Issue 25

Anno VII Numero 25 Luglio 2013

Rivista Internazionale Ufficiale del Gruppo Italiano Frattura Fondata nel 2007

Editor-in-chief:

Francesco Iacoviello

ISSN 1971-8993

Guest Editors: Francesco Iacoviello M. Neil James Pablo Lopez-Crespo Luca Susmel

Associate Editors:

Luca Susmel John Yates

Editorial Advisory Board:

Harm Askes Alberto Carpinteri Andrea Carpinteri

Donato Firrao M. Neil James Gary Marquis

Robert O. Ritchie Darrell F. Socie Cetin Morris Sonsino Ramesh Talreja David Taylor

Frattura ed integrità strutturale The International Journal of the Italian Group of Fracture

www.gruppofrattura.it

Frattura ed Integrità Strutturale, 25 (2013); Rivista Ufficiale del Gruppo Italiano Frattura

Table of Contents

D. Nowell, M.E. Kartal, P.F.P. de Matos Characterisation of crack tip fields under non-uniform fatigue loading ……………………………… 1 G. Qian, C. Zhou, Y. Hong Crack propagation mechanism and life prediction for very-high-cycle fatigue of a structural steel in different environmental medias ………………………………………………………………………... 7 A.S. Chernyatin, Yu.G. Matvienko, I.A. Razumovsky Combining experimental and numerical analysis to estimate stress fields along the surface crack front …... 15 Yu. G. Matvienko, V.S. Pisarev, S. I. Eleonsky Determination of fracture mechanics parameters on a base of local displacement measurements ………… 20 D. A. Hills, R. C. Flicek, D. Dini Sharp Contact Corners, Fretting and Cracks …………………………………………………... 27 A. Shanyavskiy Subsurface metals fatigue cracking without and with crack tip …………………………………….. 36 J. Tong, Y.-W. Lu, B. Lin, Y. H. Tai, J.R. Yates Near tip strain evolution under cyclic loading …………………………………………………… 44 J. Lachambre, J.-Y. Buffiere, J. Réthoré, A. Weck In situ 3D characterization of fatigue cracks displacement fields …………………………………... 50 F.V. Antunes, A.G. Chegini, L.M. Correia, A.L. Ramalho Effect of crack propagation on crack tip fields …………………………………………………... 54 P. Lazzarin, M. Zappalorto, F. Berto Recent developments in multi-parametric three-dimensional stress field representation in plates weakened by cracks and notches …………………………………………………………………………... 61 V. Veselý, J. Sobek, L. Šestáková, P. Frantík, S. Seitl Multi-parameter crack tip stress state description for estimation of fracture process zone extent in silicate composite WST specimens …………………………………………………………………… 69 J. T. P. de Castro, M. A. Meggiolaro Is notch sensitivity a stress analysis problem? …………………………………………………… 79 H. Askes, L. Susmel Gradient enriched linear-elastic crack tip stresses to estimate the static strength of cracked engineering ceramics …………………………………………………………………………………… 87

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Frattura ed Integrità Strutturale, 25 (2013); ISSN 1971-9883

A. Spagnoli, A. Carpinteri, S. Vantadori On a kinked crack model to describe the influence of material microstructure on fatigue crack growth …... 94 F. Iacoviello, V. Di Cocco, A. Rossi, M. Cavallini Pearlitic ductile cast iron: damaging micromechanisms at crack tip …....................................................... 102 F.A. Díaz, E.A. Patterson, J. R. Yates Application of thermoelastic stress analysis for the experimental evaluation of the effective stress intensity factor ….................................................................................................................................................. 109 E. Maggiolini, P. Livieri, R. Tovo On numerical integration for effective stress assessment at notches .............................................................. 117 J. Toribio, V. Kharin Plastic zone evolution near a crack tip and its role in environmentally assisted cracking ............................ 124 J. Toribio, V. Kharin Role of plasticity-induced crack closure in fatigue crack growth .................................................................. 130 P. Lorenzino, A. Navarro Initiation and growth behavior of very-long microstructurally short fatigue cracks ...................................... 138 B. Moreno, P. Lopez-Crespo, J. Zapatero High magnification crack-tip field characterisation under biaxial conditions ............................................. 145 P. Lopez-Crespo, P.J. Withers, J. R. Yates, A. Steuwer, T. Buslaps, Y. H. Tai Study of overload effects in bainitic steel by synchrotron X-ray diffraction .................................................. 153 C. J. Christopher, G. Laboviciute, M. N. James, E. A. Patterson Extension of the CJP model to mixed mode I and mode II ...................................................................... 161

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Frattura ed Integrità Strutturale, 25 (2013); Rivista Ufficiale del Gruppo Italiano Frattura

Editor-in-Chief Francesco Iacoviello

(Università di Cassino e del Lazio Meridionale, Italy)

Associate Editors Luca Susmel

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

John Yates

Guest Editors Francesco Iacoviello

(Università di Cassino e del Lazio Meridionale, Italy )

M. Neil James

(University of Plymouth, UK) (University de Malaga, Spain) (University of Sheffield, UK)

Pablo Lopez-Crespo

Luca Susmel

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

(Helsinki University of Technology, Finland)

Robert O. Ritchie Darrell F. Socie Cetin Morris Sonsino

(University of California, USA)

(University of Illinois at Urbana-Champaign, USA)

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

Ramesh Talreja David Taylor

Journal Review Board Stefano Beretta

(Politecnico di Milano, Italy)

Nicola Bonora Lajos Borbás Francesca Cosmi

(Università di Cassino e del Lazio Meridionale, Italy) (Budapest University Technology and Economics, Hungary)

(Università di Trieste, Italy) (EADS, Munich, Germany)

Claudio Dalle Donne Vittorio Di Cocco Josef Eberhardsteiner Giuseppe Ferro Tommaso Ghidini Mario Guagliano Carmine Maletta Liviu Marsavina

(Università di Cassino e del Lazio Meridionale, Italy)

(IMWS, Wien, Austria)

