Duration of the load

Products that are subjected to a load have to be analyzed carefully with respect to the type and duration of the load, the temperature conditions under which the load will be active, and the stress created by the load. A load can be defined as continuous when it remains constant for a period of 2 to 6 hours, whereas an intermittent load could be considered of up to two hours duration and is followed by an equal time for stress recovery. The temperature factor requires greater attention than would be the case with metals. The useful range of temperatures for plastic applications is relatively low and is of a magnitude that in metals is viewed as negligible. 

The approach to the problem is to make the best analysis of the product requirements, including what at first appear to be intangible requirements, and then to determine what are the important elements in the design. Using these as the guide, several types of structural possibilities are examined with different materials to see if they meet the performance requirements of the application. The loads, the duration of the loads, the environment,... 

One of the most profound specifics of polymers is the strong dependence of their mechanical response on the external conditions, especially temperature, strain, strain rate, duration of the loading, etc. A single polymer can show all the features of a glassy brittle soHd or an elastic rubber and, eventually, viscous liq-... 

The dispersion effect is dependent on the shear stress as well as on the duration of the load. Which of the two variables has the most influence depends on the material to be dispersed [16]. On the basis of tests on a single screw extruder, Martin has demonstrated that the... 

These considerations may explain why, in buying power, rates may equitably vary according to the load factor proposed. A load which comes mainly at light-load periods is especially attractive to a company selling power, and it will ordinarily make low rates for such service. In all cases the effort is made to have the consumer pay his proportion of the constant costs of the generating plant, as well as those running costs which correspond with the incidence and duration of the load. 

Creep krep [ME crepen, fr. OE creopan akin to ON krjupa to creep] (before 12c) vt. Due to its viscoelastic nature, a plastic subjected to a load for a period of time tends to deform more than it would from the same load released immediately after application. The degree of this deformation increases with the duration of the load and with rising temperature. Creep is the permanent deformation resulting from the prolonged apphcation of a stress below the elastic limit. This deformation, after any time under stress is partly recoverable (primary creep) upon the release of the load and partly unrecoverable (secondary creep). Creep at room temperature is sometimes called cold flow. Elias HG (2003) An introduction to plastics. John Wiley and Sons, New York. Rosato DV (ed) (1992) Rosato s plastics encyclopedia and dictionary. Hanser-Gardner Publications, New York. 

During the start-up of a fuel cell system, the batteries provide all the power needs only for a few seconds. Afterward, the fuel cell stack begins supplying some power, and the output power from the batteries reduces correspondingly. With time, the stack supplies more and more power and the batteries supply less and less power, and finally, all the power is supplied by the stack. The duration of the load-sharing process by the stack and the battery depends on the discharging capacity of the batteries, the output voltage setup of the DC-DC converter, and how fast the stack can reach its nominal power output. For an air-cooled stack, the stack can reach the nominal power output in about 1 minute. For a liquid-cooled stack, the stack can reach the nominal power output in less than 5-10 minutes. But this does not necessarily mean that the load will be fully powered by an air-cooled stack within 1 minute, because the... 

If the slenderness ratio exceeds the above specified limits, the effects can be approximated by the use of moment magnification factor. If the slenderness ratio KLJr exceeds 100, however, a more detailed second-order nonlinear analysis will be required. Any detailed analysis should consider the influence of axial loads and variable moment of inertia on member stiffness and forces and the effects of the duration of the loads. 

Impact stress It occurs from loads that are transient with time. The duration of the load application is of the same order of magnitude as the natural period of vibration of the test specimen. 

In Fig. II-5 the static pressure requiring the same load capacity as is required by a triangular shaped dynamic forcing function applied to a one-degree-of-freedom ductile system (dynamic load factor) is shown as a function of its ductility and duration divided by period of response. Parameters typically necessary to define the response of a particular structure include the duration of the load and the natural period of the structural response, as well as the damping and maximum level of ductility exhibited by the structure during the... 

FIG 8.14. Feedforward control is capable of reducing both the area and the duration of the load-response transients. 

This is a sequence of identical duly cycles, each consisting of a period of operation at constant load and a period of operation at no-load. The repeat load and no-load periods are just adequate to attain thermal equilibrium during one duly cycle. There is no rest and de-energizing period, (Figure 3.6). Unless otherwise specified, the duration of the duty cycle will be 10 minutes. 

Starting current (/, ) Can be controlled to any desired level subject to the minimum required Cannot be controlled. Although duration of the starting inrush is very short because of no-load start Can be controlled to any desired level. Normally up to 1.5 times the rated current... 

Auxiliary brake chain drives have been omitted because of the Indefinite nature of the load. Each manufacturer has established successful drives, but these are suited to specific conditions which cannot be reduced to simple terms. Most such drives violate the rules of chain speeds and loads and are successful only because of the short duration of use and deviation from calculated loads. It is recommended that the drawworks manufacturer be consulted. 

These loads, the expected duration of emergency loads and the required switching regime and number of operations must be specified at the time of order placement. 

Allow the plant to run automatically for a period of some days either with no artificial load or with a small one of fixed value. If no means of recording are built into the plant then recorders should be introduced and placed in representative positions in the various zones for the duration of the tests. Periodically examine the traces or monitor printouts. These will give an indication as to whether the plant is controlling correctly on light load. 

Recovery is the strain response that occurs upon the removal of a stress or strain. The mechanics of the recovery process are illustrated in Fig. 2-34, using an idealized viscoelastic model. The extent of recovery is a function of the load s duration and time after load or strain release. In the example of recovery behavior shown in Fig. 2-34 for a polycarbonate at 23°C (73°F), samples were held under sustained stress for 1,000 hours, and then the stress was removed for the same amount of time. The creep and recovery strain measured for the duration of the test provided several significant points. 

The tensile data can be applied to the design of short-term (such as 1 or 2 hour duration) or intermittent loads in a product provided the use temperature, the humidity, and the speed of the load are within 10% of the test conditions outlined under the procedure. The intermittent specification merely indicates that there be sufficient time for strain recovery after the load has been removed. 

Figure 24. Drawing of the continuous leaching model used by Vigier et al. (2001) to estimate the residence time of particles in the soils of the Mackenzie Basin, and the related equations. This model assumes that particles are continuously leached in the soil before leaving to the river. Dissolved load of the river integrates the present leaching of the whole soil profile. 238, 234, 230 and 226 are the leaching coefficients of h and Tla nuclides, respectively, and x is the duration of the chentical...

Impulse or duration of the blast load (Impulse is the area under the pressure-time curve. Duration is the length of time of the overpressure phase of the blast wave.)... 

For blast waves from relatively small explosion sources, the diffraction phase of the loading may dominate, and the drag phase may be relatively or entirely unimportant, because the diffraction times may be as long as or greater than drag pressure durations. 

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