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stress intensity factor decreases. Thus, a point is reached where the crack stops growing because the stress intensity factor falls below the threshold value. The experimental set-up for these tests is shown in Figure 21. The specimen itself is not visible in the photograph since it is inside the plastic environmental enclosure, but the pins in the loading device which apply the loads to the ends of a machined slot in the specimen can be seen. This slot was about one quarter of an inch wide and parallel to the direction of loading. Stress raisers were initiated in the sides of the slot by a saw cut and were sharpened to cracks by fatigue loading prior to the sustained static load test.
In this experiment, it was found that the material was not susceptible to crack growth in 100 percent humidity air, but that it was quite susceptible to crack growth in the hydrogen sulfide and salt solution, based on data from the surface-flawed specimens. The indicated threshold value of the stress-intensity factor was 15 ksi. $\sqrt{\text{in}}$ . The wedge loaded specimens failed to show any flaw growth at the stress intensities used, even in the severe environment. This was attributed to the difference in microstructure in the material in the interior of the specimen in which the "through the thickness" cracks were located.
Crack growth rates were also measured on the surface flawed specimens and were found to be an exponential function of the stress intensity factor. This implies that, for very long service lifetime, even very small cracks would result in sustained load crack growth in this hydrogen sulfide and salt solution environment.
e. In Section V of the Battelle report, the results of fractographic examination and microprobe analysis of one of the pre-existent cracks on the pinhole surface of the C11 head of eyebar C9-C11, north chain, south bar, are discussed. The portion of eyebar head C11 containing the cracks was cut from the eyebar at the National Bureau of Standards and shipped to the U. S. Steel laboratory. It was broken open in the presence of representatives of U. S. Steel and Battelle Memorial Institute, who were part of a special test group appointed on May 23, 1969, to attempt to resolve the question of what mechanism of crack growth was responsible for extending such cracks. After breaking open, one side of the fracture was retained by U. S. Steel and the other taken to Battelle for independent evaluation. The results of the investigation of the fracture surface by the U. S. Steel laboratory are discussed in the next subsection of this report, and the report of the chairman of the task group, is discussed in Section IV.
The examination of this fracture at Battelle disclosed the following:
(1) The origin of the pre-existent crack was a small corrosion pit about 0.01 inch in diameter.