US2856538A - Ultrasonic penetrant method of flaw detection - Google Patents

Description

United States Patent Office 2,856,538 Patented Oct. 14, 1958 ULTRASONIC PENETRANT METHOD OF FLAW DETECTION No Drawing. Application July 27, 1955 Serial No. 524,805

11 Claims. (Cl. 250--71) Ohio, assignor to Cleveland, Uhio, a corporation This invention relates to improvements in penetrant methods of detecting flaws having surface openings which may exist in parts and structures (hereinafter referred to as test bodies). More particularly the invention relates to improvements in such methods by the use of highfrequency vibrations, usually in the near ultrasonic range, i. e., from approximately 10,000 cycles per second to 500,- 000 cycles per second. However, one may often use vibrations down to the higher audible frequencies (i. e., to frequencies as low as 6,000 to 8,000 C. P. S.) and occasionally one way use vibrations in the high ultrasonic range, i. e., in excess of 500,000 C. P. S. The term sonic is usually understood to be synonymous with audible, but as a term to designate the range of frequencies broadly contemplated by this invention, the term sonic will be used hereinafter.

Ever since the introduction to the art of the fluorescent penetrant method of flaw detection disclosed in the United States patent to Robert C. Switzer, No. 2,259,400, for Flaw Detection, penetrant methods of flaw detection have become increasingly significant methods in the field of non-destructive testing. Essentially the method is limited to the detection of cracks, tears, blow-holes, laps, and the like having surface openings, as contrasted with other methods such as X-ray, magnetic flux, reflected ultrasonic pulse, and like methods which are generally employed to locate flaws in the interior of the test body, although some, such as the magnetic flux method, are also frequently effective, when they are operative to any appreciable degree, in locating flaws having surface openings as well as purely internal flaws. The reason for the increasing use of the penetrant method of inspection is that a minute surface discontinuity, especially at or near areas of maximum stress, is often likely to be the source of failure in service whereas purely internal flaws, unless quite gross or in a part designed with a very small factor of safety, may be relatively harmless.

The penetrant methods of inspection, in general, comprise the steps of applying a penetrant liquid to the surface of the test body, allowing portions of the penetrant to enter the flaws, removing the remaining penetrant from the surface of the body, allowing the penetrant retained in the flaws to appear at or in flaw openings, and then inspecting the body for the indication made on the surface by the retained penetrant. Where the penetrant is fluorescent and inspection is carried out under fluorescigenous radiations (e. g., black light) in accordance with the aforesaid Switzer process, very minute flaws are readily located. Since the introduction of the Switzer method, various and significant improvements have been made, as in methods of removing the penetrant from the surface of the article without removing appreciable quantities from the flaws, in aiding the development of the flaw indication, increasing the fluorescent efiiciency of the penetrants, and the like. Also, various techniques in removing the penetrant from the surface and developing subsequent flaw indications at or around the flaw openings have permitted the use of non-fluorescent penetrants whose indications are observable in ordinary visible light.

With relatively few exceptions, such as in the U. S. patent to Richard A. Ward, No. 2,405,078 for Methods and Compositions for Locating Surface Discontinuities," which teaches the art that a proper sequence of heating and cooling the test body can aid the actual penetration of the flaws by the penetrant, those skilled in the art may seem to have generally overlooked the fact that, regardless of how the characteristic of the penetrant may be improved, or how precisely the penetrant may be removed from the surface of the test body without removal from the flaws, or how efiectively the indication may be developed, the success of the operation of any of the penetrant methods of flaw detection depends upon getting the penetrant into the flaws and subsequently having retained penetrant appear at least at, .and generally preferably around, the flaw opening.

Actually, the interest in and development of aspects of penetrant inspection methods other than procedures to promote penetration and expulsion of the penetrant has not been due to an oversight by the art. Rather, heretofore, there did not appear to be much of anything which could be done to improve penetration and expulsion of the penetrant beyond careful formulation of the penetrant'to obtain optimum penetrability with respect to the materials of the bodies to be tested and, of course, to thoroughly clean the test body of extraneous matter which might clog the flaw openings, as taught by the above Switzer and Ward patents. Such apparent expedients as application of pressure, lowering the viscosity of the penetrant with solvent, et cetera, made no appreciable improvement in the aspect of the process. In the first place, lowering viscosity does not necessarily improve penetrability; in the second place, the surface areas of the flaw openings to be located are usually very small; thus, enormous pressures, in terms of pounds per square inch, become, at the flaw openings, very insignificant forces.

