at N = 9–10 × 10 6 cycles, which is almost identical to the defined d a /d N threshold for a non-propagating crack for S25C steel. In the S10C steel case, secondary hydrogen charging was not conducted, because the hydrogen content was high (5.8 mass ppm), even after 5 × 10 6 cycles of the fatigue test. At the beginning of the fatigue test of the hydrogen-charged S10C specimen, d a /d N was greater than 10 −10 m/cycle, indicating crack propagation. Fig. 10 shows that d a /d N was decreased by increases in both time and N , while Fig. 12 shows photographs of the crack for N = 1.10 × 10 7 and 1.24 × 10 7 cycles after hydrogen charging of the S10C steel. Crack propagation was only clearly observed in this specimen. In the S10C prestrained specimen case, the hydrogen content was higher than that of the S25C prestrained specimen. However, crack propagation was arrested at a hydrogen content value of 15.6 to 10.7 mass ppm. Hence, 15.6 mass ppm hydrogen is the threshold value beyond which hydrogen enhances the propagation of an arrested crack. It has been reported that the fatigue limit of carbon steel with HV ≤ 200 is not reduced by hydrogen (S25C steel; HV = 129; C H = 0.31 mass ppm) [3]. However, in the presence of higher hydrogen content, it is possible for hydrogen to enhance the propagation of an arrested crack even for these metals, as large amounts of hydrogen decrease the fatigue limit. It seems that the difference in the hydrogen content threshold values for S10C (15.6 mass ppm) and S25C (4.8 mass ppm) steels is due to the densities of the small cracks initiated during the torsional prestraining. Because the pearlite content of S25C steel is greater than that of S10C, a greater number of pearlite cracks were initiated for S25C steel than for S10C. This problem requires more investigation, in order to clarify the effects of the small cracks initiated during torsional prestraining on the non-propagating crack.

(a) N = 1.10 × 10 7 ( C H,R

(b) N = 1.24 × 10 7 ( C H,R = 150 MPa; f = 30 Hz).  pre

= 15.6 mass ppm)

= 10.7 mass ppm)

Figure 12 : Non-propagating crack (S10C;  pre content by the equations in Fig. 11.

= 45.0 deg/mm;  a

: Specific angle of twist. C H,R

: Hydrogen

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Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01

C ONCLUSIONS

T

he effects of hydrogen on the fatigue crack growth rates of torsional prestrained ferritic–pearlitic low-carbon steels (JIS-S10C and JIS-S25C steels) were investigated. The following conclusions were obtained. 1. Hydrogen content increased with the torsional prestrain for both S10C and S25C steels. The hydrogen content of hydrogen-precharged torsional prestrained S10C steel was lower than that of S25C steel at the same torsional prestrain. No clear difference in the maximum hydrogen contents between the hydrogen-precharged torsional fractured specimens of S10C and S25C steels existed. 2. With respect to crack initiation, there was no obvious difference between the uncharged and hydrogen-precharged specimens in spite of the large amount of C H induced by torsional prestrain. The acceleration of fatigue crack growth by hydrogen was the main cause of the decreased fatigue life. 3. For the virgin material, no obvious effect of hydrogen on the fatigue crack growth rate was observed. In contrast, for torsional prestrained materials, the acceleration ratios, {(d a /d N ) H /(d a /d N ) U }, increased with the torsional prestrain and hydrogen content. However, an upper bound of {(d a /d N ) H /(d a /d N ) U } of approximately 30 was observed, even when large amounts of hydrogen were charged (10.0 ≤ C H ≤ 30.3 mass ppm). 4. A hydrogen content threshold was found; hydrogen content above this limit enhances the growth of the non- propagated crack, even for metals with lower hardness (HV < 200).

A CKNOWLEDGMENTS

T

his research has been supported by the NEDO project “Fundamental Research Project on Advanced Hydrogen Science (2006e2012)”.

