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    ASTM D8255 - 19

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

    Active Standard ASTM D8255 | Developed by Subcommittee: D02.F0

    Book of Standards Volume: 05.05

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    Significance and Use

    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 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.

    1. Scope

    1.1 This guide provides general tutorial information and best practice for measuring the work of fracture on manufactured graphite and carbon specimens. Although applicable to all carbon and graphite materials, this guide is aimed specifically at measurements required on nuclear graphites, where there may be constraints on the geometry and/or volume of the test specimen.

    1.2 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.

    1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

    1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

    2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.

    ASTM Standards

    C559 Test Method for Bulk Density by Physical Measurements of Manufactured Carbon and Graphite Articles

    C651 Test Method for Flexural Strength of Manufactured Carbon and Graphite Articles Using Four-Point Loading at Room Temperature

    C695 Test Method for Compressive Strength of Carbon and Graphite

    C747 Test Method for Moduli of Elasticity and Fundamental Frequencies of Carbon and Graphite Materials by Sonic Resonance

    C749 Test Method for Tensile Stress-Strain of Carbon and Graphite

    C769 Test Method for Sonic Velocity in Manufactured Carbon and Graphite Materials for Use in Obtaining an Approximate Value of Youngs Modulus

    D7775 Guide for Measurements on Small Graphite Specimens

    D7779 Test Method for Determination of Fracture Toughness of Graphite at Ambient Temperature

    D7972 Test Method for Flexural Strength of Manufactured Carbon and Graphite Articles Using Three-Point Loading at Room Temperature

    E4 Practices for Force Verification of Testing Machines

    E399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials

    ICS Code

    ICS Number Code 71.060.10 (Chemical elements)

    Referencing This Standard
    Link Here
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    DOI: 10.1520/D8255-19

    Citation Format

    ASTM D8255-19, Standard Guide for Work of Fracture Measurements on Small Nuclear Graphite Specimens, ASTM International, West Conshohocken, PA, 2019, www.astm.org

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