Creep environmental factors

A typical environmental factor which effectively limits the use of possible adhesives for a certain application is the service temperature. In outdoor applications the existence of high humidity may also restrict the number of possible adhesives. Environmental factors such as temperature, humidity and chemicals may have a severe degrading effect on the adhesive properties and, in particular, the creep properties. 

The data mentioned previously in Table 13.4 give short-term mechanical properties under very simple loading conditions. These values are typical of commercially available PSF, PES, and PPSF of practical molecular weight. When one is considering a polymer for a particular end-use apphcation, it is very important to also consider other service life factors such as creep and fatigue properties as well as temperature and other environmental factors. 

To account for the material evolutions, guidelines dedicated to the external or internal reinforcements of concrete structures using FRP composites usually introduce substantial durability parameters (reduction factors on FRP tensile properties and on shear characteristics of FRP/concrete bonded interfaces, creep stress levels and fatigue limits) see, for instance, ACI 440.2R-08 (2008) and ACI 440.1R-06 (2006). However, further research is still needed to refine the design codes and better calibrate the durability reduction factors taking into account synergistic effects between various environmental factors. 

Fulmer (1967) demonstrated that for a specific polymer every environment (liquid) shows a constant shift factor versus another environment. Next to his own experiments on (filled) polyethylene he used data obtained by Bergen (1964) in creep investigations. Table 26.9 lists some of these environmental shift factors for different polymers in two liquids. Comparative data for other polymer-liquid combinations are scarce. 

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. 

Additional factors which must be taken into account are environmental effects (thermal as well as chemical), effects of defects, statistical variability of the material, long-term behavior, and cyclic versus static loading effects. Assessment of these effects requires the end user to conduct a large series of tests using multiple specimens. A typical series will examine a unidirectional material in tension in the 0, 90, and cross-ply directions 0, 90, and cross-ply in compression and 1-2, 1-3, and 2-3 shear at different temperatures ranging from —54°C to the expected service temperature creep rupture at temperatures up to the expected service temperature and fatigue at room and elevated temperature. This series of tests, shown in Table 12.1, may require over 400 specimens. 

Calculated CGR versus ECP correlations for ECLs ranging from 0.1 to 50 cm and for a stress intensity factor of 27 MPa.m1/2 and for the other, constant conditions, as stated in the figure, are shown in Fig. 13. In the low potential limit, no ECP dependence of the CGR is noted, because crack extension occurs only because of creep. At higher potentials, environmental effects become dominant such that, at sufficiently high potentials, the CGR increases in a... 

A. Ata and C. Vipulanandan, Factors Affecting Mechanical and Creep Properties of Silicate Grouted Sands, Journal of Geotechnical and Environmental Engineering, Oct. 1999, pp 868-876, ASCE, Reston, VA. 

The common factor to the subjects considered in this chapter is that they arc all concerned with evaluating the resistance of materials to exposure to some environmental agent other than temperature (and air). Durability involves many factors mechanical, thermal, and electrical stresses including creep, fatigue, and abrasion. These can be difficult enough to measure and are greatly complicated by the effects of nonambient temperatures, but when the effects of environments are superimposed, the problems can increase in a quantum leap. 

In conjunction with the environmental effect, the loading frequency is an important factor. Since the environmental effect is due to the reaction between the environment and the freshly exposed material t the crack tip, the period of time for reaction is important. The degree of crack extension due to environmental crack tip damage during each cycle has been studied and reported by Wei [ ] and Bradshaw and Wheeler [" j. Also, test frequency can have a marked effect on results, if creep is a contributor to the fatigue processes [" ]. 

Adhesives have either ductile or brittle shear behaviour, as shown in Figure 5.32 of the EUROCOMP Design Code. Ductile adhesives are preferred in structural joints because of their better resistance to peel and impact loads. However, brittle adhesives typically have better creep and environmental resistance than ductile adhesives and are therefore used in applications where these factors are critical. 

For plastics parts prone to creep and environmental influences, increased safety factors in addition to materials-dependent reduction factors need to be applied. 

Long-term tests under static load are performed under a variety of conditions e.g., under tensile, compressive, bending and flexural loads, under uni- or multi-axial load, under combined load, and as a function of temperature, processing influences, environmental conditions and depending on design features (notch factor). In practice, creep tests are typically performed under tensile load. 

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