Short-term loads

The obvious question is Ts there an optimum design for the corrugations Unfortunately the answer is No because if one wishes to increase transverse stiffness then the obvious thing to do is to increase D up to the point where buckling problems start to be a concern. Usually this is when D/h = 10, for short-term loading and less than this for long term loading because of the decrease in modulus of viscoelastic materials. 

This modulus value is often arbitrarily chosen, although several methods have been suggested for arriving at a suitable value. One is to plot a secant modulus based on 1% strain or that is 0.85% of the initial tangent modulus (Chapter 2, SHORT-TERM LOAD BEHAVIOR). However, for many plastics, particularly the crystalline TPs, this method is too restrictive, so in most practical situations the limiting strain is decided in consultation... 

The tensile modulus is an important property that provides the designer with information for a comparative evaluation of plastic material and also provides a basis for predicting the short-term behavior of a loaded product. Care must be used in applying the tensile modulus data to short-term loads to be sure that the conditions of the test are comparable to those in use. The longer-term modulus is treated under the creep test (Chapter 2). 

More or less implicit in the theory of materials of this type is the assumption that all the fibers are straight and unstressed or that the initial stresses in the individual fibers are essentially equal. In practice this is quite unlikely to be true. It is expected, therefore, that as the load is increased some fibers will reach their breaking points first. As they fail, their loads will be transferred to other as yet unbroken fibers, so that the successive breaking of fibers rather than the simultaneous breaking of all of them will cause failure. As reviewed in Chapter 2 (SHORT TERM LOAD BEHAVIOR, Tensile Stress-Strain, Modulus of elasticity) the result is usually the development of two or three moduli. 

We should realize that the values of the moduli of elasticity, as discussed so far, are only applicable to short-term loading situations. The creep, already mentioned several times before, renders these values unsuitable to characterize the behaviour of a polymer under stress over a longer time. In 4.5 we already met the example of two polymers, where POM, at a certain stress, initially deforms less than PC, but, later on, its deformation exceeds that of PC. 

Researchers [6] showed that the microstructure of RubCon has elastic, elastic-plastic, and viscous phases. The amount of elastic in the composite is less in comparison with others, and consequently deformability of RubCon at action a long-term and a short-term loadings in the greater degree is determined by elastic and elastic-plastic deformations. The increase of the strength of RubCon and the modulus of elasticity at compression and the decrease of its ultimate deformations at negative temperatures can be explained by increases in viscous phase viscosity and partial transformation of an elastic-plastic phase of a composite in elastic. The increase in the elastic phase results in embrittlement of the composite but no changes in its stress-strain state. 

MacLean JJ, Lee CR, Alini M, latridis JC. The effects of short-term load duration on anabolic and catabolic gene expression in the rat tail intervertebral disc. J Orthop Res 2005 23 1120-1127. 

This section reviews the static property aspects that relate to short-term loads (Figure 7.15 and Table 7.8). As reviewed with RTFs the TPs being viscoelastic respond to induced stress by two mechanisms viscous flow and elastic deformation occurs. Viscous flow ultimately dissipates the applied mechanical energy as frictional heat and results in permanent material deformation. Elastic deformation stores the applied mechanical energy as completely recoverable material deformation. The extent to which one or the other of these mechanisms dominates the overall response of the material is determined by the temperature and by the duration and magnitude of the stress or strain. The higher the temperature, the most freedom of movement of the individual plastic molecules that comprise the TP and the more easily viscous flow can occur with lower mechanical performances. Reinforcements in TPs significantly reduce this situation compared to UTPs. 

Nature of loading Static short-term loads 2... 

ETFE foil is a flexible material which can take high short term loading. This makes it a good product for use where there is a risk of explosions While ETFE can be easily damaged if there is intent, the system can be fitted with a pressure monioring system such that, in the event of a dramatic loss of pressure an alarm will sound. However, it is recommended to place ETFE systems out of public reach. Glazed systems can also be easily damaged, if there is intent... 

Specialized applications of batteries cover military and space technology and some scientific instruments. Examples of such battery-powered devices are satellites, space probes, rockets, bathyscaphes and other submersible craft, electrically driven torpe-dos, meteorological balloons, and so forth. Specialized batteries may be divided into two subdivisions power sources for short-term loads (typically for a single discharge) and those for long-term low-drain discharge. 

Under quasi-static loading, or in conditions under which short-term loading responses are expected to occur [27], both meniscal and discal tissues may be modeled as linear elastic and orthotropic. Under a constant load rate, the non-linear behavior may be described by an exponential stress-strain relationship given by... 

Operating design temperature (°C) HPT (°C) Short-term loading Long-term loading... 

Long-term loading Short-term loading... 

It is a fundamental assumption of material behaviour that for short term loading the material is linear elastic to ultimate failure. Under long term loading the material is linear visco elastic with recovery on removal of load, providing the strain does not cause permanent material deterioration. 

Often times, vessels are subjected to short term loadings that overstress some portion of the vessel for a very short duration. This short duration, overstressed condition, may in fact be acceptable providing the number of cycles at that overstressed condition is acceptable. A fatigue evaluation can validate this situation. The Code allowable stresses are based on continuous service up to 100,000 hours. If the vessel or component is going to be exposed to the overstressed condition for less than 100,000 hours, then the acceptability hinges on the number of allowable cycles rather than allowable stress. 

Unlike wind, seismic events are short term loading conditions. As a result, the ASME Code Section VIII, Division I allows for an increase in the allowable stress of 1.2. Section VIII, Division 2, building codes, and design standards (such as ASCE/SEI 7) use load combinations and typically do not allow for an increase in allowable stress, however the seismic load is usually reduced when combined with other types of loads and so the effect is similar. The vessel may only experience an earthquake several times during the life of the equipment,... 

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