Relative Creep Research Articles

Creep of four tropical hardwoods from Cameroon

The knowledge of the behaviour of wood over time is important in the design of timber structures. This study, which is part of a program to better understand the behaviour of tropical woods, aims at showing the effect of creep in wood. Observations were made on the development of the deflection in a uniformly stressed beam under controlled temperature and humidity conditions. The results obtained show that for stresses not greater than 35% of the stress at failure, the behaviour of the species tested is linear. In these cases, the total relative creep is not more than 35%. Similar results have been obtained for four cameroonian species: Azobe, Tali, Sapelli and Movingui.

Effect of the orientation of the loading trajectory on the minimal intensity of the creep rates of materials under conditions of plane stress

1. On the basis of the experimental investigation of the regularities governing creep of steels 15Kh2MFA and 08Kh18N9 under conditions of a state of plane stress we suggested an equation which makes it possible to determine the minimal creep rate intensity of material in a state of plane stress. As basic data we used the results of creep tests obtained, e.g., under conditions of uniaxial tension and torsion at equal stress intensity levels and at constant test temperature. 2. We suggested a criterion of creep of material in the case of a state of plane stress, experimentally substantiated on steels 15Kh2MFA and 08Kh18N9. 3. We furnished a quantitative evaluation of the effect of the type of state of stress on the relative minimal creep rate intensity.

Microstructural aspects of long-term creep ductility of AISI 316 LN steel

Microstructural aspects of the creep plasticity and impact strength of non-stabilized austenitic CrNiMo steels with two different nitrogen contents have been evaluated. During creep exposure an intergranular precipitation of the sigma phase, M23C6 or/and Cr2N particles produce favourable conditions for transition from typically intergranular fracture to ductile shear fractures or to intergranular ductile fractures with characteristic dimple morphology. A given type of fractures is accompanied with high values of the relative creep rupture elongation (up to 100%) dependent on temperature and time to rupture.

Creep in Ordered Nickel Base Alloys

ABSTRACTThis paper examines the fundamentals of deformation behavior in ordered y′ (Ni3Al), β (NiAl) and β′ (Ni2AITi) phases, and specific two phase y/y′ and B/B′ alloys. The relative creep strengths of these ordered nickel-base alloys are discussed. Differences in creep behavior are explained in terms of the effects of temperature, crystallographic orientation and alloying on creep deformation mode. In particular, it is shown that trace additions of boron and carbon to y/y′ alloys, or deviations from stoichiometry in β′ -type alloys can exert a strong influence on creep behavior.

Creep Resistant Elastomer Formulations

Abstract Over 150 elastomer formulations of NR, DPNR, CR, IIR, AU, EU, EO, ECO, SBR, EPM, EPDM and a silicone rubber have been evaluated to identify the most creep resistant materials. Selected data are presented. Three of the best formulations (DPNR compound 15, CR compound 27 and EPDM compound 49) were found to give excellent creep performance extrapolating to less than 28% relative creep for a 15 year service life. As a result of a requirement for 15 year resistance to 25 pphm O3 concentration, EPDM formulation 49 was selected, and pads were molded and compression tested. Because of the O3 resistance of EPDM, there should not be a significant increase in creep rate.

Influence of carbon on the static and cyclic creep behaviors of alpha iron

The plastic deformation of both ZrH 2-treated iron and iron with a controlled amount of carbon is examined under constant load and repeated loading. The pure iron exhibits cyclic creep acceleration under cyclic loading but the magnitude of the creep enhancement is small and is relatively insensitive to applied stress, a result in contrast to the behavior of f.c.c. metals. The presence of carbon, even in small concentration (65 at. ppm), enhances the relative cyclic creep acceleration when the specimens are aged after carburization. In carburized and quenched specimens, however, both static and cyclic creep are suppressed. The observed difference in cyclic creep behavior is correlated to the difference in the nature and distribution of the carbon and the relative ease of pinning of dislocations by the carbon atoms.

Effects of time, temperature and curing on the stiffness of epoxy laminating systems

Relative creep moduli of a series of epoxy laminating resins were found to be the same in uniaxial tension and in torsion when measured under loads of short duration. However, their tensile creep moduli decreased with time and temperature at different rates, changing their relative stiffness. For one typical resin the short-term tensile and shear moduli decreased with cure temperature reaching minima and then increased slightly. Deflection temperature under load determined by standard tests correlated inversely with the short-term tensile modulus for the typical resin considered and failed to provide a basis for determining the relative stiffness of the different resin systems.

