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Accessible Unlicensed Requires Authentication Published by De Gruyter June 19, 2017

Residual stress state in pipe cut ring specimens for fracture toughness testing

Der Eigenspannungszustand in Ringproben aus Rohrsegmenten für Bruchzähigkeitsprüfungen
Darko Damjanović, Dražan Kozak, Stefan Marsoner and Nenad Gubeljak
From the journal Materials Testing

Abstract

Thin-walled pipes are not suitable for measuring fracture toughness parameters of vital importance because longitudinal crack failure is the most common failure mode in pipes. This is due to the impossibility to manufacture standard specimens for measuring fracture toughness, such as SENB or CT specimens, from the thin wall of the pipe. Previous works noticed this problem, but until now, a good and convenient solution has not been found or developed. To overcome this problem, very good alternative solution was proposed, the so-called pipe ring notched bend specimen (PRNB) [1–5]. Until now, only the idealized geometry PRNB specimen is analyzed, i. e., a specimen which is not cut out from an actual pipe but produced from steel plate. Based on that, residual stresses are neglected along with the imperfections in geometry (elliptical and eccentricity). The aim of this research is to estimate the residual stress state(s) in real pipes used in the boiler industry produced by hot rolling technique. These types of pipes are delivered only in normalized condition, but not stress relieved. Therefore, there are residual stresses present due to the manufacturing technique, but also due to uneven cooling after the production process. Within this paper, residual stresses are estimated by three methods: the incremental hole drilling method (IHMD), X-ray diffraction (XRD) and the splitting method (SM). Knowing the residual stress state in the ring specimen, it is possible to assess their impact on fracture toughness measured on the corresponding PRNB specimen(s).

Kurzfassung

Dünnwandige Rohre eignen sich nicht für die Messung von Bruchzähigkeitsparametern. Sie sind aber von entscheidender Bedeutung, da das Längsrissversagen der häufigste Fehlermodus in Rohren ist. Standardproben zur Messung der Bruchzähigkeit, wie SENB- oder CT-Proben, können aus der dünnen Rohrwand nicht hergestellt werden. Dieses Problem wurde bereits vielfach angemerkt, jedoch fehlen hierfür bislang entsprechende Lösungsansätze. Um dies zu überwinden, wird ein alternativer Weg vorgeschlagen, die sogenannte Pipe Ring Notched Bend Probe (PRNB) [1–5]. Bisher wird nur die idealisierte Geometrie-PRNB-Probe analysiert, d. h. eine Probe, die nicht aus dem eigentlichen Rohr herausgeschnitten, sondern aus Stahlblech hergestellt wird. Dabei werden Eigenspannungen im Zusammenhang mit geometrischen Imperfektionen vernachlässigt (elliptisch und exzentrisch). Ziel dieser Forschung war es, den Eigenspannungszustand in realen warmgewalzten Rohren des Kesselbaus abzuschätzen. Diese Rohrtypen wurden nur normalisiert, sind aber nicht eigenspannungsreduziert. Daher liegen fertigungs- und abkühlbedingte Eigenspannungen nach der Herstellung vor. In dieser Arbeit werden die Eigenspannungen mit drei Methoden abgeschätzt: das inkrementelle Bohrlochverfahren (IHMD), die Röntgenbeugung (XRD) und das Zerlegeverfahren (SM). Mit Kenntnis des Eigenspannungszustands in der Ringprobe ist es möglich, deren Einfluss auf die Bruchzähigkeit zu bewerten, gemessen an der/den entsprechenden PRNB-Probe(n).


*Correspondence Address, Dr. sc. Darko Damjanović, J. J. Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski Brod, Trg Ivane Brlić Mažuranić 2, HR-35000 Slavonski Brod, Croatia, E-mail:

Dr. Darko Damjanović, born in 1987, graduated from J. J. Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski Brod (MEFSB), Croatia, in 2010. He continued his studies at the same University, where he completed PhD in the Mechanical Engineering Faculty in Slavonski Brod in 2016. The theme of the PhD thesis was “Determination of fracture behavior of the pipe-ring specimen with axial crack under the influence of residual stresses”. His main research area is numerical analysis of components as well as numerical and experimental fracture mechanics.

Prof. Dr. Dražan Kozak, born in 1967, graduated from J. J. Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski Brod (MEFSB), Croatia, in 1991. He continued his studies at University of Zagreb, Croatia, where he completed his MSc and PhD in the Faculty of Mechanical Engineering and Naval Architecture in 1995 and 2001, respectively. He was Dean of MEFSB in the period from 2009 to 2013. In 2014, he became Full Professor at MEFSB where he is Head of the Department for Mechanical Design today. His main research area is numerical and experimental analysis of fracture behavior of welded structures, especially pressure vessels and pipelines.

Dr. Stefan Marsoner, born in 1975, graduated from University of Leoben, Austria, as Dipl.-Ing. in the field of Material Science, in 1998. He finished his PhD in the field of high strength PM cold work tool steels in 2002. Since 2003, he is working as a business area manager and key researcher at the research company Materials Center Leoben in Austria. His main research focus lies on mechanical properties and structure-property relationships of highest strength materials, the heat and surface treatment and the microstructural characterization of high alloyed steels.

Prof. Dr. Nenad Gubeljak, born in 1963, graduated as BSc from University of Maribor, Faculty of Mechanical Engineering, Slovenia, in 1988. He continued his studies in the same faculty and completed his PhD in April 1998. He spent a year as guest researcher at GKKS Research Center Geesthacht in Germany in the years 2000–2001. He is employed as Head of the Institute of Mechanics and Chair of Mechanics at the Faculty of Mechanical Engineering of the University of Maribor. His main research areas are fatigue and fracture testing, analysis of fracture behavior of welded joints and structure integrity assessment.


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Published Online: 2017-06-19
Published in Print: 2017-06-01

© 2017, Carl Hanser Verlag, München