(Politecnico di Torino, Italy)

(European Space Agency - ESA-ESRIN) (Politecnico di Milano, Italy) (Università della Calabria, Italy) (University of Timisoara, Romania) (University of Porto, Portugal)

Lucas Filipe Martins da Silva

Marco Paggi

(Politecnico di Torino, Italy) (Università di Parma, Italy)

Alessandro Pirondi

Ivatury S. Raju

(NASA Langley Research Center, USA) (Univ. Telematica Guglielmo Marconi )

Giacomo Risitano Roberto Roberti

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

Marco Savoia

Andrea Spagnoli

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Frattura ed Integrità Strutturale, 25 (2013); ISSN 1971-9883

Descrizione e scopi Frattura ed Integrità Strutturale è la rivista ufficiale del Gruppo Italiano Frattura . E’ una rivista open-access pubblicata on-line con periodicità trimestrale (luglio, ottobre, gennaio, aprile). Frattura ed Integrità Strutturale riguarda l’ampio settore dell’integrità strutturale, basato sulla meccanica della fatica e della frattura, per la valutazione dell’affidabilità e dell’efficacia di componenti strutturali. Scopo della rivista è la promozione di lavori e ricerche sui fenomeni di frattura, nonché lo sviluppo di nuovi materiali e di nuovi standard per la valutazione dell’integrità strutturale. La rivista ha un carattere interdisciplinare e accetta contributi da ingegneri, metallurgisti, scienziati dei materiali, fisici, chimici e matematici. Contributi Frattura ed Integrità Strutturale si prefigge la rapida disseminazione di contributi originali di natura analitica, numerica e/o sperimentale riguardanti la meccanica della frattura e l’integrità strutturale. Si accettano lavori di ricerca che contribuiscano a migliorare la conoscenza del comportamento a frattura di materiali convenzionali ed innovativi. Note tecniche, lettere brevi e recensioni possono essere anche accettati in base alla loro qualità. L’ Editorial Advisory Board sollecita anche la pubblicazione di numeri speciali contenenti articoli estesi presentati in occasione di conferenze e simposia tematici. Istruzioni per l’invio dei manoscritti I manoscritti devono essere scritti in formato word senza necessità di utilizzare un particolare stile e devono essere inviati all'indirizzo iacoviello@unicas.it. Il lavoro proposto può essere in lingua Italiana (con riassunto in inglese di almeno 1000 parole e didascalie bilingue) o Inglese. La conferma della ricezione avverrà entro 48 ore. Il processo di referaggio e pubblicazione on-line si concluderà entro tre mesi dal primo invio. 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. The paper may be written in English or Italian (with an English 1000 words abstract). 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.

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

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Frattura ed Integrità Strutturale, 25 (2013); Rivista Ufficiale del Gruppo Italiano Frattura

Characterisation of Crack Tip Stress Field

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ingle parameter characterisation of the crack/notch tip field using fracture mechanics parameters like K, J or CTOD has been extremely powerful in advancing predictive technologies for critical or sub-critical crack growth. It has also become clear over the last 40 years that single parameter approaches have limitations particularly in dealing with crack growth phenomena arising from crack tip shielding, often resulting from the plastic enclave surrounding a crack. Influences of this enclave on the crack tip stress field ahead of the crack are maximised during cyclic loading. In the case of a parameter like stress intensity factor, K, which characterises the crack tip field via an elastic approximation, it is not surprising that any set of plasticity-induced circumstances which perturb the size of the plastic enclave and its associated strain field lead to predictive difficulties. Over the last 30 years, notable areas of activity related to such difficulties include short cracks, plasticity-induced closure, variable amplitude and multiaxial loading and notch effects. Thus an increasing number of authors and research groups, particularly in Europe, are working on the topic of characterisation of crack tip stresses using more than one fracture mechanics parameter. Attention has been directed, for example, towards incorporating the T-stress into life prediction methods. The T-stress is the second term in a Williams type expansion of the crack tip stresses and it affects the extent and shape of crack tip plasticity. It would therefore be expected to be influential in plasticity-related crack growth phenomena and a number of publications have demonstrated this to be true. The situation is further complicated where a crack experiences multiaxial loading and Modes II and III fracture mechanics parameters are also necessary. Other research groups have focussed attention on incorporating additional elastic fracture mechanics parameters into crack/notch tip characterisation, which describe the effects of an Eshelby-type ‘plastic inclusion’ on an elastic stress field. The first highly successful workshop on this topic was held in Forni di Sopra, Udine, Italy in March 2011 and the proceedings were published as a joint-Special Issue of IJFatigue and FFEMS.

Francesco Iacoviello (Università di Cassino e del Lazio Meridionale, Italy) M. Neil James (University of Plymouth, United Kingdom) Pablo Lopez-Crespo (University de Malaga, Spain) Luca Susmel (University of Sheffield, United Kingdom)

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Frattura ed Integrità Strutturale, 25 (2013); Notiziario

Notiziario n. 24 - Luglio 2013

I N QUESTO NUMERO 1) Editoriale del Presidente IGF; 2) Verbale dell’Assemblea ordinaria dei Soci 2012; 3) Resoconto delle attività IGF 2012-13; 4) Prossime attività IGF; 5) Servizi IGF disponibili nel sito www.gruppofrattura.it; 6) ESIS Procedures and Documents (www.esisweb.org); 7) Modulo di iscrizione IGF-ESIS.

