necessarily apply to them all. In the Summary (d) on p. 71, it is stated th a t:
“ The types o f fracture were almost identical with those obtained for the slow test.” In making an ordinary tensile test o f mild steel, you know that it will start to fail with a wedge o f deformation—Liiders line or whatever you like to call it—which starts from the concentration o f the stress, and generally, because the test-piece cannot be pulled truly axially, it will start as shown in Fig. A , and then there will be others and a criss-cross appearance is obtained on the surface, though finally the test-piece will break in the centre. Those Liiders lines occur as curves when a bullet is fired at a mild steel plate. B y firing bullets at different velocities— this was done many years ago, and we repeated it not many years ago—
two pictures are obtained, as shown in Pig. B (a) and (b). In (a) the bullet has gone right through; it has made a hole right through the plate, and the deformation lines are short. A slower bullet, which merely dents the plate, produces the same shape o f deformation, the same Liiders lines, but, as in (6), they extend further; the slower speed has allowed a greater ab
sorption o f energy in deforming the material, whereas the faster has used itself up in punching a hole through the centre, and the yielding or straining o f the surrounding metal is less.
The other conclusion is “ The yield-point was increased very con-Fic.. A.
(a) Fig. B(a).—Fast
Bullet H ole:
Little Defor
mation of Mild Steel Plate.
Fig. B(6).— Slow Bullet Indentation: Defor
mation Lines Spread Wider.
siderably.” That is only another way o f saying that the yielding has de
creased, and it is said that with the high speed the amount o f yielding before rupture which has taken place in the metal is very much less than with the low speed. Fig. B (a) with the high-speed bullet shows smaller Liiders line curves, i.e. less yielding, i.e. a higher yield-point than Fig. B
(b),
in which a slower speed bullet has produced more yielding, i.e. longer Liiders lines, although stressing the metal below the point o f rupture. That is something which anyone can repeat on a piece o f mild steel by using Fry etching after firing bullets at different speeds, and that is an easy way o f obtaining very rapid failure.The A u th o r (in reply) : Regarding Professor Thompson’s remarks as to the yield-point and maximum stress being the same for some materials, I would point out that work, which has been done at slower speeds on the effect o f the rate o f loading on tensile test results, tends to show that for some materials the yield-point is increased at a greater rate than the maximum stress, and this result would appear to be the final outcome o f this tendency.
Dr. Hatfield referred to the pressure material which was used. This material is used commercially in a high-speed pressure recording device, and its properties have been carefully investigated.
The fact that the percentage elongation, percentage reduction o f the arc*,
and the typo o f fracture arc the same as for the slow test is confirmed by Mann, to whose work Dr. H oyt refers in his discussion on this paper. Below the “ transition velocity ” o f Mann these properties are unaffected.
The acceptance o f the mean lino drawn through the vibration for measure
ment o f the maximum stress is doubtless open to criticism, but as I had only a limited time at m y disposal for this work, the late development o f the technique gave mo no time to try to eliminate this obvious defect. I hope that at some future date the method will be refined and diagrams will be obtained giving a load elongation diagram free from any distortion.
CORRESPONDENCE.
P r o fe s s o r D o n a ld S. C la r k * : It should bo pointed out that the speeds employed in this work can scarcely be classified as “ ultra-high speeds,”
since they correspond approximately to the speeds that are used on our present- day impact testing machines, which are in the region o f 15-20 ft. per second.
Attention is directed to work being carried out at Watertown (Mass., U.S.A.) Arsenal at velocities up to 300 ft. per second, j
It has been m y opinion for some timo that a thorough understanding of the fundamentals o f impact loading under tension could be obtained only through an investigation o f the stress-strain relations which exist during the application o f loads o f short duration. Mr. Ginns has made an excellent start, and has developed a method which seems to have possibilities. It would seem very desirable to have a continued and more thorough investiga
tion of this work. Additional work should include an improvement o f the force-measuring system in order to decrease the oscillations in the diagrams.
It is my opinion that the oscillations which appear in the force deformation diagrams could be reduced by a simple redesign o f the equipment. These oscillations tend to give some uncertainty to the exact nature o f the diagrams.
It would have been o f particular interest to have compared the energies represented by the area under these force deformation diagrams with determina
tions made with the standard tension impact machine. Further, in view o f the work o f H. C. Mann o f the Watertown Arsenal, it would be interesting to see how the energy absorption determined by Mr. Ginns’ method compared with the energy required to cause failure under static conditions.
