FIELD FOR RESEARCH

Evening Post, Volume CXIX, Issue 72, 26 March 1935, Page 10

FIELD FOR RESEARCH

STRENGTH OF MATERIAL

THEORY AND PRACTICE

Sir William Bragg, 0.M., F.R.S., gave a discourse at the Koyal Institution on one of the main subjects now engaging the attention of crystallographers, "The Theoretical Strength and Practical Weakness of Materials," states "The Times." He said that the results of this study could not fail to be of interest and importance, not least because of the light they might throw on,the living organism.

It has long been understood that the forces which hold materials together were due to the combined action of the forces exerted on one another by the atoms and molecules of which the material was composed. But as long as there were no means of examining the fine details of the structure of a solid, nothing could be done to connect the large-scale forces with the small. It would be impossible to explain the working of a machine without information about the sizes, positions, connections, and mutual relation of its parts, wheels, axles, levers, pipes, containers, and so on. In the last 20 years the situation has been entirely transformed. To begin with the X-rays now provided a means of distinguishing the atoms and molecules of the solid, not individually, indeed, but when they were arranged in the regular array of the crystal, as was generally the case, even in bodies which did not seem to be crystalline. Besides the X-rays there were now other means of investigation.

The chemist could now, in a sense, "see" the molecule which had for so long been the invisible subject of his researches. It was presented to him on the results of the X-ray researches as a shadow picture. It was as if a light shone through a glass tank containing a transparent liquid, in which various clouds remained stationary and cast their shadows upon a screen. These were the electron, clouds of the atoms. Each cloud was densest.: at the centre and faded to invisibility at its edges. The picture covered generally a single unit of the pattern, which when multiplied and repeated in all directions formed the crystal. The positive core of the atom cast no shadow as it had no effect on the X-rays. The scale employed was about a hundred million to one. Such pictures were quite new to science and in combination with other important sources of information opened up a fascinating field of research. It began to be possible to relate the jjn-operties of solid bodies to the properties of the atoms forming them. This could only be done in simple cases at present, or to a moderate extent in bodies tif greater complexity, including the extremely interesting compounds of living tissues. As an example the shadow of a re-cently-discovered body which was violently explosive showed a central hexagon of three carbon atoms and three nitrogen atoms arranged alternately round the ring, while a set of three nitrogen atoms, known as the triazide group, was flung out in a line from each carbon atom. The whole was rather like the arms of the Isle of Man. Its fragile appearance seemed to explain its instability, and the known "urge" of nitrogen atoms to combine in pairs, accounted for the violence of the action when the molecule broke. \' V. This was but one example of a great number which linked up the newly-ac-quired knowledge with the facts of experience. There could be no doubt of the general Tightness of the whole movement. Yet in some respects there appeared, at first sight, to be disco/dance. It was now known that the forces binding atoms and molecules together were essentially of an electric origin. They were extremely strong. For instance, a calculation suggested by Debye showed that if the sodium atoms in a couple of ounces of salt were placed at the North Pole and the chlorine atoms at the South Pole, and if each chlorine atom could be made to retain the excess negative electrical charge (that of one electron) which it had in the crystal, while each sodium atom was correspondingly positive, the electrical attraction between the two electrical charges would be equal to the weight of 50 tons. Needless to say, such a separation could never be effected, but the conception gave an idea of the magnitude of the forces to which cohesion was due. Where the strength of a piece of rock salt to resist, rupture was calculated from a" knowledge of its structure and the behaviour of the eleetricar forces the result was hundreds of times as great as that which the testing machine gave. This discordance was almost certainty, explained by a suggestion made by Dr. A. A. Griffith, of the Royal Aircraft Establishment at South Farnborough, who was engaged in research connected with aeroplane design. It seemed that there must be minute flaws or cracks which escaped the microscope because they were too small and the X-rays because they were too large. Material under tension gave way because the presence of cracks caused excessive: stress at their edges and so the cracks spread. The housewife snipped the edge of a piece of material when she wished to tear it. It was only when the electrical forces between the positive and negative atoms all acted together that the force resisting rupture was so great; and that, which was assumed in the calculation, was never realised. Just so the soft roots of the ivy when they, all acted together could tear apart the stones of a building. Rock salt was chosen for illustration, but other materials hehaved the same way. '

There were several suggestions, but there was no general agreement about the origin of the cracks or flaws. Their existence was doubtless connected with other properties of solids, such as the well-known hardening of metals by cold working. There was much work ahead, and the results of it were sure to "be of great interest and importance. This was clear, since they would have to do with the solids, which were handled so often and so freely; most of all, perhaps, because they would throw light on the behaviour of the materials of the living organism.