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Finding Flaws by "Black Light"

11th September 1942
Page 31
Page 31, 11th September 1942 — Finding Flaws by "Black Light"
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In the "Glow-Crack" System Fluorescent Solution Retained in Crevices and Crannies Becomes Brightly Luminous Under the Ultra-violet Ray

ONE of the problems of both mannfacture and repair is crack detection. The inclusion of one faulty part, when assembling a unit of mechanism, may lead to a Very expensive breakdown. Yet it may be most difficult, if not impossible, to pass or to reject items under investigation with complete certainty that a right decision has been taken.

Ferrous materials can be inspected for flaws by magnetic means and the process used is 'quite satisfactory. Non-ferrous parts, however, present a difficulty. Methods exist—such as anodizing or using oil or dye which seeps out of the crack and stains a chalk-whitened exterior—but close visual inspection under a light which illuminates the whole area in the vicinity of the fissure is entailed, mistakes are easy, and considerable strain is suffered by the persons carrying out the detection which, in time, impairs their efficiency.

Now a scheme has been evolved by which the crack alone is made to shine.

" Black light," that is, ultra-iziolet rays are cast upon the area under examination, so that, all surroundings remain unilluminated, but the crack or flaw glows brightly.

A good impression of this phenomenon is conveyed by the accompanying illustration which depicts a bearinghalf, under investigation by this new process, in which there is bad adhesion between the bronze lining and the steel housing, continuing as a crack in the liner.

Named the Glow-Crack system, the scheme has been developed by Colloidal Research Laboratories, Ltd., 66-70, Petty France, London, S.W.1, and is an application of the B.A.B. system of fluorescent lighting to crack detection. Another application is the easy examination during the blackout of works passes. By coating these with special transparent lacquer, which is invisible in ordinary light, but which shows up brilliantly under " black light," it is possible to detect quickly and certainly whether persons are authorized or otherwise to gain access to factories.

A pass of a certain colour by day or artificial light may appear to have a totally different colour under ultraviolet light. A variation is to apply the lacquer as a stripe across the pass, this stripe being quite invisible except when made to glow by the " black light," the rays of which in the atmosphere are, of course, invisible, unlike those of an ordinary electric torch. Alternatively to a stripe, lettering or a distinctive mark may be applied.

It occurred to us that a further application might be to bus tickets, the object being to facilitate punching them in the dark.

For crack detection the process to which the parts under investigation are submitted is straightforward and short. The specimens—pressings, stampings, etc.—are immersed, for an automatically controlled period, in a bath of fluorescent solution at a temperature of 75 degrees C. Then they are with.drawn and washed in a cleansing solution which removes all fluorescent material that has not become anchored to the edges of any crevices that may exist.

Next the specimens are examined under ultra-violet rays, produced by a special lamp, or by an ordinary lamp coated externally with a special preparation devised to pass only ultraviolet rays together, perhaps, with some dark-violet light which hardly illuminates ordinary surfaces at all. It is of interest that an ordinary drybattery "flash lamp" can be employed for the purpose.

It is stated that the total time required to examine quite a large specimen is only three minutes. Current consumption is low. The space occupied by equipment which can keep pace with an average production line is so small as to constitute no problem. Operatives can be trained in a matter of a few hours.

A small loss of solution is involved by the fluorescent treatment. It amounts, however, to no more than 0.23 gallon per 1,000 sq. ft. of specimens examined. The washingsolution loss is 0.17 gallon per sq. ft. At present prices, this works out at a cost of abiout one penn■4 per sq. ft. By the use of additional plant, solvent can be recovered.

As the process also degreases the specimens, a separate operation is eliminated, which constitutes an economy. The main advantages, however, include the facts that the process is applicable with equal efficiency to ferrous and non-ferrous materials, that detection is quick, positive and without eyestrain, and that it is within the capacity of unskilled labour, whilst not entailing additional treatment such as demagnetizing.

At a recent demonstration of the Glow-Crack system; an air-screw blade, having a hollow shank, was under investigation. The axial tunnel ran a long way up into the blade, and to have detected the presence of an internal crack remote from the mouth of the hole by other means would certainly have been difficult. In this case, however, in the dark interior, the luminous flaw attracted the •eye at once, showing like a small pale-blue lightning flash against a leaden thunder cloud.

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People: mark may
Locations: London

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