It is an object and advantage of this invention to improve the penetration of liquid penetrants into flaws in penetrant inspection methods, and by the same forces, namely, sonic forces, to aid in the expulsion of such retained penetrants to or around flaw openings.

A further object and advantage of this invention is to improve the sensitivity of penetrant inspection methods and to increase the capacity of existing penetrant inspection facilities by shortening the time required for penetration and expulsion.

Other objects and advantages of this invention will be apparent from the following general and detailed disclosure and the appended claims.

As indicated above, for satisfactory and reliable detection of flaws in a test body it is necessary to remove any foreign matter which may block or clog the flaw openings. As one procedure for effecting cleaning, the so-called ultrasonic procedure Was tested and found, in general, to offer no particlar advantage over other methods except for small parts, which can be cleaned en masse in a tank of ultrasonically vibrated liquid. In ultrasonic cleaning the cleaning liquid is vibrated ultrasonically either by a transducer submerged in the bath or by an ultrasonically vibrated bottom and/ or walls of the vessel containing the liquid. It has been proposed to submerge a suspected part in an ultrasonically vibrated bath, not of the usual light hydrocarbon or chlorinated hydrocarbon employed for ultrasonic cleaning, but of a preferred fluorescent penetrant. Where the suspected part had a tightly closed cold shut extending to its surface, improved penetration was observed. This result is assumed to follow from two concepts. One concept is that, with the high proportion of kerosene or like light hydrocarbon in many fluorescent penetrants and the emulsifying agent often present to render the penetrant self-emulsifying, the penetrant vehicle can itself function as a cleansing liquid. The second concept is: that possibly one reason ultrasonic cleaning was not particularly effective in cleaning tightly closed flaws is because normal cleaning liquids may remove foreignmatter from such flaws but, in doing so, the cleaning liquids may remain in such flaws and become, themselves, foreign matter resisting penetration. and displacementby subsequently applied fluorescent penetrants; by using thelfluorescent penetrant itself as the cleaning liquid, any; such. residual cleaning. agent would reveal itself in subsequent inspection under. fluorescigenous radiations, provided a sufficient volume of penetrant was retained in the flaw to permit detectable amounts to creep to and appear at. or. around the flaw opening at the time of inspection. Unfortunately, except in the foregoing instances where the above postulates would have an opportunity: to be effective, ultrasonic cleaning or immersion in a. vibrating bath. of fluorescent penetrants offered no noticeable improvement in ultimate results over those obtainable by standard procedures of cleaning by any suitablemeans, application and removal of penetrant after allowance of: time for penetration and reappearance of retainedpenetrant, and inspection. for. retained penetrant atandaroundflaw openings. 1

It;had also beennoted that when parts which had been subjected to. penetrant inspection methods were subsequently inspected. by reflected ultrasonic pulses to locate internal laminations and the like, such ultrasonic pulses did not noticeably promote the expulsion of the penetrant from fiaws which had surface openings.

In view of the foregoing experiences, it was, therefore, with". considerable surprise that an ultrasonically driven magnetostrictive transducer was observed (when it happended' to beremoved while still being driven ultrasonieally-atabout 10,500 C. P. S. from a liquid havingrather poor penetrating characteristics) to expel, nonetheless, liquid with considerable force from a minute fatigue crack thatwas apparently developing in theweld securing the face plate to the armature of the transducer. Since the transducer with its incipient surface defect had only been immersed for a very short period of time, several conclusions could be drawn from this chance observation: (1)The volume of liquid expelled from the slight opening indicated that the void behind the opening was much larger than would be expected from the size of the opening. (2) The volume of liquid which had penetrated into the void through the opening was much greater than would be'expected to have penetrated in the period of time the transducer was immersed if the penetration was due solely to undisturbed'surface active forces, the forces presumably relied on in prior penetrant inspection methods. (3) The liquid was being expelled from the flaw at'a vastly greater rate than the slowrate usually allowed form prior penetrant inspection methods. (4) The-liquid which had penetrated into the'void' had somehowhad its penetrating characteristic greatly improved. (5) The flaw" in the transducer was behaving entirely differently-from' behavior which would be expected from previous experiences in subjecting articles having surface flaws to sonic vibrations.

