M.K. Sahu

Evaluation of Material JR and Fracture Toughness Transition Curves using Micro‐Mechanical Modeling

Local approach has been used to compute a) Jinitiation and JR curves at different temperatures and b) fracture toughness transition curves at different probability of failure. The material under investigation is the Reactor Pressure Vessel Steel 22NiMoCr37. Ductile fracture has been analyzed using Gurson material constitutive model and probability of cleavage failure is calculated using Beremin’s model. A variation of Gurson parameter q2 near crack tip region as a function of charpy energy is suggested to obtain Jinitiation as well as complete JR curve accurately at different metal temperatures.

Temperature Dependency of Beremin’s Parameters for 20MnMoNi5-5 Material

The mechanism of cleavage fracture in reactor structural material is significantly different from ductile fracture. The cleavage fracture in a material usually originates from micro-cracks, which are formed by different mechanisms. The micro-cracks are formed due to non-homogeneous distribution of plastic deformation within the grains, called slip-initiated cleavage. The cracked grain boundary carbides also are the sources to originate the micro-cracks. This occurs when the stress normal to the planes of carbide particles is sufficiently high. The fracture takes place by the formation of micro-cracks and their extension with little global plastic deformation. The cleavage process is stress controlled and consumes little deformation energy and hence, the crack grows unstably fast. In this case the local fracture criterion is generally based on a critical cleavage stress, which is statistical in nature. Based on the weakest link assumption and Weibull statistics, Beremin developed a model for analyzing brittle fracture process by local approach. The two parameters of the model are material properties and can be determined from notched tensile tests at low temperatures and their finite element analysis. Once these parameters are determined, these can be used to predict probability of cleavage fracture initiation in a component. In the present work, temperature dependency of Beremin’s parameters ‘m’ and ‘σu ’ has been investigated for 20MnMoNi55 structural material. Tensile tests have been conducted on specimens having central groove at two different −50 °C and −100 °C temperatures. Thirty specimens have been tested in each set. Image processing technique has been used for measuring diametrical contraction from on-line images. The analytical study has been carried out to determine Beremin’s parameters using the statistical variation in load v/s diametrical contraction. For this purpose, finite element analysis has been carried out to determine Weibull stress during fracture. The experimental probability of failure of each specimen has been calculated by ranking them based on the diametrical contraction at the time of fracture. Such a table along with the corresponding Weibull stresses have been used to find out Beremin’s parameters using “maximum likelihood method”. The temperature dependency of these parameters is then investigated for the present material.© 2006 ASME

Continuum Damage Mechanics From Specimen to Component Level

Continuum damage mechanics is applied from specimen to component level to compute fracture behaviour. The computed parameters are load-displacement and J-Δa curves. A parametric study is done to show the effect of variation of f0 and fn on J-R curve. It is suggested to use a variation of q2 parameter near crack tip instead of a constant value in Gurson constitutive model. Such variation helps to obtain Jinitiation as well as complete J-R curve in close agreement with experimental values. The Gurson material parameters obtained from specimen level analyses are then used to analyze reactor grade straight pipe and elbows with circumferential flaws. The close agreement between computed and measured parameters show the importance of continuum damage mechanics in real life applications.© 2005 ASME

Analytical Determination of Material JR and Fracture Toughness Transition Curves Using Micro-Mechanical Modelling

Local approach has been used to compute a) Jinitiation and JR curves at different temperatures and b) fracture toughness transition curves for German Reactor Pressure Vessel Steel 22NiMoCr37. Ductile fracture has been analyzed using Gurson material constitutive model and probability of cleavage failure is calculated using Beremin’s model. A variation of Gurson parameter q2 near crack tip region as a function of charpy energy is suggested to obtain Jinitiation as well as complete JR curve accurately at different metal temperatures.Copyright