Early in March, 1917, when it appeared that the United States would be drawn into the European War, a proposal was made by Prof. P. L. Bridgman, of the Jefferson Physical Laboratory, Harvard University, to the Bureau of Ordnance to construct a 3-inch gun by a new method. As outlined in his plan, Professor Bridgman stated that if adopted it would produce precisely the same distribution of stress which is found in a built-up gun, but by a much simpler and less expensive process than the method generally in use.
The principle involved in the method was clearly outlined in the proposed plan as follows:
If a hollow thick tube is exposed to an internal pressure great enough to stress all the fibers to the elastic limit, when the pressure is removed the external layers will shrink back on the internal layers and leave them in a state of compression. It may be proved that the final state of stress in such , a tube, which has been permanently stretched, is precisely that which would be obtained under the best possible condition in a built-up gun; but these conditions are never realized in practice.
The proposition of constructing guns is, therefore, as follows: A single piece of steel of the outside dimensions of the finished gun is roughly bored to approximately the required diameter; it is then stretched by one or more applications of sufficiently high internal pressure; the pressure is then released and the inside is machined to the size necessary to take the liner; the liner is inserted, thereby finishing the gun except for mountings and fittings. The very great economy of this method, both in time and in money, as contrasted with the old method, is apparent. The stretching of the gun might possibly be a matter of a day.
The reason that this very simple method is not at present in use is that the pressures necessary to produce the initial stretching are much higher than the pressures which can be handled by means commonly known. For example, if the steel of which the gun is to be made has an elastic limit of 4S,ooo pounds per square inch, the necessary preliminary pressure must be as high as 120,000. Such pressures in liquids cannot be dealt with by usual methods without excessive leak.
To mention a single example, several years ago a method was projected at the Watertown Arsenal for testing guns by the application of high hydrostatic pressures, but the method had to be abandoned because pressures higher than 60,000 pounds could not be retained without leak.
In this connection I am in a position to place at the disposal of the government a technique by which pressures high enough to insure the success of the proposed method may be easily handled. For over 10 years I have been engaged in the Jefferson Physical Laboratory of Harvard University, almost exclusively in research on various effects of pressures much higher than have been previously investigated. The previous range of scientific high-pressure work has been 45,000, or at the most 60,000, pounds per square inch, whereas as a matter of daily routine I have been employing up to nearly 200,000 pounds. This, of course, has involved the invention of the methods of packing and other details of technique which are novel or at least which are not employed elsewhere to my knowledge.
The proposal to construct a gun by this novel method was referred to the Special Board on Naval Ordnance. It was found that the cold stretching of steel had been a matter of daily practical application for many years, as illustrated in the manufacture of wire and cold-drawn tubing, and it had been demonstrated many years before that the cold stretching of steel and other metals beyond the elastic limit, thereby producing a permanent set, resulted in a decided increase in the elastic strength of the metal thus treated.
The board, therefore, in view of this increased elastic strength, as well as the stresses introduced in the walls of the cylinder, recognized the practicability of the proposed method when applied to symmetrical cylinders of small dimensions, but considered that the application to large irregular masses involved in gun construction would present difficulties requiring much experimental work.
At that time the government equipment, as well as at many industrial plants, had been built up to follow the present system of manufacture of built-up guns at comparatively great speed. It was not considered expedient at a time when the greatest output of standard weapons was demanded that any changes should be made in the well-established system of manufacture of built-up guns. The Navy Department had neither the personnel nor the facilities for the experimental work involved in testing the merits of this new theory of gun construction without interfering with the production of guns of standard design. Moreover appropriations had only recently been made for building a naval laboratory for testing just such theoretical questions, and while the value of Professor Bridgman’s proposals from his long experience in laboratory work was fully recognized by the bureau, it was decided to defer such experimental work until it could be undertaken with the resources to be provided in the new naval laboratory.
While investigating the proposal of Professor Bridgman, it was found from the files of the bureau that in the fall of 1914 Md. A. H. Emery, a prominent mechanical engineer, had brought to the attention of the bureau an identical process of gun construction, covered by patents from 1903 to 1908, in which he proposed to produce initial stresses in the gun metal by a process of stretching beyond the elastic limit, employing for this purpose hydraulic pressure in the interior of the bore. As these plans were evidently presented verbally, there is no record of the action of the bureau beyond a paragraph in a letter of December 9, 1914, in which the bureau informed him that it considered his method of doubtful value, as the same results could be obtained under the present method of gun construction, by using higher pressures in the gun bore and thereby raising the elastic limit of the material if this should be considered desirable.
In April, 1917, the subject was again brought before the Bureau of Ordnance by the Naval Consulting Board, which recommended the construction of two 5-inch guns by Mr. A. H. Emery and Professor Bridgman, working in conjunction, at an estimated cost of $85,000. The proposed method was again carefully considered by the bureau, which, while conceding the importance of the proposed method, was unwilling to undertake the extensive experiments which the proposed construction of two 5-inch guns required, on the ground that attempts at that time to change the system of gun construction in the navy, in view of the pressing needs for war material, were inadvisable. The proposal was brought to a termination by the action of the Naval Consulting Board in withdrawing their letter recommending such experimental work.