Calculation of the creep of structures with low stress levels

The standard method of testing cylindrical specimens for creep under extension is not accurate enough at creep rates of approximately 10 -6 percent/h or less, and the long-term creep characteristics of metals given in the reference literature for boiler and turbine materials [i] usually lead to stresses at the level of the creep limit ~c (corresponding to 1% deformation in 105 h) or higher. Moreover, the stresses acting in structures operating under conditions of creep, e.g., in boiler turbine construction, are usually set below Zc, and their average level amounts to approximately 0.4-0.7 ~c" Extrapolation of the reference data is used to calculate the creep in a structure operating under such low working stress levels. Thus, data on the minimal creep rates for low stresses are obtained by linear extrapolation against double logarithmic coordinates, which corresponds to the assumption of a power relationship between the stress z and the relative creep rate }:

The mechanical properties of two uranium alloys and their role in the oxidation of the alloys

The creep and tensile properties of two uranium alloys (U-21 at.% Nb and U-16.6 at. % Nb-5.6 at. % Zr) were determined in the temperature range 750–900°C. The creep data were fitted to an equation of the form\(\dot \varepsilon = k\sigma ^n\)exp (-Q/RT)which in turn was used to calculate stresses in oxidizing alloy specimens on the basis of elongation measurements made on the specimens during oxidation. The variation of the average stress in the oxide and in the substrate metal is given as a function of time. The details of the mechanism by which stresses are generated during the oxidation of the alloys are discussed. Comparisons of the relative creep and oxidation rates of the alloys with the stress levels observed during oxidation lead to the conclusion that at least for certain specimen geometries the differences in the mechanical properties of the oxide scales and of the parent alloys account in large measure for the differences in the oxidation characteristics of the alloys.

Contact creep as the principal source of the losses associated with the rolling of a plastic ball

An engineering method is proposed for calculating the contact creep of plastic balls used in rolling bearings. The method is based on empirical expressions relating the relative creep strain with the specific load and its duration of action. The complex contact-creep characteristics of the material are obtained by means of simple laboratory tests.

Comparative creep-rupture properties of tungsten-25% rhenium consolidated by arc-melting and powder-metallurgy techniques

The creep-rupture properties of W-25% Re materials consolidated by arc-melting and powder-metallurgy techniques have been compared at 1650 ° and 2200 °C for rupture times up to 1000 h. At comparable stresses, powder-metallurgy material has a shorter rupture life and lower rupture ductility than arc-melted material. On the basis of secondary creep rate, powder-metallurgy material is weaker than arc-melted material at 1650 °C but stronger at 2200 °C. Optical and electron metallography showed that voids form at grain boundaries in both materials. In the powder-metallurgy material, grain boundaries appear to be immobile, and massive growth of voids initiates fracture with very little deformation. In the arc-melted material, grain boundaries appear to be mobile, and fracture seems to be forestalled by their ability to move. The relative creep rates can be explained on the basis of void growth and grain size.

Thermoelastic Expansion and Creep of Polyethylene Terephthalate and Polypyromelitimide Film and Polyethylene Terephthalate Fibers from 20 to 295 K

A quartz tube dilatometer was used to measure the lineal thermal expansion and creep of single lengths of polyethylene terephthalate (PETP) film, polypyromelitimide (PPMI) film, and PETP multifiber yarn, while stressed under constant tension. Tensions below and above the conventionally defined yield strength were used and the sample temperature ranged from 20 to 295 K. Relative creep strain measurements, taken at the constant temperatures 77, 195, and 295 K were found to obey the equation ε=exp[−2.3 exp(A′y)]where y is a function of stress, time, and temperature and A′ is a constant depending on the material. This equation was used to correct the thermoelastic expansion measurements for creep at the higher stresses. PETP multifiber yarn subjected to a slight tension was found to elongate during cooldown from 293 to 20 K. Higher stresses caused less elongation; i.e., the coefficient of expansion increased with stress. This result is believed to be due to changes in crystallinity at the higher stresses. A similar stress effect was found with PETP film but not with PPMI film. The thermoelastic expansion of the film samples was also found to be sensitive to the thickness.

