定荷重フラクトグラフィ試験によるＫＩ ＳＣＣ，ＫＩ Ｈおよび割れ進展速度の評価

Abstract

The threshold stress intensity factor, KISCC for stress-corrosion cracking or KIH for hydrogen assisted cracking, is generally determined by one of the cantilever constant-load test, constant-displacement test and rising-load test. In the present paper, problems encountered with these tests, especially on the accuracy, test duration, and/or measurement of crack growth rate, were discussed first. Then the holding-load and fractography test was proposed as a way to improve the existing procedures. An outline of the new testing procedure is as follows: Several precracked specimens exposed in test environments or precharged with hydrogen are loaded constantly at different KI values for a relatively short predetermined test duration. The specimens are then unloaded and separated apart by cleavage fracture or fatigue loading. The amount of subcritical crack growth during the test, ΔaH, is measured by an electron microscopic observation of the fracture surface. The value of KISCC or KIH at the onset of the crack growth and the growth rate are determined from the relation of ΔaH and its rate per test duration to KI value.The test procedure was used to predict the values of KIH and growth rate for hydrogen assisted cracking of 1/2 Mo steel and 2-1/4Cr-1Mo steel. The results indicate that the usual test procedure estimates higher KIH than the proposed one. Thus, it is considered that the reexamination of the existing KISCC and KIH data is required.