Fracture of High-Strength Sheet Steel Specimens Containing Small Cracks

A procedure is described for introducing small surface cracks into high-strength steel sheet specimens. The effect of crack size on tensile strength of the specimens is illustrated for two high-strength steels with comparable yield strengths but different levels of fracture toughness at room temperature. The effect is also illustrated for one of the steels at lower temperatures, down to −280 F, covering a wide range of fracture toughness for this material. The results are also discussed in relation to an equation developed by G. R. Irwin which expresses the stress intensity factor for a semi-elliptical surface crack in terms of the length and depth of the crack. This is used to calculate a lower limit for the $Kc$ fracture toughness value from the initial crack dimensions, $Knom$⁠. The calculated values of $Knom$ are almost independent of the crack dimensions over a wide range, indicating a relationship between initial crack dimensions and slow crack growth. It is shown that the effect of a small surface crack is more drastic than might be expected from the results of tests on centerslotted specimens, such as are used to determine $Kc$ values of sheet materials. The results of tests in which water or india ink was introduced into the cracks showed that these fluids had a distinctly detrimental effect on the fracture strengths of the steels used.