Contact Traction and Creep of Lubricated Cylindrical Rolling Elements at Very High Surface Speeds

This paper discusses test results obtained on a single contact, cylindrical, traction test machine operating at rolling speeds ranging from 5,000 fpm to 25,000 fpm and at maximum Hertz stresses from nearly zero to 200,000 psi. Effects of speed, load and contact length on traction and relative creep between two plain cylinders lubricated with a naphthenic mineral oil were investigated. Test results indicate that coefficient of traction and creep were greatly affected by all three variables tested. Interestingly, the increase in normal load always resulted in an increase in coefficient of traction. However, beyond the contact stress of 200,000 psi, the load effect practically disappeared. No discontinuities were discovered in the coefficient of traction curves in the speed and load range tested. However, boundary lines were established separating the regions of complete hydrodynamic lubrication (where traction is nearly zero), from elasto-hydrodynamic regions (where the traction rises with load). The obtained data paves the way for the design of traction drives capable of operating at very high speeds, such as might be encountered in gas turbine drives. Also, certain qualitative conclusions suggest several possible approaches to reducing skidding of rolling contact bearings at high speeds.

Effect of Surface Condition on Creep of Some Commercial Metals

Abstract The relative creep rate of specimens of 2S aluminum and commercial drawn copper were investigated with the oxide surface layers removed and then, under the same conditions, after the oxide was permitted to form. The oxide was removed mechanically by stripping the test specimens in an inert atmosphere furnace so equipped that an axial stress could be applied and the creep rate determined without disturbing the atmosphere. After the oxide-free specimen was observed to be creeping essentially linearly with time (i.e., the quasiviscous, secondary creep range was established), air was admitted to the furnace. The extension versus time plot was continued after admission of air and the temperature held constant. Since all of the known parameters affecting creep rate (temperature, grain size, degree of work hardening, stress, and so on) except the gaseous environment were the same before and after admitting air, the effect of this change could be isolated. A decrease in creep rate was observed after air had been admitted to the furnace. These experimental results indicate that the presence of an oxide surface layer on the commercial grades of pure copper and aluminum increases the creep strength of these metals. The manner in which this relatively thin surface layer contributes to the strength of the parent metal is not clear, but the phenomenon is consistent with the theory of dislocations as presented by G. I. Taylor if it is assumed that the oxide inhibits the generation and/or migration of dislocations.

Primary Creep and Stiffness of Vulcanized Rubbers

Abstract The addition of two compounded blacks renders induction concerning the mechanism of either black uncertain. Generally speaking, however, the black in a specimen of given size may be thought of as replacing its volume of rubber, so, when under a particular load, the rubber remaining is more highly stressed. Because the rubber is highly stressed, the relative creep is increased, but because there is less of it, the actual creep is diminished. for a given shape of specimen, all soft compounds and gum stocks have the same curve of load-area vs. strain up to 20 per cent compression, 40 per cent tension, and 40 per cent shear; the same applies to medium hard stocks. The stiffness of any such specimen can be expressed in terms of a convenient reference level, such as the load-area at 10 per cent shear 1-minute strain. Over a wide range of conditions in which polymer, compound, cure, and geometry of loading have been varied, 10-minute creep remains proportional to 1-minute deflection for each particular geometry of loading, but in tension the ratio of 10-minute creep to 1-minute deflection is greater than in shear, in compression less—an effect which is generally more marked in soft than in hard compounds. Measurements of creep on specimens in which the ratio of diameter to height is greater than 1.6 were, mainly on account of the small deflections involved, not sufficiently accurate to be included. It is reasonable to assume, however, that creep is proportional to strain under other conditions than those considered here, such as in specimens of other shapes and specimens loaded in bending and torsion.

Use of the Shore Durometer for Measuring the Hardness of Synthetic and Natural Rubbers

Abstract The manner in which the readings of the Shore durometer vary with time was studied for 16 different compounds of natural and synthetic rubbers. The data were obtained from motion pictures showing a durometer and a stop watch in the same field of view. To eliminate the personal factor, the durometer was mounted in a frame, and pressure was applied by dead-weight loading. The results indicate both the necessity and the practicability of specifying that measurements be taken at a definite time after the application of the durometer to the specimen. The results also indicate the possibility of using readings taken at two different times as a rough measure of the relative creep of different rubbers.

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.