E DITORIALE DEL P RESIDENTE

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arissimi, in questi ultimi nove mesi, tanti ne sono passati dall’ultimo notiziario, l’IGF ha continuato a tentare di offrire un servizio ai suoi Soci ed a tutta la comunità scientifica la cui attività è legata alla frattura ed all’integrità strutturale. Come ormai tradizione, cardine della nostra attività è il web, non solo attraverso la pubblicazione del proprio sito istituzionale, giunto ormai al suo settimo anno di vita, ma utilizzando il più possibile gli strumenti di divulgazione che la rete rende disponibili, primi fra tutti Google Books e iTunesU. Eccovi quindi qualche numero relativo all’utilizzo di questi due servizi: - Google Books: Nel 2012 sono state visualizzate oltre 180000 pagine, per una media di oltre 15000 pagine al mese. Nei primi cinque mesi del 2013 abbiamo costantemente superato le 28000 pagine visualizzate al mese!!! - iTunesU: circa 500 visioni al mese nei primi 4 mesi del 2013 (download e streaming) delle videoregistrazioni IGF. Si tratta di numeri assolutamente importanti che testimoniano l’interesse che le iniziative IGF riscuotono presso la comunità scientifica. Pe quanto riguarda la rivista IGF, gli ultimi numeri hanno visto la presenza di alcuni temi prevalenti: - il numero 23, Gennaio 2013, dedicato al Workshop The italian research on smart materials and MEMS , - il numero 24, Aprile 2013, dedicato alla Russian Fracture Mechanics School ; - il numero 25, Luglio 2013, dedicato al Workshop internazionale Characterisation of Crack Tip Stress Field , organizzato a Malaga dall’IGF in collaborazione con il Grupo Espanol de Fractura . Tali numeri tematici sono stati particolarmente apprezzati e vi ricordo che l’IGF e la sua rivista sono strumenti di divulgazione disponibili per tutta la comunità scientifica… se avete intenzione di organizzare una special issue tematica con noi, contattateci e sottoponeteci la vostra idea!! È importante sottolineare che la pubblicazione dei numeri tematici

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Frattura ed Integrità Strutturale, 25 (2013); Notiziario

non interferisce con il normale processo di pubblicazione: desiderando mantenere la cadenza trimestrale, i lavori sottoposti al normale processo di sottomissione sono normalmente pubblicati nei numeri sopra detti, riuscendo così a rispettare una delle principali peculiarità della rivista IGF, ovvero la possibilità di pubblicare il proprio lavoro in tempi estremamente rapidi (entro tre mesi dal primo invio). A proposito dell’apprezzamento di Frattura ed Integrità Strutturale , il numero degli accessi alla rivista è veramente importante, intorno ai 1000 articoli scaricati al mese, considerando esclusivamente il sito istituzionale. Se consideriamo poi che la rivista è anche disponibile in Google Books con gli Annals ed è direttamente disponibile in numerosi repository, la diffusione di Frattura ed Integrità Strutturale è veramente notevole. Per quanto riguarda le attività più “tradizionali”, dall’ultima Assemblea dei Soci, l’IGF ha organizzato: - una giornata di studio dal titolo Virtual testing of materials and structures (Torino, 8 ottobre 2012) - il Second IJFatigue & FFEMS Joint Workshop Characterisation of Crack Tip Stress Field (Malaga, Spagna, 15-17 aprile 2013). Nel 2013 sono state inoltre pubblicate due special issue (Int. J. of Fatigue e FFEMS) collegate con il First IJFatigue & FFEMS Joint Workshop svoltosi a Forni di Sopra nel 2011. Infine, qualche parola sul Consiglio di Presidenza “uscente”. Vi confermo le parole con cui ho concluso l’editoriale dello scorso anno: senza il continuo e costante supporto dell’intero Consiglio di Presidenza, i risultati appena elencati non sarebbero stati possibili. Probabilmente dalla lettura dei verbali delle riunioni del Consiglio di Presidenza il livello di collaborazione raggiunto non traspare. Questo probabilmente perché, per poterci incontrare, abbiamo sempre più utilizzato le nuove tecnologie ( Skypè primo fra tutti) e le comunicazioni fra i membri del Consiglio sono diventate notevolmente più frequenti. Ci sono alcuni membri del Consiglio, il nostro Tesoriere Angelo Finelli ed il nostro Segretario Luca Susmel anzitutto, con cui i contatti sono letteralmente quotidiani, ed i risultati si sono visti nell’organizzazione di eventi particolarmente complessi quale, ad esempio, il Workshop di Malaga. Come la volta scorsa, desidero ringraziare l’intero Consiglio in rigoroso ordine alfabetico: Beppe Ferro, Angelo Finelli, Donato Firrao, Carmine Maletta, Marco Paggi, Giacomo Risitano, Andrea Spagnoli, Luca Susmel. GRAZIE del vostro impegno e del vostro tempo!!! Probabilmente una menzione speciale la meritano Angelo e Luca. Angelo è un continuo “richiamo alla realtà” e ci fa sempre ricordare quali sono i problemi reali: semplicemente senza il suo impegno l’IGF non esisterebbe. Luca è per me ormai un “compagno di merende” con cui è veramente divertente inventare cose sempre nuove: per utilizzare la sua lingua d’adozione, i brain storming con lui via web sono sempre divertenti e costruttivi… aggettivi solitamente difficili da coniugare!! Tanti cari saluti, I L P RESIDENTE IGF F RANCESCO I ACOVIELLO

V ERBALI C ONSIGLIO DI P RESIDENZA

CdP del 5/11/2012 (on line) La riunione ha inizio il 5 novembre 2012 alle ore 10.00 via Skypé, con il seguente ordine del giorno: 1) Comunicazioni 2) Rivista IGF 3) Sito IGF 4) Rapporti con associazioni estere 5) Organizzazione IGFXXII, Roma 2013

6) Altre attività 2013 7) Varie ed eventuali Risultano presenti i seguenti consiglieri: Angelo Finelli (Tesoriere), Giuseppe Ferro, Donato Firrao, Francesco Iacoviello (Presidente), Carmine Maletta, Marco Paggi, Giacomo Risitano, Andrea Spagnoli, Luca Susmel (Segretario).