It is indeed interesting to note that Mr. Ginns reports that the yield- points o f the materials tested under impact are considerably higher than when tested under static conditions. I am a little doubtful o f the accuracy of the determinations o f these values, because o f the oscillations which exist in the diagrams. Some work which has been done under m y direction would seem to substantiate the order o f magnitude o f Mr. Ginns’ findings.
Dr. S. F. D o r e y ,{ W h.Ex. (Member o f Council): The experimental results given in this paper are an indication o f what is to be expected in high
speed stressing, such as occurs in ship collisions, and at the firing point in internal combustion engines.
I feel that a more detailed description o f the apparatus used would be welcome, particularly an explanation o f why the vibration diagram, after complete release o f the load on the cast iron specimen, has not the datum line as its axis. Further, it is stated that the natural frequency o f vibration
* Assistant Professor of Mechanical Engineering, California Institute of Technology, Pasadena, Cal., U.S.A.
t Proc. Amer. Soc. Test. Mat., 1936, 36, 85-109.
| Chief Engineer Surveyor, Lloyd’s Register of Shipping, London.
o f fclic spring was 35*7 per second, and that a mathematical investigation allowed that tho assumption that the presence o f the specimen made no change in this frequency was justified. It is also stated that the time to fracture a specimen was calculated on this basis, but as it appears that the average time to fracture a specimen was 0-005 seconds, and the stress—strain diagrams show several complete vibrations, it is assumed that the vibration was o f a higher order than tho fundamental. It is also difficult to see how this order and frequency were ascertained.
I f the design o f the apparatus could be altered to give different rates of stressing, by some such method as connecting the centre o f the plate spring to an oil cylinder with a small orifice, the size o f the orifice being adjusted to give different rates o f stressing, a very useful field o f research could bo de
veloped, and the variation o f yield-point and maximum stress with speed o f stressing fully investigated.
Dr. E. W . F e l l * (M em ber): Knowledge o f the behaviour o f metals j when subjected to stresses varying in magnitude at high rates is o f especial value in itself, and also in supplementing existing data about their behaviour at stresses much less rapidly applied. The author’s conclusion that the yield- point at ultra-high speeds was very considerably increased in value, over j 100 per cent, increase being recorded for some materials whilst the other properties show no unfavourable change, implies a much extended elastic j range, and is therefore o f great importance.
The ordinary yielding phenomena in soft steel and annealed Duralumin, under stresses varying in the elastic range at speeds used in general testing to-dav, require time to reach completion. W hen the rate o f loading is great, such as about 0-005 second to reach fracture, it would appear that there is no time for yielding to occur, and so it seems that a much higher stress is necessary to permit yielding in the shorter time available. It is possible that there is a quite simple relation between the elevation o f tho yield-point and the rate o f loading— perhaps a linear one.
A t slow rates o f loading the distortion at the remarkable yield-pomts in these materials for tensile bars is found to bo propagated along the bar.
It would bo o f interest to ascertain any changes in the distortion, such as in propagation, due to the high rates o f loading. Bullet and impact tests on soft- steel plates suggest that a marked change occurs in the distribution o f the distortion occurring at the yield-point, for the distortion (lines o f Hartmann) extends to a greater distance from the centre o f the disturbance than it would do if the penetration were very much slower, e.g. in the case o f a hole made by a stamp slowly pressed. In carrying out such tests, it seems as well to remark that the mode o f support o f tho plate o f material should receive attention, otherwise misleading results may occur as a result o f reactions set up at various points at the back o f the plate.
As explained in the text, the curve EB o f Fig. 3 for a type o f stress-strain curve found for soft steel is very erratic beyond the stage at which yielding starts, and similar remarks apply to the other curves. Still further investiga
tion o f testing methods might result in stress-strain curves being produced j more representative o f the properties o f the material beyond the yield-point J at these high speeds.
Dr. S. L. H o y t t (M em ber): Experimental data on the behaviour of j metals when strained and broken at high speeds are very welcome. Mr. j Ginns describes a most ingenious dcvice for recording stress-strain diagrams,
* Magnesium Elektron, Ltd., Clifton Junction, nr. Manchester.
f Director of Metallurgical Research, A. 0 . Smith Corporation, Milwaukee, Wis., U.S.A.