Volume 4, Issue 3, 01 March 1933
Index of content:
4(1933); http://dx.doi.org/10.1063/1.1745167View Description Hide Description
Beams of cement‐stone and cement‐mortar‐stone with the ratio of cement to sand of 1 : 3 were allowed to bend under their own weight, the cross section of the beams being 2.27 cm square and the distance between the supports 76.1 cm. The deflections were determined at the central point between the supports by means of an Ames gauge reading to 1μ; the experiments were extended over a period of seven months. The curve of the deflection vs. time generally starts as a parabola having a vertical tangent but beyond a certain point the curve becomes linear. The slope of the curve is a measure of the mobility of the material. The parabolic curve is due largely to the hardening of chemical origin, resulting from the absorption of water, which causes a gradual decrease in the mobility, but to a smaller extent to an elastic after‐effect. The mobility of the cement is much greater than that of the mortar and it decreases with the degree of curing. The yield value of the mortar is smaller than 45×105 dynes/cm2 (65 lbs./in.2) and the yield value of the cement was not perceptible with the apparatus used. For all practical purposes, cement‐mortar‐stone may be regarded as a solid of very low rigidity and cement‐stone as a very viscous liquid. The curvature of the stress‐strain curve, usually observed, is due to elastic after‐effects arising from the fact that the experiments require time. The modulus of elasticity is smaller in the case of cement (230–260,000 kg/cm2) than in the case of mortar (270–300,000 kg/cm2); it increases with the number of curing days and approaches a constant value.
4(1933); http://dx.doi.org/10.1063/1.1745168View Description Hide Description
4(1933); http://dx.doi.org/10.1063/1.1745163View Description Hide Description
The problem of the flow of materials at stresses far below their normal yield‐values is discussed, and it is pointed out that the sharpness with which yield‐values can be measured depends on the grouping of the relaxation times for the different strains set up within the material, an uneven distribution making for a sharper definition. Any sharp and drastic change in the relaxation time of the system as a whole may justifiably be said to constitute a yield‐value, the question as to which of these points is actually taken as the yield‐value depending on the conditions of the experiment. These considerations are reinforced by a discussion of the results (to date) of certain experiments on flour doughs, and it is claimed that flour dough is a peculiarly suitable material for such investigations. A new rapid method for studying flow in flour doughs, recently described in the literature, is discussed. The dangers of classifying materials in hard‐and‐fast rheological divisions is emphasized, while it is pointed out that for practical purposes, and given adequate safeguards, such classifications may be extremely useful.
4(1933); http://dx.doi.org/10.1063/1.1745164View Description Hide Description
The properties of materials at high temperatures are receiving considerable study in the design of much industrial equipment. The present paper describes some fundamental considerations in this field by discussing the creep of metals in shear. Some theoretical and practical reasons for investigating creep in shear are given, test apparatus for this purpose is outlined, and some experimental information is included. Much more work in this type of study is desired and the paper may help to stimulate further research in this direction.
4(1933); http://dx.doi.org/10.1063/1.1745165View Description Hide Description
Use is made of geometrical constructions to demonstrate the conditions under which a plot of V/πR 3 against ½pR gives a unique curve independent of the value of R, and also to show how account can be taken of discrepancies due to modified flow near the wall of the tube. In a similar way, the reasoning from which the velocity gradient GW at the wall of the tube can be deduced from experimental figures for V, p and R has been set out in a geometrical form, which should be helpful to those to whom a pictorial representation makes a ready appeal. The deductions, though simple, involve no loss of generality. The data of Farrow, Lowe and Neale for two percent starch paste are considered by way of example, and it is shown that their formof the equation for the velocity gradient at the wall has special advantages. Later work, by disclosing a wider basis, has shown that N need not be constant as they supposed, and also that, where modified flow occurs near the wall of the tube, V/πR 3 becomes V β/πR 3, the limiting value for large radii.