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Frattura ed Integrità Strutturale, 25 (2013); Notiziario

1) Comunicazioni Il Presidente comunica che l’IGF è stato probabilmente vittima di un probabile “furto di identità”. Un tal prof Igor Gruchetsky (nel 2008!!!) ha firmato con la EBSCO Publishing un accordo a nome dell’IGF, autoproclamandosi Editor in Chief della rivista IGF e attribuendosi le eventuali royalties!! L’eventuale danno economico non è al momento quantificabile ed il problema è in corso di chiarimento con la EBSCO Publishing. Si spera di utilizzare questo “incidente” per convertire l’ “accordo Gruchetsky” in una forma per noi valida! Fortunatamente l’accordo stipulato non è di esclusività e non ci crea nessun problema. 2) Rivista IGF Sono in cantiere tre differenti special issue: - Numero “russo” (seguito da Giacomo). Il Presidente infomra il Consiglio che sono arrivati solo tre lavori. Giacomo si occupa di sollecitare i propri contatti russi per incrementare il numero di lavori (probabilmente da pubblicare ad aprile 2013) - Numero The italian research on smart materials and MEMS (seguito da Carmine). Carmine conferma l’arrivo dei lavori in tempo per il numero di gennaio 2013. - Crack Tip Stress Field II (seguito da Luca). Nel 2013 avrà luogo la seconda edizione a Malaga (aprile). Considerato il livello degli interventi, i lavori saranno pubblicati nel numero della rivista per luglio 2013 3) Sito IGF: Grazie all’impegno di Andrea, stiamo raccogliendo gli atti dei Convegni ICMFF (International Conference on Multiaxial Fatigue & Fracture, di cui abbiamo già on line il numero 9, svoltosi a Parma nel 2010). In alcuni casi si devono risolvere i problemi di copyright. 4) Rapporti con associazioni estere: Il Presidente informa il Consiglio che è stato firmato l’addendum all’accordo con ICF riguardante gli atti dei Convegni IGF Il Presidente informa il Consiglio che è stata terminata la nuova digitalizzazione di volumi ECF trovati in originale da James Marrow (Segretario ESIS). La pubblicazione on line sarà a carico dell’ESIS. 5) Organizzazione IGFXXII, Roma 2013: Sede: Sapienza, Facoltà di Ingegneria. Costo: ad offerta (duemila euro dovrebbero bastare) Docente invitato. Harm Askes (University of Sheffield). Su proposta del Presidente e del Segretario, il Consiglio approva all’unanimità l’attribuzione del titolo di Socio onorario ad Harm Askes. Sono da prevedere 4 coffee break, un pranzo a buffet, 1 cena sociale. Atti su penne ed on line (già durante convegno) Il Presidente, ricordando le date limite (invio abstract 10/03/2013; invio lavori 31/05/2013 ), sollecita tutti i membri del Consiglio a contribuire alla pubblicizzazione dell’evento. 6) Altre attività 2013 - Il Presidente informa il Consiglio della possibilità di organizzare una Scuola di Dottorato con l’AIAS (data presumibile, inverno 2013). Dopo ampia ed approfondita discussione il Consiglio, alla luce dei molteplici impegni già previsti, preferisce rimandare l’attivazione di questa collaborazione con l’AIAS. L’IGF pubblicizzerà l’iniziativa che attiverà l’AIAS attraverso i suoi canali. - Organizzazione evento Forni 2013. Sollecitato da numerosi soci, il Consiglio, dopo ampia ed approfondita discussione, decide di soprassedere per il 2013 all’organizzazione dell’ormai tradizionale Workshop di Forni, riservandosi di analizzare l’organizzazione dell’evento per il 2014. - XI edizione Convegno Bioingegneria “Aspetti clinico-fisici ed ingegneristici applicati alle Scienze della vita” che si terrà il 5 luglio 2013 presso l'Aula Magna della Facoltà di Ingegneria dell'Università di Messina. L’evento si svolge quasi contemporaneamente al Convegno Nazionale IGF. In ogni caso, Giacomo, Luca, Francesco assicurano la massima collaborazione possibile all’evento. 7) Varie ed eventuali Non ci sono varie ed eventuali. Il Presidente termina la riunione alle ore 12.00.

R ESOCONTO DELLE ATTIVITA ’ IGF 2012-2013

Convegni e Workshop - giornata di studio dal titolo Virtual testing of materials and structures (Torino, 8 ottobre 2012) - Second IJFatigue & FFEMS Joint Workshop Characterisation of Crack Tip Stress Field (Malaga, Spagna, 15-17 aprile 2013).

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Frattura ed Integrità Strutturale, 25 (2013); Notiziario

Sito IGF - Continua la ristrutturazione sito IGF. Rivista IGF “Frattura ed Integrità Strutturale”

La rivista è arrivata al numero 25 (luglio 2013), ed è disponibile on line secondo due differenti formati: singolo pdf e versione sfogliabile. Inoltre la rivista è stata indicizzata Scopus a partire dal numero di Gennaio 2012 (numero 19).

P ROSSIME ATTIVITÀ IGF

Minisimposio IGF in XXI Congresso Associazione Italiana di Meccanica teorica ed applicata (AIMETA) Torino, 17-20 settembre 2013.

ECF21 Ex Fractura Unitas Catania, 19-24 giugno 2016.

S ERVIZI IGF DISPONIBILI NEL SITO WWW . GRUPPOFRATTURA . IT

APP IGF Da aggiungere al vostro browser (IE, FF, Chrome) http://apps.conduit.com/IGF_Gruppo_Italiano_Frattura-IGF?appid=c9eb6320-1cca-4472-a477-1eaedf7fd956&lang=it "IGF Persona" Personalizzazione Firefox http://www.getpersonas.com/en-US/persona/178855 IGF Toolbar Per IE, FF e Safari http://apps.conduit.com/IGF_Gruppo_Italiano_Frattura-IGF_toolbar?appid=f38b5860-5625-411e-8bdf 8c07880840d6&lang=it IGF Calendar Inserendo l’indirizzo gruppofrattura@gmail.com nella sezione "altri Calendari" del proprio Google Calendar è anche possibile scegliere le notifiche preferite! Sono inserite anche tutte le deadline importanti (invio abstract, invio paper etc ..). E’ possibile controllare il sito dell'evento utilizzando il collegamento con Google map! IGF su Facebook http://www.facebook.com/GruppoItalianoFrattura IGF su iMechanica http://imechanica.org/node/6970 IGF su iTunesU http://itunes.apple.com/it/institution/gruppo-italiano-frattura-igf/id531986833 IGF su Google Books http://books.google.com/books?uid=116456810689061243520&as_coll=1001&hl=it&source=gbs_lp_bookshelf_list

