Published: Jan 1956
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This paper covers a few of the points thought to be especially significant in handling the practical phases of brittle fracture. When metallurgists and engineers were first confronted with the brittle fracture of steel, it was thought that impact must be the cause. But leading experimenters soon found this concept to be inadequate and, at the turn of the century, the notch was recognized in its true light and was introduced into formal tests of steel quality. At the same time, the energy absorbed in breaking the test bar was universally adopted as the measure of quality; this led to the simple device of breaking the test bar by means of a swinging pendulum. Doubtless due to the earlier notions, the test thus became commonly known as the “impact test” and so perpetuated the idea that brittle fracture was the result of impact. This was in spite of demonstrations to the contrary, such as the work of Considere, who found that increasing the strain rate simply raised the temperature at which brittle fracture occurred. Two other misconceptions impeded understanding of notch brittleness and delayed the general acceptance of notched-bar testing. First was the failure to fully recognize the rigidity or the stiffening effect, at the notch section, which is caused by the third dimensional stress. This is the stress, amplified by the stress concentration, which must exceed the cohesive or brittle strength for brittle fracture to result. Attention became centered on stress raisers, and the theory seems to have been that, since steel is ductile, a slight deformation at the root would relieve the stress, and that nothing more was important. An example which brings out the effect of rigidity is the failed forging, which while ductile with the single or standard-width test bar was brittle, with the same notch, when tested with the double-width bar. Another is the Navy Tear Test, which gives a much higher transition temperature than the Charpy keyhole test bar does with the same notch acuity. Another misconception has been that steels having the same tensile properties would all perform similarly in the presence of a notch. This was negated at an early date by a formal and authoritative series of tests conducted by the German Society for Testing Materials, the results of which were published in 1907. In more recent times, the fallacy of this assumption has been demonstrated many times.
Hoyt, S. L.
Metallurgical Consultant, Columbus, Ohio