Fractures fibers

The fibers fracture followed by fracture of the composite material... 

Failure of the composite material can then occur in two ways. First, the matrix shear stress around the fiber could exceed the allowable matrix shear stress. More precisely, the bond between the fiber and the matrix might be broken due to high shear stress in the aforementioned mechanism for transfer of stress between broken fibers. Second, the fiber fracture could actually propagate across the matrix through other fibers and hence cause overall fracture of the composite material. If a... 

Figure 3-52 Cumulative Number of Fiber Fractures versus Percentage of Ultimate Composite Load (After Rosen, Dow, and Hashin [3-29])...

An implication of Eq. (6.3) is that debonding only occurs when < trj , or Gic < (7 d/SE[, otherwise the fiber will break prior to debonding. Other criteria for fiber fracture in single fiber pull-out test, refer to Section 4.2.4. 

When these specimens are stressed in tension, the fibers fracture first. If the fiber-matrix bond is weak, the resulting stress is relieved by debonding on either side of the break. These debonded fibers slide back into their holes in the resin, against considerable friction, and the load is built up again in the fibers. 

With a strong interfacial bond, when a fiber fractures, the high stresses in the matrix near the broken ends are relieved by the formation of a short radial crack in the resin. There is no interfacial debonding and corresponding friction at a sheared interface, but rather, the load is transferred to the fiber by elastic deformation of the resin. The lack of adhesive failure in this case is responsible for the relatively low emission observed. 

Metals and carbon fibers fracture at grain boundaries... 

In addition to the direct observation of fiber fragmentation through the optical microscope, acoustic emission accompanying fiber fracture was also monitored by a piezoelectric transducer positioned at the center of the SFC specimen. Both the number of acoustic events and the energy of the pulses were measured. Thus, the correspondence between fiber fracture, observed microscopically, and the AE events accompanying it could be established as shown in Fig. 10. 

It is clear that the acoustic emission technique is capable of providing valuable information on the in situ strength of surface treated fibers, especially at very small gauge lengths approaching the critical aspect ratio of the fiber in the composite. The AE method should also be useful in extending the SFC test to opaque matrix resins, through which fiber fracture cannot be observed by the optical microscope. 

Figure 11. AE energy distribution of fiber fracture in SFC specimens (1) untreated, and (2) A0750 (APS), (3) PS076.5, and (4) PS076 treated fibers.

Figure 4. Polarized transmitted light photomicrograph at x 70 of a typical single bare (water-sized) E-glass fiber embedded in a coupon of DER 383/DACH epoxy matrix. The specimen was strained in a direction parallel to the fiber axis until the fiber started to fracture in the coupon, showing the region around the break taken directly after the fiber fractured. There is a small matrix crack that extends outward from the fiber into the matrix.

Fig. 11.28 The number average fiber length at various axial positions of an injection molding screw. DuPont Alathon 2010 pellets preblended with 3.2 mm chopped fibers and extruded in a 0.75 in diameter, 20.1 L/D, with 3 1 compression ratio extruder. [Reprinted by permission from R. von Turkovich and L. Erwin, Fiber Fracture in Reinforced Thermoplastic Processing, Polym. Eng. Sci., 23, 743 (1983).]... 

R. Turkovich and L. Erwin, Fiber Fracture in Reinforced Thermoplastic Processing, Polym. 

The length of the fracture surface or degree of taper of cotton fibers broken in tension is an index of their brittleness (33). For example, cotton fibers fractured in water or with standard moisture-regain produce... 

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