On account of the very small estimated cost of the experiments proposed by Professor Bridgman, he was entrusted by the chief of the bureau with the application of his ideas to the construction of a 3-inch 23-caliber gun, similar to the Mark III, Model 4, landing gun. Experiments were to be conducted under the general supervision of the Special Board of Naval Ordnance, and the superintendent of the naval gun factory was requested to give Professor Bridgman all possible assistance in working up the design for the forging and the necessary machinery required for producing the hydraulic pressure. Although the naval gun factory early in June took up the design of the accessories necessary for the construction of the 3-inch gun, in accordance with his ideas, it was not found practicable to complete the working drawings until the following October, when it was estimated that the forging for the press could not be made before the end of March, 1918, and its completion not before June 30. This delay was due to the urgent demands upon the gun factory for supplying standard materials which had preference over any experimental work.
The gun forging and accessories were finished early in June and taken to the Bureau of Standards, where arrangements had been made for utilizing its two-million pound Emery testing machine as the source of pressure for the experiment. It was designed to use this powerful machine, for producing the hydraulic pressure in the gun, by forcing a cylindrical 3-inch steel piston terminating in a packing into a specially constructed cylinder of 3-inch bore, thereby compressing the fluid in the interior of the cylinder, in the further end of which was a fixed packing. To this latter was attached the end of a system of hollow tubes, 1/8-inch internal diameter, communicating with the interior of the gun, and conveying the pressure from the cylinder to the gun. The pressure packings in the gun were constructed on the same general design as the packings of the pressure cylinder, one fixed in the breech end of the forging, and a movable one to be shifted as desired.
The first pressure experiments were begun July 20, on the larger section of the gun tube, 7.5 inches external and 2.5 inches internal diameter. There was evidently very great friction in the movable packing of the pressure cylinder, and further difficulties in the congealing of the liquid in the tubes connecting with the gun, as the final pressure recorded by the Emery testing machine was 1,300,000 pounds which, without these two causes, should have given 170,000 pounds per square inch in the bore of the gun. At this point the connecting pipe burst, and the pressure in the gun was thereby relieved. It was found upon examination that the 3-inch steel piston for forcing the movable packing into the pressure cylinder was deformed, necessitating the removal of cylinder and piston to the navy yard for repairs.
The records of diametral expansion, as the pressure was gradually increased, indicate a temporary enlargement of .016 inch, or .0021 per linear unit. This elongation per unit of length of the outer layer corresponds to the elastic limit of the metal about 60,000 pounds, which is determined by the ratio of this elastic limit to the modulus of elasticity of steel, 30,000,000 pounds, about .002. Although the electric gauge for reading the pressures at a point in the connecting pipe just before reaching the gun recorded a pressure of 95,000 pounds per square inch at that point, it is certain that the pressure in the gun was considerably less. It may be said parenthetically at this point that, from the mathematical theory of the strength of hollow cylinders stretched by interior pressures beyond the elastic limit, knowledge of which was obtained later in the experimental work along these lines, this pressure and resulting radial expansion were sufficient to give practically all the benefits to be obtained by the process of radial expansion.
The experiment was resumed July 26, with a lighter oil, and the pressure gradually increased up to 94,000 pounds, recorded as before on the shunt pressure gauge, when the connecting pipes again burst. The mean diametral expansion in this case was .042 in, or .0056 per linear unit. The pressures and corresponding elongations are recorded on the stress-strain diagram, Fig. 1, from which it will be seen that the metal of the gun worked elastically up to about 78,000 pounds. This great elastic strength was undoubtedly due to the cold stretching to which the gun had been subjected on the 20th.
The following day the movable packing was shifted to a point near the muzzle, thus exposing the whole length of the bore to the pressures which were effected on the 27th and 29th. The pressures were carried to 78,000 pounds, producing a permanent elongation of the smaller external diameter (6.3 inches) of .075 inch, or .012 per linear unit. (See Fig. 2, Curve A-B, for stress-strain diagram.) The permanent elongation of the larger section was only .0004 per linear unit, the point A in the diagram, Fig. 1, showing that the metal worked elastically up to about 70,000 pounds. Although the experiment was now complete as far as the elastic strength of a gun of such dimensions could be carried, it was again subjected on the 29th to a pressure of 82,000 pounds, the progressive elongations showing that the metal of the gun walls was acting within the limit of its elastic strength, up to 72,000 pounds for both sections.
The cold-stretching process having terminated satisfactorily, the gun was sent to the Naval Gun Factory for machining, boring and rifling, after which the finished gun was mounted on a regular 3-inch mount for that type of gun, and sent to the Naval Proving Ground for firing tests, which were terminated November 30, 1918. Only 14 rounds were fired, the first five the regular proving rounds for the 3-inch landing gun, the remainder with increasing pressures until the last three reached 21.8 tons. The test was then stopped on account of damage to the firing pin, which required the return of the gun to the Naval Gun Factory for repairs of this element. The gun upon examination showed no injurious effects from the high pressures in the last few rounds.
