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IN THE SPIRIT OF BOD AND JENKS Editorial +44 (0) 20 7349 8484 [email protected] 18-20…
[The failure of a racing engine is invariably an expensive business and any means that can be taken to negative such a happening are worthy of close study. Mention has recently be made in MOTOR SPORT of Crack-Testing apparatus, which was used in Prince Chula’s stable, and by Peter Clark during the resurrection of the 1914 Mercedes. In this article a metallurgical expert outlines the possibilities of Radiographical Inspection in connection with the racing-car. Radiography has the advantages over other material-testing methods that parts can be tested in the rough state before machining, that the test is very thorough, and that the part being tested is not destroyed in the process, so that it can be used if desired, no matter what its condition. A friend who got this article written for us jokingly suggests that if every racing-car designer was sufficiently conscientious with his margins of safety and if he constantly resorted to radiographical inspection, pit work in the longest races would reduce itself merely to a matter of refuelling and wheel-changing. In fact, cars might well become so fast by grace of X-rays that the drivers, too, would need X-raying—in hospital! Apart from that, radiography should be an invaluable means of pre-race inspection in the future and the big stables and more ambitious amateurs might well consider its installation or utilisation.—Ed.]
EVER-INCREASING demands of the Aircraft Industry for materials of higher performance characteristics and lower weight have resulted in much research being carried out in three directions: first, in the development of improved aluminium and magnesium alloys; secondly, in the technique of casting these alloys so that the excess metal in built-up parts as well as the labour and time of assembly involved may be eliminated; thirdly, in Radiological inspection, to ensure that only perfect castings are passed out.
These demands are echoed by those whose business it is to design racing cars, and the results of these researches are likely to be applied in this direction after the cessation of hostilities. It is the aim of this article to describe briefly one of these lines of development, namely, Radiological methods of testing.
Light alloys have already been used for cylinder blocks, crankcases, pistons, connecting rods, cylinder heads, brake drums arid body work, etc., etc., and it is likely that they will be employed much more extensively, particularly for highly stressed components, when their complete reliability is assured by Radiological examination.
The justification for this method of testing lies in the fact that all metals, but particularly the light alloys, are difficult to cast satisfactorily. Without showing any visible evidence of trouble they may lose up to 95 per cent. of their strength and fail in service through internal blowholes, porosity cracks, cold shuts, shrinkage, or inclusions. It is the aim of Radiological inspection to eliminate faulty castings, and, by revealing the hidden source of trouble, to assist the foundry in the production of better castings before damage is done by the failure of some vital part.
In this connection, it is interesting to recall the experience of Prince Chula of Thailand, as recounted in the July issue of MOTOR SPORT. He points out that he “had countless failures in the first three years of racing” due to unreliability. Even after the appointment of his expert mechanic Stanley Holgate, he still had four retirements in sixteen races due to faulty material but, after adding a crack-tester to his equipment, he went through the 1939 season without a single retirement and “Bira” was never once placed lower than third. Radiography can do what crack-testing machines do more thoroughly and without the necessity for destroying faultless parts in testing.
Before dealing with the subject-matter proper, it will be as well to review briefly what X-rays are and how they are produced. X-rays are of the same nature as light but they have a very much higher frequency, which enables them to penetrate metals and many other substances in much the same way that light penetrates glass. Broadly speaking, the lower the density of the metal, the more easily it is penetrated. Being of the same nature, X-rays affect photographic film in much the same way as light although, in actual practice, films are specially designed for the job.
An X-ray tube consists essentially of a rod known as the cathode, in the end of which is mounted a small spiral filament, and another rod known as the anode, generally of solid copper fitted at the end with a tungsten button, both rods being mounted an inch or so apart in a highly evacuated glass tube.
A current of the order of 240v., 2-20 ma. is passed through the cathode filament, causing a stream of electrons to flow towards the anode. When a voltage of the order of 100,000 to 500,000 is applied to the anode, the electrons are attracted to the anode with terrific force. It is this bombardment of the tungsten button in the anode by the stream of electrons which causes the production of X-rays, which pass out of the tube at right angles to its axis.
The larger the filament current, the greater the number of electrons produced and, therefore, the greater the quantity of X-rays evolved. Raising the anode potential, on the other hand, increases the penetrating power of the rays or, as is generally said in X-ray parlance, the rays become harder.
Efficient means for cooling the tube have to be introduced. In the smaller tubes, this is done by drawing a strong current of dust-free air past the ends of the electrodes. Water is used in the larger outfits and may either run through continuously, or else travel in a closed circuit, being itself cooled in a condenser cooled by the main supply.
