Fatigue rate dependence

The rate dependence of fatigue strength demands careful consideration of the potential for heat buildup in both the fatigue test and in service. Generally, since the buildup is a function of the viscous component of the material, the materials that tend toward... 

Because the manner of breakdown and the rate at which it occurs in a fatigue test depend on so many factors, notably the mode of stressing, whether stress or strain cycles are used, the frequency, the shape of the deformation cycle (c.g.. smooth sinusoid or sharp pulse), and the environment, it is clear that a single test can only relate to the actual conditions used. One of the problems with the traditional standard tests is that the results are specific to the particular conditions, and these are arbitrary. 

The rate dependence of fatigue strength demands carefiil consideration of the potential for heat buildup in both the fatigue test and in service. Generally, since the buildup is a function of the viscous component of the material, the materials that tend toward viscous behavior will also display sensitivity to cyclic load frequency. Thus, TPs, particularly the crystalline polymers like polyethylene that are above their glass-transition temperatures, are expected to be more sensitive to the cyclic load rate, and highly crosslinked plastics or glass fiber reinforced TS plastics are much less sensitive to the frequency of load. 

The action of cyclic fatigue and of SCCG can be assumed to be independent, and the crack advance due to both mechanisms can be simply added. However, as both growth rates depend heavily on the applied load, and since the SCCG exponents are, in general, different, it is quite likely that the growth rates are very different. The simplified model description can then be used whereby only one of the two mechanisms is dominant. 

The rate dependencies of the ferroelectric material properties are also reflected in the dynamics after fatigue. Initially, most of the domain system will be switched almost instantaneously [235], and only a small amount of polarization will creep for longer time periods [194]. A highly retarded stretched exponential relaxation was observed after bipolar fatigue treatment [235], and these observations correlated well with the thermally activated domain dynamics. If the overall materials response was represented in a rate-dependent constitutive material law 236], however, then a growing defect cluster size would retard the domain dynamics considerably. Hard and soft material behaviors were also representable as different barrier heights to a thermally activated domain wall motion, as demonstrated by the theoretical studies of Belov and Kreher [236]. 

Harel H, Aronhime J, Schulte K, Friedrich K, Marom G, Rate-dependent fatigue of aramid-fibre carbon-fibre hybrids, J Mater Sci, 25(2b), 1313-1317, 1990. 

Aronhime J, Harel H, Gilbert A, Marom G, The rate-dependence of flexural shear fatigue and uniaxial compression of carbon-fibre and aramid-fibre composites and hybrids, Composites Sci Technol, 43(2), 105-116, 1992. 

Hysteresis is reduced with increasing cross-link formation. Hysteresis is the ratio of the rate-dependent or viscous component to the elastic component of deformation resistance. It is also a measure of deformation energy that is not stored (or home by the elastic network) but is converted to heat. Vulcanization then causes a trade-off of elasticity for viscous or plastic behavior. Tear strength, fatigue hfe, and toughness are related to the... 

Figure 5 Illustration of the strain rate dependence of the crack propagation rate due to the slip dissolution model, and the additive properties of the mechanical and environmental components of crack advance during corrosion fatigue.

G. Marom, H. Harel, S. Neumann, K. Friedrich, K. Schulte, H. D. Wagner (1989) Fatigue behaviour and rate-dependent properties of aramid fibre/carbon fibre hybrid composites. Composites 20,537. 

Figure 6.27 Fatigue crack propagation rate dependence on cyclic frequency and stress intensity factor range for generic PC.

Work by Schapery, Saxena, Wilhams, and others details the analysis of cracks in creeping, strain rate dependent materials, and provides a predictive basis for the apparently brittle nature of FCP in UHMWPE [43-48]. Particularly usefid are the models developed by Schapery and Wilhams, which directly link the intrinsic, constitutive viscoelastic relaxation behavior of the material to the advance of a stable crack tip [46,48]. The power of these models is the predictive nature of the mechanics in relating ECP dynamics to the material s viscoelastic behavior that is eashy measured in a simple one-dimensional creep test The elementary consequences of the models result in the static mode fatigue crack propagation behavior that is observed in UHMWPE, and thus potentially provide a first-principles explanation of the fatigue and fracture behavior by the material. 

The maximum test frequencies for cryogenic fatigue measurements can be much higher than at RT because of very low mechanical damping (tan Sm < 10 at 4.2 K) and absence of rate dependence below 1 kHz. The heat power generated per sample volume V can be estimated from the loss tangent. Young s modulus E iT), strain amplitude e, and test frequency/ ... 

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