Environmental Engineering Reference
In-Depth Information
P
P
P
P
a
P
P
(a)
(b)
(c)
P
M
M
M
M
a
(d)
(e)
(f)
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1
M+M
12
M
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(h)
(g)
(i)
Figure 8: Overview of fracture mechanics test methods: (a) CT specimen, (b)
DCB specimen loaded by wedge forces, (c) thin fi lm on a substrate
loaded in tension or bending (d), (e) the peel test; (f) DCB sandwich
specimens loaded with pure bending moments, (g) the end lap shear
(ENS), (h) the MMB specimen, and (i) the DCB loaded with uneven
bending moments (DCB-UBM).
on the crack length; for the determination of the energy release rate, the crack length
must thus be measured experimentally and correlated with the applied force.
To determine the Mode II fracture energy of composites (pure tangential crack
opening displacements) the end-notched specimen (ENS) is commonly used [22].
The ENS specimen is loaded by compressive forces; part of the applied force is
transmitted from one beam to the other by contact between the beams. However, this
method does not provide stable crack growth, so the value of the fracture energy may
be overestimated due to crack initiation and due to friction between the crack faces.
A mixed mode bending (MMB) specimen, also loaded by transverse forces, was
proposed by Reeder and Crews [23]. The MMB specimen allows the entire range
of mode mixities, from pure mode I to pure mode II, for the same specimen geom-
etry. The energy release rate depends on the crack length. For Mode II dominated
loading the crack propagation can be unstable. As shown in Fig. 8, another
approach is to load DCB specimens with uneven bending moments (DCB-UBM)
[24, 25]. For the DCB-UBM specimen confi guration, the energy release rate is
independent of crack length and stable crack growth occurs for all mode mixities
making the specimen well suited for measuring the interface fracture behavior of
various materials, including laminates and sandwich structures.
For large-scale bridging problems, such as cracking with cross-over bridging, it
is of relevance to determine cohesive laws which can be used to describe the
mechanical response of the bridging zone and thus represents large-scale bridging
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