Fiber-Matrix Debonding

Fig. 3 Refraction values of both (0°+ 90°) fiber directions with respect to impact energy per layer. The fiber/matrix debonding of CFRP laminates correlates significantly to the impact energy per volume (energy density).

Figure 5.96 Distribution of fiber tensile stress and interfacial shear stress after fiber-matrix debonding has commenced. Reprinted, by permission, from N. G. McCrum, C. P. Buckley, and C. B. Bucknall, Principles of Polymer Engineering, 2nd ed., p. 280. Copyright 1997 by Oxford University Press.

For example, consider the crack tip as it intersects a fiber (Fig. 16). The local stresses at the tip can cause fiber-matrix debonding. The crack tip continues to open causing the interfacial debonded region to extend. The fiber continues to interact with the matrix through a frictional sliding force even after the initial bond fails. The distance over which the force acts is the debonded length times the difference in strain between the fiber and the matrix. 

Fig. 16. Interaction of a crack with a fiber through fiber-matrix debonding...

The tensile properties of isotactic polypropylene materials reinforced with continuous nylon fibers were measured. Less than 10 vol % of the fibers leads to an increased yield strength and yield elongation. As little as 3 vol % of the nylon fibers increased the elongation at necking from 10 to 20%. This retarded necking arises from the fiber-matrix debonding which delocalizes the microscopic yielding processes. 

Composites show extended damage zones (e.g., fiber-matrix debonding) in front of crack tips comparable to plastic zones of homogeneous materials. This necessitates the above indicated switch from LEFM to J-integral and / -curvc concepts. There has been, however, some satisfactory work conducted where the damage zone has been treated in the same way as the plastic zone in metallic materials as far as the fracture mechanics approach is concerned [111]. 

The development of ceramic oxide composites has lagged behind the development of non-oxide composites because of the poor creep resistance of oxide fibers (compared to SiC fibers) and because of the lack of adequate oxide fiber coatings that promote fiber-matrix debonding. Recent advances in creep-resistant oxide fibers and progress on interface control has improved the potential for oxide ceramic composites in industrial and defense applications. However, an effective coating for oxide fibers that provides a weak fiber-matrix interface (and therefore tough composite behavior) remains to be demonstrated. As was discussed in Chapter 6, all oxide coating concepts discussed in the literature have been demonstrated with model systems rather than actual composite systems. 

The use of various coupling agents, which build chemical and hydrogen bonds between the matrix and fibers, has also been shown to reduce the moisture absorption of cellulose fibers in LPCs. The improved fiber-matrix adhesion generated by the coupling agent reduces the moisture absorption that results from fiber-matrix debonding. 

Figure 17.8 The different WPG fracture mechanisms, (a) fiber puU out, (b) fiber breakage (c) fiber/ matrix debonding, (d) The formation of micro-voids between wood fiber and polymeric matrix [104],...

Defects Defect particle Deformation band Diffraotion pattern Dimple struoture Domains Fatigue fracture Fiber-matrix adhesion Fiber-matrix debonding Fibrillation... 

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