Environmental consideration creep

Hoop-stress equations for press-fit situations are used. Allowable design stress or strain will depend upon the particular plastic, the temperature, and other environmental considerations. Hoop stress can be obtained by multiplying the appropriate modulus. For high strains, the secant modulus will give the initial stress the apparent or creep modulus should be used for longer-term stresses. The maximum strain or stress must be below the value that will produce creep rupture in the material. There could be a weld line in the hub that can significantly affect the creep-rupture strength of most plastics. 

Materials selection process can be depicted in terms of Figure 1.40. Materials selection involves many factors that have to be optimized for a particular application. The foremost consideration is the cost of the material and its applicability in the environmental conditions so that integrity can be maintained during the lifetime of the equipment. When the material of construction is metallic in nature, the chemical composition and the mechanical properties of the metal are significant. Some of the important mechanical properties are hardness, creep, fatigue, stiffness, compression, shear, impact, tensile strength and wear. 

The theoretically estimated and experimentally determined CGRs agree well over a considerable temperature range after calibration at one temperature and specification of an appropriate activation energy for the crack-tip strain rate (Fig. 10). The numerical solution employed in the version of the CEFM described here yields very reasonable results for the environmentally assisted and creep fracture of sensitized... 

Fig. 2 shows the different pathways in which chemical elements contained in rocks are released to the different environmental compartments. Five main processes are responsible for their dispersion into the different ecosystems (1) Weathering, either directly by rain water on rock outcrops, by soil percolation water or by root exsu-dates, which interact with rock fragments, contained in the soil cover (2) Down hill mechanical transport of weathered rock particles, such as creep and erosion and subsequent sedimentation as till material or alluvial river and lake sediments (3) Transport in dissolved or low size colloidal form by surface and groundwater (4) Terrestrial and aquatic plants growing in undisturbed natural situations will take up whatever chemical elements they need and which are available in the surface and shallow groundwater. Trace elements taken up from the soil will accumulate in the leaves and will possibly enrich the soil by litterfall (5) Diffuse atmospheric input by aerosols and rain rock particles from volcanic eruptions, desertic areas (Chester et al., 1996), seaspray and their reaction with rain water. A considerable part of this can be anthropogenic. 

---