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Frattura ed Integrità Strutturale, 25 (2013); Notiziario

IGF su Research Gate http://www.researchgate.net/topic/Gruppo_Italiano_Frattura-Italian_Group_of_Fracture_IGF/ IGF Channel

Il palinsesto è costituito dagli eventi registrati dall’IGF dal 2007 in poi, dai video e dalle animazioni inviate da chiunque desideri contribuire e da video pubblici di interesse IGF (ad esempio prelevati da You Tube). Il canale è disponibile nella home page del sito IGF, nella pagina Livestream dedicata e nella pagina FB dell’IGF. http://www.livestream.com/igfchannel IGF Tube Spazio di condivisione video ed animazioni su Youtube. http://www.youtube.com/user/IGFTube WebTV on demand Sezione dedicata nel sito IGF Rivista IGF Frattura ed Integrità Strutturale La rivista IGF è gratuita per autori e lettori, open access, peer- reviewed, prestigioso Editorial Advisory Board, pubblicazione dei lavori in tre mesi dal primo invio. I lavori pubblicati hanno assegnato il loro DOI e sono automaticamente indicizzati in numerosi motori (ISSN 1971-8993; Reg. Trib. di Cassino n. 729/07, 30/07/2007). I lavori sono disponibili come singolo pdf, come portfolio pdf ed in formato sfogliabile. Motore di ricerca IGF Il motore è indicizzato in base a titolo del lavoro ed agli autori, indicizza tutti i lavori pubblicati nel sito IGF, sia nella sezione pubblica che in quella riservata. IGF mobile Web app IGF, consente la navigazione del sito IGF anche su smartphone (ottimizzato per iphone). http://www.gruppofrattura.it/iphone/ Archivio IGF (sezione pubblica) - Rivista IGF Frattura ed Integrità Strutturale ; - Atti convegni e giornate IGF; - Tesi di dottorato; - Atti convegno 11 th International Conference on Fracture ( ICF XI , Torino 2005); - Rivista AIM La Metallurgia Italiana , dal 2000; - Rivista Teksid Metallurgical Science and Technology , dalla fondazione. Archivio IGF (sezione riservata ai soci) - Atti convegni International Congress on Fracture (ICF) ; - Fatigue Crack Path ( FCP2003 , Parma); - Crack Path ( CP 2006 , Parma); - Crack Path ( CP 2009 , Vicenza); - Crack Path ( CP 2012 , Gaeta); - Ninth International Conference on Multiaxial Fatigue & Fracture ( ICMFF9 2010 , Parma); - Atti eventi ESIS .

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Frattura ed Integrità Strutturale, 25 (2013); Notiziario

ESIS Procedures and Documents (www.esisweb.org)

Two kinds of documents are produced by ESIS Technical Committees with the following designatory system: ESIS P2-92 or ESIS P4-92D, where: 1) P means "Procedure", and 2 and 4 are the current numbers, while 92 is the year of issue.

2) D following the year (eg: 92D) means "draft", ie: not yet approved, while 3) D prior to the year (eg: D1-92) means "Document" other than test methods.

P1-92 ESIS RECOMMENDATIONS FOR DETERMINING THE FRACTURE RESISTANCE OF DUCTILE MATERIALS. Responsible body: TC1 Subcommittee on Fracture Mechanics Testing Standards.

P2-92 ESIS PROCEDURE FOR DETERMINING THE FRACTURE BEHAVIOUR OF MATERIALS. Responsible body: TC1 Subcommittee on Fracture Mechanics Testing Standards.

P3-03D DRAFT UNIFIED PROCEDURE FOR DETERMINING THE FRACTURE BEHAVIOUR OF MATERIAL. Responsible body: TC1 Subcommittee on Fracture Mechanics Testing Standards (UNDER PREPARATION NOT AVAILABLE). P4-92D ESIS RECOMMENDATIONS FOR STRESS CORROSION TESTING USING PRE-CRACKED SPECIMENS. Responsible body: TC10 Committee on Environmental-Assisted Cracking. P5-00/VAMAS PROCEDURE FOR DETERMINING THE FRACTURE TOUGHNESS OF CERAMICS USING THE SEVNB METHOD . Responsible body: TC6 Committee on Ceramics. P6-98 ESIS PROCEDURE TO MEASURE AND CALCULATE MATERIAL PARAMETERS FOR THE LOCAL APPROACH TO FRACTURE USING NOTCHED TENSILE SPECIMENS. Responsible body: TC8 Committee on Numerical Methods.

P7-00 ESIS PROCEDURE FOR DYNAMIC TENSILE TESTS Responsible body: TC5 Subcommittee on Dynamic Testing at Intermediate Strain rates.

P8-99D ESIS DRAFT CODE OF PRACTICE FOR THE DETERMINATION AND INTERPRETATION OF CYCLIC STRESS-STRAIN DATA. Responsible body: TC11 Committee on High Temperature Mechanical Testing.

P9-02D GUIDANCE ON LOCAL APPROACH OF RUPTURE OF METALLIC MATERIALS. (UNDER PREPARATION NOT AVAILABLE).

P10-02 A CODE OF PRACTICE FOR CONDUCTING NOTCHED BAR CREEP RUPTURE TESTS AND INTERPRETING THE DATA. Responsible body: TC11 High Temperature Mechanical Testing Committee. P11-02 TECHNICAL RECOMMANDATIONS FOR THE EXTREME VALUE ANALYSIS OF DATA ON LARGE NONMETALLIC INCLUSIONS Responsible body: TC20 Committee on Inclusions. D1-92 FRACTURE CONTROL GUIDELINES FOR STRESS CORROSION CRACKING OF HIGH STRENGTH ALLOYS. Responsible body: TC10 Committee on Environmental Assisted Cracking. D2-99 FRACTURE TOUGHNESS OF CERAMICS USING THE SEVNB METHOD; ROUND ROBIN, TEST PROGRAMME. The ESIS TC6 and VAMAS TWA3 developed a test method and conducted a round robin for its validation. D2-99 presents a detailed documentation of this activity. The final form of the test method has appeared as P5-00. Responsible body: TC6 Committee on Ceramics.

