Duration of load

Establish final design criteria Allowable stresses, strains, deflections Margins of safety against local and overall instability, vibrations, etc. Take into account type and duration of load, service environments, process effects, equality expectations... 

Because products tend to deform in time at a decreasing rate, the acceptable strain based on the desired service life of the product is determined. The shorter the duration of load, the higher the apparent modulus and thus the higher the allowable stress. The apparent modulus is most easily explained with an example. As long as the stress level is below the elastic limit of the material, its modulus of elasticity E can be obtained from the usual equation ... 

Thermoplastics have a viscoelastic behaviour that increases with temperature their properties are a function of the duration of load application. 

Load Factors. Duration of Load. The ability of wood to re-... 

Wood LW (1951) Relation of strength of wood to duration of load. USDA, Forest Service, Forest Products Laboratory, Report No. 1916... 

Where Young s modulus is needed in the above stress anafysis, the tensile creep modulus is inserted, using the value for the appropriate temperature and duration of loading, and for the maximum tensile strain in the component (as determined by the linear elastic anafysis). 

This is an example of strain-limited design. We apply the pseudo-elastic design method, specifying the duration of loading as S hours (which is 18000 seconds). In order to determine the modulus, we need the isochronous stress-strain curve for 18000 s. Substituting in the equation,... 

According to CEN EN 1426 (2007), the testing temperature for bituminous binders with penetration above 330 dmm is reduced to 15°C, while the load and duration of loading remain the same. 

Unlike many traditional construction materials such as iron, cement and so on, whose mechanical behaviour is almost independent of temperature and duration of loading, the behaviour of bitumen is not and it changes from elastic to viscous. When the load (stress) is applied for a very short time and at the same time the temperature is very low, the bitumen behaves as a purely elastic material. Conversely, when stress is applied for a very long time and the temperature is high, the behaviour of the bitumen is viscous. 

The total duration of loading in case the uniaxial static (creep) test is employed is 1 h. 

The increase of the number of layers to more than three represents the real situation of the pavement but requires more complicated calculations for the determination of stresses and strains developed at critical interfaces. When also considering variations of the properties of the materials that constitute the layer owing to the duration of loading or temperature, variation of loading area and applied load and non-truly elastic behaviour of the materials, the use of nomographs or tables for the determination of stresses and strains is almost impossible. Today, such calculations are carried out by software programs. 

For cold-curing epoxides wide variations in adhesive material properties are possible, with different combinations of resin, hardener, filler, and the multitude of modifiers. Products which cure at ambient temperature cannot achieve the same performance as is obtained by curing at elevated temperature. For products cured at room temperature their TgS, at 40-50 °C initially, are relatively low and may be lowered even further by absorbed water, in liquid or vapour form. This may also be accompanied by a reduction in strength and modulus. Thus the use of materials with a slow and small water uptake is to be preferred, which implies a fairly highly cross-linked formulation. Such considerations do of course depend upon the performance and durability expectations in service. Whilst the environmental durability of joints can often be improved enormously by the surface pretreatment methods employed (see Chapters 3 and 4), the adhesive must be selected carefully to ensure long term durability in consideration of the modes and duration of loading, and the environmental conditions. Ideally the adhesive should be fairly tolerant of poor surface pretreatment procedures. 

From isochronous stress-strain curves relating to endurance times of, say, 1, 10, 100, and 1000 hours, and so on, the magnitude of the stress to give the critical strain at each duration of loading can be easily deduced. This procedure can be repeated for different selected levels of strain. In general, the more critical the application and the longer the time... 

Riisch (1960) has already shown the influence of the duration of loading on the strain-stress curve and strain development over time. In Figure 11.19 a few curves represent the results of axial compression imposed by constant strain rate on concrete specimens. For slow loading the influence of creep and propagation of microcracks is important, while for higher loading rates strength is apparently increased (cf. Section 11.2.1). 

Figure 11.21 Influence of ratio ajf and duration of load on concrete strain under compression, after Riisch (1960).

The viscosity behavior of plastics makes them sensitive to strain rates as well as temperatures (see Chapters 2, 3, and 4). It therefore becomes important to define the rate, magnitude, duration, and type of mechanical stress and strain loading (i.e., tension, compression, flexure, and shear) along with temperatures during loading. The rate and duration of loading also determine whether creep or impact will be a factor in a given part s mechanical response [1, 2, 5-14, 29, 33,40-43, 55-68, 152, 202, 225, 235, 250, 270-74, 808]. 

The intensity of energy input and the duration of loading determine the load on a plastic material. Average shear load in the screw channel amounts to only 5 to 10% of peak load in the shear gap. Fig. 4.10 [35]. 

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