Determining the extent of a lateral shadow zone in an ultrasound inspection method

1. A method of determining the extent of a lateral shadow zone corresponding to an edge of a part, in a method of inspecting the part in immersion with at least one ultrasound beam emitted by a transducer oriented perpendicularly to a surface of the part, the transducer being at a predetermined distance from the surface of the part, the method comprising:

measuring the amplitude of an echo of the ultrasound beam as a function of the distance of the axis of the ultrasound beam from an edge of the part or from an edge of a beam deflector element disposed on the surface of the part;

determining the effective section of the ultrasound beam from variations in the amplitude of the echo of the ultrasound beam as a function of the distance of the axis of the ultrasound beam from the edge of the part or from the edge of the beam deflector element; and

determining the extent of the shadow zone from the effective section of the ultrasound beam, the extent of the shadow zone for the determined distance between the transducer and the surface of the part being equal to the radius of the effective section of the beam at the surface of the part.

2. A method according to claim 1 , further comprising measuring the variation in the echo amplitude at the reflection of the beam on the surface of the part as a function of the distance between the axis of the beam and the edge of the part, the distance between the transducer and the surface of the part being constant.

3. A method according to claim 2 , further comprising:

measuring the distance d 1 between the axis of the beam and the edge of the part from which the echo amplitude begins to decrease when the distance d 1 from the edge of the part decreases;

measuring the distance d 2 between the axis of the beam and the edge of the part at which said amplitude becomes zero; and

obtaining the size of the shadow zone from the edge of the part by calculating the value (d 1 +d 2 )/2.

4. A method according to claim 3 , further comprising:

measuring the distance of the axis of the beam from the edge of the part when the echo amplitude is equal to a small fraction of the maximum value of said amplitude; and

determining the distance d 2 between the axis of the beam and the edge of the part at which said amplitude becomes zero by extrapolation.

5. A method according to claim 4 , wherein said measuring of the distance of the axis of the beam from the edge of the part is performed when the echo amplitude is equal to a few percent of the maximum value of said amplitude.

6. A method according to claim 1 , wherein the transducer is oriented perpendicularly to the surface of the part away from the shadow zone, and the method further comprises:

moving the beam deflector element in translation on said surface along an axis that intersects the axis of the ultrasound beam; and

identifying the positions of said element at which the echo amplitude of the beam at the reflection on the surface of the part varies between a maximum value and a zero value.

7. A method according to claim 6 , wherein the deflection element is a prism presenting at least one reflection face that is inclined to the surface of the part and to the axis of the beam.

8. A method according to claim 7 , further comprising:

identifying the two diametrically opposite positions of the prism about the axis of the ultrasound beam for which the echo amplitude of the beam at the reflection on the surface of the part begins to decrease from a maximum value on moving the prism closer to the axis of the beam; and

calculating half of the distance between these two positions in order to obtain the extent of the lateral shadow zone.

9. A method according to claim 7 , wherein said at least one reflection face is inclined at 45° to the surface of the part and to the axis of the beam.

10. A method according to claim 1 , further comprising:

aligning the emission axis of the transducer on a calibration hole formed in the part outside the shadow zone; and

measuring the variation in the echo amplitude of the beam at the reflection on the end wall of the calibration hole while the beam is being progressively intercepted by the deflector element.

11. A method according to claim 1 , further comprising:

determining the size of the lateral shadow zone on a calibration part having identical flat-ended holes formed to the same depth and at different distances from the edges of the part, one of the holes being outside the shadow zone and the other holes being in said zone;

aligning the transducer on each hole in succession and in measuring the echo amplitude of the beam at the reflection on the end wall of the hole;

plotting the curve showing variation in said amplitude between a maximum value and a zero value as a function of the distance between the axes of the holes and the edges of the part; and

deducing the size of the shadow zone as being equal to the value of the distance at which said amplitude begins to decrease with decreasing distance between the axes of the holes and the edges of the part.

12. A method according to claim 11 , wherein the end wall of each hole is situated beside the surface of the part and each hole opens out into the opposite surface of the part.

13. A method according to claim 1 , further comprising determining a linear variation in the radius of the effective section of the ultrasound beam as a function of the penetration depth of said beam in the part on the basis of:

a maximal value for the radius corresponding to a zero distance between the transducer and the part at which the radius of the effective section at the surface of the part is equal to the radius of the transducer; and

a minimal value of the radius at a depth in the part corresponding to the focal length of the transducer.

14. A method according to claim 13 , further comprising, for a given distance between the transducer and the part and for a given depth within the part, calculating the extent of the lateral shadow zone by taking the difference between the radius of the effective section of the beam at the surface of the part and the radius of the effective section at the depth under consideration.

15. A method according to claim 13 , further comprising:

determining the minimal value of the radius of the effective section of the beam by placing the transducer at a distance from the surface of the part that corresponds to its focal length; and

measuring the radius of the effective section of the beam at the surface of the part.

16. A method according to claim 1 , wherein said measuring of the amplitude of the echo of the ultrasound beam is performed as a function of the distance of the axis of the ultrasound beam from the edge of the part, and wherein said determining of the effective section of the ultrasound beam is performed from variations in the amplitude of the echo of the beam as a function of the distance of the axis of the ultrasound beam from the edge of the part.

17. A method according to claim 1 , wherein said measuring of the amplitude of the echo of the ultrasound beam is performed as a function of the distance of the axis of the ultrasound beam from the edge of the beam deflector element disposed on the surface of the part, and wherein said determining of the effective section of the ultrasound beam is performed from variations in the amplitude of the echo of the ultrasound beam as a function of the distance of the axis of the ultrasound beam from the edge of the beam deflector element.