Method for the non-destructive testing of a blade preform

1. An automated method for the non-destructive testing of a woven preform, for manufacturing a turbine engine part and comprising a plurality of first marker threads intertwined with second marker threads, the first and second threads having light-reflection properties different from those of the threads of the preform and being woven with the threads of the preform so as to form a surface grid on a given area of the preform, the method comprising successively of:

a) placing the preform in a predetermined position so that the grid of first and second marker threads is situated opposite at least two image sensors aimed at the grid and having optical axes forming an angle with each other;

b) illuminating the given area of the preform and acquiring, with each image sensor, an image of the grid of first and second marker threads;

c) determining for each image the coordinates in the reference frame of the image of the points of intersection of the first and second marker threads;

d) deducing the actual position in space of each point of intersection of the first and second marker threads on the preform by a triangulation calculation using the coordinates of the point of intersection in question in each of the images obtained with the image sensors and by means of the actual positions of the sensors in space and the orientation of their respective axes in space;

e) comparing the actual positions of the points of intersection of the first and second marker threads with three-dimensional theoretical positions of these same points of intersection.

2. A method according to claim 1 , wherein step c comprises:

determining on each image the contours of areas corresponding to the visible parts of the first and second marker threads;

determining for each image regions comprising aforementioned areas aligned on the first marker threads and the regions comprising aforementioned areas aligned on the second marker threads;

determining in each aforementioned image region the coordinates (x i , y i ) of the centre of barycentre of the various areas corresponding to the visible parts of the first and second marker threads and generating a mathematical curve best passing through these coordinates (x i , y i );

determining the coordinates of the points of intersection between the mathematical curves extending along the first marker threads and the curves extending along the second marker threads.

3. A method according to claim 2 , wherein the contours of the areas corresponding to the visible parts of the first and second marker threads are determined by dynamic thresholding according to their form and the light-reflection level compared with the rest of the image.

4. A method according to claim 2 , wherein the centre of each visible-part area of the first and second marker threads is determined by assimilating each part area to an ellipse.

5. A method according to claim 1 , wherein steps b and c are repeated n times and in that the following steps are initiated according to the calculation of a required criterion of precision of measurement of the coordinates of each intersection.

6. A method according to claim 5 , wherein the precision criterion comprises a standard deviation calculation on the coordinates of each intersection in each image, the steps following steps b and c being initiated for a standard deviation below a given threshold.

7. A method according to claim 5 , wherein the number n is greater than 10.

8. A method according to claim 1 , wherein the preform is a fan blade preform and wherein the grid of first and second marker threads is formed on the convex face or the concave face of the blade preform.

9. A method according to claim 7 , wherein the preform is woven with carbon threads and wherein the marker threads are produced by an assembly of glass threads and carbon threads, the glass threads having a lighter colour than the carbon threads.

10. A method according to claim 1 , wherein the preform is mounted on a support conformed so as to support the preform in a predetermined position and is then moved opposite the image sensors inside a chamber with walls absorbing light rays and housing means for illuminating the preform.

11. A method according to claim 1 , wherein the image sensors are cameras with a matrix of photodetectors of the CCD or CMOS type having a resolution of approximately 10 megapixels and a focal length of approximately 8.5 mm.