WO1990013024A1

WO1990013024A1 - Calibration standard for measuring surface cracks by ultrasound and method

Info

Publication number: WO1990013024A1
Application number: PCT/BR1989/000008
Authority: WO
Inventors: Dafley Ferreira; Guilherme Henrique Cunha De Oliveira
Original assignee: Companhia Siderurgica Paulista - Cosipa
Priority date: 1989-04-19
Filing date: 1989-04-19
Publication date: 1990-11-01

Abstract

The invention consists in a method for calibrating and checking the calibration of an ultrasonic instrument for evaluating the depth of surface cracks on steel parts, by using a slotted pentagonal gaging block (2) as a support for the straight beam transducer (1) and surface wave (3 and 4) transducers, speeding-up the use of Rayleigh surface waves and rendering it feasible, either in laboratory or during infield use, for evaluating depth of cracks, independently of their location and angular position.

Description

CALIBRATION STANDARD FOR MEASURING SURFACE CRACKS BY ULTRASOUND AND METHOD

The invention refers to a method for calibrating and checking a standard ultrasonic instrument, which can be employed by any industry concerned with non-destructive testing for determination of depth of surface cracks.

At the present status of technique, ultrasonic wave non-destructive methods for crack size evaluation make use of the amplitude of the echo related with standard surface discontinuity, or the amplitude decay, such as 6 dB or 20 dB decay, depending on the position of the ultrasonic probe in relation to the crack. The 6 dB and 20 dB technique can be employed with either straight beam or angle beam probes in evaluating the size of large surface discontinuities and consists in determining the point of highest reflectivity of the discontinuity, which means to maximize the echo. This technique does not render the evaluation any easier, so the professional involved must be very skilled to achieve.,,, accurate measurements, in. most cases resulting in approximate measurements. In spite of being imposed by international standards, such techniques are somewhat questionable as to their reliability. This has caused the appearance of new techniques and, among them, the transit time technique, which was introduced by the middle of the seventies. By the transit time technique, the crack is measured essentially by the time taken by the ultrasonic wave to go from the emitter crystal to the bottom of the crack and back to the receiver crystal, and either Rayleigh surface waves or the conversion of mode of the ultrasonic wave or even the difraction at the bottom of the crack etc, can be employed. When two Rayleigh wave probes are placed on the face of any steel part, with a known distance between them, one of them being an emitter and the other being a receiver, an echo peak will appear on the ultrasonic instrument screen, in a position which corresponds to the path between the two probes. When a surface crack exists between the two probes, the peak will be shown on the screen, shifted to the right hand margin. This happens because the Rayleigh wave has traveled along the contours of the surface crack before reaching the receiver probe and, of course, the deeper the crack the more the peak on the screen will appear shifted to the right. The screen is calibrated for the distance traveled by the Rayleigh wave on the steel part face, 'so the crack depth cait be known, since the crack is the cause of the wave peak to be displaced on the screen. However, when the Rayleigh wave hits the bottom of the crack, part of the energy turns round the crack and another part is converted mainly into a shear wave, which travels to the backwall of the steel part, is reflected back and returns to the bottom of the crack, where it is converted back into a Rayleigh wave, and proceeds to the receiver probe. As a consequence, two distinct peaks appear on the screen, one corresponding to the Rayleigh wave which turned round the crack and another corresponding to the shear wave reflected on the backwall of the part. This non-destructive test should be carried with the steel part under tractive effort. The difficult is knowing to what extent should the part be loaded so as to permit a good evaluation, and how accurate and reliable such evaluation will result. Experiments using the Rayleigh wave, in conjunction with the wave mode convertion technique, show that a good evaluation of the surface crack can be achieved if the cracked zone is under tractive effort, mostly at the elastic zone of the material.

