ASTM WK67023

    Revision of E399 - 17 Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIc of Metallic Materials

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    Active Standard: E399 - 19

    Developed by Subcommittee: E08.07 | Committee E08 | Contact Staff Manager



    WK67023


    Keywords


    Rationale

    KQ/ KIc toughness increases with specimen size, especially for high toughness materials. This revision to E399 proposes to introduce a new analysis method for the standard force-displacement data collected as part of the standard E399 test method. The new method is believed to produce approximately size-insensitive, linear-elastic plane-strain fracture toughness. The new nomenclature is KIsi that is, Mode-I, size-insensitive toughness. The approach taken to integrate the new method into E399 is to add a non-mandatory appendix to E399 with the new method. The intent is that for an E399 test, the user will always produce a KIc data analysis and can optionally produce the KIsi data analysis. This approach provides a reasonable balance to continue evaluation of this new analysis procedure versus minimal disruption of the existing commercial user base that relies on E399 KIc for testing specifications. The KIc test data will be a mandatory result of the E399 test method with KIsi as an optional result. In this way the new procedure can be compared with KIc data to gain experience with the approach without disrupting existing commercial and research activity that relies on KIc. The approach leverages significant commonalities across the apparatus, specimen configuration, and test procedure between the two methods and prevents duplication of these in a completely new test method. A key difference between the KIc data analysis and the KIsi data analysis is that the KQ/ KIc data analysis is based on a 95% secant offset from the linear region of the data test record, which corresponds to 2% crack extension. As a result, when specimen size increases, the amount of crack extension at which KIc is evaluated also increases, leading to the possibility of an R-curve effect for a data set with increasing specimen size. In contrast, the KIsi data analysis is based on a fixed amount of crack growth independent of specimen size. For fixed crack growth the compliance offset is variable, and equations are provided to calculate the appropriate compliance offset as a function of remaining ligament for each of the specimens in the annexes. A finite element study has been completed by Wells et al. to evaluate this interpretation of the force-COD record and the influence of crack tip plasticity on compliance change. This study is published in the ASTM journal Materials Performance and Characterization, Vol. 7, No. 2, 2018. The study showed that LEFM assumptions remain valid but a revision of the proposed validity criteria was recommended to ensure physical crack extension occurs at the crack tip. A KIsi test program was also completed by Will Tilson on a high fracture toughness aluminum alloy product, 7475-T7351 plate. A summary of the test program can be found in the Nov 2018 E08.07 Subcommittee meeting minutes. The results of this test program verified that for this high toughness aluminum product, physical crack extension did indeed occur with the revised validity criteria, and that the KIsi results were much less sensitive to specimen size than KQ/ KIc as expected. This is the second subcommittee ballot on this work item. The many negatives and comments in the previous ballot have been addressed to the extent possible based on guidance from the E08.07.01 task group and the revised validity criteria has been adopted. This ballot is only for the new non-mandatory appendix and the specific indicated changes in the main body and annexes. This is not a ballot for the full text of E399. Only the revised sections are shown. For any other sections refer to E399-17.

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    Work Item Status

    Date Initiated:
    02-08-2019

    Technical Contact:
    Mark James

    Item:
    016

    Ballot:
    E08 (19-03)

    Status:
    In Balloting