Testing creep measurement

Stress relaxation. In a stress-relaxation test a plastic is deformed by a fixed amount and the stress required to maintain this deformation is measured over a period of time (Fig. 2-33) where (a) recovery after creep, (b) strain increment caused by a stress step function, and (c) strain with stress applied (1) continuously and (2) intermittently. The maximum stress occurs as soon as the deformation takes place and decreases gradually with time from this value. From a practical standpoint, creep measurements are generally considered more important than stress-relaxation tests and are also easier to conduct. 

Creep and stress-relaxation tests measure the dimensional stability of a material, and because the tests can be of long duration, such tests are of great practical importance. Creep measurements, especially, are of interest to engineers in any application where the polymer must sustain loads for long periods. Creep and stress relaxation are also of major importance to anyone interested in the theory of or molecular origins of Viscoelasticity. 

The presumed effect of this pressure difference on the zero-creep measurements was rarely mentioned in the literature, but its existence would not be too difficult to test. If Pc were as important as believed71, then the W0 of a prismatic wire would have been twice that of a cylindrical wire of an identical cross section, and the WQ of a wire with a flat horizontal end would be quite different from that of a wire ending in a hemisphere. Neither of these differences ever was observed. 

Foamed blends of ethylene-styrene interpolymer and LDPE were subjected to a range of mechanical tests, including compressive impact testing, Instron compression and Poisson s ratio measurements, compressive creep measurements and compression set and recovery measurements. The data obtained were compared with those for EVA and the suitability of these foamed blends as replacements for EVA in the manufacture of soccer shin guards and midsoles for sports shoes was evaluated. 20 refs. 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

The four commonly used techniques to extract information on the viscoelastic behavior of suspensions are creep-compliance measurements, stress-relaxation measurement, shear-wave velocity measurements, and sinusoidal oscillatory testing (25-27). In general, transient measurements are aimed at two types of measurements, namely, stress relaxation, which is to measure the time dependence of the shear stress for a constant small strain, and creep measurement, which is to measure the time dependence of the strain for a constant stress. 

These authors also highlighted the fact that the reduction in free volume on aging must lead to shrinkage of the sample during a creep measurement. Consequently, this effect opposes the deformation and negative creep may be seen in low-stress tests. When these were carried out the master curve constructed from the data at various T deviated from that expected, see Figure 14.10. Because aging effects led to an over-prediction of the strain when short-term data... 

Concrete has a viscous behaviour when it is loaded with a constant stress it shows a strain that increases with time. Conventionally an elastic deformation is considered when it occurs during application of the load, while subsequent deformation is attributed to creep. It is possible to define a modulus of elasticity for concrete that can be evaluated with short-term tests [9]. Similarly as for the tensile strength, empirical formulae are available that give an approximate correlation of the modulus of elasticity with the compressive strength [1,9]. A dynamic modulus can also be estimated with non-destructive tests that measure the rate of propagation of ultrasonic vibrations through concrete [1],... 

Stress relaxation experiments involve the measurement of the force required to maintain the deformation produced initially by an applied stress as a function of time. Stress relaxation tests are not performed as often as creep tests because many investigators believe they are less readily understood. The latter point is debatable, and it may only be that the practical aspects of creep measurements are simpler. As will be shown later, all the mechanical parameters are in theory interchangeable, and so all such measurements will contribute to the understanding of viscoelastic theory. Whereas stress relaxation measurements are useful in a general study of polymeric behavior, they are particularly useful in the evaluation of antioxidants in polymers, especially elastomers, because measurements on such systems are relatively easy to perform and are sensitive to bond rupture in the network. 

Wong performed creep measurements on 10,15, and 20 % PVA-C of cycle 6 by applying a constant stress of 0.05 MPa over 1 h The test was done in phosphate-buffered saline. She found that the samples did not relax completely after 1 h and that increasing the PVA concentration decreased the amount of creep [23]. 

Creep measurements involve measuring a constant tensile or flexural load to a respective specimen (as discussed previously) and measuring the strain as a function of time. In a typical creep plot, percentage creep strain is plotted against time. The apparent creep modulus at a particular time can be calculated by dividing the stress by strain at that particular time. Creep compliance is determined by dividing the strain by stress. For a tensile test, the simplest way to measure extension is to make two gauge marks on the tensile specimen and note the distance between the marks at different intervals. However, accurate measurement of extension requires an optical or laser extensometer. In a flexural measurement, the strain is usually calculated with the help of a linear variable differential transformer system. 

Materials received by Dr. K. Liu at Oak Ridge National Laboratories (ORNL) exhibits the same high creep resistance as the ones that have been exposed for 1500 hr in the current tests. Also, the specimens tested by ORNL do not exhibit extensive transient creep, which suggests that the creep measurements are being affected either by minor differences in the microstructure of the two batches of materials, or by some difference in test procedure. To further study the creep process, a set of specimens are being annealed at 1350°C before being crept at high temperatures. Also, set of specimens prepared from the same billet will be tested both at ORNL and at NIST to determine if some difference in test procedure can account for the differences in creep behavior,... 

Compressive creep tests allow measurement of strain as a function of time when a constant stress is applied. These can be conducted at several stress levels for aerogel of various densities. Loads are removed at the end of the creep test, and strains as a function of time are monitored to determine the recovery behavior. Compressive relaxation tests can be conducted at different strain levels. The relaxation functions determined at the same strain level at different temperatures can be shifted horizontally to determine whether a master curve can be formed for use to determine the long-term behavior. Recovery behavior after relaxation can also be characterized by monitoring the stress as a function of time after removing partially the step strain. For aerogels that contain polymers such as X-aerogels... 

EN 1542 Products and systems for the protection and repair of concrete structures. Test methods. Measurement of bond strength by pull-off. EN 1544 Products and systems for the protection and repair of concrete structures. Test methods. Determination of creep under sustained tensile load for synthetic resin products (PC) for the anchoring of reinforcing bars. 

Creep measurements bring in the dimension of time by characterizing the extension with time of a test specimen subjected to a constant load. Loads are applied in tension, fiexure, or compression. Classical creep is extremely time consuming because it involves subjecting the test specimens to the desired load for the period of time of interest, which can be several months or even years. Consequently, creep data are not common. Sometimes, it is possible to apply time-temperature superposition to generate a master curve... 

The mechanical behaviour of coatings can also be measured using transient methods, such as creep and stress relaxation testing. Creep tests record strain under constant load while stress relaxation measurements record stress under constant strain. In both cases, the modulus as a function of time is calculated. 

The resin is generally the weaker constituent of a composite, and so will set the upper limit to the service temperature. This arises because of its inability to transfer the load effectively as the service temperature approaches the heat distortion temperature of the resin. As discussed subsequently, it is for this reason (amongst others) that the acceleration of creep measurements by undertaking shorter tests at higher temperatures (time-temperature superposition) must be undertaken with caution. 

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