Stress-controlled load

Thermal fatigue is observed mainly in stress-controlled loading because the strain amplitude increases in this case due to the reduction of stiffness with increasing temperature. The heat generated per cycle thus increases with time. If the loading is strain-controlled, thermal fatigue is usually not problematic because the stress decreases in this case. 

Fig. 13.2. stored elastic strain energy in the rubber bands in strain- and stress-controlled loading... 

Conclusion In strain-controlled loading (booth A), increasing the stifTness increases the elastically stored energy, whereas decreasing the stiffness increases the elastically stored energy in stress-controlled loading (booth B), see figure 13.2. 

Tests using a constant stress (constant load) normally by direct tension have been described in ISO 6252 (262). This test takes the specimen to failure, or a minimum time without failure, and frequently has a flaw (drilled hole or notch) to act as a stress concentrator to target the area of failure. This type of testing, as well as the constant strain techniques, requires careful control of specimen preparation and test conditions to achieve consistent results (263,264). 

Upon exceeding a critical stress, ductile surfaces deform plastically. Often in nanoindentation experiments under controlled load, the indenter moves in a... 

The mechanical behaviour of oxide scales has been investigated by a complex of appropriate techniques (cf. for example [12]), where the attention has been focused on the analysis of the stress development in the scale, as well as the measurement of the scale adherence. In the case of weak scale adherence, spontaneous scale failure is often observed during oxidation or cooling of specimens. For systematic investigations of the fracture-mechanical properties of oxide scales, scale failure is induced by a controlled loading of the scale which is produced by an appropriate deformation of the whole specimen. 

We will now consider the special problems in tall tower design which are not described in the ASME Code for Unfired Pressure Vessels. As discussed previously, circumferential stresses control the design of cylindrical vessels if external loads are of small magnitude. In tall vertical vessels, four major factors (wind load, seismic loads, dead weight and vibration) may contribute to axial stresses — in addition to axial stress produced by the operating pressure or vacuum of the vessel. 

For the lower portion of tall towers, where the combined axial stress controls the design of the shell, there is the problem of selecting the maximum allowable axial compressive stress. The combined axial tensile stress presents no problem. The tensile stresses produced by internal pressure, bending stress of wind loads or bending stress firom seismic loads may be combined by simple addition of the stresses. The thickness of the shell may be calculated so that the combination of axial tensile stresses is equal or less than the maximum permissible value specified by the ASME Code. 

Figure 3.14 Setup used for the stress-controlled progiaimiiing consisting of an LVDT, fixture, and weights. A static load was used to compress the specimen and the deformation measured using the LVDT. Source [41] Reproduced with permission om Elsevier...

Figure 3.15 Four-step thermomechanical cycles (step 1 high temperature loading—> step 2 cooling step 3 room temperature unloading step 4 free shape recovery) for the pure SMP and syntactic foam programmed under a stress-controlled condition with a pre-stress of 263 kPa at 79 °C followed by free recovery. Source [41] Reproduced with permission from Elsevier...

Fully constrained recovery was performed using the MTS Q-TEST 150 machine and the associated furnace. Once the specimens were programmed under stress-controlled conditions, as described above, they were placed in the fixture shown in Figure 3.9 such that the strain was fixed and the stress was initially zero. Heating was performed at an average rate of 0.3 °C/min from room temperature until 79 °C and then held for approximately 20 minutes (some specimens were held for over 24 hours in order to investigate the stress relaxation behavior). The load cell of the MTS machine was used to record the recovered force as a function of time and temperature. 

Loading can be either load (or stress) controlled, displacement (or strain) controlled, or something in between. Examples include aerodynamic loads on an aircraft (see Aerospace applications), which tend to be load controlled, and the displacement of a sealant between relatively stiff adherends, which is displacement controlled. Because average adhesive strain, in its simplest form, is defined as displacement divided by bond thickness, strains and resulting stresses are higher in thin bondlines subjected to displacement-controlled loading scenarios. Joints loaded in such a manner often perform better with thicker bondlines. Displacement-controlled situations include thermal expansion/shrinkage of adherends, mismatched adherend expansion, and attachments bonded to pressure vessels or other adherends that are stressed. 

Fi. 37. Fatigue curves for rolled Cu foils (99.99% purity) that were measured in the stress-controlled tension-tension loading mode parallel and perpendicular to the rolling direction. Thickness 25 pm. 

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