Before the firing tests were completed confidential information was received by the bureau that this system had been in use elsewhere in the construction of experimental guns, for a period of about 20 years. The tests of these guns had been of such a conclusive nature that it was considered unnecessary to continue further high-pressure firings, as owing to its small caliber and the short travel of the projectile it would not have been practicable to subject it to sufficiently high pressure to tax the great elastic strength of the gun, which the experiment indicated ought to be approximately 72,000 pounds.
During the design and construction of the 3-inch gun on the plans proposed by Professor Bridgman, Mr. Emery had frequently appeared before the Special Board on Naval Ordnance advocating the extensive use of his process, on which he had obtained letters patent in 1903, and inviting attention to the fact that the construction of the 3-inch gun under Professor Bridgman’s plans was an infringement of his patent rights. The specifications of his patent, the original application of which was made in 1897, embraced many practical details of accessory apparatus, and appeared to cover the basic principle of cold stretching by which the elastic limit of the metal was increased, as well as the auto-hooping which occurred when the metal of the gun was released from the high internal pressures which had produced the radial expansion of the tube. His process included a further element by which the increased special elasticity, introduced by the cold stretching, was made permanent by a mild-heat treatment with live steam at a temperature of 250° F. The claim was advanced that this had the effect of relieving the fatigue of the metal which, without this treatment, would require quite long periods of time for its elimination—a process of accelerating the ageing of the metal. He presented, at the same time, detailed plans for the application of his process to various types of guns involving both one-piece guns and guns composed of several tubes and jackets to be fitted by appropriate hydraulic pressures.
As the method of construction utilized by Professor Bridgman was described in great detail in the specifications of the Emery patents, which were apparently valid, and had been called to the attention of the Bureau of Ordnance in 1914, Mr. Emery was authorized early in September to proceed with the construction of a 4-inch 50-caliber one-piece gun, from designs which he had prepared and submitted to the bureau, shortly after the withdrawal of its proposal by the Naval Consulting Board.
The Tioga Steel & Iron Company was authorized to manufacture a special 4-inch forging in accordance with drawings furnished by Mr. Emery, which was completed, inspected and accepted, and sent by express to the Naval Gun Factory on December 10, 1918. The boring and machining of the forging to the required dimensions for the application of the stretching process was completed, and the gun sent to the Bureau of Standards, where the large Emery testing machine was to be utilized in producing the required pressures. Test specimens were taken from the breech and muzzle sections of the forging, representing the inside, middle, and outside metal. The elastic and tensile strengths of these specimens were determined for comparison with similar specimens after the process of cold stretching. The forging was so designed that the application of the required pressure for cold-stretching the gun to the elastic limit of the exterior layer would be effective at the same time for all sections. This was accomplished by making the ratio of the exterior to the interior diameter of the tube as near as practicable the same for all sections, since the interior pressure for this purpose was assumed to be a function of the ratio of the inside to the outer diameter. (See Fig. 3 for these dimensions.)
Without going into too minute descriptions of the details of the various accessories, it will be sufficient to explain that they were designed to produce the interior pressure desired, by forcing into the breech end of the gun, through a stuffing-box packing, a cylindrical steel piston compressing the liquid within the bore of the gun. The design for sealing the bore of the gun against the high internal pressure was very simple, consisting of two fixed packings at the breech and muzzle ends respectively. The one at the muzzle contained a steel valve block for admitting the hydraulic medium, “ Vedol ” oil, into the gun from the hand force pump which was used to raise the preliminary pressure to 6000 pounds per square inch; the breech plug, screwed into the breech of the gun, was fitted with a simple stuffing-box packing through which the 4-inch steel piston moves With a relatively small friction, compressing the liquid in the gun and producing the desired hydraulic pressure. This limit was determined at intervals, the total pressure required to overcome the friction varying from 18,000 pounds at the beginning to 5000 pounds after a few tests.
The amount of radial expansion was determined by the readings of 11 pairs of Ames dial gauges set concentrically with the gun by means of 11 rings arranged at equal intervals along the gun, and held in position by three spring-bearing screws. The arrangement of the gun and accessories, with reference to the testing machine, is sufficiently indicated in the cut from a photograph taken during the operation (Fig. 4). Readings of these gauges to the nearest ten-thousandth of an inch were made and recorded as the pressure progressively increased or decreased, and subjected to careful inspection by differences to determine critical points in the progress of radial expansion.
In addition to the gauges the rings carried light wires running parallel to the axis of the gun, for the purpose of indicating by their measured distance from the outside of the tube whether any curvatures of the tube were caused by the internal pressure.
The setting up and adjustment of the many accessories were completed, ready for the application of pressures, December 30, 1918. The work progressed without accident of any kind from start to finish, in accordance with the carefully arranged plans, and requiring no alterations of the accessories as originally designed.
