STP995V2: An Analytical and Experimental Comparison of Rectangular and Square Crack-Tip Opening Displacement Fracture Specimens of an A36 Steel

    Sorem, WA
    Graduate research assistant and professor, University of Kansas, Lawrence, KS

    Dodds, RH
    Associate professor, University,

    Rolfe, ST
    Graduate research assistant and professor, University of Kansas, Lawrence, KS

    Pages: 25    Published: Jan 1988


    Abstract

    Analytical and experimental investigations of both rectangular-cross-section (t by 2t) and square-cross-section (t by t) three-point-bend crack-tip opening displacement (CTOD) fracture specimens were conducted on an A36 steel. This study is part of a long-range investigation aimed at developing criteria to evaluate the susceptibility to fracture in specific structural situations, that is, pressure vessels and structural steels with short cracks (a/W 0.15), where the square CTOD specimen must be used. Generally, the rectangular CTOD specimen is used in laboratory testing, but for weldment testing, where shallow surface cracks may exist, or for other structural applications with crack depth ratios other than a/W = 0.5, the square specimen is often required. The objective of this study is to compare the CTOD fracture toughness results of square specimens with those of rectangular specimens using equivalent crack depth ratios (a/W = 0.5).

    Both analytical and experimental studies were conducted on full-size [31.8 by 31.8-mm (1.25 by 1.25-in.)] and sub-size [12.7 by 12.7-mm (0.50 by 0.50-in.)] square specimens. Static CTOD tests were conducted in the lower shelf and lower transition regions where structural steels such as A36 are typically used. Three-dimensional elastic-plastic finite-element analyses were conducted to compare the effect of specimen size and geometry on the specimen stress field.

    The results of the square specimens were compared with the results of previously tested rectangular specimens that were tested and analyzed by Wellman et al. following the same procedures. In comparing these four specimen sizes, it was shown that the lower-bound CTOD values obtained experimentally were independent of either specimen size or geometry in the lower shelf and lower transition regions prior to stable crack growth. The near-crack-tip opening mode stresses at the center plane of each specimen were shown to be essentially equal for the four specimen sizes analyzed. This implies that the square and rectangular specimens with a/W = 0.5 may be used interchangeably to characterize the notch toughness prior to stable crack growth. It may also be possible to obtain the same lower-bound results using sub-size specimens which are less than the full thickness of the actual structure.

    Keywords:

    elastic-plastic fracture mechanics, crack-tip opening displacement, size effect, toughness, finite element, Weibull distribution, constraint, weakest link, fracture mechanics, nonlinear fracture mechanics


    Paper ID: STP27724S

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP27724S


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