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D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6; DOI: 10.3221/IGF-ESIS.25.01

Special Issue: Characterization of Crack Tip Stress Field

Characterisation of crack tip fields under non-uniform fatigue loading

D. Nowell, M.E. Kartal University of Oxford, Department of Engineering Science, Parks Road, Oxford, OX1 3PJ, UK. P.F.P. de Matos U3Is, Unidade de Investigação e Internacionalização do ISVOUGA, Santa Maria da Feira, Portugal.

A BSTRACT . The paper analyses previously reported work, which uses digital image correlation to measure fatigue crack closure. As well as determining crack opening loads, the information on crack shape may be used to estimate the stress intensity factor, as well as other parameters in more complex models of crack tip fields. A number of specimens were subjected to single overload cycles, which produced a significant retardation in crack growth rate. The method previously applied to the analysis of constant amplitude loading is here used to analyse the single overload case. The stress intensity factor history is found to be very different in the two cases and the consequences of this observation for analysis of fatigue crack propagation are discussed. K EYWORDS . Digital image correlation; Displacement measurement; Crack-tip fields; Elastic-plastic fatigue crack.

I NTRODUCTION

T

he study of crack tip stress, strain, and displacement fields has a long history. One hundred years ago, Inglis recognized that a sharp elastic crack would exhibit a stress (and strain) singularity [1]. Twelve years later Westergaard [2] produced a stress function solution for a crack in a biaxial stress field which formed the basis for the later concept of the stress intensity factor. An excellent compilation of seminal papers in the field has been provided by Sanford [3,4]. The stress intensity factor has proved to be an extremely useful concept for the assessment of cracks and it has been widely used in industry. Paul Paris’ extension of the concept to its use in fatigue crack propagation [5] is particularly worthy of mention. However, it is clear that the stress intensity approach has many limitations. In particular, all fatigue cracks must exhibit some irreversibility at or close to the crack tip, otherwise they would not propagate. The stress intensity approach attempts to characterize this by a similitude argument, which depends on the assumption of small scale yielding (i.e. that the process zone of the crack tip is contained within a wider ‘K-dominant’ region ) . Use of the stress intensity approach has particular limitations for the case of non-uniform loading, since load history effects are difficult to capture. Hence, there has been increasing interest in improved characterization of crack tip fields. In parallel, new experimental techniques, such as digital image or volume correlation (DIC/DVC); X-ray tomography; and large scale synchrotron facilities have provide tools which allow various models of crack tip behavior to be tested. A significant number of papers were presented at the First I.J. Fatigue & FFEMS Joint Workshop on the Characterisation of Crack Tip Stress Fields held in Forni di Sopra in 2011. This resulted in the publication of two special issues of the journals [6, 7], but the workshop showcased only a small proportion of the work in the field. On the experimental side, other recent papers of note include tomographic studies of cracked specimens under live loading [8] and in-situ SEM

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D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6; DOI: 10.3221/IGF-ESIS.25.01

studies of deformations in the crack tip region. The latter work has led to a suggested life prediction approach which obviates the need to characterize the number of cycles experienced by the crack [9]. Despite the existence of rainflow and similar approaches, cycle counting can be a difficulty in non-uniform loading situations. The experimental work reported in the current paper springs from an investigation of crack closure originally carried out by de Matos [10]. However, the data was later re-processed to compare with a two-parameter crack tip field model proposed by Pommier and co-workers [11]. This work was reported at the Forni di Sopra workshop [12] and in the subsequent special issue [13]. However, the majority of the work reported was concerned with constant amplitude loading. A few experiments on simple overload cycles were carried out and the results of one of these experiments were presented in [13]. The displacement fields were converted into the two parameter description ( K,  ) and provided an interesting contrast to the results obtained from constant amplitude loading. The current paper therefore sets out to explore these results in more detail and to analyse a wider range of variable amplitude experiments. E XPERIMENTAL WORK he fatigue experiments carried out by de Matos have been reported elsewhere, but a brief summary is given here for clarity. Fig. 1 shows the main experimental set-up, which employs a CT specimen manufactured from 6082 T6 aluminium alloy. The specimen is loaded in fatigue and the area along the crack flanks is monitored using a Questar long range microscope and a digital camera (a low-cost USB webcam). A second camera is used to measure the crack length on the opposite side of the specimen, but this does not provide sufficient resolution for displacement measurement. The use of the microscope means that only a small area (approx. 600 x 400  m) close to the crack tip is imaged, but this permits high resolution and high accuracy evaluation of displacements. It should be noted that, because the primary focus of the investigation was crack closure, most of the image is along the crack wake, and there is very little information collected from the region ahead of the crack tip. T

Figure 1 : Experimental configuration: side and top views.

Images were collected during fatigue loading at 30fps, whilst the specimen was loaded in a servo-hydraulic test machine at a frequency of 0.25 Hz. A set of 360 images was collected over four loading cycles at a number of discrete points during crack propagation. The loading frequency was increased between these measurement steps in order to propagate the crack in a reasonable time interval. Each set of images was analysed using a public domain Matlab script [14]. Rather than analyse a full displacement field, the relative displacement of five pairs of points along the crack flank was determined (Fig. 2). This information proved useful in investigating crack closure directly, but may also be employed to determine crack tip loading history. The elastic model for a sharp crack predicts that the displacement along the crack faces is given by:

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D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6; DOI: 10.3221/IGF-ESIS.25.01

K r

4

 

u r

( )

(1)

I

i



E

2

so that a plot of log(relative displacement) against log(r) may be used to estimate the stress intensity factor at any particular time during the crack loading history.

