Measurements constant stress

It may be of interest to note here that the failure of glass exhibits a dependence on time. For example when a constant stress of the order of magnitude of K/c is applied, the time-to-failure varies inversely as the applied stress. On the contrary if the applied stress is continuously increased such as to keep a constant strain rate, stress-to-failure varies directly as strain rate. These are manifestations of delayed failure which is due to the phenomenon of fatigue. Observations made under constant stress measures the static fatigue while the observations under constant strain rate conditions measure dynamic fatigue. The difference in... 

The above measurements should be supplemented by particle size distribution measurements of the diluted dispersion (ensuring that no floes are present after dilution) to assess the extent of Ostwald ripening. Another compHcation may arise from the nature of the flocculation which, if it occurs in an irregular fashion (producing strong and tight floes), may cause i (0) to be increased while cr may show some decrease, and this wiU complicate the analysis of the results. Yet, in spite of these complications, constant stress measurements may provide valuable information on the state of the dispersion on storage. 

Set can be measured after the test piece has been subjected to constant stress or constant strain, but the latter is by far the most widely used, and constant stress measurements are virtually never seen for quality control purposes. 

Constant stress measurements are usually made using a jig incorporating a calibrated spring to apply the specified stress. 

Creep can be defined as a progressive inerease in strain, observed over an extended time period, in a polymer subjected to a constant stress. Measurements are carried out on a sample clamped in a thermostat A constant load is firmly fixed to... 

Despite the above complications, constant stress measurements may provide valuable information on the state of the suspension on storage. Carrying out creep experiments and ensuring that a steady state is reached can be time consuming. One usually carries out a stress sweep experiment, whereby the stress is gradually increased (within a predetermined time period to ensure that one is not too far from reaching the steady state) and plots of versus a are established. 

Extensional viscosity based on constant stress measurements [88] has also been reported [89,90]. In one case [89], the filament is extended vertically on top of a bath whereas in the other case [90], the vertical sample is immersed in the bath. The commercial equipment available for the measurement of extensional viscosity from rheometrics is based on the latter [90]. 

The situation is not so simple when these various parameters are time dependent. In the latter case, the moduli, designated by E(t)and G(t), are evaluated by examining the (time dependent) value of o needed to maintain a constant strain 7o- By constrast, the time-dependent compliances D(t) and J(t)are determined by measuring the time-dependent strain associated with a constant stress Oq. Thus whether the deformation mode is tension or shear, the modulus is a measure of the stress required to produce a unit strain. Likewise, the compliance is a measure of the strain associated with a unit stress. As required by these definitions, the units of compliance are the reciprocals of the units of the moduli m in the SI system. 

A method for measuring the uniaxial extensional viscosity of polymer soHds and melts uses a tensile tester in a Hquid oil bath to remove effects of gravity and provide temperature control cylindrical rods are used as specimens (218,219). The rod extmder may be part of the apparatus and may be combined with a device for clamping the extmded material (220). However, most of the mote recent versions use prepared rods, which are placed in the apparatus and heated to soften or melt the polymer (103,111,221—223). A constant stress or a constant strain rate is appHed, and the resultant extensional strain rate or stress, respectively, is measured. Similar techniques are used to study biaxial extension (101). 

As the current pulse is largely dominated by the stress differences, a short duration current pulse is observed upon loading with a quiescent period during the time at constant stress. With release of pressure upon arrival of the unloading wave from the stress-free surface behind the impactor, a current pulse of opposite polarity is observed. The amplitude of the release wave current pulse provides a sensitive measure of the elastic nonlinearity of the target material at the peak pressure in question. 

The torsion-tube test described by Whitney, Pagano, and Pipes [2-14] involves a thin circular tube subjected to a torque, T, at the ends as in Figure 2-29. The tube is made of multiple laminae with their fiber directions aligned either all parallel to the tube axis or all circumferentially. Reasonable assurance of a constant stress state through the tube thickness exists if the tube is only a few laminae thick. However, then serious end-grip difficulties can arise because of the flimsy nature of the tube. Usually, the thickness of the tube ends must be built up by bonding on additional layers to introduce the load so that failure occurs in the central uniformly stressed portion of the tube (recall the test specimen criteria). Torsion tubes are expensive to fabricate and require relatively sophisticated instrumentation. If the shearing strain y 2 is measured under shear stress t.,2, then... 

Creep rupture. Creep-rupture data are obtained in the same way as creep data except that higher stresses are used and the time is measured to failure (Figs. 2-28 and 29). The strains are sometimes recorded, but this is not necessary for creep rupture. The results are generally plotted as the log stress versus log time to failure (110). In creep-rupture tests it is the material s behavior just prior to the rupture that is of primary interest. In these tests a number of samples are subjected to different levels of constant stress, with the time to failure being determined for each stress level. General technical literature and product data sheets seldom provide a complete description of a material s behavior prior to rupture. It should include the development of any crazing and stress whitening, its strain-time... 

Its validity at normal temperatures was shown for more than 60 materials, ranging from pure metals to glassy polymers. Obviously, the polymers of the present study are good examples for Barker s rule. The product ot2E is linked to the difference of the two heat capacities c0 and cE, measured under constant stress and under constant strain, respectively [58], Also, a2E is linked to the difference of two Young s moduli Es, and ET measured adiabatically and isothermally [59]. 

Except at very low strains, up to about 15%, the ratio of stress to strain in vulcanised rubber is not a constant. Modulus is the tensile stress (measured in MPa, lb/in2 or kg/cm2) required to stretch the rubber to a given strain (or elongation) the elongation must always be stated, otherwise the expression is meaningless. A more precise expression is stress per square unit (in2 or cm2) at the given strain . 

As previousiy discussed, a meit indexer measures the flow property at a constant stress. The stress at the die waii can be caicuiated using a modification of Eq. 3.34. The pressure change for this caicuiation is simpiy the ioad weight divided by the area of the piunger. The modified equation is as foiiows ... 

Note 2 Viscoelastic properties are usually measured as responses to an instantaneously applied or removed constant stress or strain or a dynamic stress or strain. The latter is defined as a sinusoidal stress or strain of small amplitude, which may or may not decrease with time. 

Some applications require the material to remain under constant stress for years, yet it is often not reasonable to conduct such extended time measurements. One approach which circumvents this employs time-temperature superposition. Measurements are obtained over a shorter time span at differing temperatures. A master curve of C as a function of a reduced time tl a where a is a shift factor, is generated, and this allows the results to be extended to longer times. The shift factor is obtained by employing the Williams, Landel, and Ferry (WLF) relationship... 

Experimental Measuring the strain behavior (creep) at a given constant stress. At time t = 0 we subject our sample to an external stress a0, connected to an elastic potential A = o0rq/3 at every flow element. This elastic deformation is a disturbance of the thermodynamical equilibrium. At the same time this stress creates a purely elastic deformation y0 = o0/G0 of the whole body. 3... 

Set tests are made in either tension or compression and for their prime use, quality control, the choice of mode can be made according to the convenience of the test piece available. If intended to simulate service conditions, e.g. indentation of flooring, the most relevant mode of deformation would be used. Tests can be carried out in which the test piece is subjected to either constant stress or constant strain but, as the latter is by far the most widely used, the illustration of set measurement given in Figure 11.3 is based on constant strain in the compression mode. 

A servo hydraulic dynamic fatigue machine can, clearly, be used for testing under constant strain as well as constant stress conditions and this would be preferable to the traditional mechanical instruments. Interestingly, a comparison has been made of dynamic mechanical properties measured on a new version of the Goodrich flexometer and a servo hydraulic tester48. 

---