Stress cycle fluctuating

Fatigue is another property of considerable interest to the design engineer. CycHc deflections of a predeterrnined ampHtude, short of giving immediate failure, are appHed to the specimen, and the number of cycles to failure is recorded. In addition to mechanically induced periodic stresses, fatigue failure can be studied when developing cycHc stresses by fluctuating the temperature. 

Technological advances to provide cheap, reliable power focus on power electronics, materials, structure design and manufacturing, and transmission with a continuously fluctuating input power. Sandia National Laboratories engineer Paul Veers summarizes the challenges this way, The airfoils must work in a dynamic stall environment. The structure must withstand billions of stress cycles with low cost materials.. .. A control system should smooth all power fluctuations.. .. It must be accomplished in an environmentally non-intrusive way (Veers, 1996). 

Maximum, mean and minimum values of the fluctuating normal and shear stresses in a stress cycle (tension positive)... 

Failure Mode Under Dynamic Load. Whenever a pipe is subjected to a pressure fluctuating between an upper limit Pg nd a lower limit P, the pipe will fail after a number of stress cycles N which will essentially depend on the geometrical layout of the defects in the pipe and the magnitude of the fluctuation AP = Pg - P of the pressure. 

Fig. 5.30 a Fully reversed cycle with zero mean stress ( 0 and minimum stress 0) c cycle fluctuating from zero or from the origin ( 0,... 

It must be underlined that the reduction of the mean stress just described with the hypothesis of cycles fluctuating from zero (R — 0) remains valid also in the more general case of g > 0. As to these cases, it is worth noting that a very low stress amplitude superposed to a very high mean stress may not get any benefit (no mean stress reduction) if condition 2. in Eq. (5.57) ffa + Cm > Oy is not verified. If... 

Also indicated in Figure 8.17b are several parameters used to characterize the fluctuating stress cycle. The stress amplitude alternates about a mean stress o- defined as the average of the maximum and minimum stresses in the cycle, or... 

A very wide sheet of grp which is known to contain intrinsic defects 1 mm long, is subjected to a fluctuating stress which varies from 0 to 80 MN/m. How many cycles would the sheet be expected to withstand if it is made from (a) chopped strand mat (CSM) and (b) woven roving (WR) reinforcement. The crack growth parameters C and n, and the critical stress intensity factors, Kc, for these materials are... 

It should be stressed that the reversible chemical reactions give us better chance to observe many-particle effects since there is no need here to monitor vanishing particle concentrations over many orders of magnitude. Indeed, the fluctuation-controlled law of the approach to the reaction equilibrium similar to (2.1.61) was observed recently experimentally [85] for the pseudo-first-order reaction A + B AB of laser-excited ROH dye molecules which dissociate in the excited state to create a geminate proton-excited anion pair. The solvated proton is attracted to the anion and recombines with it reversibly. After several dissociation-association cycles it finally diffuses to long distances and further recombination becomes unobservable. 

During cell/stack operation, water content in the membrane is affected by the local intensive variables, such as local temperature, water vapor concentration in the gas phase, gas temperature and velocity in the channel, and the properties of the electrode and gas diffusion media. The power fluctuation can result in temperature variation inside the cell/stack, which will subsequently change the local membrane water content. As the water content in the membrane tends to be non-uniform and unsteady, this results in operation stresses. When the membrane uptakes water from a dry state, it tends to expand as there is no space for it to extend in plane and it can wrinkle up as schematically shown in Fig. 4 when the membrane dries out, the wrinkled part may not flatten out, and this ratcheting effect can cause the pile up of wrinkles at regions where membrane can find space to fold. The operation stress is typically cyclic in nature due to startup-shutdown cycles, freeze-thaw cycles, and power output cycles. 

In the wall thickness fluctuations up to 5 % may occur. As a result of the uneven temperature in the molten polymer during rotation, and also by the not always exactly reproducible rate of cooling, deviations in the dimensions of the finished product may amount to 5 %. Requirements are, that the materials can be molten completely, that the melt is sufficiently low-viscous, and that the molten polymer does not degrade too rapidly. Besides plasticised PVC, HDPE and LDPE are often used, as well as copolymers of PE such as EVA (ethylene - vinyl acetate copolymerj.Because the shear stresses in this process are extremely low, a narrow molar mass distribution is to be recommended, as discussed in 5.4. Cycle times vary between 3 and 40 minutes, dependent on the wall thickness. Cycle times can be reduced considerably by using machines with multiple moulds, since the cycle time... 

Continuum Depiction Local oscillation of the melt-wall boundary condition in the exit region causes peturbations on the exit stress and die swell Oscillation of the overall stress due to unstable boundary condition produces cycles of melt compression and decompression in the barrel and fluctuations in the extrude swell The extrudate distortion arises from formation of secondary flow (vortices) in the barrel due to the strong converging flow near the die entry... 

---