Critical fiber loading

The fundamental problem with this type of model is the presence of so many unknown empirical quantities that need to be evaluated for a given glass fiber loading (critical fiber length), (average fiber... 

Fiber reinforcements are compacted in a consolidation procedure in which external loads are applied to compress fibers, to squeeze air and resin out, to suppress voids, and to increase the fiber volume fraction. Before compaction, the fiber reinforcement networks are unable to carry traction stresses at/or below a certain initial critical fiber volume fraction,. As the fiber volume fraction, pj, increases under compression, the network can carry a rapidly increasing load. Eventually, the fiber volume fraction of the network approaches a theoretical maximum based on the relevant close-packed geometry, and cannot increase without an enormous increase in load. The compressibility of the fiber reinforcement network is dependent not only on the elastic properties of fibers, but also on the configuration of the fiber reinforcement network as well, that is... 

When a stress equal to cr is applied to a fiber having just this critical length, the stress-position profile shown in Figure 16.7a results—that is, the maximum fiber load is achieved only at the axial center of the fiber. As fiber length I increases, the fiber reinforcement becomes more effective this is demonstrated in Figure 16 Jb, a stress-axial position profile for l>l, when the applied stress is equal to the fiber strength. Figure 16.7c shows the stress-position profile for I <... 

Typically, polyester resins are used for high-end applications that require excellent electrical and thermal resistance. When dimensional stability under load is more critical, glass fibers are incorporated to increase the heat distortion temperature and the stiffness of the part. Examples of glass fiber reinforced parts include electrical housings, electrical adapters, computer components, telephone housings, and light bulb sockets. When impact modified, polybutylene terephthalate can be injection molded to make car bumpers. 

The [10°] off axis tension specimen shown in Fig 3.23 is another simple specimen similar in geometry to that of the [ 45 ]s tensile test. This test uses a unidirectional laminate with fibers oriented at 10° to the loading direction and the biaxial stress state (i.e. longitudinal, transverse and in-plane shear stresses on the 10° plane) occurs when it is subjected to a uniaxial tension. When this specimen fails under tension, the in-plane shear stress, which is almost uniform through the thickness, is near its critical value and gives the shear strength of the unidirectional fiber composites based on a procedure (Chamis and Sinclair, 1977) similar to the [ 45°]s tensile test. 

One of the major differences between the results obtained from the micromechanics and FE analyses is the relative magnitude of the stress concentrations. In particular, the maximum IFSS values at the loaded and embedded fiber ends tend to be higher for the micromechanics analysis than for the FEA for a large Vf. This gives a slightly lower critical Vf required for the transition of debond initiation in the micromechanics model than in the FE model of single fiber composites. All these... 

In contrast, the single fiber composite model predicts that the IFSS concentration becomes higher at the embedded end than at the loaded end if fiber Kf is greater than a critical value, suggesting the possibility of debond initiation at the embedded fiber... 

Figs. 4.44 and 4.45 show the increase in the debond length, f, and displacement, as a result of the reduction of p (from Po = 0.22 to p = 0.07) under cyclic loading. It is interesting to note that both I and 5 remain constant until the coefficient of friction, p, is reduced to a critical value p. (= 0.144 and 0.166, respectively for fiber pull-out and fiber push-out). The implication is that the debond crack does not grow... 

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