Loading, short-term

Nonsteady loads—Short-term duration variable. 

Long-term loading Short-term loading... 

For ultimate fatigue failure, testing on several FRP composites has shown the fatigue strength to be directly related to the characteristic strength of the material in the direction of the applied loading (short term UTS). 

Polymer Upper service temperature without mechanical load Short-term I Long-term ... 

Ta b I e 5.40 Service temperatures of stabilized polyolefins (without mechanical load) short-term loaded once or several times for less than one hour, long-term continuous load for several years... 

Table 5.42 Service temperatures for PS polymers (without mechanical load) short-term loaded ...

Table 5.44 Service temperatures for polyvinyl chloride (no mechanical load) short-term loaded once or repeatedly for less than an hour long-term permanent load over years VA vinyl aoetate, DOP dioctyl phthalate [86]...

Creep. The creep characteristic of plastic foams must be considered when they are used in stmctural appHcations. Creep is the change in dimensions of a material when it is maintained under a constant stress. Data on the deformation of polystyrene foam under various static loads have been compiled (158). There are two types of creep in this material short-term and long-term. Short-term creep exists in foams at all stress levels however, a threshold stress level exists below which there is no detectable long-term creep. The minimum load required to cause long-term creep in molded polystyrene foam varies with density ranging from 50 kPa (7.3 psi) for foam density 16 kg/m (1 lb /ft ) to 455 kPa (66 psi) at foam density 160 kg/m (10... 

Long-term failure. A sample under continuous load for a year may break at a stress about half of that required in a short-term tensile test (Figure 19.6). 

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. 

Turndown on a PAN type tray should be limited to 2 1 (ratio of high to low flow rates), which results in a reasonable design. Bonilla [131] points out that it is cosdy to design for short-term high turndown rates such as start-up, shutdown, or for other short term periods, because it is better to increase reflux ratio to increase internal loads for such periods rather than design the distributor for large turndown. 

Loading levels above about 8 N/mm for short term and 4N/mm for an indefinite period are, therefore, not recommended for bitumen emulsion-modified cementitious floors. 

First the sample, that was loaded to about 20% of its short-term yield strength or 13.8 MPa (2,000 psi), recovered almost completely one hour after the release of the load, the net strain being 0.03%. Second, the sample loaded to 66%of its short-term yield strength, or 41.4 MPa (6,000 psi), retained a strain of 0.8% at 1,000 hours after the release of the load. The initial strain was 2.8%, the strain from the 1,000 hour creep an additional 1.7%. Thus only about one-half the creep strain was recovered. Visually extrapolating the recovery curve reveals that even after a year (104 hr.), about one-third of the creep strain (0.6%) will remain. 

Viscoelastic and rate theory To aid the designer the viscoelastic and rate theories can be used to predict long-term mechanical behavior from short-term creep and relaxation data. Plastic properties are generally affected by relatively small temperature changes or changes in the rate of loading application. 

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... 

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