Incremental stress test

When the uncrosslinked self-assembled collagen fibers tested above were crosslinked by removal of water (dehydration) the resulting incremental stress-strain curves approached those observed for tendons (see Figure... 

Figure 7.7. Total, elastic, and viscous stress-strain curves for uncrosslinked self-assembled type I collagen fibers.Total (open squares), elastic (filled diamonds), and viscous (filled squares) stress-strain curves for self-assembled uncrosslinked collagen fibers obtained from incremental stress-strain measurements at a strain rate of 10%/min. The fibers were tested immediately after manufacture and were not aged at room temperature. Error bars represent one standard deviation of the mean value for total and viscous stress components. Standard deviations for the elastic stress components are similar to those shown for the total stress but are omitted to present a clearer plot. The straight line for the elastic stress-strain curve closely overlaps the line for the viscous stress-strain curve. Note that the viscous stress-strain curve is above the elastic curve suggesting that viscous sliding is the predominant energy absorbing mechanism for uncrosslinked collagen fibers.

Figure 8.5. Plots of elastic modulus versus mineral content (b) and days of mineralization (a) from incremental stress-strain tests performed on mineralized self-assembled type I collagen fibers. Slopes were obtained from the straight portions of the elastic and viscous stress-strain curves.

Stress testing is likely to be carried out on a single batch of material and to include the effect of temperatures in 10 °C increments above the accelerated temperature test condition (e.g., 50 °C, 60 °C, etc.) humidity where appropriate (e.g. 75% RH or greater) oxidation and photolysis on the drug substance plus its susceptibility to hydrolysis across a wide range of pH values when in solution or suspension. 

In a broader unselected population referred for nuclear stress testing, Sharir et al. (6) found that the extent of reversible perfusion defect (as expressed by the summed difference score) was the best predictor of subsequent nonfatal myocardial infarction, and was best fit by an exponential curve. Among these patients, 26% had a history of myocardial infarction, and patients with nonischemic cardiomyopathies, valvular disease, or who underwent revascularization within 60 days were excluded. Importantly, even though ejection fraction most powerfully stratified the risk of cardiac death, in patients with an ejection fraction >30% the amount of perfusion defect provided incremental prognostic information. In patients with an ejection fraction of <30%, the rates of cardiac death were high (>4% per year) regardless of the amount of ischemia. 

Consolidation tests are commonly performed to (1) evaluate the compressibOity of soil samples for the calculation of foundation settlement (2) investigate the stress history of the soils at the boring locations to calculate settlement as weU as to select stress paths to perform most advanced strength tests (3) evaluate elastic properties from measured bulk modulus values and (4) evaluate the time rate of settlement. Consohdation test procedures also can be modified to evaluate if foundation soils are susceptible to coUapse or expansion, and to measure expansion pressures under various levels of confinement. Consohdation tests include incremental consohdation tests (which are performed at a number of discrete loads) and constant rate of strain (CRS) tests where load levels are constantly increased or decreased. CRS tests can generally be performed relatively quickly and provide a continuous stress-strain curve, but require more sophisticated equipment. 

To reduce the experimental efforts in measuring cyclic stress-strain curves, the incremental-step test can be used. In this test, the strain amphtude is varied block-wise between zero and a maximal value as sketched in figure 10.30. After the block has been repeated several times, the material behaviour does not change an5rmore and a stationary state is arrived at. If the stress is measured at each of the strain maxima, the... 

Fig. 1.26 Cyclic stress-strain curve obtained on Cr-Mo-V steel of 660 MPa yielding strength and 820 MPa ultimate with the incremental step test [32]...

The vessels will be pressurised up to 1,1 - 1,25 fold of the normal service pressure of the drums, within stress increments of 50 kPa (0,5 bar) and a constant and smooth slope of less than 10 kPa.min . During the complete test the data will be acquired, stored and analysed. 

These latter curves are particularly important when they are obtained experimentally because they are less time consuming and require less specimen preparation than creep curves. Isochronous graphs at several time intervals can also be used to build up creep curves and indicate areas where the main experimental creep programme could be most profitably concentrated. They are also popular as evaluations of deformational behaviour because the data presentation is similar to the conventional tensile test data referred to in Section 2.3. It is interesting to note that the isochronous test method only differs from that of a conventional incremental loading tensile test in that (a) the presence of creep is recognised, and (b) the memory which the material has for its stress history is accounted for by the recovery periods. 

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. 

Effect of Beam Current. The relationship between beam current and the incremental creep rate owing to the radiation is shown in Figure 5. The incremental creep rate is the increase in creep rate which can be attributed to the radiation exposure. The data on Figure 5 are plotted with applied stress as a parameter. No distinction is made between the 3-m.e.v. and the 8-m.e.v. electron tests, although the points are coded so that the legend on the graph indicates the beam energy. 

There has been some controversy over the definition of creep which should be used. Traditionally, creep was defined in the rubber industry as the increase in deformation after a specified time interval expressed as a percentage of the test piece deformation at the start of that time interval. In other industries creep is normally defined as the increase in deformation expressed as a percentage of the original unstressed dimension of the test piece. Consequently, care has to be taken when comparing creep values obtained from different sources. ISO 8013 has both definitions, calling them creep index and creep increment respectively. The definition of creep increment in the standard refers to the original dimension as thickness, which would not apply to tension. ISO 8013 also defines a compliance index which is the ratio of the increase in strain to the constant applied stress. 

The fiber modulus and matrix shear modulus are also required for the analysis. The fiber s coordinates are recorded directly from the stage controllers to the computer. The operator begins the test from the keyboard. The x and y stages move the fiber end to a position directly under the debonder tip the z stage then moves the sample surface to within 4 yum of the tip. The z-stage approach is slowed down to 0.04 jan/step at a rate of 6 steps/s. The balance readout is monitored, at a load of 2 g the loading is stopped, and the fiber end returned to the field of view of the camera. The location of the indent is noted and corrections are made, if necessary, to center the point of contact. Loading is then continued from 4 g in approximately 1 g increments. Debond is determined to have occurred when an interfacial crack is visible for 90-120° on the fiber perimeter. The load at which this occurs is used to calculate the interfacial shear stress at debond. 

Interpretation A peak serum cortisol concentration of >20pg/dL is a normal response. The peak value is more important than the incremental change. The incremental change may not be seen in patients who are tested at times of stress, when then-adrenal output of cortisol is already maximally stimulated by endogenous ACTH. 

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