Determination And Prediction Method Of Fracture Toughness For Low-Alloy Steel Of Nuclear Power Equipment

Fracture toughness plays a significant role in the structural integrity assessment of nuclear power equipment.However,it is difficult to choose which fracture toughness test standard is more suitable for various materials,especially for Chinese SA508-Ⅲ steel at different test temperatures.Moreover,fracture behavior of Chinese SA508-Ⅲ steel mechanism is not clarified and the fracture toughness prediction model which should consider the effect of temperature has not yet been established.In this research,the principle of establishing blunting line was deduced.Furthermore,this research also puts forward a constitutive curve method(CCM)and a normalized J-R curve method to deal with the JIC test data.The effect of temperature on the Chinese SA508-Ⅲ steel fracture behavior was systematically studied.As well known,high temperature fracture toughness tests are sophisticated and frequently invalid.Therefore,a modified correlation between the KJIC-CVN model and considering the effect of temperature was established.The main research contents and conclusions are as follows.In order to clarify how to choose the standard of fracture toughness tests,the theory of blunting line(BL)was deduced in this research.The results showed that the ideal elastic-plastic and the power law hardening material constitutive assumptions was chosen for BL in ASTM E1820 and ISO 12135,respectively.Therefore,based on the stress-strain curve of metallic material whether exists a distinct yielding plateau under various test temperatures,namely,the constitutive curve method(CCM),the fracture toughness of SA508-Ⅲ steel at different temperatures was obtained.Moreover,in order to characterize the influence of geometry constraint,within ASTM E1820 standard,lots of Compact Tension(CT)specimens with different geometric parameters were manufactured for fracture toughness tests.This research showed that the higher geometry constraint conducted a smaller fracture toughness.The ASTM E1820,ISO 12135 and CCM were chosen to determine the fracture toughness of SA508-Ⅲ at various temperatures.The results showed that the processing method has a significant effect on the fracture toughness at different temperatures.Moreover,the SEM results showed that the fracture morphologies revealed that the Chinese SA508-Ⅲ steel presented ductile fracture at 20~150 °C.While the ductile and cleavage fracture mixed mode was appeared at 200~320 °C.The 3D X-ray computed tomography was addressed to depict the initiation,growth and coalescence of ductile voids and approach to the calibration of the GTN model by developing a new understanding of the ductile fracture mechanism in Chinese SA508-Ⅲ steel.The calibrated model was implemented to predict ductile tearing behaviour in CT specimens.The results showed good agreement with test data and also reproduced the morphology and branching of crack extension observed in practise.This research enabled new insights to be gained concerning the development of non-uniform void volume fraction distribution in the almost crack growth area.The effect of temperature(T)was directly considered in KJIC-CVN model for predicting fracture toughness.To assess the accuracy of fracture toughness when calculating from the value of Charpy-V notch impact energy by using the modified correlation,both the Charpy-V notch impact tests and fracture toughness tests for Chinese SA508-Ⅲ steel were performed at different temperatures.The results showed that the modified correlation exhibited a small percentage of error(within 8%),which is lower than that of other correlations for estimating fracture toughness of Chinese SA508-Ⅲ steel.Based on the above theoretical analysis and experimental research,the selection principle of fracture toughness test standard and proposes the corresponding data processing method was clarified,the fracture mechanism and behavior of Chinese SA508-Ⅲ steel at high temperatures were expounded.This research also established the fracture toughness prediction model which considering the effect of temperature.The results from this research will strengthen the guidance provided to structural integrity engineers in the industry regarding the calibration and application of ductile damage mechanics models such as the GTN model or modified KJIC-CVN model for predicting fracture toughness in SA508-Ⅲ steel.