Figure 2 : Typical image showing the crack tip, together with the five pairs of measurement points.

R ESULTS W

hilst the main experimental programme concerned the investigation of behaviour under constant amplitude loading, a small number of specimens were subjected to a single overload in an otherwise constant amplitude load history. Analysis of this situation represents a useful first step towards a full theory which can be applied in any general loading case. Fig. 3a shows the schematic loading history, whereas Fig. 3b shows crack growth rate data (plotted against nominal  K) for a number of 3mm thick specimens, including two (CTF6 and CTF8) which were subjected to an overload. The resulting retardation in crack growth rate can clearly be seen.

(a)

(b)

Load

Time

Figure 3 : (a) Schematic load history for single overload specimen; (b) Variation of crack growth rate with nominal elastic  K for 3mm thick specimens, including two with single overloads.

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D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6; DOI: 10.3221/IGF-ESIS.25.01

The experimental data is processed as described above and used to determine the stress intensity factor history for the crack. In the case of specimen CTF6, the variation of K with load, just before the overload, is shown in Fig. 4(a). It can be seen that the behaviour is quite similar to that predicted by an elastic analysis of the specimen geometry. However, there is an offset between the theoretical and experimental lines, caused by crack closure. Alternatively, this phenomenon may be thought of as a residual (negative) K caused by the residual stress (and displacement) field present. The two lines have similar slope since, once the crack is fully open at a load of approximately 0.5 kN, the behaviour is predominantly elastic, with only a small zone of cyclic plasticity close to the crack tip.

(a)

(b)

Figure 4 : Variation of measured stress intensity factor with load for specimen CTF6 (a) Before the overload and (b) Immediately after the overload. Fig. 4b shows equivalent data just after the overload. A cursory inspection suggests that the behaviour appears as expected. The slope of the curve remains approximately parallel to the elastic line, and the offset has increased (perhaps as a result of the increased magnitude of residual stress resulting from the overload). However, closer inspection reveals some unexpected features. The analysis yields negative stress intensity factors, which we would not expect to be physically admissible on their own. This must mean that the relative displacement between the crack flanks is varying as -  r , where  is a positive constant. Further, this would imply that the stress field at the tip of the crack is compressive and reasonably well described by the singular Westergaard solution. One possible explanation for these observations is that the crack is being held open by the additional crack opening displacement caused by the overload. Investigation of the crack profile broadly confirms this view. As the crack propagates after the overload, the behaviour gradually returns to that which exists previously. Fig. 5 shows a similar plot of load against K taken 23,500 cycles after the overload. The negative offset is substantially reduced, although negative stress intensity factor values still exist. Moreover it is apparent that the variation is no longer linear over the range of load plotted. The deviation from linearity at lower load values is almost certainly caused by the onset of closure, which was absent immediately after the overload. The discussion above suggests that a single parameter, K , may be insufficient to fully characterize the crack tip environment, particularly under non-uniform loading conditions. An improvement can almost certainly be obtained by explicitly acknowledging the role of crack tip plasticity. A simple model has been proposed by Pommier and Hamam [11] which relies on partitioning the displacement field into elastic and plastic components. The elastic component, u el , is the usual field associated with K, whereas the plastic field, u pl , is associated with crack tip opening. To a first approximation it may be taken as that given by a unit dislocation located at the crack tip. Hence

I el pl u K u u   

(2)

As far as the relative displacements between the crack faces are concerned, this leads to

K r

8 ( )







u r

(3)

I

y



E

2

4

D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6; DOI: 10.3221/IGF-ESIS.25.01

Hence, a plot of u y /(E√(2  )) and an intercept of  , and values of K and  may be obtained from the experimental results in a similar manner to the purely elastic case. Fig. 6 shows the variation of K with  for the cycle soon after the overload (i.e. corresponding to the elastic analysis shown in Fig. 5b). This shows evidence of the hysteresis in of K /  space, which Pommier and Hamam suggest is associated with fatigue crack propagation. According to their hypothesis, absence of a cyclic variation in  corresponds to loading beneath the threshold value of  K. against √r should give a straight line with a gradient of 8K I

Figure 5 : Variation of measured stress intensity factor with load for specimen CTF6, 23,500 cycles after the overload.

Figure 6 : Variation of K with  when the elastic/plastic approach is used to analyse a cycle after the overload.

C ONCLUSIONS

he results presented here have demonstrated that displacement information collected in the crack wake can be analysed using digital image correlation in order to give reliable values of the stress intensity factor. For the case of constant amplitude loading, the results are broadly as expected and show a linear relationship of K with load, suggesting that the behaviour is predominantly elastic. There is, however, an offset to the experimental curve, compared with the linear elastic model, which can be attributed to the existence of crack closure. The situation when a non-uniform load history is examined appears more complex. Here, a simple case of a single overload in an otherwise constant amplitude loading has been examined. Even this relatively simple load history gives rise to some surprising results. For some time after the overload, negative stress intensity factors are measured when using a purely elastic model. Normally it is only possible to obtain positive values, as the crack closes and allows transmission of compressive forces across the T

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D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6; DOI: 10.3221/IGF-ESIS.25.01

crack faces. The values measured here can only be possible when a significant crack tip opening displacement is also present. This suggests that a more sophisticated description of crack tip fields is necessary, perhaps including elastic and plastic terms as suggested by Pommier and Hammam [11]. Future work will include the comparison of the measured results with strip yield and finite element models. Further experiments are also planned, which will look at the region ahead of the crack tip as well as along the crack flanks. This should allow correlation of the results obtained in the crack wake with conditions in and around the process zone itself.

A CKNOWLEDGEMENT

P

.F.P. de Matos would like to gratefully acknowledge the support of the Portuguese Fundação para a Ciência e a Tecnologia (FCT) for providing a D.Phil. scholarship (reference SFRH/BD/12989/2003, financed by POSI).

