Fiber buckling

Experimental Results for Fiber Buckle Wavelength versus Fiber Diameter (After Dow and Rosen [3-28])... 

Figure 3-55 Extensional Mode and Shear Mode of Fiber Buckling...

The analysis to find the fiber buckling load in each mode is based on the energy method described by Timoshenko and Gere [3-31], The buckling criterion is that the change in strain energy for the fiber, AUf, and for the associated matrix material, AUf, is equated to the work done by the fiber force, AW, during deformation to a buckled state, that is,... 

Because the buckle wavelength is L/m, the second term in Equation (3.134) is small when the buckle wavelength is large relative to the fiber diameter, h. Thus, the fiber buckling stress is approximately... 

Poisson s ratio of nonporous and porous matrix Poisson s ratios of a three-dimensional layer Major and minor Poisson s ratios for a single ply Fiber buckling stress... 

Figure 20.27 SEM micrograph of the fracture surface of a carbon fiber epoxy resin laminate after a fiber buckling mode failure due to a longitudinal compression stress viewed under high magnification showing both tension and compressive fracture in a single fiber. Source Reprinted with permission from Ewins PD, Potter RT, Phil Trans R Soc London, A294, 507-517,1980. Copyright 1980, The Royal Society of Chemistry.

It should be mentioned that compressive strength of carbon fiber is low compared to its tensile strength. The ratio of compressive strength to tensile strength for carbon fibers may vary anywhere between 0.2 and 1 (Kumar, 1989). High-modulus PAN-based carbon fibers buckle on compression, forming kink bands at thinner surface of the fiber. A crack initiates on the tensile side and propagates across the fiber (Johnson,... 

Fiber buckles Fiber breaks Slot/crack... 

Fiber buckling occurs due to the in-plane compression of the fibers which causes fiber wrinkling. 

Out of all the different deformation modes, fiber buckling is the only unfavorable one since the material movement through this mode creates what is considered as defects, although it is quite difficult to observe with complex stmc-tures, such as knits and braids. Out-of-plane buckling usually occurs when the inplane modes cannot accommodate the required deformation. In-plane buckling can also occur, but is less likely due to in-plane geometric constraints. Figure 9d. 

The buckling of fibers under a longitudinal compressive force is a major issue in fiber applications. Figure 15.26 shows the possible response of an unconstrained fiber under longitudinal compressioa Under the longitudinal compressive force, the fiber buckles easily even at low strains if there is no lateral support. The critical compressive force above which the fiber buckling occurs, is... 

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