No abstract
A multilaboratory blind comparison testing program evaluated the accuracy of the short rod method of measuring the fracture toughness of metallic materials. Valid comparisons between values measured according to ASTM Standard Method of Test for Plane-Strain Fracture Toughness of Metallic Materials (E 399-78) and values of the plane-strain critical stress intensity factor as measured by the short rod method of fracture toughness measurement (KIcSR) were obtained for several steels, aluminum alloys, and titanium. The KIcSR values measured by the short rod method were consistently low, averaging 6% below the measurements according to ASTM Standard Method E 399. A 4% adjustment in the short rod calibration constant that had been previously evaluated only to ±7% brings the two sets of measurements into very good agreement. The short rod method thus appears to be a viable alternative for measuring the fracture toughness of metallic materials.
Mode I and Mode II stress intensity factors (SIF) are presented for three-point bend specimens with misaligned locations of loadings and cracks. Such stress intensity factors were obtained through the application of the weight function technique and the particular stress distribution applicable to a three-point eccentrically loaded beam. Mode I and Mode II stress intensity factors for an eccentrically loaded four-point beam having a misaligned crack were obtained by superposing the data applicable to the load and crack on a misaligned three-point loaded beam. The results of these analyses were then employed to evaluate the effect of load and crack misalignment on Mode I and Mode II stress intensities of specimen configurations included in ASTM E 399 for Plane-Strain Fracture Toughness of Metallic Materials and in ASTM C 1421-99 for the Determination of Fracture Toughness of Advanced Ceramics at Ambient Temperature. The SIF ratios for Mode II-to-Mode I were also determined for both the eccentrically loaded three-point beam and the four-point beam having misaligned cracks. Differences in the Mode I stress intensities were determined for the cases where the load and crack misalignments are taken into account and those idealized cases where they are not. Finally, recommendations are presented to reduce, where necessary, these differences to an acceptable degree.
A method is presented for approximating stress intensity factors for notched-configurations without cracks. It is expected that estimates obtained from such analyses, which are outlined in the text and detailed in the Appendix, should be useful in estimating the failure load in a notched structure behaving in an idealized elastic manner. Additionally, this approach will allow an estimate of the magnitude of load required to precrack fracture toughness specimens having crack starter notches. Several notch geometries and loadings are investigated, such as a V-notch, to first illustrate the method of analysis and compare results with those in the literature and to also establish merit and accuracy limits of the method. Two additional cases are then solved, both for finite width tensile sheets, one having symmetric U-notches and the other employing a central slot with finite radii at each end. Both cases are reduced to their end limit: a U-notch in a sheet of semi-infinite width and a circular hole in an infinitely wide sheet. The resulting approximations agree well with limited available data. Finally, to aid the experimentalist who either uses the compliance approach or a photoelastic analysis to obtain stress intensity factors, the question of how deep or how long a side-edge U-notch or a central slot compared to its radius should be to simulate a crack, is provided.
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