In order to determine the critical point in the process of cold-stretching the gun tube, the data of pressures and corresponding radial expansions were recorded on large sheets and carefully differenced as the experiment proceeded. A more convenient means of studying and analyzing the data, however, is given by the stress-strain diagrams constructed from the tabulated data at the conclusion of the work. Three of the typical diagrams are given in Figs. 5, 6 and 7, and for Sections 1, 5 and 8, respectively, which show with sufficient clearness the results of the experiment. The time element, which is not shown in the diagrams, does not play a very important part, but it may be stated here that the application of the progressive pressures began at 9.35 on January 2, and were terminated at 5.15 the same afternoon, when the pressure recorded for the interior of the gun was 92,000 pounds. The pressure was then rather rapidly reduced until at 5.30 p. m. the gun was without pressure. The following morning, January 3, at 9.45, the pressure was again applied and at 12.15 had reached 107,000 pounds per square inch, the highest pressure employed. This pressure was progressively diminished until at 3.45 the pressure was reduced to zero and the oil withdrawn from the interior. Immediately after live steam was introduced and maintained up to a pressure of 15 pounds, giving a temperature of 250° F., and this was maintained until 8 p. m.
In order to test the results of the experiment upon the elastic strength of the gun, pressures were again applied and run up progressively to 80,000 pounds, at which point it was indicated that the elastic strength of the gun had been reached.
A study of all the diagrams constructed from the data show very clearly that at the first introduction of pressure the metal of the gun, assumed to have been without initial stresses, worked elastically up to from 40,000 to 60,000 pounds pressures, after which the more rapid expansion shows that the metal was elongating beyond the elastic limit. The amount of temporary and permanent expansion acquired is shown clearly in the diagrams, which also show that when pressure was applied the following day the tube acted elastically up to 80,000 pounds, and that the subsequent permanent elongations given by increasing the pressures up to 107,000 pounds did not apparently give any additional elastic strength. It is plainly indicated that if the heat treatment had been applied at the end of the first expansion, due to 92,000 pounds pressure, the gun would have had all the elastic strength shown in its final pressure test.
At the conclusion of the pressure experiments, the forging was sent to the naval gun factory, where accurate measurements of outside and inside diameters were made, for comparison with similar measures before the cold-straining process. The results of these measurements are given in Fig. 3, a rough drawing to scale of the gun forging. The excess metal of the breech and muzzle sections, about 4 inches long, was cut from the forging, and taken by Mr. Emery to his laboratory at Glenbrook, Conn., for the preparation of specimens, the measurements and tests of which would show the effects of the radial expansion process.
These tests were the more important, as the generally accepted theories of the elastic strength of guns, founded upon the relation of stress and strain within the elastic limit, could not be considered as applying to a case where the greater part of the metal constituting the walls of the gun had been stretched beyond the elastic limit, acquiring a permanent set.
For the purpose of ascertaining the initial stresses introduced in the gun a set of 11 rings was cut from the inner face of the breech and muzzle sections, equally spaced from the interior to the exterior of the gun. The rings, 0.2 inch square section, were cut into the face of the section, and accurately measured across several diameters while still held in place; they were then cut off and again accurately measured. The results of the measurements expansion of the ring, determined from the difference in diameter before and after cutting off, by the modulus of elasticity, which in the results given was taken to be about 28,500,000.
In order to determine the effect of cutting down the muzzle section to dimensions of finished gun, the extra metal was turned off the exterior, and a representative section thus obtained 3.8 inches long, by 7”.8013 outer and 4”.0034 inner diameter, respectively. From this outer and inner test rings were cut 0.2 wide by 0.1 thick (measured radially), and the measurements resulted.
The two test rings of the size of the finished gun at the muzzle were for the purpose of determining beyond question that the metal of the cylinder walls, after cold stretching, is in a condition of initial strain represented by an infinity of hoops with just the right amount of shrinkage to give the maximum elastic strength; and that the removal of metal from the outside or from the inside leaves the remainder in a proper state of initial strain. It may be noted in the same connection that the inner ring of the breech section, shown in the preceding paragraph, is 5| inches in diameter, approximately the dimensions of the finished powder chamber section.
The initial strains found in the various rings cut from both sections show that the metal of the finished gun is in an ideal condition of initial stress, varying progressively by infinitesimal graduations from 25,700 to 35,300 pounds per square inch tangential compression at the interior layer of the bore, to from 26,000 to 45,000 pounds tangential tension at the exterior layers. As these stresses vary progressively from compression at the interior to tension at the exterior, it is evident that the process has resulted in a condition of self-hooping in which the hoops are the successive infinitely thin layers from exterior to interior, each one shrunk upon the one next interior with a shrinkage such that all the layers (hoops), under the maximum interior pressure design, reach their elastic limit in tension together.
The initial strains thus shown are most satisfactory, not only in respect to distribution but also in respect to the amount.
The improvement in the quality of the metal is equally apparent from an examination of results yielded by test specimens taken from the forging before and after the application of the process. These were taken from breech and muzzle sections, tangentially in planes at right angles to the axis and at such distance from the center as to have the o'.'505 square sections shown as closely as possible the quality of the metal at the bore, the middle and the outside. The testing was carefully done in each case on an Emery testing machine, taking extensometer readings in ten- thousandths of an inch for the extension under load. In the following table are given the elastic strength E and the ultimate strength R in thousand pounds per square inch:
Specimens Before After Before After
Muzzle, outside ...................................... 60 65 94 97.5
Muzzle, middle ........ 57-5 62.5 514.5 96
Muzzle, inside ......... 62 72 95 105
Breech, outside ...................................... 56 64 91 100.5
Breech, middle ........ 52 55 91 89
Breech, inside ......... 40 65 91 94
The inside specimens, having experienced the greatest permanent elongation, show the greatest improvement; their stress- strain diagrams are shown in Fig. 9, where the greatest elastic strength belongs to the specimens after cold working.
