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FLAW DETECTION Non-Destructive Testing The use of higher frequencies such as 25 Mc/s and Inspection makes possible a variant of the method, in which the display shows a resonance pattern of the waves within the material. This gives a very An Account of Recent American Experience with sensitive measure of the thickness of material, or the depth of a crack, as the pattern obtained Various Techniques is characteristic of the thickness. One application of this method is to measure the thickness of skin remaining on aircraft where corrosion pro ducts have had to be sanded away. This applies particularly to 75 ST material. The thickness produces its characteristic resonance pattern on N June 2 a small group of people interested Fluorescent Penetrants the screen, and this is then matched by the in non-destructive testing assembled in This method is based on the fact that certain operator on a calibrated tapered specimen of the London to hear an informal and most in oils will penetrate into any surface cracks and same material. With experience the patterns formative lecture on the use of such methods in that when viewed by ultra-violet light they obtained can be interpreted to give a measure of the U.S. aircraft industry. The speaker was Mr fluoresce. The success of the method depends on the porosity or other defects in new material. William C. Hitt, Assistant Chief, Quality Control, the sensitivity of the penetrants, but one known Douglas Aircraft Co. Inc., Santa Monica, Cali Because it requires a smooth surface on the as a 'post-emulsifier' has proved very satisfactory, fornia. The meeting was organized by Inspection specimen, and because it is less easy to direct the even on rough surfaces, the only pre-treatment Equipment of 19 Drury Lane, London, W.C.2. impulses in a direction inclined to the surface, being degreasing in a trichlorethylene vapour bath. the contact method is not used as extensively as Mr Hitt first showed a film which gave a general the water-coupled method. Tanks up to 40 ft. picture of inspection methods at Douglas, the long have been built for the inspection of large principal techniques shown being X-ray, magnetic Ultrasonic Inspection slabs and the crystal head is traversed over the crack detection, fluorescent penetrants and ultra specimen automatically. Flaws detected are sonics. After the film Mr Hitt showed a large The most powerful of the methods described marked for closer inspection later. Forgings of number of slidcs. by Mr Hitt was flaw detection by the use of ultra suitable shape may be inspected on rotating sonics. In this method a pressure or sound wave turntables under water. For repetition work is applied to the surface of the part and travels X-ray Examination permanent installations are used, but much can through the material, being reflected back from be done with improvised facilities. A children's Apart from the direct radiography of parts, the far surface. Any flaw in the wave path will rubber paddling pool has been used, and the particularly castings, X-rays are being used for also give an echo. A measure of the relative tim part scanned by hand; and a plate to be inspected the examination of small assemblies, detecting ing of the reception of the echoes gives an indica formed the bottom of a tank made by clamping such faults as the maladjustment of contacts in tion of the position of the flaws. wooden borders to its edges. It is also possible sealed switches. From this has developed the use In practice a crystal transducer acts as both to have the crystal head surrounded by a stream of radiography for precautionary inspection in vibration generator and receiver. This may be of water from a hose playing on the surface of airframe maintenance. Sections of the airframe applied directly to a smooth surface coupled with the part to be inspected. This method has been which - known to be prone to fatigue damage a little oil, or, in a more flexible arrangement, the applied to ferrous structures where a trolley with can be X-rayed without dismantling or even, in part is immersed in water and the crystal tra magnetic wheels traverses itself over the specimen, the case of fuselages, without taking down furnish versed over it, also in the water but not in contact crawling up vertical surfaces. Another develop ings and insulation. Radiographs of very good with the part. The water carries the pulses to the ment for speeding up the inspection process is definition can be obtained and cracks detected. surface of the part. Frequencies of 5 Mc/s are the use of wider crystals made up of a mosaic of Care and practice are needed in the interpretation in common use, but for some purposes 25 Mc/s smaller ones so as to give a wider coverage at of the negatives as the fact that several materials is a more useful frequency. The presentation of each pass. having differing densities may be present can be the received signal on the oscilloscope can be in confusing to those used to seeing radiographs of One technique for helping the inspector to the form of A, B or C seans, to borrow terms components of only one material. Mr Hitt decide whether a crack or other flaw will machine from radar practice. On the A scan echoes are showed examples of such radiographs in which out, particularly on a complex forging, is to take represented as vertical displacements at points cracks could be seen, and in one case the absence a rejected but fully machined part and section it along a horizontal base representing the time of a short section of stringer which had fractured in strips. An unmachined forging is then sectioned base. This gives the position of the flaws, if any, out completely due to vibration. As this failure on the same planes and a standard of comparison on the axis of the crystal head, i.e. in one dimen was behind an ice shield and covered from within for the amount of metal machined away at each sion. On the B scan the thickness of the material the fuselage by toilet fixtures, the saving in using station is then readily available. is represented in the vertical direction and by radiography was considerable. At least three U.S. using a tube with an appreciable delay time, a airlines now use X-ray inspection at maintenance The development of these ultrasonic techniques two-dimensional picture of a cross-section, show checks, and Douglas are recommending X-raying has made possible a very good assessment of the ing the extent of flaws, in that plane, can be soundness and strength properties of material as an alternative to stripping out and inspecting obtained as the head traverses over the part. The and forgings, and has much reduced the amount when checks on various sections of the structure C scan, which corresponds to the P.P.I, of radar, of machining and other work put into material of the aircraft are found to be necessary. It is not gives a circular plan view of a part which is subsequently found defective and scrapped. a perfect tool but it will detect dangerous cracks rotating past the sensing head. if properly applied. There is no question of cover ing the whole aircraft with X-ray film, but certain A considerable amount of work has been done Sandwich Panels selected locations can be examined from a num at Douglas and in association with other com ber of viewpoints, and for a fleet of aircraft of panies on the evaluation and calibration of the An inspection problem of increasing import the same type the co-ordinates of the point from ultrasonic method, using test pieces with known ance is the checking of the bond between the core, which the view is taken can be standardized holes. It has been found that a flaw 1/64 in. in especially honeycomb, and the skin of sandwich relative to aircraft datum. The method is applic diameter can be detected and that the signal is panels. One method used by Douglas in the case able to all types of aircraft and missiles. substantially proportional to the size of the flaw. of magnesium alloy sheets bonded to paper The accuracy of location is of the same order as honeycomb was to sprinkle sand on the panel the size of the smallest flaws detected. By calibra and put it in a chamber in which there was a Magnetic Crack Detection tion against sectioned material specimens which source of variable frequency sound waves. Loose This method is useful for detecting cracks on a have a known gradient of mechanical properties, patches resonated at some frequency and this surface of the part. It is useful for applications in owing to increase of porosity in some direction, was clearly shown by sand patterns. This tech the field, and the lecturer gave an instance of it has been found possible to derive from the nique was not found so suitable for aluminium display a fair estimate of the properties of material how undercarriage drag struts on a number of alloy and steel sandwiches, and an instrument presented for test. In agreement with material aircraft of one type were checked for cracks quite consisting of a plain gear wheel giving point simply with a powder magnetic medium sprayed suppliers, standards of acceptance based on the contact, connected to an electric guitar pick-up, on. The method, however, only finds cracks, findings by the ultrasonic method have been was made. When the gear wheel was run over the giving no indication of their severity. A device worked out, and at Douglas all large slabs for panel the vibrations of loose sections could be known as the Magnaflux Sedac has now been integral panels, and other materials also, are heard in the headphones. This is a method for developed to determine the depth of a crack once inspected ultrasonically before machining is detecting lack of bond rather than quantitative it has been found. begun. strength of the bond. July 1955 227
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jul 1, 1955
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