Assessment of Ductile-to-Brittle Transition Behavior of Localized Microstructural Regions in a Friction-Stir Welded X80
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10.1007/s11661-016-3473-z The Minerals, Metals & Materials Society and ASM International 2016
JULIAN A. AVILA, Ph.D. Candidate, is with the School of Mechanical Engineering, University of Campinas, Rua Mendeleyev 200, Campinas, SP, 13083860, Brazil, and also Research Engineer with the Brazilian Nanotechnology National Laboratory, Rua Giuseppe Ma´ximo Scolfaro 10000, Campinas, SP, 13083970, Brazil. ENRICO
METALLURGICAL AND MATERIALS TRANSACTIONS A
LUCON, Research Engineer, is with the Applied Chemicals and Materials Division, Structural Materials, National Institute of Standards and Technology, Boulder, CO, 80305, and also with Protiro Inc., Denver, CO, 80207. JEFFREY SOWARDS, Metallurgist, is with the Applied Chemicals and Materials Division, Structural Materials,
VOLUME 47A, JUNE 2016—2855
I.
INTRODUCTION
THE Charpy V-notch test is the most common test to assess impact toughness, since the Charpy V-notch specimens are easy and inexpensive to machine; also, performing the tests and analyzing the results are relatively quick and simple. Results are not completely equivalent to those from fracture toughness tests, but are expected to show similar trends, as well as provide an accurate ductile-to-brittle transition temperature (DBTT). The Charpy V-notch (CVN) specimen does not reproduce the same crack tip stress triaxiality state or strain conditions during service as actual pipelines defects,[1] and the large radius of the notch does not guarantee a path for the crack growth across a specific microstructure, unless the specimen is fatigue precracked. For quantitative measurements, it is recommended to perform conventional fracture toughness tests,[2] such as crack tip opening displacement (CTOD) tests. The Charpy V-notch test has often been used to generate ductile–brittle transition temperature curves for impact energy, which gives information not only on the value of the DBTT, but also on the upper and lower shelf energies. Charpy tests performed with an instrumented striker allow one to obtain the conventional impact energy by use of the machine encoder to measure the pendulum displacement, and to measure the force on the instrumented striker with strain gages. The force vs. displacement data provide additional information besides the total impact energy (Wt), including the energy at maximum force (Wm), the crack initiation energy (Wiu), the crack arrest energy (Wa), and the percentage of shear fracture area (% shear).[3,4] The small inherent size of miniaturized Charpy V-Notch (MCVN) specimens allows for the evaluation of localized regions, and as a result, the use of this type of specimen provides a chance to assess materials with size restrictions[2] and small regions of interest. There are two standardized MCVN specimens; the reduced half-size (RHS) specimen with 26.6 mm 9 4.8 mm 9 4.8 mm dimensions[5] and the Kleinstprobe specimen (KLST) with 27 mm 9 4 mm 9 3 mm dimensions.[4] These types of specimens have been used in the analysis of localized regions of weld joints[6] and working elements of machine
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