At the naval gun factory the gun was given the usual finishing and rifling of the standard 4-inch guns of the same type. After mounting on the standard mount for this type of gun, it was sent to the Naval Proving Ground for test firings early in March, 1919. In order to furnish comparison with the best type of standard 4”/50 gun, the firings were to be carried on in conjunction with a standard Mark IX-5 gun No. 3479, made by the American & British Manufacturing Company, Bridgeport, Conn. In the inspection of the two guns previous to the firing, by members of the Special Board on Naval Ordnance, it was noted that rifling of the Emery guns was much rougher in appearance than that of the standard gun, and had several small gouges and evidence of tool chattering in the rifling, while that of the standard gun was very smooth and highly polished.
Investigation in the gun shop disclosed that the rifling of contract guns in general is admitted to be superior in appearance to rifling done in the gun shop, but not in efficiency. This is due to the fact that the gun shop does not put in the time on the operation that outside contractors do, since it has been determined that a lesser polish is satisfactory for service purposes. Normally, the gun is lapped after rifling by both a lead slug and a copper slug, which are forced into the rifling by spring pressure. It is the copper lapping which puts on the final brilliant finish, and in the case of the Emery gun the urgency of the order made it seem expedient to the gun shop to omit the copper lapping and lap out only with the lead slug.
Further inquiry brought out the information that representatives of the American & British Ordnance Company, the manufacturers of the standard gun noticed above, had stated that they had found great difficulty in satisfactorily machining gun forgings made by the Tioga Steel & Iron Company; that they had tried varying the speed of the lathe, together with the temper and hardness of the tool used, and that, notwithstanding their best efforts, the machined gun forgings of this company always had a rough appearance. It would seem, therefore, that the rough appearance noticed in the rifling of the 4-inch Emery gun was due partly to some characteristics of this steel.
It is further to be noted, in connection with the quality of the steel employed in the construction of this gun, that the microscopic photographs of sections taken from the gun forging, both before and after straining, show a marked difference in the granular appearance of the steel between the Tioga Steel & Iron Company forging and a properly heat-treated specimen, indicating that the forging had been over-heat treated.
Both guns were star-gauged before the test, and after every 60 rounds thereafter. The first star-gauging measurement brought out a further imperfection in the Emery gun, its diameter being larger by .021 inch at the origin of the rifling.
Emphasis is laid on these relative imperfections of the Emery gun, as the theory of the process indicates not only improvement in elastic strength but in other qualities of the metal, especially at the interior of the bore, which on account of the high compression to which the interior is subjected ought to offer greater resistance to erosion.
The firing test of the two guns was continued from April 16 to about September 1, 1919, on which date each gun had fired 599 rounds; the following measurements give the enlargement in thousandths of an inch at and near the origin of rifling, resulting from careful star-gauge measurements made after the completion of the endurance tests:
| Distance from |
|
Emery | breech | Standard |
136 | 42.13 | 161 |
136 | 42.25 | 154 |
128 | 43.13 | 139 |
71 | 52.00 | 77 |
A visual inspection of the bore of the two guns shows the lands at the origin of the rifling nearly worn away for about 8 inches or a foot. At the muzzle, however, the lands are still in serviceable condition, although the gouges and tool markings in the rifling of the Emery gun still persist. Both appear still fit for further service, especially when compared with a standard Mark IX-5 No. 1302, in which, after 544 rounds, the lands are almost gone throughout the bore from origin to muzzle.
The extent to which the accuracy life of the guns has been reduced may be inferred from the results of firing the last five rounds of the series, using A & B shell, in comparison with a new Mark IX-5 standard gun:
Gun Velocity Pressure Range
Emery ...................................... 2759=13 14.32 = .33 921511±90
Standard .................................. 2768 = 25 14.61 = .24 9225 ± 98
New .......................................... 2862=11 16.43 = .21 9522 ± 40
The loss of about 100 f. s. in velocity, two tons in maximum pressure, and 300 yards in a range of 9500 yards, would not preclude their further use, although the accuracy of fire is considerably less.
Notwithstanding the further endurance indicated for both guns, it was decided to make thorough star-gauging and taking of impressions of their bore at the naval gun factory, where they were sent on September n, 1919; and upon the completion of this work to subject them to high-pressure firing for the purpose of testing the apparent superior elastic strength of the gun constructed by the Emery process. The design of the standard gun calls for a maximum elastic strength of 56,000 pounds, while the construction of the Emery gun shows that it should have a maximum elastic strength of 80,000 pounds. As these tests may be somewhat delayed, permission has been given by the bureau to publish a brief description of the process.
During the progress of the experiments resulting in the construction of these guns, the general principles of the proposed method, as well as the various practical questions involved, were examined and discussed at length by the Special Board on Naval Ordnance, to which were added the superintendent and other officials of the naval gun factory, as well as officers of the bureau and the Naval Proving Ground, having general experience and knowledge of the subject of gun construction.
