A new approach to detect and size closed cracks by ultrasonics

The effect of crack on time-of-flight of shear waves (4.5 MHz) polarized in perpendicular directions to the crack surface, propagating parallel to the direction of crack growth is investigated. The first and second back-wall echoes instead of weak crack-tip echo are used for the measurement of time-of-flight. The time-of-flight is measured for fatigue cracks grown by different loading histories in ferritic steel (pressure vessel steel A533B-1) under the condition of no loading. A correlation between the change Alpha Tau of the normalized time-of-flight at the crack position from the value for the initial notch in the material and the ratio of the crack depth alpha, to the specimen width, W, is found as Alpha Tau = -2.0 x 10-3 (alpha/W). This relationship is explained on the basis of the effect of plastic deformation occurring around the crack. It is shown that the depth of tightly closed fatigue crack in austenitic stainless steel (AISI 304) also can be evaluated under the condition of no loading using this realtionship.