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.
Methods for determining the J integral from an experimental load versus load point displacement curve for the compact specimen are discussed. The original analysis by Merkle and Corten, which accounted for the tension component in the compact specimen, is presented along with a simplified version (of the analysis) that is shown to be essentially equivalent to the original formulation. Based on experimental results from Landes, Walker, and Clarke, a further simplified expression is recommended as the best expression to use for determining J for the compact specimen.
The calibrated strain on the back face (the face opposite that from which the slot is machined) of compact tension (CT) and T-type wedge-opening-loading (WOL) specimens provides a method for measuring crack length when the load is known or for measuring load when the crack length is known. The method is simple, reliable, sensitive, and inexpensive. A good correlation was achieved between strain measurements on a CT specimen and values computed from a two-dimensional finite element analysis. The method has good potential for developing into a more sensitive crack length measurement technique than has previously been achieved. Calibration tables and graphs are reproduced that describe the relationship between crack length, back-face strain, and load for CT and T-type WOL specimens of any size and thickness and for any linear elastic material. The method has several advantages over the closely related crack opening displacement (COD) technique for some test situations and these are described. In particular, the back-face strain increases linearly with crack length for constant stress intensity conditions except for very deep cracks in CT specimens. The overall characteristics render the technique ideal for incorporation into computerized/automated crack growth testing. For constant back-face strain, the stress intensity was shown to decrease with increase in crack length for both CT and T-type WOL specimens. This decrease is more pronounced than for corresponding constant COD testing and this provides a good technique for obtaining threshold fatigue or stress corrosion conditions.
Finite element procedures are used to optimize the efficiency of the electrical potential technique for monitoring the initiation and slow growth of cracks, as applied to the compact tension fracture test piece. An analysis of various configurations of current input and potential measurement lead placement is performed to optimize the accuracy, sensitivity, and reproducibility of measurement and to maximize output voltages. Numerical calibration curves are computed for selected configurations and are confirmed by experimental measurements.
An investigation of the initiation and growth of erosion and of the effect of velocity and pressure on erosion in a rotating disk is presented. Also, the role of an intervening noncavitating period on erosion is studied. The results indicate that at high intensities the peak rate of erosion decreases with increases in pressure. The erosion rate/time curves obtained for metallic materials are explained by the eroded particle distribution and the cavity size. The average size of the eroded particles decreased when pressure and tensile strength of the material were increased. The erosion rate peaked after an intervening noncavitating period. The use of the rate of erosion, defined as an average over the entire test duration, in the equation governing the theory of erosion resulted in reasonably good correlations. The correlations reveal that it is possible to predict the length, width, and area of a cavity when the cavitation parameter σ is known. The normalized width of a cavity may be estimated if its normalized length is known.
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