The conclusions of the board, unanimously adopted at its last meeting on March 5, 1919, recommended the immediate construction at the naval gun factory of type 6"/53 guns, utilizing the facilities at the Bureau of Standards, and at the same time the development of plans for installation of the necessary facilities at the naval gun factory for construction of guns of this type as well as larger guns. The Bureau of Ordnance has approved these recommendations and has adopted for the official designation of the process the name of “ Radial Expansion.”
A brief historical resume of the basic principles involved in the proposed method will show that it is not such a startling innovation as might be imagined from the description of the two experiments already described. These principles are:
1. Special elastic strength induced by cold working.
2. Fixing or ageing of the increased elastic strength by a mild- heat treatment after cold working.
3. Radial expansion; resulting in a condition of initial strains, which are only partially realized in the methods now in general use; that is, shrinking on jackets and hoops previously expanded by heat, or wire winding.
1. Special Elasticity.—The development of a special elastic strength in metal by cold stretching was first definitely demonstrated and announced by Colonel Rosset, as a result of experiments at the Turin Gun Factory in 1874, upon test specimens cut from Petin-Godet hoops and other selected specimens of steel for gun construction. A summary of these experiments is contained in “ Notes on Construction of Ordnance ” (Army Ordnance Bureau No. 11), published in 1882. The results showed that if a test bar be subjected to a tensile stress greater than its limit of elasticity, it will not undergo, when subjected to a subsequent stress less than the above, any permanent elongation and, therefore, its elastic power has been increased. It was further indicated by the great regularity in the elastic elongations, and their proportionality to the stresses, that this special elasticity might be increased with the increase of the stresses nearly up to the point of rupture.
A clear idea of this development of special elasticity will be readily appreciated from the stress-strain diagram of test specimens subjected to cold stretching. When a test bar is subjected in a testing machine to a gradually increasing tension, there is a measurable elongation of the bar at each increment of tension, which is proportional to the tension until the elastic strength of the metal is reached. Within this limit, if at any instant the tension is removed, the bar recovers wholly its original dimensions. If the tension and corresponding elongation are plotted, the former as ordinates and the corresponding elongations per unit of length as abscissae, the resulting diagram will be a straight line, up to a certain point defined as the elastic limit, as illustrated in Fig. 10, in the sections OA, OB and OC; but as the load is gradually increased beyond the elastic limit, this regularity of behavior suddenly ceases, the elongation for a given increment of tension increases more rapidly than the tension, and if the load is now removed the bar contracts somewhat, but is not restored to its original dimensions—it has acquired a permanent elongation or set.
If the bar, after a period of repose, is again subjected to the load which resulted in the permanent set, it will be found that the elastic limit of the specimen has been increased—that the constant ratio of load to elongation will hold up to the load which produced the permanent set.
The dotted curve B, from Colonel Rosset’s data, is formed from the mean values of stress and corresponding elongations of several specimens which were subjected to loads leading to rupture. Other specimens of the same material were subjected to a stress sufficient to cause a permanent elongation, and the increase in the elastic limit of these specimens is shown in curve A, by the increase of the ordinate for the point A over that of the point B.
As these experiments were conducted for the purpose of studying the best method of constructing built-up guns, Colonel Rosset naturally inquired if the same results, in regard to special elasticity, would be obtained with specimens in which the permanent set had been produced by elongations due to heating and subsequent cooling, the contraction being restrained in order to produce the permanent set. The explicit problem was to determine if the contraction due to cooling of a heated hoop upon an interior tube, thereby introducing initial strains in the hoop and tube, would result in increased elastic strength as in the case of cold stretching.
Curve C corresponds to curve A—in A the special elasticity induced by cold stretching, in C the corresponding permanent set having been produced by heating and restrained contraction. It is evident at once from a comparison of the two diagrams that neither the elastic strength nor the ultimate strength of the latter metal has been favorably affected.
His conclusions, in regard to the general subject, especially with reference to built-up guns, in which the jacket or hoop is expanded by heat and contracted upon the interior members, are so important that they are quoted:
From these experiments it may be inferred that the elastic power is not increased by a tensile strain, when that strain is obtained by expanding the specimen by heating, and restraining the contraction consequent on cooling;
Wishing to develop the elastic power in order to obtain greater elastic elongation, it will be necessary to have recourse to mechanical tensile strains, and not to the method of restraining the contraction consequent on cooling.
In Fig. 11 is illustrated the effect of cold stretching upon the development of special elasticity in the case of two specimens of mild steel tested by Mr. A. H. Emery. These two diagrams are selected from a large number of similar ones, the results of Mr. Emery’s experiments made on the bridge links used in the construction of the St. Louis Bridge. The diagram shows that in the dotted curve the metal reached the elastic limit at the point F, corresponding to about 40,000 pounds of tension, and began to show the more rapid elongation beyond this point up to G, 46,000 pounds, when the load was removed. The permanent elongation for this load was about .009 per linear unit. After a brief period of time, the load was again applied and the strength line H-I shows that the limit of elastic strength has been increased up to the stress which produced the permanent set. From that point on the metal gradually elongates, and rupture takes place at 77,300 pounds. In the second diagram the elastic limit was reach at a slightly lower point, and the load was continued with the elongations indicated up to the point C at 62,000 pounds per square inch, with momentary elongation of .0413 and a permanent elongation of .040. In this specimen the load was allowed to remain for a period of a week, when it was removed; upon the gradual application of the load, the curve clearly shows the increased elastic limit up to 70,000 pounds.
