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There has been reported the occurrence of cracks in bolts in nuclear and thermal power plants. This paper proposes a new method for depth sizing of crack in bolts using ultrasonic phased array technology. The finite element method is used to predict sectorial views for various crack depths when a linear array transducer is used to inspect bolts. Numerical results show that a peak associated with the vicinity of crack tip can be observed in the curve of echo intensity versus refraction angle for deep cracks. The refraction angle with respect to this peak decreases with crack depth increasing, and it is larger than the refraction angle corresponding to central beam impinging the crack tip. Therefore, crack depth will be underestimated if it is determined directly from this peak observed in a sector view by ultrasonic phased array technology. To verify numerical results, experiments are performed for bolt specimens where EDM notches with different depths are introduced. Experimental and numerical results are in good agreement with each other. Based on the refraction angle associated to the peak, a new method for depth sizing of crack in bolts is proposed. Comparison between determined and actual depths shows that it is possible to determine a crack depth accurately from inspection results by proposed method.

Proposal of a new method for depth sizing of crack in bolts based on ultrasonic phased array technology