Creep fibre type

Thermosetting polymers are viscoelastic at ambient temperature and unreinforced are susceptible to creep. Fibre reinforcements are generally elastic and creep resistant but some types may show the phenomena of stress rupture if exposed to high stresses in adverse environments. Composites reinforced with unidirectional stiff fibres and stressed in the fibre direction can be highly creep resistant as the fibre attracts a high proportion of the load in line with its stiffness relative to the polymer matrix. The behaviour of composites under static loading is a function of many material variables polymer type... 

Examples of creep effects covering environment, fibre type, resin type, effects of temperature, effects of strain and effects of overload are given in a variety of references at the end of this section. Creep effects are also affected by cyclic and intermittent loading. At cyclic frequencies above a few hertz the effects of heating of the material from hysteresis can be a factor in increasing creep. 

The creep and fatigue behaviour for natural fibre-reinforced plastics is less weU understood than for glass fibre-reinforced plastics because of the lack of systematic and detailed information. Limited information is currently available oti the effects on the fatigue behaviour of natural fibre-reinforced plastics of different composite parameters such as fibre type, the quality of fibre-matrix adhesion and fibre properties and their content. 

Except for the small amounts of boron-based additives in the enhanced matrix, the standard SiCf/SiC and enhanced SiCf/SiC samples are very similar, with the fibre types, volume fractions and architecture being virtually identical. Because the longitudinal fibres control the rates of creep strain accumulation, the initial portions of the creep strain/time curves are therefore indistinguishable. However, the point of... 

The creep rate of the principal fibre types has been compared by testing in tension at 50% of their breaking strength (Figure 10.3). 

Certain types of laminate are susceptible to creep, as discussed in Chapter 1. The use of 0° plies greatly reduces the creep rate and the problem is not usually severe with long fibre composites, although laminates consisting entirely of 45° plies can show a scissor-closing motion, that is, a reduction in the angle to 43° or thereabouts if held under a sustained load with a sufficiently ductile matrix. 

Fig. 13. The creep rates of the Nicalon Type S fibres are lower than the other near-stoichiometric fibres which contain sintering aids.

As can be seen from Fig. 13 the creep rate of the Hi-Nicalon Type S fibre is one order of magnitude lower than that of the other two near-stoichiometric fibres because of the lack of sintering aids which facilitate creep at high temperatures. Growth of a... 

Fin- 15. The. surface of a Hi-Niealon Type S fibre showing the layer of silica which has formed during creep at 1400°C. 

Fig. 15 shows the silica layer formed at the surface of a Hi-Nicalon Type S fibre after creep at 1400°C... 

The time-dependent behaviour is different in the two types of highly oriented, chain-extended polymer fibres. Table 1 gives the results of studies in FIBRE TETHERS 2000 (1995), which were made because creep rupture is a concern in deep-water mooring of oil-rigs. The low-load creep in aramid fibres is due to a straightening of the initial. structure. It reduces in rate, even on a logarithmic scale, with time and is not a source of creep rupture. In Vectran, the creep is less and is absent after 10 days under load. 

In long-term applications and when subjected to constant loads, FRP bars are susceptible to creep rupture. This phenomenon, which does not occur in steel rebars, depends on the load level, environmental conditions and type of fibre reinforcement, with carbon fibres being the least and glass fibres the most susceptible (ACI Committee 440,2006). Also regarding their long-term performance, like FRP pultruded profiles FRP bars do not corrode and are... 

Creep strain/time curves have been determined for a range of SiCf/SiC type composites, " for various types of SiC fibre and for other CFCMCs. However, the faetors eontrolling the creep properties of different composites have usually been diseussed by reference to only a few standard parameters, such as the minimum creep rate (Sm), and the creep rupture life (tf). For this reason, the present projeet seeks to demonstrate that evidence derived from analyses of creep curve shape is relevant to interpretation of the detailed manner in which specific material variables influence strain aecumulation and damage evolution during tensile creep of CFCMCs. 

For a variety of SiCf/SiC type composites, it has been stated that the SiC matrices are more creep resistant than the Nicalon fibres. " This conclusion was reached on the grounds that the creep rates reported for silicon carbide produced by chemical vapour deposition appeared to be lower than the rates predicted by extrapolation of results obtained for Nicalon fibres. However, it is a simple matter to demonstrate that the fibres rather than the matrices control the creep strength of both the standard SiCf/SiC and enhanced SiCf/SiC composites. 

Figure 2. Stress/minimum creep rate relationships for (a) SiCf/SiC type composites and Nicalon NLM202 fibres at 1573K and (b) the AbOsf/SiC material and alumina fibres at 1373K.

With the standard SiCf/SiC product, creep of the longitudinal fibres was accompanied by crack formation in the brittle matrix. Microcracks originated at pores within the transverse and longitudinal fibre bundles (Fig. 5a), with some longer cracks (Fig. 5b) forming parallel to the stress axis (presumably because of the complex stress states introduced by straightening of the interwoven fibre bundles during tensile creep). However, these types of crack rapidly became arrested, with the... 

Creep is low in fibre-reinforced plastics at room temperature and comparable with or better than steel depending upon the type of fibre and steel. The stiffest fibre (carbon) has the lowest creep rate. 

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