2. Tempering or Ageing.—The question of the permanence of the special elasticity induced in steel by cold working has been a subject of much experimental investigation during the past 25 years. Some investigators have claimed, notably Prof. J. B. Johnson, of the University of Wisconsin, that the gain in special elasticity, due to tension, is accompanied by a loss of elasticity in compression. These conclusions are stated in a text-book for engineers recently published by Professor Johnson, under the title “The Materials of Construction, a Treatise for Engineers on the Strength of Engineering Material,” wherein he stated:
Both wrought iron and rolled steel, in their normal state, have “ apparent elastic limits” in tension and in compression numerically about equal. If this material be stressed much beyond these limits, however, in either direction, its elastic limit in this direction is numerically raised to about the limit of its greatest stress, while the elastic limit in the opposite direction is greatly lowered or even reduced to zero.
In the specifications of Mr. Emery’s patent, it was claimed that the gain in elastic strength by cold stretching could be rendered permanent by a mild-heat treatment, the claim being based upon the result of numerous experiments establishing this principle; an examination of numerous diagrams of these early experiments shows that the claim was well founded. Confirmation of his results are contained in a long and carefully conducted series of experiments by Prof. J. A. Van Den Broek, at the University of Michigan, which were published in the “ Journal of the Iron and Steel Institute,” at London, England, May, 1918.
These experiments show that metal, strained beyond the elastic limit, requires some time in which to recover from the disturbance of the molecular or crystalline structure; but this recovery of the metal may be accomplished by a mild-heat treatment from 100 degrees to 300 degrees centigrade, which seems to render permanent the condition introduced by cold straining.
His statement of conclusions is so important that they are quoted below without change. He first distinguished the several kinds of cold working, and the resulting special elasticity as being of different direction and sense, i. e.,
Tension and compression, same direction Tension and compression, opposite sense.
Positive torsion (+shear), same direction.
Negative torsion (—shear), opposite sense.
Torsion and tension, respectively, of different directions.
(1) When mild steel is cold-worked and properly aged or tempered, and subsequently tested in the same sense as that of cold working, its elastic limit may be raised more than 100 per cent and from 10 per cent to 20 per cent beyond the stress at which cold working was discontinued.
(2) When mild steel is cold-worked in one direction and properly aged or tempered, but tested in either one of two senses of a different direction, then its elastic limit may be raised some 50 per cent.
(3) When mild steel is cold-worked in one sense and properly aged or tempered, but tested in the opposite sense, then the elastic limit remains at the value of the original elastic limit, but the yield point is raised.
(4) When mild steel is cold-worked in any direction or sense, without ageing or tempering, then the elastic limit falls below the value of the original elastic limit, often down to zero.
(5) Tempering cold-worked steel at temperatures from 100° C. to 300° C., or ageing it, has a tendency to perfect its elastic properties. Tempering merely accelerates the effects of time.
3. Self-Hooping Due to Radial Expansion.—The first reference to the subject of cold-stretching a cylinder beyond the elastic limit, in order to realize the initial strains due to the subsequent contraction upon the release of the pressure, appears to be contained in the second volume of “ Limite D’Elasticity et Resistance a la Rupture,” by Captain Charles Duguet, published in 1885.
In the study of the ordinary method of built-up guns by hooping, he was led to consider the means of attaining a “ cylinder of equal resistance,” defining it as one in which each layer of the cylinder, under the maximum load within the elastic limit, should be brought at the same instant to its limit of elasticity. Such a cylinder would have a greater resistance than any other cylinder of the same dimensions and weight of metal.
The method of wire-winding approached this idea, but a more perfect realization could be obtained by cold-stretching a given cylinder of homogeneous material, by the application of increasing interior pressure until each concentric layer had acquired a permanent deformation up to the outside layer, which would be stretched just to its elastic limit. He assumes, in the dilatations of such a cylinder, as in all deformations of a non-porous solid, in which the density varies very little, that we may consider the volume constant without any appreciable error, so long as the dilatations do not exceed 0.03 per unit length. As a result of this assumption, the relative dilatations of the various layers are inversely proportional to the square of the radii.
He describes clearly the elastic resistance of such a tube primitively deformed:
When a tube has been sufficiently deformed by an interior pressure Pc, the elastic limit is passed at every one of its points. If the pressure ceases to act the tube is deflated, contracts upon itself; but the total deformations produced in the different layers being nowhere proportional to the elastic forces which accompanied them, all these forces cannot vanish at the same time; the exterior layers remained stretched and exert upon the interior layers a radial pressure; inversely, the interior layers react upon the others and are compressed tangentially, just as happens in a hooped cylinder. Under these conditions, a new pressure inferior to P0 will produce only elastic deformations
Under the new action of the pressure Po, each layer suffers a deformation corresponding to its actual elastic limit; and p and t being the pressure and principal tension developed at any point whatever, we shall have at each point the following relation:
mp + nt—G.
