Amount Of Creep Research Articles

An Analytical Investigation of Creep under Combined Loadings

BISPLINGHOFF has given a comparative graph showing the relative importance of the parameters of design at various Mach numbers. At Mach numbers above five, i.e. in the range of missile flight, creep becomes the critical design factor. Even though the time of flight of the missile is very short, and not enough to cause any considerable amount of creep, the deflexions caused will be enough to disturb the stability of compression members. Also the aerodynamic shape can be distorted with serious effects on the flow characteristics of the profile.

A Simple Cold Test for Vulcanized Polymers

Abstract The freeze resistance of vulcanized polymers has been evaluated for the past two years at this laboratory by a method which involves measurements of primary creep at successively lower temperatures. This method has been found to be particularly useful when it is desired to know, within 5° or 10° F, the temperature at which the polymer ceases to function as a rubberlike material. In a laboratory of this kind, where considerable work is done in evaluating new polymer types and where the polymerization variables studied often are of such a nature as to produce relatively large changes in the freeze resistance of the vulcanized polymer, a tolerance of 5° to 10° F has been found to be a workable value. The test is simple and can be carried out with the Shore A durometer or the A.S.T.M. hardness tester, one or both of which are generally available in a rubber laboratory. Its simplicity notwithstanding, the test is reasonably objective. Both the durometer and the A.S.T.M. tester are handled in the conventional manner, and readings are taken five and 30 seconds, respectively, after the pressure plate or presser foot contacts the specimen. The degree of creep is recorded as the difference in hardness points between these two readings. It is a matter of experience that, for most polymers, the amount of creep or flow exhibited at successively lower temperatures increases gradually, reaches a maximum, and then begins to decrease at a relatively high rate. The temperature at which this rapid drop-off occurs is considered significant because at this point the polymer begins to lose, at an accelerated rate, the fluidity which it had at higher temperatures and is being transformed into a hard, nonrubberlike material.

The Creep of Natural and Snythetic Rubber Compounds in Shear

Abstract Creep tests, extending in some cases as long as 900 days, indicate that both natural and synthetic rubbers such as Neoprene and butadiene copolymer can be compounded to give satisfactory service in shear mountings. At 140° F, creep is from two to nine times greater than at 80° F, depending on the compound. Tests at room temperature do not indicate either the amount of creep or the life to be expected at higher temperatures. Actual creep (measured in inches) increases with stress, but when expressed at percentage of initial deflection, it may be independent of stress. Creep curves are linear over a considerable range of time when plotted on log-log scales. However, extrapolation of such curves to predict results after very long times is not justified, because the curves may not continue to be linear, or failure of the mountings may occur, particularly at high temperatures. Short time tests of any sort are not necessarily indicative of the relative creep or life of compounds in long-time service. The tests reported here are a small portion of a large number which is being continued and augmented. It is hoped that these and other investigations now under way may contribute to clearing the picture of the complex interrelations between the many physical properties of compounds of rubberlike materials. The rubber technologist uses his specialized knowledge to develop a wide variety of compounds, making use of several types of basic rubberlike materials. He chooses whichever fits service requirements best from performance and economic viewpoints. Modern materials and recent developments in processing technique have made possible compounds suitable for a wide range of service conditions.

Effect of Temperature on Physical and Optical Properties of Photoelastic Materials

Abstract This paper reports the results of tension tests, to determine the suitability for photoelastic work, of five samples of Bakelite and two of Marblette at temperatures between 32 and 140 F. These results show the variation, of Young’s modulus, Poisson’s ratio, stress-optical coefficient, and strain-optical coefficient with temperature. Stress-strain curves of the materials are also given showing the relative amounts of creep. The effect of heat-treatment on Marblette is shown by tests before and after annealing.

Creep in Cellulose Acetate Filaments

The elongation of a series of single cellulose acetate filaments stressed for a period of 29 days with a constant load has been measured at relative humidity values ranging from 0 to 100% in nominal steps of 10%. The creep (elongation under constant stress) continues indefinitely but at a rapidly decreasing rate in all the fibres. The rate of creep is found to decrease with the total time of stressing with certain superimposed discontinuities which scem to indicate transitions frorn one manner of shear slip to another. The amount of creep at the end of any arbitrarily fixed time increases slowly between 0 and about 60% relative humidity, but above 60% the increase is very rapid. Rupture seems to be governed by a limiting elongation (maximum strain).