Factors Effecting Composite Strength

Johnson [20] has suggested that the tensile failure of carbon fibers is due to the presence of defects and the structure suggests that anisotropic effects may be ignored and the size of the basal plane cracks estimated from a simple Griffith relation  

Theoretical strength Vs defect size for Type I (HM) and Type II (HT) carbon fibers. Source Reprinted with permission from Reynolds WN, Proc 3 ° Conference Industrial Carbons and Graphites, SCI, London, 427-430, 1971. Copyright 1971, The Society of Chemical Industry. 

There are three factors which are particularly important in controlling the strength of composites, apart from the basic fiber and matrix properties namely  

Critical length (the minimum length of fiber embedded in the resin to avoid pull-out when an axial load is applied, calculated to be about 0.1 mm) 

The tensile properties of carbon fibers have been investigated [28-32]. Tibbets et al [33] determined the properties of VGCF. 

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Finally, of great importance are finite width effects. The strength of a composite laminate in the presence of a hole, a crack, or impact damage is drastically reduced as the size of the notch covers a significant percentage of one of the dimensions of the structure. Finite width correction factors have been obtained by Tan for holes [19]. 

The orientation of fibres is an important factor in the design of a composite material strncture. Strong anisotropic effects may be created by fibres and there have been several attempts both to determine the influence of fibre orientation on mechanical properties and to optimize it. Much of the research was concentrated on advanced composites with ductile matrices and the objective function for design or optimization was the composite strength. These composites were extensively applied not just in aircraft and rockets, but also in the construction of cars the review of these investigations is published by Ashby and Jones (2005). 

There is an extensive amount of data in the literature on the effect of many factors (e.g. temperature, monomer and surfactant concentration and types, ionic strength, reactor configuration) on the time evolution of quantities such as conversions, particle number and size, molecular weight, composition. In this section, EPM predictions are compared with the following limited but useful cross section of isothermal experimental data ... 

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