Time filter

Source Type

Winterthur, Switzerland

Schindler H.-J.,Materials Tec AG | Kalkhof D.,Swiss Federal Nuclear Safety Inspectorate ENSI
Journal of Testing and Evaluation | Year: 2015

An increased loading rate causes not only a shift of the ductile-to-brittle transition curve, but also a slight change of the shape of this curve. The latter tends to become steeper as the loading rate increases. This effect was observed even at loading rates that are considered to be quasi-static according to ASTM E1921 [ASTM E1921-13: Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2013. Actually, the coefficient 0.019 in the exponent of the MC turned out to be substantially higher at elevated loading rates. This means that a basic assumption of the evaluation procedure of ASTM E1921 is not met, which leads to an increased uncertainty of T0 or T0,x, respectively. This effect of is most pronounced in testing at elevated loading rates using the single-temperature option. An improved method to determine the reference temperature is proposed, where the exponent in the median transition curve in ASTM E1921 is considered to be an open parameter. The resulting reference temperatures, denoted as T100 and T100,x, respectively, are expected to be more accurate than standard T0, which is confirmed by comparison with experimental data. Furthermore, the validity requirements for T100 are less restrictive and more suitable than those in ASTM E1921, since they do not depend on the outcome of the tests. Based on the improved data, an improved estimation formula for the rate-induced shift of T0 is derived. Suggestions are made for improvement of ASTM E1921. Copyright © ASTM Int'l (all rights reserved).

Schindler H.-J.,Materials Tec AG | Kalkhof D.,Swiss Federal Nuclear Safety Inspectorate
13th International Conference on Fracture 2013, ICF 2013 | Year: 2013

The inherent scatter of fracture toughness of ferritic steels in the brittle-to-ductile transition regime require statistical methods to be applied for testing and evaluation. However, for engineering purposes lower bounds of KIc such as the ASME-reference curve are often preferred since they allow deterministic worst-case predictions to be made. So the question is how to derive lower bounds of a quantity that is governed by weakest-link-statistics. Actually, neither the MC-approach nor the empirical ASME-reference curve deliver well-founded lower bounds for components of relatively small thicknesses. A theoretical model is suggested to fill this gap. The key element of the approach is the hypothesis that the weakest-link-effect is saturated at a certain thickness. The corresponding upper limit of size-dependence turned out to be close to the minimum thickness required for plane-strain conditions at the crack-front. The derived mathematical relations enables KIc to be calculated from KJc as measured on a smaller specimen. In reverse, from a lower-bound KIc as provided by the ASME-code a thickness-dependent lower bound of KJc can be obtained. The proposed model is shown to yield predictions that are consistent with experimental data as well as with the ASME-lower bound. Copyright © (2013) by International Conference on Fracture.

Schindler H.J.,Materials Tec AG
Proceedings of the Biennial International Pipeline Conference, IPC | Year: 2010

Under special circumstances a fast propagating crack can be triggered in pipelines by just a small local damage. In order to assess the safety with regard to this type of catastrophic failure, an analytical model was established to describe the process of large ductile tearing mathematically. Using a simplifying kinematical model and the hypothesis that the tearing fracture process is governed by a constant CTOA, the total energy dissipation rate can be quantified and compared to the available fracture energy. Since the Charpy fracture energy KV is often the only available toughness-related material parameter of existing pipelines, the toughness parameters used in the model, particularly CTOA, has to be determined from KV. This was achieved by using the same 2-parameter model of ductile tearing to analyse the fracture process in bending. A closed form relationship was obtained between the minimum pressure required for fast ductile crack propagation and the system and material parameters. It could be shown that rapid ductile tearing requires the hoop stress to exceed a certain limit, which depends on the geometrical parameters of the pipeline and KV of its material. The analysis was verified by comparison of the results with the experimental data of full- scale burst tests available in the literature. Unlike empirical correlations, the derived analytical formula seems to be universally valid, regardless of pipe dimensions or steel grades. Copyright © 2010 by ASME.

Schindler H.-J.,Materials Tec AG | Kalkhof D.,Swiss Federal Nuclear Safety Inspectorate ENSI | Viehrig H.-W.,Helmholtz Center Dresden
Engineering Fracture Mechanics | Year: 2014

The fracture behaviour of a component or specimen that contains a sharp notch is governed essentially by the same theoretical relations known from cracks. The blunt notch root only causes an increase of the resistance against crack initiation, which depends on the fracture mechanism. In the present paper, the relation between fracture toughness and notch toughness is investigated by simple analytical models. The derived formulas were compared with experimental results obtained from fracture toughness tests on RPV-steel 24 NiCrMo 3-7 at various temperatures. 1T-CT- and 0.4T-SEB-specimens that contained a sharp notch with a root radius of 0.06mm introduced by spark erosion (EDM) instead of the standard fatigue crack were used. The predictions were found to agree well with the experimental data. The effect of the notch radius on fracture toughness is most pronounced in the brittle to ductile transition regime, where fracture toughness can be characterized by the master curve and the corresponding reference temperature T0 according to ASTM E1921. Accordingly, the effect of the notch radius can be quantified by a shift of T0. Since the shape of the transition curve depends on the notch radius, the procedure of ASTM E1921 to determine T0 is not applicable. An alternative is suggested. As limiting cases, ductile tearing and brittle fracture are also considered. © 2014 Elsevier Ltd.

Schindler H.-J.,Materials Tec AG | Leinenbach C.,Empa - Swiss Federal Laboratories for Materials Science and Technology
Journal of ASTM International | Year: 2010

The fatigue crack growth behavior of a brazed joint is characterized by a Paris-exponent that is much higher than the one of bulk metals, which means that brazed components have only a short residual fatigue life after the initiation of a fatigue crack. Therefore, the threshold of fatigue crack growth of brazed joints is of particular importance in the fatigue analysis of brazed components and needs to be understood well. Residual stresses have to be considered in crack growth analysis in the threshold regime. The corresponding effects are explored experimentally and theoretically. The stress intensity factors due to the residual stresses in a brazed T-joint of compressor impellers were measured by using the cut compliance method. It was found that residual stresses are present but relatively small. They do not affect the endurance stress significantly. A concept for an endurance analysis of a brazed T-joint that includes imperfections and residual stresses is suggested. Copyright © 2010 by ASTM International.

Discover hidden collaborations