Stress time diagram

Fig. 10.26. Stress-time diagram and Smith s fatigue strength diagram. As shown, the points in the fatigue strength diagram can be taken directly from the different stress-time diagrams. In metals, the compressive yield strength Rc is usually the same as Rp...

Other ways to present creep data include compilation to stress-time diagrams with continuous lines indicating time-dependent strain limits. Therefore, this way of plotting creep data is frequently referred to as an iso-strain diagram. The schematic approach for converting creep data into an iso-strain digram is illustrated in O Fig. 34.8. 

It is important to differentiate between brittie and plastic deformations within materials. With brittie materials, the behavior is predominantiy elastic until the yield point is reached, at which breakage occurs. When fracture occurs as a result of a time-dependent strain, the material behaves in an inelastic manner. Most materials tend to be inelastic. Figure 1 shows a typical stress—strain diagram. The section A—B is the elastic region where the material obeys Hooke s law, and the slope of the line is Young s modulus. C is the yield point, where plastic deformation begins. The difference in strain between the yield point C and the ultimate yield point D gives a measure of the brittieness of the material, ie, the less difference in strain, the more brittie the material. 

As an example, for room-temperature applications most metals can be considered to be truly elastic. When stresses beyond the yield point are permitted in the design, permanent deformation is considered to be a function only of applied load and can be determined directly from the stress-strain diagram. The behavior of most plastics is much more dependent on the time of application of the load, the past history of loading, the current and past temperature cycles, and the environmental conditions. Ignorance of these conditions has resulted in the appearance on the market of plastic products that were improperly designed. Fortunately, product performance has been greatly improved as the amount of technical information on the mechanical properties of plastics has increased in the past half century. More importantly, designers have become more familiar with the behavior of plastics rather than... 

For practical applications empirically determined creep data are being used, such as D(t) or, more often, E(t) curves at various levels of stress and temperature. The most often used way of representing creep data is, however, the bundle of creep isochrones, derived from actual creep curves by intersecting them with lines of constant (log) time (see Figure 7.7). These cr-e-curves should be carefully distinguished from the stress-strain diagram discussed before, as generated in a simple tensile test ... 

Fig. 8.15. Stress-(drying) time diagram of boehmite gel layers dried at 60% RH at 25°C (dotted line) and 40°C (solid line) with a wind velocity of 3.25 ms in the drying chamber. From Voncken et al.

Fig. 8.20. Deflection (stress) versus time diagram during the cyclic heat treatment (calcination) of boehmite membranes converted to y-alumina at 600°C. Heating and cooling rates were 60°C/h. From Kumar [13]. Curve a (dotted) blank run, support only curve b actual run with supported membrane curve c deflection of b corrected for support effects.

Stress-Strain-Time Diagrams, Including Failure Envelopes, for High-Density Polyethylenes of Different Molecular Weight... 

SCC materials data have often been presented in the form of diagrams showing time to fracture as a function of nominal stress. The diagrams show that a minimum (threshold stress) is necessary to eause SCC. However, a better measure of eritieal stress is the critical stress intensity factor (see Section 7.12.3). 

Isochronic stress/strain diagrams are more informative than creep strength diagrams. They are obtained by holding samples under a constant load for various lengths of time. Finally, a stress/strain diagram is obtained for each load time. 

Engineering materials are generally referred to as metallic and nonmetallic (ceramics and high pol5nners) materials, which are further classified as ductile or brittle. As shown in the stress-strain diagram in Figure 1.1, the strain of ductile materials is 100-1000 times larger than that of brittle materials. The... 

Figure 5.12 Thermomechanical behavior of SMPFs by both cold and hot tension programmings, (a) Stress-strain-time diagram for Sample 2. Steps 1 to 5 complete programming and Step 6 completes stress recovery, where step 1 is to stretch the fiber bundle to 100% strain at a rate of200 ram/min at 100 °C step 2 is to hold the strain constant for 1 hour step 3 is to cool the fiber to room temperature slowly while holding the pre-strain constant step 4 is to release the fiber bundle from tbe fixture (unloading) step 5 is to relax the fiber in the stress-free condition until the shape is fixed and step 6 is to recover the fiber at 150 °C in the fully constrained condition (adapted from Reference [20]) (b) Stress-strain-time diagram for Sample 3. Steps 1-4 complete programming and step 5 completes stress recovery, where step 1 is to stretch the fiber bundle to 100% strain at a rate of 200 mm/min at room temperature step 2 is to hold the strain constant for 1 hour step 3 is to release the fiber bundle from fixtures (unloading) step 4 is to relax the fiber in the stress-free condition until the shape is fixed and step 5 is to recover the fiber at 150 °C in the fully constrained condition (adapted from Reference [20]) (c) Stress evolution with time for Sample 2 (d) Stress evolution with time for Sample 3.

Strain, stress to failure and modulus values acquired for the 30 specimens with holes are listed in Table 1 along with time-dependent strain and exposure time that each specimen witnessed during exposure testing. The time under exposure varied from 10 hours to 2400 hours. Sample of stress strain diagrams is shown in Figure 1 and 2. 

Fig. 5. Changes with time of the stress-strain diagram of sulphur prepared at different temperatures.

Fig. 14.12 shows the stress—strain—normalized resistance plot for the specimen with 12-mm notch spacing. We know that the nominal strain at fracture for the composite material is around 0.0147. Due to the notches, the strain concentration would be three times that of the smooth specimen. Thus any damage around the notch should start at one-third of the applied strain on the smooth specimen. This is indeed the case as seen in Fig. 14.13. A sharp change in the slope of the stress—strain curve is seen at a nominal strain of 0.005 = l/3eu- Before this knee the resistivity variation is nonlinear with respect to the applied strain. With the onset of damage at e = 0.005 (at the edges of the notches) a sharp increase in resistivity is seen. After this point the resistivity response is linear with the applied strain. Another jump in resistivity can be seen, probably due to damage initiation at the other notch, however, the stress—strain diagram does not detect this. After the second jump the sensor responds very...

Fig. 30. Elongation and stress-relaxation of a semicrystalline PTMT film stress-strain/time diagram...

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