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Fatigue of Metals in Relation to Accidents
this laboratory undertakes all types of scientific photography including pliotography using Infra-red and Ultra-violet rays. On the first of December, 1959, all the three laboratories mentioned above have been merged to form the State Forensic Science Laboratory with the Chemical Examiner to Government, Dr. A. R. Natarajan, M.Sc., M.B., B.S., D.I.C., F.R.I.C., F.I.C. (I)., as the Director and Mr. N. Pitchandi, M.Sc., A.I.I.Sc., A.R.I.C., who was in charge of the police laboratory, as Additional Director. This integrated laboratory is now functioning independently and directly under the Ministry of Home Affairs in the Government of Madras. Proposals are under consideration for the further development of this laboratory in order that it might give all the necessary assistance to the police investigating officers even during the initial stages of investigation. The three integrated laboratories are to be housed in a modern building planned on the Madras Marina. Instruction to police officers in scientific crime detection is also given by the laboratory officers. I t may also be mentioned that the University of Madras has decided to institute a post graduate course in Criminology and Forensic Sciences and a part of the training of thc candidates will be in this State Forensic Science Laboratory. The need to develop the practice of I:orensic Science has been felt by the Government of India and a Central Forensic Institute comprising of tlie Central Forensic Science Laboratory, thc Ccntral Finger Print Bureau and the Central Detective Training School has already been established a t Calcutta. Two other States have also started their own Forensic Science Laboratories. Recently steps have also been taken to form the Indian Academy of Forensic Sciences with the object of promoting the advancement of forensic science in the country. I t is hoped that this important branch of science will develop rapidly and be utilised more and more by the Police forces of the country to assist them in their fight against crime.
Fatigue of Metals
Relation
Accidents
W. R. BERRY
University of Leeds, England. (Synopsis of a Public Lecture of the Forensic Science Society at the Univesvity of Leeds, December l l t h , 1959, under the clzairmanship of Dr. D. Patterson) A fatigue failure in a metal is a gradual tearing of the metal from a nucleus of intense local stress concentration which is propagated by the repeated application of cyclic tensile stresses. The nucleus itself which is essential before a fatigue failure can occur is usually what is known as a "stress raiser" and such stress raisers occur a t changes in section and anything which can form a sharp re-entrant angle, often very minute, in the region subjected to cyclic tensile stresses. Once a minute crack is started from such a nucleus, the end of the crack itself becomes a marked stress raiser and on repeated application of cyclic stresses, will extend gradually into the metal until eventually the amount of sound metal remaining becomes insufficient to withstand further application of service stress and sudden and complete rupture occurs. In accidents involving fatigue failure therefore, the incident a t the time of the accident is merely the proverbial " last straw " and does not require to be in any way abnormal. I t is very much to the credit of engineering designers that accidents involving fatigue rarely occur due simply to design. When they do occur due to design, particularly in mass produced parts, they are generally due to unsuspected stress raisers and are usually catastrophic since the development of 10
the first failure inevitably takes considerable time in service, by which time there will usually be many similar parts in service liable to the same trouble. When it is realised that with the modern thirst for high speeds and fast acceleration combined with lightness for power and fuel economy, it is rarely possible to make the designed stresses sufficiently low, to ensure that the part concerned will have infinite life and since service conditions must vary enormously, i t is remarkable that designers have succeeded in so many cases in designing parts which, while their life will probably be finite, it is sufficiently long for the service expected from the mechanism. The many millions of highly satisfactory motor vehicles is outstanding proof of the accuracy of the designers' forecast. Albeit, the older the vehicle becomes the greater must be the risk of fatigue cracks starting up particularly with the ravages of corrosion which imposes a very heavy responsibility on people who have to certify old cars as roadworthy. This is particularly the case since fatigue cracks before the final complete rupture occurs are extremely difficult to detect. The reason for this is that the uncracked portion of the material holds the two surfaces of the crack in perfect register so that when the service stress is removed (which is generally the only condition under which examination can be made) the uncracked portion will return to its normal free position and hold the two faces of the crack so tightly together that it is often virtually impossible to see them with the naked eye, especially on dark or rough surfaces in the poor light usually only possible when the part is on the machine. While " stress raisers " can sometimes exist in the metal itself, such as sharp pointed non-metallic inclusions and these can form the nucleus for fatigue failures, accidents due to such causes are comparatively rare. By far the greatest bulk of stress raisers which cause accidents due to fatigue, however, have nothing to do with the metal itself or the design of the mechanism. They arise either during manufacture, maintenance operation, or in service. In manufacture they arise from inadequate radii a t fillets and threads, keyways, etc., and from deep sharp machining grooves, nicks, hammer marks, etc., and the inevitable surface irregularities when parts are used in the as-forged, or, as-rolled condition. In maintenance operations they often arise due to hammer marks, cracks produced in cold straightening, inadequate tightening of nuts, etc. In service they arise principally from corrosion, particularly the so-called fretting corrosion produced by rubbing causing isolated deep sharp pointed corrosion pits. Mechanical damage in service can also form the nucleus for fatigue failures to start. I t behoves users and Service people, therefore, to keep close watch for signs of rubbing and localised corrosion in parts subjected to repeated cyclic stresses (that is alternating stresses, vibration, etc.) and mechanical damage to the surface of such parts, particularly as the age of the mechanism increases, and not to flog their machines by excessive running under maximum conditions. Service people and manufacturers should be particularly careful to ensure that they do not produce localised surface damage on such parts. In view of the many millions of autoinobiles and the thousands of aeroplanes and many other mechanisms subject to fatigue conditions giving completely satisfactory life, it will be realised that accidents in which fatigue of metals plays a part must be comparatively rare and users of such mechanisms should keep a sense of proportion and not be unduly alarmed. After all, some risks must be taken in the modern world-for instance, chimney pots have been known to blow off and injure people walking in the street, but there is no undue alarm a t the possibility. (The lecture was illustrated by about a dozen slides of actual service failures, ranging from a large stainless steel ship's propeller to a small steering drop arm from a motor car, and the specific causes of each were discussed).