R EFERENCES

[1] Saburo, M., Noriko, T., Yukitaka, M., Effects of Hydrogen on Fatigue Crack Growth and Stretch Zone of 0.08 mass%C Low Carbon Steel Pipe, Trans. Jpn. Soc. Mech. Eng. A, 74 (2008) 1528–1537. [2] Michio, Y., Takashi, M., Noriko, T., Hisao, M., Saburo, M., Effects of hydrogen gas pressure and test frequency on fatigue crack growth properties of low carbon steel in 0.1–90 MPa, Trans. Jpn. Soc. Mech. Eng. A, 80 (2014) SMM0254–SMM0254. [3] Yoshiyuki, K., Masanobu, K., Keiko, S., Jun-ichiro, Y., Effect of Absorbed Hydrogen on the Near Threshold Fatigue Crack Growth Behavior of Short Crack: Examination on Low Alloy Steel, Carbon Steel and Heat Resistant Alloy A286, Trans. Jpn. Soc. Mech. Eng. A, 74 (2008) 1366–1372. [4] Hiroshi, N., Ryota, K., Takayuki, F., Effects of Hydrogen on Tensile and Torsional Strength Properties of Torsional Prestrained Ferritic-Pearlitic Carbon Steel, Proceedings of International Hydrogen Conference (IHC 2012). [5] Yukitaka, M., Stress Intensity Factors Handbook (In 2 Volumes), Committee on Fracture Mechanics, Pergamon Press, Japan, (1987) 659–662. [6] Takai, K., Watanuki, R., Hydrogen in Trapping States Innocuous to Environmental Degradation of High-strength Steels, ISIJ international, 43 (2003) 520–526. [7] Tanaka, H., Homma, N., Matsuoka, S., Murakami, Y., Effect of Hydrogen and Frequency on Fatigue Behavior of SCM435 Steel for Storage Cylinder of Hydrogen Station ( Strength Problems of Materials Used for Hydrogen Energy System), Trans. Jpn. Soc. Mech. Eng. A, 73 (2007) 1358–1365.

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W. Ozgowicz et alii, Frattura ed Integrità Strutturale, 35 (2016) 11-20; DOI: 10.3221/IGF-ESIS.35.02

Focussed on Crack Paths

Relation between the plastic instability and fracture of tensile tested Cu-Sn alloys investigated with the application of acoustic emission technique

W. Ozgowicz, B.Grzegorczyk Silesian University of Technology, Poland wojciech.ozgowicz@polsl.pl A. Pawełek, W. Wajda, W.Skuza. A.Piątkowski Institute of Metallurgy and Materials Science of Polish Academy of Sciences, Poland a.pawelek@imim.pl Z. Ranachowski Institute of Fundamental Technological Research of the Polish Academy of Sciences, Poland zranach@ippt.pan.pl

A BSTRACT . The work concerns the application of the acoustic emission (AE) method in testing the mechanical properties of continuously cast industrial tin bronze CuSn6P, which reveals tendencies to instable plastic flow connected particularly with the Portevin-Le Chatelier (PLC) effect. The relations between the jerky flow connected with the PLC effect, AE intensity and the evolution of a fracture of the investigated alloy subjected to the tensile test at a strain rate (   ) of about 1.2·10 -3 s -1 in the range of temperatures (20÷400  C) has been analyzed. It has been found that the highest intensity of the oscillation of stresses, corresponding to the instability of plastic deformation PLC occurred at 200  C, whereas the maximum of the AE activity is at about 200÷250  C. The brittle intergranular fracture starts in the range of equicohersive temperature (T E ) of about 200  C. Plastic deformation of the investigated alloy in the range of the temperature of minimum plasticity, amounting to about 400  C, results in intercrystalline fractures on the entire surface of the stretched samples. K EYWORDS . Copper alloy; Portevin-Le Chatelier phenomenon; Tensile test; Acoustic emission; SEM; Intercrystalline fracture. ok. 10 -2 s -1 is a complex process, occurring most often in a heterogeneous way, due to the simultaneous effect of several mechanisms of deformation. The knowledge of these mechanisms and structural processes encountered in the given P I NTRODUCTION lastic deformation of copper alloys, particularly those with a small energy of the stacking fault (SFE), for instance tin bronzes and brasses at elevated temperature (about 0.4 to 0.6 T t ) and a strain rate (   ) amounting to about 10 -5 s -1 ÷

