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Unformatted text preview: ferable because substantial energy is absorbed both during plastic deformation of the
matrix and during delamination. These processes contribute to the overall toughness of
the composite materials.
In the previous example widespread damage develops in the composite prior to fracture, leading to a tough material. On the contrary, if the interfaces are so strong that no
σ σ Fracture
Fracture Localized damage Distributed damage ε ε
(a) (b) | v v FIGURE 14.5–6 (a) Schematic of a tensile stress-strain diagram and the progress of damage in ductile composites,
and (b) the same as part a but in a brittle system. | e-Text Main Menu | Textbook Table of Contents 13.01.98 plm QC1 rps MP 601 pg602 [V] G2 7-27060 / IRWIN / Schaffer 602 Part III iq 13.01.98 plm QC2 rps MP Properties delamination is possible and the matrix is not ductile enough to effectively blunt the ﬁber
cracks, then the progression of damage remains localized and a low-energy fracture
results. This process is shown schematically in Figure 14.5–6b.
The same principles apply to ceramic-matrix composites. Typically the cracking begins in the matrix and the toughness is enhanced by ﬁber bridging and ﬁber pullout, as
described in Section 14.4. 14.5.5 Fatigue Behavior of Composites
As discussed in Chapter 9, fatigue failures in metals generally initiate at the specimen
surface because of microplasticity, which leads to crack formation. These cracks propagate and become larger, causing the ﬁnal fracture. Fatigue mechanisms in composites
are considerably different. The different stages of fatigue of laminates consist of ply
cracking, delamination, and ultimately ﬁber fatigue. This process is illustrated in Figure
14.5–7 in a laminate with ﬁbers in the 0 and 90 orientations. The number of applied
fatigue cycles is divided by the number of cycles of failure to derive a cycle ratio. The
microstructural changes that result from fatigue damage are then correlated with the cycle
ratio. The extent of ply cracking is best represented by the crack density in the laminae
oriented at 90 to the loading axis. As shown in Figure 14.5–7a, the crack density
increases rapidly at ﬁrst and then reaches a constant value. Delamination does not occur
initially, but occurs rapidly after saturation of ply cracking (Figure 14.5–7b). Delamination saturates when the last stage of composite fatigue, ﬁber fatigue, begins. As shown in
Figure 14.5–7c, when enough ﬁbers have fractured because of fatigue, the composite
90 0 0 Delamination Crack density 0 Ply
cracking 0 90 Delamination 1 0 1
Cycle ratio Cycle ratio
(a) (b) Modulus 0 90 0 Fiber
fatigue 0 1
(c) | v v FIGURE 14.5–7 Schematic representation of stages of fatigue damage accumulation: (a) ply cracking, (b) delamination, and (c) ﬁber cracking.
(Source: H. T. Hahn and L. Lorenzo, Advances in Fracture Research, Pergamon Press, 1984.) | e-Text Main Menu | Textbook Table of Contents 0 5
= (σ max/ σ uts) E0
E/ during a fa...
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