13. Effect of Notch Acuity on Critical COD

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Conventional COD bend testing is widely used to find the critical COD (δ_c) of materials. It is also well-known that sinceδ_c is dependent on the notch acuity, there is a considerable spread ofδ_c data obtained in the testing. Therefore, it is required in ASTM-E399 and BS-5762 that notched specimens be precracked in fatigue before conducting the fracture toughness test for brittle fracture initiation, to account for natural defects in severe cases as sharp cracks. Finite element analyses were carried out for two types of notched specimens: one with a sharp notch to represent a fatigue crack and the other with an idealized machined notch of rectangular shape. The calculation indicates that the displacement at the notch tip of the idealized machined notch is almost unrelated to the size of plastic zone, from which it is considered that this has nothing to do with the fracture initiation. Based on the above results, a method of estimating the δ_c value when fatigue-precracked from the test results with a machined notch only is proposed. Two sets of equations are introduced for this estimation to cover the two different test methods: three-point bending COD testing and center-notched tension testing. Large numbers of three-point bending COD tests and several center-notched tension tests were conducted on notched specimens with and without fatigue precracking, which were cut from the same plate, for several materials. The analysis of these test data has shown that the probability of occurrence ofδ_c as estimated from the test results on the specimens with a machined notch only agrees with that of δ_c obtained on the fatigue-precracked specimens.
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13. Effect of Notch Acuity on Critical COD

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