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W. Ozgowicz et alii, Frattura ed Integrità Strutturale, 35 (2016) 11-20; DOI: 10.3221/IGF-ESIS.35.02

conditions of plastic deformation is indispensable for an optimal formation of the structure and properties of the investigated alloys and for programming the technology of the industrial plastic working of the products [1÷3]. Many alloys of iron and nonferrous metals indicate the phenomenon of heterogeneous deformation (Cu, Al, Ti, Ni) at elevated or high temperature in the course of tensile or compression tests in the form of irregularities on the work- hardening curve. This effect of plastic instability is often determined as “jerky flow” or “serration” and in literature it is called Portevin – Le Chatelier effect (PLC) [4, 5]. The characteristic oscillations of the stress on the work – hardening curve in the range of the plastic flow differ in their shape and size, depending mainly on the temperature and strain rate [6]. The PLC effect has been know for many years, although so far it has not been fully explained [7]. This effect is mainly investigated from the viewpoint of material factors, taking into account the microstructural conditions of the initiation of the localized plastic deformation resulting from the formation and propagation of the shearing bands and rheological factors, connected with the mechanics of plastic deformation in various thermodynamic and physico-chemical conditions. Investigations concerning or the PLC effect base both on traditional methods and mechanical tests of uniaxial stretching or compression, and also on modern methods , as for instance the digital correlation of the image or acoustic emission AE [8,9]. The method AE belongs to the group of passive methods, because the apparatus of AE does not emit signals, nor does it affect the physical state of the tested objects. It depends on the detection of automatically occurring effects in the monitored object and analyzes this acoustic signal, resulting from the propagation of elastic waves generated in the mechanically loaded material due to the fast release of the energy accumulated in them. The release of elastic energy is connected with the formation of instantaneous local metastable states caused by various phenomena is a sub- microscoping scale, as for instance the diffusion of atoms in the crystallographic lattice, or in macroscopic scale – the formation of twins of deformation or nucleation of cracking. The shape of the signal AE is influenced by many factors, namely the chemical composition and microstructure of the investigated alloys, the size of the grains, heat treatment, the temperature of the process and the strain rate as well as the state of precipitations and the texture of the material. AE measurements are in comparison with other methods, characterized by a high sensitivity in recording the physical phenomena [10]. The aim of the present paper is to apply methods of acoustic emission in mechanical tests of uni-axial stretching of tin bronze of the type CuSn6P from industrial smelting, indicating distinctly a tendency to instability of plastic flow at elevated temperature of deformation due to the effect PLC. The integral purpose of these investigations is to determine the relation existing between the PLC effect and the AE generation and the phenomena of intercrystalline cracking of the investigated alloy at an elevated temperature of the tensile test.

E XPERIMENTAL PROCEDURE

T

he investigated material was standardized tin bronze CuSn6P provided from industrial smelting, in the form of a rod cast continuously (Wertli’s process) with a diameter of 11.6 mm and the chemical composition presented in Tab. 1.

Denomination of the alloy and kind of analysis

Chemical composition in % of mass

No.

Sn

P

Bi

Pb

Sb

As

S

Fe

Cu

CuSn6P ladle analysis

1.

6.70

0.42

0.010

0.080

0.010

0.025

0.003

0.018

bal.

CuSn6P PN-EN 1982:2008

2.

5.5÷7

0.01÷0.4

-

0.02

-

-

-

0.1

bal

Table 1 : Chemical composition of the bronze used (mass %).

Static tensile tests were carried out at an elevated temperature and a strain rate (   ) amounting to about 1.2·10 -3 s -1 , applying for this purpose the testing machine ZWICK Z 1200 in the range of loading up to 100 kN, making use of the digital recording of the tensile curves. The values of the force was recorded in the entire range of measurements with an accuracy of 0.5 %. The samples were preheated within the range of the temperature of stretching (20÷400  C) in a MAYTEC furnace, recording the temperature with an accuracy of ± 4  C. The temperature chamber permits to run the tests at a temperature from 123 K to 873 K. For testing spherical samples were used 4 mm in diameter and 62 mm long, with threaded heads. The deformation was recorded on a length of measurements amounting to l o = 27 mm. Measurements and AE recording were accomplished in the course of tensile testing, by means of a piezoelectric sensor,

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