Fatigue of Metals
Relation
Accidents
W. R. BERRY
University of Leeds, England. (Synopsis of a Public Lecture of the Forensic Science Society at the Univesvity of Leeds, December l l t h , 1959, under the clzairmanship of Dr. D. Patterson) A fatigue failure in a metal is a gradual tearing of the metal from a nucleus of intense local stress concentration which is propagated by the repeated application of cyclic tensile stresses. The nucleus itself which is essential before a fatigue failure can occur is usually what is known as a "stress raiser" and such stress raisers occur a t changes in section and anything which can form a sharp re-entrant angle, often very minute, in the region subjected to cyclic tensile stresses. Once a minute crack is started from such a nucleus, the end of the crack itself becomes a marked stress raiser and on repeated application of cyclic stresses, will extend gradually into the metal until eventually the amount of sound metal remaining becomes insufficient to withstand further application of service stress and sudden and complete rupture occurs. In accidents involving fatigue failure therefore, the incident a t the time of the accident is merely the proverbial " last straw " and does not require to be in any way abnormal. I t is very much to the credit of engineering designers that accidents involving fatigue rarely occur due simply to design. When they do occur due to design, particularly in mass produced parts, they are generally due to unsuspected stress raisers and are usually catastrophic since the development of 10
the first failure inevitably takes considerable time in service, by which time there will usually be many similar parts in service liable to the same trouble. When it is realised that with the modern thirst for high speeds and fast acceleration combined with lightness for power and fuel economy, it is rarely possible to make the designed stresses sufficiently low, to ensure that the part concerned will have infinite life and since service conditions must vary enormously, i t is remarkable that designers have succeeded in so many cases in designing parts which, while their life will probably be finite, it is sufficiently long for the service expected from the mechanism. The many millions of highly satisfactory motor vehicles is outstanding proof of the accuracy of the designers' forecast. Albeit, the older the vehicle becomes the greater must be the risk of fatigue cracks starting up particularly with the ravages of corrosion which imposes a very heavy responsibility on people who have to certify old cars as roadworthy. This is particularly the case since fatigue cracks before the final complete rupture occurs are extremely difficult to detect. The reason for this is that the uncracked portion of the material holds the two surfaces of the crack in perfect register so that when the service stress is removed (which is generally the only condition under which examination can be made) the uncracked portion will return to its normal free position and hold the two faces of the crack so tightly together that it is often virtually impossible to see them with the naked eye, especially on dark or rough surfaces in the poor light usually only possible when the part is on the machine. While " stress raisers " can sometimes exist in the metal itself, such as sharp pointed non-metallic inclusions and these can form the nucleus for fatigue failures, accidents due to such causes are comparatively rare. By far the greatest bulk of stress raisers which cause accidents due to fatigue, however, have nothing to do with the metal itself or the design of the mechanism. They arise either during manufacture, maintenance operation, or in service. In manufacture they arise from inadequate radii a t fillets and threads, keyways, etc., and from deep sharp machining grooves, nicks, hammer marks, etc., and the inevitable surface irregularities when parts are used in the as-forged, or, as-rolled condition. In maintenance operations they often arise due to hammer marks, cracks produced in cold straightening, inadequate tightening of nuts, etc. In service they arise principally from corrosion, particularly the so-called fretting corrosion produced by rubbing causing isolated deep sharp pointed corrosion pits. Mechanical damage in service can also form the nucleus for fatigue failures to start. I t behoves users and Service people, therefore, to keep close watch for signs of rubbing and localised corrosion in parts subjected to repeated cyclic stresses (that is alternating stresses, vibration, etc.) and mechanical damage to the surface of such parts, particularly as the age of the mechanism increases, and not to flog their machines by excessive running under maximum conditions. Service people and manufacturers should be particularly careful to ensure that they do not produce localised surface damage on such parts. In view of the many millions of autoinobiles and the thousands of aeroplanes and many other mechanisms subject to fatigue conditions giving completely satisfactory life, it will be realised that accidents in which fatigue of metals plays a part must be comparatively rare and users of such mechanisms should keep a sense of proportion and not be unduly alarmed. After all, some risks must be taken in the modern world-for instance, chimney pots have been known to blow off and injure people walking in the street, but there is no undue alarm a t the possibility. (The lecture was illustrated by about a dozen slides of actual service failures, ranging from a large stainless steel ship's propeller to a small steering drop arm from a motor car, and the specific causes of each were discussed).