1993

Rintamaa, Rauno

Quantification of the dynamic fracture toughness of structural materials is essential to a wide range of problems from nuclear power plant integrity assessments to structural engineering applications. The difficulties associated with accurate determination of the fracture resistance under dynamic loading are considerable. Although a lot of research work has been directed to the problem, there are no standardized procedures for the precracked Charpy type specimen. In this study a new single specimen test method and testing facility for evaluating dynamic fracture toughness has been developed. The method is based on the application of a new pendulum type instrumented impact tester equipped with an optical crack mouth opening displacement (COD) extensometer. The fracture toughness measurement technique uses the Double Displacement Ratio (DDR) method, which is based on the assumption that the specimen is deformed as two rigid arms that rotate around an apparent centre of rotation. This apparent centre moves as the crack grows, and the ratio of COD versus specimen displacement changes. As a consequence the onset of ductile crack initiation can be detected on the load-displacement curve. Thus, an energy-based fracture toughness can be calcu- lated. In addition, the testing apparatus can use specimens with the double ligament size as compared with the standard Charpy specimen which makes the impact testing more appropriate from the fracture mechanics point of view. The novel features of the testing facility and the feasibility of the new DDR method have been verified by performing an extensive experimental and analytical study. An experimental comparison of the inertia effects and specimen oscillations revealed that from the measurement point of view the registered load-time curve of the conventional testing geometry has more oscillation effects than the inverted testing geometry. The experimental results indicated quite clearly that the oscillation amplitude from the first to the third oscillation was slightly less in the new impact tester. The new DDR method gave comparable values for the ductile fracture initiation toughness as could be obtained by applying different methods based on the multi-specimen technique. Fracture resistance curve information, stretch zone width measurements and specimen instrumentation were used for determining the ductile fracture initiation toughness. Good agreement was observed for three different materials and two specimen sizes. The DDR method was successfully applied in studying the effects of side grooves, specimen size and irradiation embrittlement on the ductile fracture initiation toughness.