Compaction fiber tension

Regardless of matrix or fiber materials, the key process variables for filament winding are temperature, compaction pressure/fiber tension, and laydown rate. Typical measures of final cylinder quality include degree of cure/crystallinity, void volume fraction, fiber volume fraction, and residual stresses and strains. 

Basic Structure Crystal orientation Crystallite content Homogeneity Precursor Polymer composition Appropriate fiber tension High density and compactness Prevent filament coalescence... 

Fig. 12.12 Results of compact tension specimen showing increase in toughness as crack goes past fiber, and crack opening displacement from which closure stresses can be determined.

Compared with the three other semi-finished materials, the tapes are fully consolidated (Fig. 7.16), that is, the micro and macro impregnation are completely done. FuUy consolidated tapes are available on the market with fiber volume contents of up to 60% and even higher. Due to the not applicable impregnation process the winding speed can be higher in comparison to the other winding patterns. The heating zone melts the polymer and the tension (Fig. 7.6) or compaction roller (Rg. 7.7) presses the filaments on the mandrel. 

In addition, as a typical failure mechanism, fiber/matrix debonding occurs due to the shear and tension type loading. If fiber/matrix debonding has taken place, the local separation initiates additional fiber cracking, wear debris formation, and a more intensive wear process. In the steady state wear process, a so-called compacted wear debris layer (CWDL) covers the surface it is composed of pulverized wear debris and matrix material. During the wear process, this layer is continuously formed and removed by the surfaces sliding over each other. 

The thread tension set during the stitching assists to improve the compaction and fixation of the reinforcing fiber bundles. Figure 5 shows the compaction instrument which is developed at the Institute for Composite Materials Ltd., and... 

Figure 6. Influence of thread tension on the compaction behavior of a dry lay-up based on CF-woven 5H satin, [0/90]4, polyester long fiber spun 73 tex, stitch length 2.5 mm, seam distance 40x40 mm, stitching direction 45 , presser-foot height such that the foot rests on...

Experimental results of a quasi-unidirectional CMC revealed different creep behavior in tension and compression. Due to the porous matrix, the samples with 90° fiber orientation showed the largest creep deformation rates. In tension creep the fracture strain was 0.4-1% for 90° samples whereas in 0° fiber orientation creep strain above 8% was feasible. At the beginning of compression experiments, the absolute creep rate was highest and decreased continually. Creep parameters, i.e. temperature and stress dependencies, were deteimined in case of compression stress. Thereby, the activation energy averaged to 700 kJ/mol and the stress exponent varied with the fiber orientation from 3 to 1.9. The variation in the stress exponent was presumably caused by different stress exponents of fibers and matrix and quite likely due to the effect of matrix compaction. Latter one could be visualized by optical micrographs. 

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