Standard Guide for Work of Fracture Measurements on Small Nuclear Graphite Specimens

Abstract

5.1 Structural integrity assessments typically use values of strength and elastic modulus to predict crack initiation in graphite components and there is a suite of ASTM standards (Section 2, Test Methods C651, C695, C747, C749, C769, and Guide D7775) to cover the measurement of these properties. 5.2 The graphite component behavior after crack initiation depends on fracture mechanics parameters, such as fracture toughness and the work of fracture. Test Method D7779 provides the specification and requirements for measuring the fracture toughness of graphite based on linear-elastic stress analysis. Moreover, Test Method D7779 applies to cases where there are no restrictions on specimen size and on applicable machining and specimen preparation techniques. 5.3 Most polycrystalline graphites are non-linear elastic, non-uniform, quasi-brittle materials. For such materials, an effective approach for the determination of fracture properties is the analysis of the global energy balance associated with crack extension, similar to Griffith's theory of brittle fracture. This approach does not have the mathematical complexity of the non-linear elastic fracture and is easier to implement in practice. 5.4 Work of Fracture, γf (J/m2), is defined as the energy required to form a crack divided by the cross sectional area of the crack. It is assumed that the energy per unit area is constant during crack propagation. In general, components that have an excess of strain energy to the point of fracture, compared to the work needed to extend the crack to full dimension, fail by fast fracture. Any excess energy is converted into kinetic energy through a process that generates stress waves. If the amount of excess energy is sufficiently large, the stress waves will have peak magnitudes greater than the material strength, leading to the initiation and propagation of secondary cracks that could result in the fragmentation of the component. 5.5 However, some components that have less strain energy at the point of fracture than the work needed to extend the crack to full dimension, fail in a quasi-brittle manner and result in stable cracks, crack bridging and distributed micro-cracking. Graphite components are generally tested in their as-manufactured state and fail somewhere between these extremes showing fast fracture with relatively minor amounts of secondary cracking and little tendency to fragment. The change in the WoF and strain rate of graphite components in a reactor environment is important in assessing the component’s tendency for secondary cracking and fragmentation.