A description is given of the experimental technique devised to apply the method outlined theoretically in part I to the measurement of the dynamic compressive yield strength of various steels, duralumin, copper, lead, iron and silver. A polished piece of armour steel was employed as a target, and cylindrical specimens were fired at it at various measured velocities from Service weapons. The distance between the weapon and target was made short to ensure normal impact, and apparatus was devised for the precise measurement of striking velocity over this short range. The dynamic compressive yield strength was computed from the density of the specimen, the striking velocity, and from measurements of the dimensions of the test piece before and after test. Details are given of the accuracy of the various measurements, and of their effect on the values of yield strength. The method was found to be inaccurate at low and high velocities. For instance, with mild steel, satisfactory results were only obtainable within the range 400 to 2500 ft. /sec. The range of velocities within which satisfactory results could be obtained varied with the quality of the material tested, soft metals giving results within a much lower range than that necessary for harder materials. Because of its failure at low velocities, the method could not be employed to bridge the gap between static and dynamic tests. The rate of strain employed in the dynamic tests could not be measured, but was estimated to be of the order of 10,000 in. /in. /sec. With the materials tested little change of dynamic strength occurred within the range of striking velocities employed, probably because the rate of strain did not vary to any great extent with the striking velocity. Within the range of weapons available, that is, from a 0·303 in. rifle up to a 13 pdr. gun (calibre 3·12 in.), little change of dynamic strength occurred with alteration of the initial dimensions of the specimens, probably because the corresponding change of rate of strain was not large. In general, the dynamic compressive yield strength S was greater than the static strength Y represented by the compressive stress giving 0·2% permanent strain. For steels of various types, regardless of chemical composition and heat treatment, there was a relation between S / Y and the static strength Y , the ratio decreasing from approximately 3 when Y was 20 tons/sq. in. to 1 when Y was 120 tons/sq. in. A similar relation occurred with duralumin, S / Y varying from 2·5 at Y = 8 tons/sq. in. to 1·4 at Y = 25 tons/sq. in. Dynamic compressive yield values were obtained for soft materials such as pure lead, copper and Armco iron, which, under static conditions, gave no definite yield values. A plot of the unstrained length of the specimen X , expressed as X / L (where L = initial overall length), versus the final overall length L 1 , expressed as L 1 / L , was made for the various materials. Any specified value of X / L was associated with greater values of L 1 / L for the more ductile materials, such as copper and lead, than for the brittle materials, such as armour plate and duralumin.
Summary: An experimental vibrator, operating on the top surface, has been used to compact freshly placed concrete (mix 1:2½:5; water-cement ratio 0·60) of low to very low workability. A study has been made of the effects on the compaction of slabs 18 in. thick of varying the frequency of vibration within the range 1,500-6,000 v.p.m., the acceleration within the range up to 12g, the amplitude within the range 0·004 to 0·064 in., and forward speeds of travel within the range 1-8ft per min. The density of the concrete was measured after test, while measurements were made during the experiments of the vibration occurring at various depths in the concrete. A small variation in the workability of the concrete was found to have a considerable efIect on the surcharge required and the depth of compaction achieved. Consequently the effect of the method of compaction has only been studied so far for concrete of one workability, only those tests where the compacting factor was within a narrow range around 0·80 being considered. When the vibrator operated at constant acceleration and constant number of vibrations per foot of travel, increase of amplitude of the beam increased the depth of compaction. At constant acceleration and amplitude of the beam, slower speeds of travel gave increased depth of compaction. When both the amplitude of the beam and number of vibrations per foot run were constant, the depth of compaction was unaffected by change of acceleration. The total work done on a particle at a given depth was found to be related to the density attained at that depth. A connexion has been found between equivalent depth of compaction and the product of the amplitude of the beam and the number of vibrations per foot run. The depth of compaction increases with this factor but attains a limiting value, probably dependent on the concrete mix and the characteristics of the vibrator. It seems desirable, with vibrators designed to operate at constant acceleration, to operate them with a large amplitude and low frequency.
Descriptions are given of the electronic apparatus and techniques developed at the Road Research Laboratory for measuring vibrations. The equipment had been used, in the first instance, for measuring the movements produced in freshly placed concrete by an experimental vibrator operating on the surface. Details are given of the circuits, the calibration arrangements, and the type of information obtainable.
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