Radiological inspection resolves itself into four distinct operations—hand-picking, screening, radiography, and fracture.
In screening, X-rays are allowed to pass through the casting on to a fluorescent screen which lights up with a shadowgraph of the casting, in which gross defects show as light or dark patches.
Radiography, in which the fluorescent screen is replaced by a special photographic film, is essential to detect the finer sources of trouble, in particular, porosity, which may result in the loss of up to 50 per cent. strength. Except in the developing process, radiographical operations do not need to be conducted in the dark, since the film may be wrapped in light-proof paper or placed in aluminium containers through which the X-rays will pass unchanged. Development is substantially the same as for photographic negative; positives are unnecessary.
The relation between time of exposure and voltage and current employed is important. The higher the voltage and current the lower the time of exposure; the finer porosity tends to be lost if the voltage is too high. Exposures of the order of 30 seconds to 5 minutes are common.
Continued exposure to X-rays, either direct from the tube or reflected from the floor, walls or ceiling, is very dangerous to operators and may result in severe skin diseases or acute anæmia. To prevent this, the apparatus is generally placed in .a small room lined with sheet lead to absorb the scattered rays and the controls are placed outside the room so that the operator is well away from the tube while the current is on.
Exposure of the body to X-rays results first of all in a change in the relative number of red blood corpuscles and white corpuscles or leucocytes. In a normal healthy person this relation is constant and varies very little from one person to another. Consequently, since it can readily be determined by a simple blood count, X-ray operators generally submit to this test at a hospital every three months or so. In modern well-designed laboratories, the danger of working with these rays is negligible.
Hand picking involves inspecting castings for signs of possible trouble which require further particular examination by X-rays. The importance of hand picking cannot be over-estimated. With experience it is possible to recognise obscure markings which are sometimes associated with certain troubles, and X-ray examination confirms or negatives this. When dealing with a large number of relatively unimportant castings, it is frequently sufficient to examine those selected on hand picking and, if these are satisfactory, to pass the batch without further examination.
The fracture test, which is carried out on only a small percentage of the castings, shows whether or not the grain structure is satisfactory. Furthermore, porosity reveals itself by a dark coloration and the fracture test is often used to supplement the radiological examination when complete freedom from porosity is important.
So far we have only mentioned castings, but the method is equally applicable to the examination of all forms of material. It has up to now been applied principally to castings, since these are, after all, the basis of all forms of metal. Sheet, for example, is rolled from a cast-billet and wrought products all start as castings. It also gives a valuable indication as to the efficiency of welded joints. Porosity, blow holes and entrapped welding flux show up well on the radiograph, and this method of testing is proving an invaluable aid in the development of welding technique. Readers who are interested in this aspect of the subject should consult the Transactions of the English and American Institutes of Welding for further details.
Equally so, radiological testing is not confined to the light alloys of aluminium and magnesium, although these are the easiest to deal with. Roughly speaking, a 100 kv. X-ray outfit is sufficient when dealing with magnesium alloys and will penetrate about 3 inches of this metal. Aluminium more than 1 inch thick is best examined on a 150 kv. set, while thick sections in the high tensile steels may require as much as 500 kv.
Nevertheless, all these outfits are now on the market complete with simple transformers for connection to the ordinary A.C. lighting circuit. The 10O kv. outfits are generally air cooled, but the larger sets require a steady flow of cooling water circulating by means of a small pump integral with the apparatus. If the cooling water fails, a warning light shows and the current is automatically cut off.
When using the higher voltages, additional precautions have to be taken against scatter. It has already been mentioned that X-ray films are used in paper wrappers or aluminium containers. Now, when X-rays strike the side of the casting, other rays of less penetrating power are given off. In the case of the lower voltages, the penetration of these secondary rays is slight and they do no harm, but when high voltages are employed, the secondary rays are able to penetrate the wrapping and cause fogging of the film. To prevent this, the sides of the casting are packed with some material which absorbs the rays. One method is to place the casting in a box and pack it round with lead shot. Another and generally more convenient method is to pack with plasticine containing powdered tungsten.
All these methods of inspection are now in actual use in the aircraft industry, and without them it would be impossible to produce warplanes with such a high power-weight ratio and yet complete reliability under all conditions. In fact, it is probably true to say that the superiority of the R.A.F. over the German Air Force is in no small measure due to this method of test, since radiological inspection is by no means so highly developed in Germany.
If X-rays can do this for the aeroplane is it not probable that they can do as much for racing-cars?
IN THE SPIRIT OF BOD AND JENKS Editorial +44 (0) 20 7349 8484 [email protected] 18-20…
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