Measuring and testing – Vibration – By mechanical waves
Patent
1994-12-22
1996-07-23
Chapman, John E.
Measuring and testing
Vibration
By mechanical waves
G01N 2910
Patent
active
055378757
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for the ultrasonic inspection of point welds of metal sheets or plates, in which a high-frequency test probe with a transducer diameter equal to the nominal diameter of the point weld to be inspected is coupled, with a lead section, to the sheets in the vicinity of the point weld and, using the pulse/echo method, the sheets are irradiated with ultrasonic signals and the echo pulse train corresponding to overall-thickness is evaluated.
The inspection of point welds is described in the book J. Krautkramer and H. Krautkramer "Werkstoffprufung mit Ultraschall" (Ultrasonic testing of materials) 4th ed., pp. 505 and 506. According to this book the inspection of point welds between two sheets involves vertical testing and acquisition of the multiple echoes of the overall thickness. If the lenticular weld joint (welding spot) is well developed, a relatively short echo pulse train results from the overall thickness of the weld jointed sheets, because of the high attenuation (damping) of sound at the coarse grain of the welding spot solidified from the melt. A cold weld on the other hand entails a longer echo pulse train from the overall thickness due to the fine grain of the sheet with significantly lower attenuation of sound; the interface between the two surfaces does not reflect in a typical cold weld. In general a very long multiple echo train results from an ideal cold weld. Too small a volume of the welding spot causes the multiple echo train to lengthen as well, as the path traveled by the sound pulse within the fine-grain sheet becomes longer and the path through the more strongly attenuating grain of the welding spot becomes shorter. At the same time a shift of the ultrasonic frequency occurs. If there is no joint whatsoever between the sheets, no echo pulse train results from overall thickness. In that case there is only the echo pulse train for the single sheet thickness of the coupled single sheet.
In passenger car manufacturing, standard spot diameters of welding spots are between 3.6 and 5.5 mm. The transducer diameters of the test probes are dimensioned accordingly. The test probe frequency is in the MHz region, for example at 15 MHz. Coupling is effected with a lead section of synthetic material or water.
Proper operation of the test probes is of special importance. Proper irradiating conditions are only obtained with a precise vertical coupling of the test probe and transmission of sound through the center of the welding spot. During practical testing this is achieved by slightly shifting the test probe laterally into the center of the welding spot and lining it up vertically, to be identified by the intermediate echo train from the single sheet thickness(es) reaching a minimum. The shortest possible echo pulse train from the overall thickness and the smallest possible echo pulse train from the single sheet thicknesses indicate that the optimal positioning and thus the optimal irradiation conditions for the test probe have been obtained. Non-optimal irradiation conditions may entail a measuring result indicating an inadequate weld joint for a possibly good weld point; or, when the test probe is improperly lined up, the absence of defect echoes resulting from strong attenuation may create the incorrect impression of a good weld point.
The evaluation of echo pulse trains on the screen of an ultrasonic device is difficult and demands great experience on the part of the examiner. This in turn means that objective results cannot always be achieved.
This is where the invention comes in. Its object is to further develop the ultrasonic inspection method of the type mentioned initially in such a way, that it becomes suitable for automatically monitoring the quality of point welds between metal sheets.
Taking as a basis the method of the type mentioned initially, the invention suggests the following solution: the frequency spectra of the echo pulse trains are calculated and for each measurement the spectral line corresponding to the overall-thickness travel time is examined
REFERENCES:
patent: 4208917 (1980-06-01), Aoyama et al.
patent: 4265119 (1981-05-01), Dubetz et al.
patent: 4428235 (1984-01-01), Sugiyama
Viehmann Hans
Volkmann Klaus
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