METHOD OF DETERMINING THE BREAKING STRESS IN SHEAR OF A PART OF DETERMINED THICKNESS
A method of determining the breaking stress in shear for a part of determined thickness and made up of two elements that are bonded together by a layer of adhesive. A plane sensor emits an ultrasound wave at the part of determined thickness. The plane sensor receives a reflected signal made up of a plurality of successive echoes. A processor unit calculates a fast Fourier transform of the reflected signal. A Gaussian envelope connecting together the peaks of all of the resonances of the fast Fourier transform is determined. A frequency at which the Gaussian envelope is at a maximum is determined. The breaking stress from a predetermined correspondence relationship between the frequency of the maximum of the Gaussian envelope and the breaking stress is determined.
1 . A method of determining the breaking stress in shear for a part of determined thickness e and made up of two elements that are bonded together by a layer of adhesive, the method comprising the following steps:
using a plane sensor to emit an ultrasound wave at a determined nominal frequency towards said part of determined thickness;
using said plane sensor to receive a reflected signal made up of a plurality of successive echoes caused by the ultrasound wave being reflected on various interfaces in said part of determined thickness;
using a processor unit connected to said plane sensor to calculate a fast Fourier transform of the reflected signal as obtained in this way;
determining a Gaussian envelope connecting together the peaks of all of the resonances of said fast Fourier transform;
determining a frequency at which said Gaussian envelope is at a maximum; and
determining said breaking stress from a predetermined correspondence relationship between the frequency of the maximum of said Gaussian envelope and the breaking stress as previously recorded in said processor unit.
2 . A method according to claim 1 , wherein said determined thickness of said part is obtained prior to emitting said ultrasound wave by means of a micrometer.
3 . A method according to claim 1 , wherein said fast Fourier transform is calculated over a spectrum band surrounding a frequency response of said plane sensor.
4 . A method according to claim 1 , wherein the calculation of said fast Fourier transform is limited to no more than the first seven echoes of said reflected signal, excluding the first echo that relates to the reflection of the ultrasound wave on the outside face of said part.
5 . A method according to claim 1 , wherein said frequency of the maximum corresponds to the derivative of a polynomial function passing through the amplitude maximums of said resonance peaks, and obtained by means of a spread sheet.
6 . A method according to claim 1 , wherein said correspondence relationship is recorded in said processor unit in the form of a calibration curve or of a table of values.
7 . A method according to claim 6 , wherein said correspondence relationship results from traction tests performed on various test pieces for which the frequency of the maximum of the Gaussian envelope has been determined.
8 . An application of the method according to claim 1 , in determining the breaking stress in shear of an aluminum TA6V leading edge of a turbine engine blade made of a composite material having an “interlock” weave, by means of a plane sensor having a nominal frequency lying in the range 10 MHz to 25 MHz.