Fiber pullout

In such a material the toughness is primarily due to bridging contribution, rather than fiber pullout. 

SEM can also be used for the interface analysis of composites. Figures 16a and 16b show the SEM of PMPPIC-treated and untreated pineapple fiber-rein-forced LDPE composites. Strong adhesion between fiber and matrix is evident from Fig. 16a, whereas Fig. 16b indicates fiber pullout [78]. 

Pitkethly, M.J. and Doble, J.B. (1990). Characterizing the fiber/matrix interface of carbon fiber-reinforced composites using a single fiber pullout test. Composites 21, 389-395. 

Fig. 4.25. Schematic presentations of applied stress versus displacement (

Fig. 4.39. Comparisons of initial debond stress,

Zbou, L.M. and Mai. Y.W. (1993), On the single fiber pullout and pushout problem effect of fiber anisotropy. J. Appl. Math. Phys. (ZAMP) 44, 769-775. 

Ha. J.S. and Chawla, K.K. (1993). Effect of SiC/BN double coating on fiber pullout on mullite fiber/ mullitc matrix composites. J. Mater. Sci. Lett. 12, 84-86. 

Bannister, D.J., Andrews, M.C., Cervenka, A.J. and Young, R.J. (1995). Analysis of the single fiber pullout test by means of Raman spectroscopy. Part 11. Micromechanics of deformation for an aramid/ epoxy system. Composites Sci. Technol. 53, 411—421. 

Figure 33. Scanning electron micrographs of the fracture surface of a 3D carbon/carbon composite (59) Fiber pullout causes increased energy consumption in fracture.

Further evidence for the lack of bonding was obtained by examination of tensile samples. Numerous pulled-out fibers were evident for billet A, and these were covered with pitch matrix with a minimum of discernible CVD. In contrast, the billet B tensile sample did not display fiber pullout and the fracture tended to occur nearer the CVD-matrix interface than at the CVD-fiber interface, which indicates good bonding and the ability of a fiber to share its load with the surrounding fibers. Consequently, fiber pullout was reduced and fracture was more uniform between the fibers, resulting in higher strength. 

Depending on x, Gf, and g, two failure modes are possible for aligned short brittle fiber composites fiber pullout and fiber fracture. Let us consider a composite in which > e. If / < 4, the average stress in the fiber will be given by Eq. (15.47), so the strength in the composite can be written... 

Tests on tin oxide fiber coatings in model composite systems indicated some crack deflection at the coating-fiber interface (Siadati et al., 1991 Venkatesh and Chawla, 1992). However, tensile tests of tin oxide coated alumina fiber-reinforced alumina matrix composites demonstrated a decrease in the extent of fiber pullout as the density of the matrix phase was increased. This led to increasingly brittle fracture behavior in these composites (Goettler, 1993). Tin oxide also has thermal stability problems at elevated temperatures (Norkitis and Hellmann, 1991). For example, in the presence of air at temperatures above 1300°C (2,372°F), tin oxide (solid) decomposes into SnO (gas) and Oj (gas). This decomposition occurs at even lower temperatures when the partial pressure of oxygen in the test environment is reduced. 

Venkatesh, R, and K.K. Chawla. 1992. Effect of interfacial roughness on fiber pullout in atumina/Sn02/glass composites. Journal of Material Science Letters 11 650-652. ... 

Material data of CVI-SiC/SiC and CVI-C/SiC composites available from MAN - Technologic, Germany, are listed in Table 7 [173]. Figure 20 shows the fracture surface and fiber pullout of a CVI-SiC/SiC composite. 

At high temperatures (> 600°C) the composites degrade in strength and toughness due to oxidation of C fibers and/or these interface layers and prevention of fiber pullout (brittle fracture mode). Studies are in progress to increase oxidation resistance by use of a CVD-SiC overlayer which seals the surface of the porous composites. 

Figure 20. Fracture surface of a CVI SiC/SiC composite showing fiber pullout (Courtesy M. Leuchs, MAN Technology/Germany).

Pan, N. Theoretical modeling and analysis of fiber pullout behavior from a bonded fibrous matrix The elastic-bond case. Journal of Textile Institute, 84, 472485 (1993). 

FIGURE 2. a) Schematic showing toughening through crack deflection at the fibei/matrix interface, b) SEM micrograph of fracture surface ofNextel 610/monazite/alumina composite tested at 1200°C showing fiber pullout and c) fracture surface ofNextel 610/alumina composite tested under the same conditions. Absence of crack deflection mechanism led to brittle failure in c). 

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