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1. (WO1998052032) METHOD FOR RESIDUAL STRESS MEASURING
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CLAIMS

1. Method for measuring residual stress in a metallic specimen wherein, by means of an ultrasound transducer an ultrasound entry wave having a ground frequency is introduced into a surface of the metallic specimen, the ultrasound entry wave is measured by means of an entry wave detector and the succeeding ultrasound exit wave is measured by means of a exit wave detector and the measured values of the ultrasound entry and exit wave are used to determine residual stress, characterized in that, for a selected entry gate time the value of the entry convolution integral of the measured ultrasound entry wave and a periodic wave of the ground frequency is calculated and during a selected exit gate time the value of the exit convolution integral of the measured ultrasound exit wave and a periodic wave of the ground frequency is calculated and the residual stress is determined using the calculated values of the entry convolution integral and the exit convolution integral.

2. Method according to claim 1, characterized in that, the value of the convolution integral for both the entry wave and the exit wave is calculated for two symmetrical "/2 shifted periodic waves and the residual stress is calculated from the ratio of the square root of the sum of the squares of the calculated values for the exit convolution integrals and the entry convolution integrals respectively.

3. Method according to claim 1 or claim 2, characterized in that, the period wave is a sine-wave.

4. Method according to any of the preceding claims for measuring the residual stress in a metallic specimen of aluminium, characterized in that, the periodic wave has a ground frequency of between 15 and 35 MHz preferably between 20 and 30 MHz.

5. Method according to any of the preceding claims, characterized in that, the entry ultrasound wave on which the calculation of the convolution integral is based is a burst of an ultrasound wave.

6. Method according to any of the preceding claims, characterized in that, the selected entry gate time is equal to the selected exit gate time.

7. Method according to any of the preceding claims, characterized in that, at least one of the electrical outputs of the detector for the entry wave and the exit wave is given a DC-offset prior to performing the convolution.

8. Method according to any of the preceding claims wherein at least one of the detectors has an electrical output, characterized in that, at least one of the electrical outputs of the detector for the entry wave and the exit wave is given DC-offset prior to performing the convolution of between 15 and 35 % preferably between 20 and 30 % of the maximum amplitude of the relevant wave when measuring the residual stress.

9. Method according to any of the preceding claims, characterized in that, the metal specimen is scanned by the ultrasound transducer and/or detector to form a plan view of the parameter distribution.