By the technique of the transit time, with the aid of the Rayleigh wave, there is a tendency for the crack to be overevaluated. The transit time technique with the aid of the wave mode conversion shows, in general, an absolute error lower than that of the Rayleigh wave, but the- crack -size can be either underevaluated or overevaluated.

Many works have been written about the transit time technique that show how to attain accuracy in measuring cracks, but they all were based on experiments using laboratory equipment, far more accurate than the commercially available ultrasonic instruments, what renders this technique unfeasible for practical purposes. In addition to the transit time technique, there are other techniques such as electrical or magnectic ones, and a technique using eddy currents; but all of them have limitations for practical purposes, for the solution special problems in crack depth evaluation.

The solution proposed by the invention consists in a method to calibrate and check the calibration of a common ultrasonic instrument of the pulse-echo type, by means of a slotted pentagonal gaging block which, together with a straight beam transducer, permits to quickly calibrate the apparatus for the surface wave velocity and, together with two surface wave transducers, to check the calibration and evaluate the crack depth.

The dimensions of the slotted pentagonal gaging block are known, the thickness serving as a standard for calibration and the slots serving as standards for checking. In the case of the present invention the block is made of type-ABNT- Brazilian Standards Association - 1020 steel. Both the straigh beam and the surface wave beam transducers operate at 2 mHz frequency.

This method provides accurecy, reliability and reproducibility of the tests, avoiding the need for a skilled professional to execute them, as well as the problem of the space necessary for probes clearance, the impossibility to attain a result if there are too many cracks close together and the need for loading the part.

Reliability can be corroboreted by a comparative study of test results with standard values, achieved through laboratory methods. Besides being reliable, the test results are repetitive, what corroboretes their reproducibility.

The simplicity of the method adopted by the invention permits to do without large professional experience, to achieve accurate results.

The method of the invention, unlike other methods known, does not require that a clearance exists between the probes to determine the crack depth, so concursing to render testing easier in cases of cracking inside a slot or a groove, or of too many cracks close together.

A cutting tool fitted to a hand device known as a chisel and employed to widen the cracks permits to do without a loading. Describing in detail and referring to Picture 1, the ultrasonic instrument is calibrated by positioning the straight -beam transducer (1) on the face of the sloted pentagonal gaging block (2) , and by positioning, on the screen, the multiple echoes in correspondence with their respective values. If the thickness of the sloted pentagonal gaging block (2) is known, a scale is selected on the instrument screen which is equivalent to four times that thickness. This means that four echoes will be seen on the screen. The straight beam transducer

(1) emits longitudinal waves, perpendicular to the face of the part beeing tested. The nominal velocity, in the steel, of the longitudinal wave, emitted by the straight beam transducer (1) is 5,900 m/s. The nominal velocity, in the steel, of the surface wave emitted by the surface wave transducers (3 and 4 ) , is about half, that is, about 2950 m/s. It means that the values obtained for the multiple echoes are equivalent to half the nominal thickness values, as compared with values that could be obtained by using surface wave transducers. Thus, as long as the thickness of the sloted pentagonal gaging block (2) is known, the ultrasonic instrument can be adjusted for any scale desired, so speeding-up the calibration with the aid of the straight beam transducer (1).

Referring still to Picture 1, the terminals of the two surface wave transducers (3 and 4) are connected to the ultrasonic instrument and those two transducers are positioned side by side on the face of the slotted pentagonal gaging block (2) , the thickness of which is known, one acting as a transmitter, the other as a receiver, keeping a minimum clearance (5) betweem them, in order to zero the ultrasonic instrument. For such, the light peak seen on the screen should be positioned completely at the left, that is, at the zero of the scale.

Referring to Picture 2, to check the ultrasonic instrument calibration, the two surface wave transducers (3 and 4) are placed in positions (6 and 7) of the slotted (8) zone of the slotted pentagonal gaging block (2) and the readout on the ultrasonic instrument screen should be made compatible with the dept of the slot, that is, if the slot (10) is Xmm deep, so Xmm should be read on the screen of the ultrasonic instrument, if the slot (11) is Ym deep, so Ymm should be read on the screen of the ultrasonic instrument, and so on.