Initially the only tenable explanation for the above observed results appeared to be that, in the prior experiences-the articles subjected to ultrasonic vibration probably did not have a natural frequency corresponding to the'frequency of the sonic vibrations to which they were subjected, whereas the frequencies imposed on the flawed transducer approximated the natural frequency of the transducer, it being a general requirement of transducer designthat its mass and shape have a natural frequency approximating the'frequency to be imposed upon it. Furthertests-of the validity of the' foregoing explanation do not appearto support a conclusion that, inorder to improve the penetrability of a liquid penetrant into a flaw having a surface opening in a solid article and to drive the penetrant into and out of the flaw ultrasonically, the sonic frequencies must approximate the natural frequency of the article. Rather, it seems to be only necessary to select a sonic frequency such that the article will absorb and vibrate in response to, i. e., resonate with, the sonic vibrations imposed upon it. It is still not understood precisely why such selected resonant frequencies produce the observed effects of drawing or forcing a penetrant liquid into a flaw when the surface opening of the flaw is covered by the liquid and Why such sonic vibration will expel penetrant when the opening is no longer covered with liquid.

Example 1.--One specific example utilizing the discovered phenomenon is as follows: A large quantity of forged aluminum airframe fittings were to be tested for flaws having surface openings. Suspected flaws in the forgings were closed forging tears. This type of flaw may be caused-by .a tear in the billet when it is subjected to the first or an early forging blow, which tear is then closed, but not welded, either in subsequent forming and shapingblows or later in theblow which caused the tear. It is difiicult to-locate this type of flaw, even with painstaking, and precise fluorescent penetrant testing procedures as heretofore conducted. In order to determine a suitable resonant sonic frequency for testing the number of forgings, a fewwere-tested by a fluorescent penetrant procedure as disclosed in the above Ward patent untila sample forging with an observable and known flaw with a surface opening was secured. The sample flawed forging was thenimmersed ina bathof self-emulsifying fluorescent penetrant, such as disclosed in-the-said-Ward patent, and a polarized polycrystalline transducer (bariumtitanate) driven at its resonant frequency of 14,000 C. P. S. was pressed momentarily, i. e., about 10 seconds, against the. sample forging while itwas immersed in the bath.. The particular frequency of the transducer was chosen because prior experience hadtaught the operator that a part having the mass and shape of the forging was likely to be resonant in the range of the chosen frequency. It should be noted that the momentary immersion of the sample contrasted with the relatively prolongedimmersion of, say, fifteen minutes, allowed in prior fluorescent penetrant procedures for penetration of the flaw. The sample forging was then removed from the penetrant bath, drained, and washed to remove from its surface the penetrant which still clung to it The selected transducer was then applied after the sample had been dried. The second application of the transducer was made in relative darkness under a black light for a momentary period of about twenty to thirty seconds. A-fluores'cent indication began to appear as the faceplate of the transducer was moved over the surfaceof the sample forging,

but not as rapidly as it should have if the samplewere resonant at 14,000 C. P. S. The sample wasreturned to the bath and the procedure was repeated, this time with a transducer vibratingat 15,500 C. P. S. Under'the black light a brilliant fluorescent indication ofthe flaw began to appear in less than five seconds. To box in the resonant transducer, the process was repeated'with' a transducer vibrating at 18,250 C. P. S., but the appearance of the indication was not as rapid as when the 14,000 C. P. S. transducer was employed; If the'"15,50( C. P. S. transducer had not indicated the'reson'ant fre quency of the forging, one vibrating'at 12,750 C. P. S. would have been tried. In this connection it should be pointed out that as the resonant frequency of anarti'cle increases, the band to which it will resonate appears to broaden. Thus, at audible frequencies of 6,000 to 8,000 C. P. S., a resonant band may be as 'narr'owas 500 C. P. 8;, whereas at high ultrasonic frequencies, say, above 1,000,000 C. P. S. the band may beas'broadas 100,000 C. P. S. Ceramic transducers are recommended for the purpose of'determining resonantfrequencies' of articles to be tested because of the universal range of ceramic material from audible to high ultrasonic. Magneto-strictive transducers appear to be etficient only below 50,000 C. P. S. But since most articles to be tested appear to have a resonant frequence in this range, and since this process is especially adapted for the production testing of mass produced articles, it has been found practical, once the resonant frequency of the article has been determined, to design magneto-strictive transducers for the article in order to take advantage of their ruggedness and the ability to weld on a new face plate after a face plate has commenced to fatigue and pit in use.