G, which depends upon the initial deformation, varies from one point to another and diminishes from the exterior to the interior.
In his theory of the resistance of metals, G is defined as the coefficient of “shear” (sliding, glissements), and is approximately six-tenths of the elastic limit of the metal for simple traction:
The interior pressure capable of deforming a tube so that the exterior layer shall be just at its elastic limit E is certainly superior to but it differs from it very little; for, at each point, even at the interior layer, the deformations produced are so small that the elastic forces developed are very little different from those which correspond to the natural limit of elasticity.
In the formula above quoted, n is equal to 0.59, m is equal to 0.41, and a is equal to 0.3. He applies his formula to an unstrained cylinder and to the same cylinder which has been strained as above described.
Let us take as an example a steel cylinder of one caliber thick, in which the metal has an elastic limit of 30 kgs. in simple traction. The natural elastic resistance is P0 equal to 1580 atmospheres; every pressure superior to this will produce a permanent deformation. A pressure slightly superior to per square millimeter, say about 4000 atmospheres, will produce permanent deformations at every point and leave the exterior layer stretched just to its natural elastic limit; its elongation will be the interior layer will suffer under the same pressure a dilatation nine times as great, or 1.35 per cent.
Thus, by a simple initial interior dilatation of 1.35 per cent, produced solely by interior pressure, we will increase the elastic resistance of a soft- steel tube one caliber thick from 1600 to 4000 atmospheres and we will obtain thus a cylinder more resistant than any other hooped cylinder of the same material and the same dimensions, as we have realized the solid of perfect equal resistance already defined.
The assumption of Duguet, given in the preceding paragraphs for the relations of the pressure and resulting tension at any point in a hollow cylinder, is equivalent to
t+0.7p=E,
where E is the initial elastic limit of the metal before cold working.
Beginning in 1909, extensive investigation of the subject has been made at the Central Laboratory of the French Navy, under the name of “ Autofrettage,” literally self-hooping or auto-hooping. The mathematical theory of " Autofrettage ” has been developed by Ingenieur-General L. Jacob, and very recently published in a two-volume work entitled “ Resistance et Construction des Bouches a Feu, Autofrettage.” He assumes that the elastic limit at any point is reached when the relation of the interior pressure and the resulting tension is
t+0.3p=E,
and calls attention to the fact that the theory of Duguet gives too small an amount for the maximum resistance of a cylinder which has been built by the process of autofrettage.
He also notes, further, that the adoption of the theory that the elastic limit of the metal is reached when at any point
t + p = E
leads to a still smaller resistance for a given cylinder.
The latter assumption is the basis of the mathematical investigations of Colonel Malaval, under whose direction the experimental work at the Central Laboratory of the French Navy has been carried on.
Each of these eminent authorities appears to establish the correctness of his respective theory upon the result of experiments and investigations made at the central laboratory. Both, however, call attention to the fact that the pressure of autofrettage is limited to such a pressure that, upon the return of the stretched cylinder to a state of rest, there shall not be developed any new permanent deformations in the interior layer.
Whichever theory is elected, the permissible pressures are considerably less than those to which the Emery gun was subjected. The examination of the initial strains of the rings cut from the metal of the Emery gun apparently does not furnish a complete criterion in this respect. If there has been introduced new permanent deformations in the interior layers upon the return to rest, they do not appear to be indicated by the initial strains found in the 24 test rings.
It is not intended in this description of the construction of the two experimental guns to discuss or to develop the mathematical theory of the elastic strength of guns constructed by the method of radial expansion, which will necessarily be deferred until the basic assumptions are more definitely established by experiment, nor is it necessary to do this in order to utilize the method in the experimental construction of either single-piece or built-up guns of several hoops and tubes.
In the design of guns to be constructed by this method, the elastic-strength curve is determined from the powder-pressure curve, just as in standard guns now in use; whether the gun shall be a single piece or a compound gun will be determined by the size of the forging which it is practicable to make from a metallurgical standpoint, and convenience in the machining of the gun. From the theoretical standpoint, the construction of a 16-inch gun should offer but few difficulties outside those to be expected in a smaller single-piece gun. The various parts are machined to a close fit with as little play as possible, the parts are assembled and the pressure is applied just as would be done with a smaller single-piece gun, as the pressure necessary to realize the elastic strength desired is a comparatively simple function of the ratio of the inside to the external radius.
Sufficient experimental work has already been carried out at the naval gun factory to demonstrate that the pressure plant installation can be constructed in a convenient form and at a relatively small cost, as the extensive experiments conducted at the French Naval Laboratory, under Colonel Malaval, have shown that all the elastic strength desirable can be obtained by pressures giving a permanent deformation of the interior layer not exceeding .03 ; an amount very much smaller than was experienced in the case of the Emery gun, due to the attempt to carry out the process by stretching the whole gun with the same pressure for all sections.