Page 261 - Engineered Interfaces in Fiber Reinforced Composites

242 Engineered interfaces in jiber reinforced composites

6.1.2. Fiber-matrix interface debonding in mode 11 shear

For a composite containing fibers whose maximum strain is greater than that of
matrix (i.e. q > ern), the crack propagating in the matrix is halted by the stiff fiber if
the current level of stress is not high enough as shown in Fig. 6.l(b). Alternatively,
the crack may pass around the fiber with little damage to the interface bond. As the
applied load is further increased, the fiber and matrix attempt to deform
differentially, causing local Poisson contraction. Relatively large local stresses are
built up in the fiber at the same time. This allows the level of shear force developed
at the interface to exceed the apparent interfacial shear bond strength, Zb, and results
in interfacial debonding at the crack plane if the maximum shear strength criterion is
employed as the failure criterion. The debond will extend some distance along the
fiber with further increase in the external load (Fig. 6.1(c)). The debonding
toughness, Rd, first proposed by Outwater and Murphy (1969) can be evaluated by
the total elastic strain energy stored in the fiber over the debond length td at its
breaking stress o: divided by the cross-section area of the composite:

It has been well established that the debonding process is often the major
contributor to the total fracture toughness of glass fiber reinforced polymer matrix
composites (GFRPs) (Murphy and Outwater, 1973), although there may be some
confusion as to the exact nature of the energy absorption mechanism. Harris (1980)
later pointed out that the debonding process implicit in the Outwater-Murphy
analysis requires that the fibers separate from the matrix in shear, and the debond
toughness, Rd, given by Eq. (6.1) appears to be a consequence of debonding rather
than the debonding itself. If Rd were to arise from the latter case, it is necessary to
consider the fiber debond stress, bd, which is a function of either the fiber-matrix
interface fracture toughness, Gi,, or the interface bond strength, q,, depending on
the debond criterion being used (see Chapter 4). The debond toughness of Eq. (6.1)
may then be modified to

An upper limit to the interface fracture toughness, Gi,, can thus be estimated from
the work of debonding divided by the cylindrical debond area

An implication of Eq. (6.3) is that debonding only occurs when CTd < of, or
Gi, < o;2d/8Ef, 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.