Once the resonant frequency of the forgings had been determined, the testing of individual pieces followed the procedure set forth in the above Ward patent, except that the pieces were subjected to resonant Vibrations in the bath of penetrant and after drying but prior to inspection under black light. Whereas about fifteen minutes would have been allowed for soaking in the bath, the pieces could be removed in a matter of seconds after being subjected to sonic vibrations. Likewise, while the pieces would have been normally allowed to stand after drying for about three-quarters of an hour to allow the indications to appear, subjecting the pieces to resonant frequencies for approximately thirty seconds fully developed the indications. After sonic development of the indications, the pieces were dusted with finely powdered chalk. Whereas such chalk is used in the Ward process to absorb penetrant from the flaw openings and provide a reflective background for the indications, in this process the chalk simply provided a reflective background to enhance the brightness of the indications. When inspected under black light, the indications were noticeably bright because of the greater amount of exuded fluorescent penetrant and inspection was, therefore, much faster. It is suspected that this process developed indications which would not have been developed without the aid of sonic vibrations, but the possible result is only speculative.

Example 2.-Iron castings were inspected with a selfemulsifying visible penetrant such as disclosed in the co-pending Ward and Switzer application, Serial No. 606,708, filed July 23, 1945, for Detection of Flaws and according to the procedure taught therein, except that, prior to inspection, the castings were subjected to resonant sonic vibrations. The expected flaws were blowholes and shrinkage cracks at the filets, relatively open and deep flaws which are satisfactorily penetrated with out the assistance of sonic vibration. The roughness of the surface warranted removal of the penetrant from the r surface of the castings with a thorough scrubbing spray. Although this tended to remove penetrant from the flaw openings as well, such removal could be tolerated because of the depth and volume of the flaws. The purpose of subjecting the washed and dried castings to resonant sonic vibrations was to accelerate the appearance of the penetrant indications and to increase their size and distinctiveness by expelling more penetrant from the flaws.

Example 3.-Ball-bearing races were inspected according to the procedure of the above Ward patent. The expected flaws were grinding cracks and possibly cracks caused by heat-treatment. These cracks may be very fine and the parts normally require lengthy immersion to insure penetration. To accelerate penetration, the parts were subjected to a resonant sonic vibration. With the cracks fully filled with penetrant, they developed distinctively and more quickly and a resonant sonic vibration was not deemed necessary at the inspection stage.

Example 4.-T-hjs modification relied on the magnetostrictive characteristics of the alloy steel wrist pins being tested and is useful only for testing parts which are made of material having such characteristics. Fine grinding and polishing cracks were the flaws most likely to be found in the wrist pins. While immersed in a bath of fluorescent penetrant as disclosed in the above Ward patent, the pins were pushed through a coil connected to a source of alternating current having a frequency equal to the resonant frequency of the wrist pins. As the pins were pushed through the coil, they were subjected to pulses of a high frequency magnetic field, the pulses being of a duration of one-sixtieth of a second, spaced one-thirtieth of a second apart. During these pulses or bursts, the pins became, in effect, the resonating armature of a magneto-strictive transducer. Penetration of the flaws by the penetrant was greatly accelerated. The pins were then washed, dried and developed in the usual manner prior to inspection under black light. The development of the fluorescent flaw indications could have been accelerated by similarly subjecting the pins, after drying, to a high-frequency magnetic flux, but, as in Example 3 above, such aid to the development of the flaws was not deemed necessary.

It is to be understood that in the foregoing examples, a non-fluorescent penetrant may be employed instead of the stated fluorescent penetrant, or vice-verse. If a fluorescent penetrant is employed, it is composed of a fluorescent dye dissolved in the penetrant vehicle to impart to the penetrant a different fluorescent hue, or at least a hue of much greater fluorescent intensity in thin films, than any actual or simulated natural fluorescence of the vehicle, such a dye being hereafter referred to as a fiuoragent. If a non-fluorescent penetrant is employed, it is comprised of a dissolved dye or finely dispersed pigment inseparable from the penetrant vehicle by the sonic vibrations employed and imparting to the penetrant a visible hue which contrasts in visible light from the hue of the background of the flaw indication; such a non-fluorescent dye or pigment is hereinafter referred to as a coloring agent. It is also to be understood that, to provide a light-reflecting background for the flaw indications provided by the exuded penetrant, one may apply to the test body a material which absorbs the penetrant and provides a contrasting, light-reflecting background for the fluoragent or coloring agent in the penetrant.

It should be apparent to those skilled in the art that the foregoing examples are illustrative only and deal with test bodies having fairly constant cross-sections and, thus, fairly constant resonant frequencies throughout. The process may be operative for testing articles which appear to have varying resonant frequencies in different portions. In testing such articles it may be necessary to transduce different frequencies in different portions of the article, care being exercised against fracturing the articles in the boundary zones between portions having different resonant frequencies.