Deferring to Picture 3, in order to evaluate the depth of craks on flat surfaces, the ultrasonic instrument is zeroed by positioning the surface wave transducers (3 and 4) on the face of the steel part, in a zone free of cracks, and then positioning the two surface wave transducers (3- and 4) in positions (12 and 13) on the crack (14) , and reading the crack depth straight from the configuration (15) on the screem of the ultrasonic instrument.

Referring to Picture 4, in order to evaluate depth of cracks (16) in a zone with a variation in cross section (17) or with a depression (18) , the surface wave transducers (3 and 4) are positioned on the zone free of craks a cross section variation or with a depression similar to the cracked zone, the ultrasonic instrument is zeroed and then transducers (3 and 4) are positioned in positions (19 and 20) , on the cracked zone (16) , keeping the same clearance as used in zeroeing the ultrasonic instrument and reading the crack depth straight from the configuration (21) on the screen of the ultrasonic instrument.

When it is knowm, through other transducers, that the crack is not right-angled to the face, the angle of inclination is determined with the aid of adequate transducers, and then the crack length is determined by means of surface wave transducers (3 and 4) .

Referring to Picture 5, when it is not possible to zero the ultrasonic instrument in a zone free of cracks, the peri etrical distance (22) between the surface wave transducer (3) in position (23) and the surface wave transducer (4) in position (24) is measu¬ red, in order to determine the depth of crack (25) . The value read from the configuration (26) on the screen of the ultrasonic instrument is applied to the following form: p ^**** 1 - P/2 in which: p- depth of the crack; 1 = value read on the ultrasonic instrument screen; P = perimeter.

Referring to Picture 6, in case a wave travels from one face directly to the other skipping the crack, a hand device known as a chisel, fitted with a cutting tool in position (27) , is used to widen the crack. Sach tool is fastened to the chisel by means of a screw in position (28) . The internal space (29) , closed at the end (30), is usefull to preserve the cutting tool, when not in use.

Claims

Claims 1 - "METHOD FOR MEASURING DEPTH OF SURFACE CRACKS BY RAYLEIGH- ULTRASONIC WAVES", characterized by an ultrasonic instrument being calibrated and checked for the velocity of the Rayleigh surface wave by means of a slotted pentagonal gaging block (2) as follows: Initially, with a straight beam transducer (1) positioned on the pentagonal face and off the slots, the scale on the screen of the instrument is adjusted to a sentable lenght, like for instance four times the thickness of the gaging block (2), and the image, on the screen, of the multiple echoes is positioned at their corresponding values; afterwards, by means of two surface wave transducers (3 and 4) positioned on the same pentagonal face and off the slots, emitter and receiver placed side by side, keeping a minimum clerance between then (5) , the light peak on the screen is adjusted to the zero point of the scale and, then, with the two transducers (3 and 4) placed on the side faces of the pentagon, in front of each slot, positions (6 and 7), each slot is measured, so checking the calibration of the ultrasonic instrument; one proceeds then to evaluate the cracks (14, 16, 25), whenever possible zeroeing the instrument on a face free of cracks, the circuntances of which are equal to those of the cracks to be measured and, if necessary widening the crack by means of a cutting tool fitted at position (27) of a hand device known as a chisel.

2 - "METHOD FOR MEASURING DEPTH OF SURFACE CRACKS BY RAYLEIGH ULTRASONIC WAVES", as claimed in 1, characterized by the slotted pentagonal gaging block (2) being manufactured in steel, in the shape of a segment of a pentagonal prism, the upper and lower sections of which are perpendicular to the prism, by having minimum gap slots with internally rounded corners, cut along the bissetrix, at the five apices, with different depths, and by the high accuracy and regularity in overall dimensions which are known but necessarily unique.