R EFERENCES

[1] Inglis, C.E., Stresses in a plate due to the presence of cracks and sharp corners, Trans of the Institution of Naval Architects, 55 (1913) 219-230. [2] Westergaard, H.M., Bearing pressures and cracks, Jnl Appl. Mech., 6 (1939) 49-53. [3] Sanford, R.J., Selected Papers on Foundations of Linear Elastic Fracture Mechanics, SEM/SPIE, USA (1997). [4] Sanford, R.J., Selected Papers on Crack Tip Stress Fields, SEM/SPIE, USA (1997). [5] Paris, P., Erdogan, F., A critical analysis of crack propagation laws, Jnl Basic Engineering, 85 (1963) 528-534. [6] James, M.N., Yates, J.R., Susmel, L., Iacoviello, F., Nowell, D., Lazzarin, P., Patterson, E.A., Carpinteri, A., Characterisation of Crack Tip Stress Fields, Proceedings of the 1 st Joint International Journal of Fatigue/Fatigue & Fracture of Engineering Materials & Structures conference, held in Forni di Sopra, UD, ITaly, 7-9 March, 2011, Int. Jnl Fatigue, 46 (2013) 1-1. [7] James, M.N., Yates, J.R., Susmel, L., Iacoviello, F., Guest Editorial: Special Issue on Characterisation of Crack Tip Stress Fields, Fatigue Fract. Engng Mater. Struct., 36 (2013) 1-2. [8] Limodin, N., Rethore, J., Buffiere, J.Y., Influence of closure on the 3D propagation of fatigue cracks in a nodular cast iron investigated by X-ray tomography and 3D volume correlation, Acta Materiala, 58 (2010) 2957-2967. [9] Zhang, W., Liu, Y., Investigation of incremental fatigue crack growth mechanisms using in situ SEM testing, Int. Jnl Fatigue, 42 (2012) 14-23. [10] de Matos, P.F.P., Nowell, D., Experimental and numerical investigation of thickness effects in plasticity-induced fatigue crack closure, Int Jnl Fatigue, 31 (2009) 1795-1804. [11] Pommier, S., Hamam, R., Incremental model for fatigue crack growth based on a displacement partitioning hypothesis of mode I elastic-plastic displacement fields, Fatigue Fract. Engng Mater. Struct., 30 (2006) 582-598. [12] Nowell, D., Kartal, M.E., de Matos, P.F.P., Measurement and modelling of near-tip displacement fields for fatigue cracks in 6082 T6 aluminium, Proc. First I.J. Fatigue & FFEMS Joint Workshop, Forni di Sopra, Italy, March 7-9, 2011, Gruppo Italiano Frattura, 2011. [13] Nowell, D., Kartal, M.E., de Matos, P.F.P., Digital image correlation measurement of near-tip fatigue crack displacement fields: constant amplitude loading and load history effects, Fatigue Fract. Engng Mater. Struct., 36 (2013) 3-13. [14] Eberl, C. Thompson, R., Gianola, R., Digital image correlation and tracking with Matlab, Matlab Central file exchange (2006) http://www.mathworks.co.uk/matlabcentral/fileexchange/12413-digital-image-correlation-and-tracking.

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G. Qian et alii, Frattura ed Integrità Strutturale, 25 (2013) 7-14; DOI: 10.3221/IGF-ESIS.25.02

Special Issue: Characterization of Crack Tip Stress Field

Crack propagation mechanism and life prediction for very-high cycle fatigue of a structural steel in different environmental medias

Guian Qian, Chengen Zhou, Youshi Hong * State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China * Corresponding author. Tel: +86 10 82543966, hongys@imech.ac.cn

A BSTRACT . The influence of environmental medias on crack propagation of a structural steel at high and very high-cycle fatigue (VHCF) regimes is investigated based on the fatigue tests performed in air, water and 3.5% NaCl aqueous solution. Crack propagation mechanisms due to different crack driving forces are investigated in terms of fracture mechanics. A model is proposed to study the relationship between fatigue life, applied stress and material property in different environmental medias, which reflects the variation of fatigue life with the applied stress, grain size, inclusion size and material yield stress in high cycle and VHCF regimes. The model prediction is in good agreement with experimental observations. K EYWORDS . Very-high-cycle fatigue; Aqueous environment; Stress intensity factor; Plastic zone.

I NTRODUCTION

V

ery-high-cycle fatigue (VHCF) [1-16] of metallic materials is regarded as fatigue failure at stress levels below the conventional fatigue limit and the corresponding fatigue life beyond 10 7 loading cycles. Lots of modern engineering structures and components, such as airplanes, turbines, automobiles and high speed trains are expected to endure the safe performance in the range of 10 7 - 10 10 load cycles. One typical feature of VHCF for high strength steels is that the S-N curve consists of two parts corresponding to subsurface and surface crack initiations, resulting in a stepwise or duplex shape of the curve [1-16]. Generally, the crack initiation in VHCF regime is observed as a fisheye pattern on the fracture surface, which is located at the specimen subsurface region and originated from a nonmetallic inclusion for high strength steels [4-11]. Since the pioneering work by Naito et al. [17, 18], there have been a variety of studies on the VHCF behavior for different materials [1-16]. Among these studies, the crack initiation mechanism in VHCF attracted most of the attention. However, the crack initiation and propagation process of high strength steels in environmental medias in VHCF is still not clear. In addition to experimental investigations, theoretical models for fatigue strength and life prediction in VHCF regime are of significant importance for both scientific and engineering applications. However, models to predict S-N curves in VHCF regime in different environmental medias are lacking due to the complicated crack initiation mechanisms. Therefore, in this paper, the process of crack initiation and propagation for a structural steel in environmental medias in VHCF was investigated based on the experiments. The fatigue test was performed in laboratory air, fresh water and 3.5% NaCl aqueous solution. The influence of environmental medias on the variation of fatigue strength and cracking process is presented. Based on the experimental observations, a model is proposed to study the S-N curves of the material in high cycle and VHCF in different medias.

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