As pointed out above, there are many possible speculative explanations for this process. At the resonant frequencies, the articles being tested may stretch and shrink, thereby opening and closing the surface openings and voids of the flaws and working the penetrant in and out as a liquid would be worked in and out of the pores of a sponge. The high frequencies of the vibrations may disturb the surface-active forces at the interfaces of the flaws and the penetrant and thereby overcome or counteract forces which might otherwise obstruct the movement of the penetrant in the flaws. There is also some evidence that such sonic frequencies radically alter the viscosities of liquids, creating minute and fleeting cavitations in which the pressure is lower than the vapor pressure of the liquid, thereby causing the liquid to behave more like vapor than a liquid within the environs of.

such cavitations. Whenever more knowledge is gained in the art regarding the correct explanation or explanations of the phenomenon involved, substantial improvements and changes may be made in this process without departing from the scope of this invention as set forth in the appended claims.

What'- is claimed is:

1. The process of ins'pecting' test bodies for flaws haw ing' surface openings comprising the steps of applying a penetrant liquid to the surface of the test body, removing' the penetrant from the surface, inspecting the body for indication of the location of such flaws by the appearance on the surface of the test body of a portion of the penetrant which was retained in the fiaw' during the removal of the penetrant from the surface, and subjecting' the test body to sonic vibrations to which the body is resonant after the penetrant has been removed from the surface thereof in order to accelerate the appearance of flaw indications.

v 2. The process of inspecting test bodies for flaws having surface openings comprising the steps of applying a penetrant liquid to the surface of the test body, removing the penetrant from the surface, inspecting the body for indication of the location of such flaws by the appearance on the surface of the test body of a portion of the penetrant which was retained in the flaw during the removal of the penetrant from the surface, and subjecting the test body to sonic vibrations to which the body is resonant when the penetrant is applied to the body in order to accelerate the penetration of flaws by the penetrant.

3. The process as defined in claim 2 in which the body is also subjected to sonic vibrations after the penetrant has been removed from the surface thereof in order to accelerate the appearance of flaw indications.

4. The process of inspecting test bodies of magnetostrictive material for flaws having surface openings comprising the steps of applying a penetrant liquid to the surface of the test body of magneto-strictive material, removing the penetrant from the surface, inspecting the body for indication of the location of such flaws by the appearance on the surface of the test body of a portion of the penetrant which was retained in the flaw during the removal of the penetrant from the surface, and subjecting the test body to sonic vibrations to which the body is resonant by placing the body in magnetic field varying at a frequency substantially equal to the frequency at which said body is resonant.

5. The process as defined in claim 4 in which the test body is subjected to said sonic vibrations after the penetrant has been removed from the surface thereof in order to accelerate the appearance of flaw indications.

6. The process as defined in claim 4 in which the test body is subjected to said sonic vibrations when the penetrant is applied to the body.

7. The process as defined in claim 4 inv which the test body'issubjected to said sonic vibrations when the penetrant is applied to the surface and after the pene trant has been removed.

8; The process of inspecting testbodies for flaws having surface openings comprising the steps of applying a penetrant liquid containing a fiuoragent rendering the penetrant fluorescent to the surface of the test body, removing the penetrant from the surface, inspecting the body under fluoreseig'enous light for indication of the location of such flaws by the appearance on the surface of the test body of a portion of the penetrant which was retained in the flaw during the removal of the penetrant from the surface, and subjecting the test body to sonic vibrations to which the body is resonant.

9. The process as defined in claim 8 in which the surface of the test body is provided with a light-reflecting background for the flaw indications by applying a material which absorbs the penetrant and reflects light emitted by the fluoragent in the penetrant, said material being applied before carrying out the inspection step.

10. The process of inspecting test bodies for flaws having surface" openings comprising the steps of applying a penetrant liquid containing a coloring agent of a color different from the surface color of the test body to the surface of the test body, removing the penetrant from the surface, inspecting the body under visible light for indication of the location of such flaws by the appearance on the surface of the test body of a portion of the penetrant which was retained in the flaw during the removal of the penetrant from the surface, and subjecting the test body to sonic vibrations to which the body is resonant.

11. The process as defined in claim 10 in which the surface of the test body is provided with a light-reflecting background for the flaw indications by applying a material which absorbs the penetrant and reflects light of the hue of the coloring agent, said material being applied before carrying out the inspection step. 7

References Cited in the file of this patent UNITED STATES PATENTS De Forest et a1 May 11, 1954