Cyclicality creep

After a time increment dt, the local stresses in the fibers and matrix can be updated to obtain (tr,-, e e,)t at any time t+ dt. Using this information, the composite stress, strain, and strain rate (ac, ec, ec), can be obtained from the constituent parameters (iterative computation, the creep behavior of the composite and constituents can be predicted for any loading history, including cyclic creep. 

Zawada et al.44 showed that the proportional limit, expressed in strain (0.3%) rather than in stress, was identical for unidirectional and cross-ply laminates of SiCf/1723. Moreover, the fatigue limit of the unidirectional composite, expressed in strain, corresponded well with the measured fatigue strain limit of the cross-ply laminates. This indicates that the fatigue limit of a cross-ply laminate is primarily governed by the 0° plies and that the influence of the 90° plies is minimal (this result is expected to hold only for room temperature fatigue—see Chapter 5 for a discussion of how transverse plies influence cyclic creep behavior). The 90° plies develop transverse cracks early... 

The creep behaviour of various vulcanised rubbers under cyclic conditions has been compared with that under constant loading. Cyclic loading is found to produce an enhancement of creep rates thought to be due to crosslinks breaking under stress concentrations caused by crystallisation. Half of the cyclic creep can be accounted for in this way visco-elasticity accounts for the remainder. 6 refs. 

In 1942 Busse et al. [72] suggested this mechanism for polyamide, cotton and rayon. Briiller et al. [147] stated cyclic creep strains were predictable from the Boltzmann superposition principle. 

In conclusion, it may be mentioned that the characterization of the mechanical behaviour of materials has many facets. Different methods of testing pertain to different aspects and conditions. The tensile properties, as determined by the tensile test, correspond to slowly applied single load applications. Rapidly applied and cyclic load applications respectively provide the impact and the fatigue properties. Hardness is an analog of the tensile strength which a tensile test measures. The creep test pertains to mechanical behaviour under long term loading at elevated temperatures. 

Some viscoelasticity results have been reported for bimodal PDMS [120], using a Rheovibron (an instrument for measuring the dynamic tensile moduli of polymers). Also, measurements have been made on permanent set for PDMS networks in compressive cyclic deformations [121]. There appeared to be less permanent set or "creep" in the case of the bimodal elastomers. This is consistent in a general way with some early results for polyurethane elastomers [122], Specifically, cyclic elongation measurements on unimodal and bimodal networks indicated that the bimodal ones survived many more cycles before the occurrence of fatigue failure. The number of cycles to failure was found to be approximately an order of magnitude higher for the bimodal networks, at the same modulus at 10% deformation [5] ... 

Fatigue or dynamic fatigue can be defined as the decrease in load bearing capacity with time under cyclic or intermittent load, the term static fatigue being sometimes used to describe creep-rupture (see Sections 4.9.2 and 6.10). 

Figure 3.1 The five most common mechanical tests (I) constant elongation for tensile strength determinations, (2) constant force for creep determinations, (3) fixed elongation for stress relaxation determinations, (4) cyclic strain for dynamic mechanical determinations, and (5) impact for impact determinations. (After J. Fried, Plastics Engineering, July 1982, with permission.)...

This boron- and carbon-doped SiC exhibits excellent strength and stiffness, extreme hardness, and thermal and chemical resistance. The strength of this system is not affected by temperatures up to 1650°C. Creep is virtually nonexistent up to 1400°C. Cyclic durability testing conducted at 1370°C in air showed no deterioration of strength after 3500 h (94). 

For the cyclic corrosion test, a layer of acrylosilane polymer coating (10-25 fim thick) was dip-coated onto the plasma-deposited substrates. The coated samples were then subjected to 25 scab cycles. The test results are plotted in Fig. 7. Corrosion performance (as described by the length of scribe creep) was correlated to the wattage used for plasma film deposition. As discussed in the previous section, the chemical structure and properties correlated with the deposition conditions, especially the power level applied. Therefore, atomic compositions for plasma polymers deposited at different power levels were also plotted in Fig. 7.A... 

These in situ generated silica fillers also give increased resistance to creep or compression set in cyclic deformations, as is illustrated in Figure 8.6.43 The in situ filled PDMS samples are seen to show very little compression set. They can also provide increased thermal stability, as demonstrated by the results presented in Figure 8.7. 44... 

Stiffness, resistance to deformation under constant applied load (creep resistance), resistance to damage by cyclical loading (fatigue resistance), and excellent lubricity are mechanical properties for which acetal resins are perhaps best known and which have contributed significantly to their excellent commercial success. General-purpose acetal resins are substantially stiffer than general-purpose polyamides (nylon-6 or -6,6 types) when the latter have reached equilibrium water content. 

Data regarding the effects of cyclic loading and creep on the life of brittle matrix composites are limited. The concepts to be developed thus draw upon knowledge and experience gained with other composite systems, such as metal matrix composites (MMCs) and polymer matrix composites (PMCs). The overall philosophy is depicted in Fig. 1.7. 

On the other hand, very high temperatures and low cyclic loading frequencies (high cycle times) promote near-tip creep-fatigue conditions where... 

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