Creep loading

Creep tests are ideally suited for the measurement of long-term polymer properties in aggressive environments. Both the time to failure and the ultimate elongation in such creep tests tend to be reduced. Another test to determine plastic behavior in a corrosive atmosphere is a prestressed creep test in which the specimens are prestressed at different loads, which are lower than the creep load, before the final creep test (11). 

Fig. 71 Lifetime curves as a function of the creep load for nylon 66 yarns with different draw ratios (d.r.). The yarn with a stretch ratio of 5.6 is from a different polymer batch [54]. The drawn lines represent the regression lines of the observed data...

PP bead foams of a range of densities were compressed using impact and creep loading in an Instron test machine. The stress-strain curves were analysed to determine the effective cell gas pressure as a function of time under load. Creep was controlled by the polymer linear viscoelastic response if the applied stress was low but, at stresses above the foam yield stress, the creep was more rapid until compressed cell gas took the majority of the load. Air was lost from the cells by diffusion through the cell faces, this creep mechanism being more rapid than in extruded foams, because of the small bead size and the open channels at the bead bonndaries. The foam permeability to air conld be related to the PP permeability and the foam density. 15 refs. 

Formation of Spatially Distributed Crazes in Amorphous Polymers under Creep Loading... 

For a more complete description of the time and the temperature dependence of the fibre strength a theoretical description of the viscoelastic and plastic tensile behaviour of polymer fibres has been developed. Baltussen (1996) has shown that the yielding phenomenon, the viscoelastic and plastic extension of a polymer fibre can be described by the Eyring reduced time model. This model uses an activated site model for the plastic and viscoelastic shear deformation of adjacent chains in the domain, in which the straining of the intermolecular bonding is now modelled as an activated shear transition between two states, separated by an energy barrier. It provides a relation between the lifetime, the creep load and the temperature of the fibre, which for PpPTA fibres has been confirmed for a range of temperatures (Northolt et al., 2005). 

Background At elevated temperatures the rapid application of a sustained creep load to a fiber-reinforced ceramic typically produces an instantaneous elastic strain, followed by time-dependent creep deformation. Because the elastic constants, creep rates and stress-relaxation behavior of the fibers and matrix typically differ, a time-dependent redistribution in stress between the fibers and matrix will occur during creep. Even in the absence of an applied load, stress redistribution can occur if differences in the thermal expansion coefficients of the fibers and matrix generate residual stresses when a component is heated. For temperatures sufficient to cause the creep deformation of either constituent, this mismatch in creep resistance causes a progres-... 

Fig. 5.1 Idealized representation of the transient change in fiber and matrix stress that occurs during the isothermal tensile creep and creep recovery of a fiber-reinforced ceramic (the loading and unloading transients have been exaggerated for clarity). It is assumed that the fibers have a much higher creep resistance than the matrix. The matrix stress reaches a maximum at the end of the initial loading transient. After full application of the creep load, the matrix stress relaxes and the fiber stress increases. Upon specimen unloading, elastic contraction of the composite occurs, followed by a time-dependent decrease in fiber stress and increase in matrix stress. Overall, creep tends to increase the difference in stress between constituents and recovery tends to minimize the difference in stress. After Wu and Holmes.15...

Fig. 5.5 Effect of changing the elastic modulus ratio and constituent creep stress exponents on the total strain rate of a 1-D composite subjected to tensile creep loading.31 In both (a) and (b), the dashed lines represent the composite behavior, and the thin solid lines the constituent behavior. In the calculations, it was assumed that the creep load was applied instantaneously.

Initial and Final Creep Mismatch Ratio At low temperatures, or during rapid loading, the stress in the fibers and matrix can be estimated from a simple rule-of-mixtures approach this gives the elastic stress distribution between the fibers and matrix. During creep, the stress distribution is time dependent and is influenced by both the initial elastic stress distribution and the creep behavior of the constituents. Immediately after applying an instantaneous creep load (i.e., at t = 0+), the CMR, =0+ can be found by substituting ef0 = Af (E/ Ec)a-C nf and emfi = Am (Em/Ec)[Pg.176]

Fig. 5.7 Macroscopic damage modes that occur during the tensile and flexural creep of fiber-reinforced ceramics. It is assumed that matrix or fiber damage is avoided during initial application of the creep load (see discussion of loading rate effects in the next section). Periodic fiber fracture can occur if the creep rate of the matrix exceeds that of the fibers. Periodic matrix fracture is common when the matrix has a higher creep resistance than the fibers. In this figure, it is assumed that initial microstructural damage is avoided during application of the creep load.

Fig. 5.8 Influence of initial loading rate on the 1200°C tensile creep life of 0° SCS-6 SiCf/HPSN composites crept in air at a nominal stress of 250 MPa. The creep curves include the elastic strain on loading. For each loading rate, duplicate tests were performed to provide a rough indication of the scatter in the results, (a) Creep load applied in 1000 s, and (b) creep load applied in 2.5 s. After Holmes et al.38...

The occurrence of frictional heating has important implications for the design of structural components. One concern is that components such as gas-turbine airfoils and combustors will be subjected to creep loading com-... 

Example 14.1 A polystyrene sample of 0.02/m cross-sectional area is subjected to a creep load of 10 N. The load is removed after 30 s. Assuming that the Maxwell element accurately describes the behavior of polystyrene and that viscosity is 5 x 10 P, while Young s modulus is 5 X 10 psi, calculate ... 

Creep loading means that the stress is given by... 

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