Rasmus ÿstergaard forside 100%.indd - Solid Mechanics

302 R.C. Østergaard, B.F. Sørensen
joints, the thickness of the adhesive is typically much
smaller than the thickness of the adherents. In the present
study we study delamination of three-layer specimens.
The results are applicable for both sandwich
structures and adhesive joints.
For sandwich structures, debonds (areas between the
face sheet and core with no adhesion) constitute an
important damage type that can lead to debonding crack
growth, i.e. crack growth in the interface between face
sheets and core. Interface cracking is generally mixed
mode cracking, i.e. both normal and shear stresses
develop just ahead of the crack tip (Williams 1959;
Rice 1988). Experiments have shown that the fracture
energy can depend on the mode mixity (Cao and Evans
1989; Wang and Suo 1990; Liechti and Chai 1992).
Consequently, a complete analysis requires the determination
of both the energy release rate and the mode
mixity.
Earlier work on sandwich structure fracture
specimens include Carlsson and Prasad (1993) and
Prasad and Carlsson (1994) who analyzed sandwich
configurations where one face sheet was loaded by a
combined axial/transverse edge load and the other face
sheet was fixed on a rigid plane. The mode mixity was
determined by numerical means for each sandwich and
each load situation. Several other sandwich specimens
also exist. For instance, the three point bend geometry
and the center notch flexure geometry (Ratcliffe and
Cantwell 2001; Cantwell et al. 2000), however these
specimens does not allow for varying the mode mixity
through the loading. Østergaard et al. (2007) carried
out experimental measurements of interface fracture
toughness of sandwich structures and found that the
mode mixity has a significant influence on the fracture
mechanism and the fracture toughness. Suo and
Hutchinson (1989) analyzed sandwich specimens with
core layer thickness much smaller than the face sheet
layers. They found that for moderate stiffness mismatch,
the difference in the mode mixity of the sandwich
and the homogeneous specimen (i.e. without the
thin core layer) was relatively small. However, the difference
increased as the stiffness ratio between the two
layers became large. Fleck et al. (1991) and Akisanya
and Fleck (1992) analyzed a sandwich with a small
core thickness and showed that under some conditions
the crack jumps between the interfaces resulting in a
wavy crack path. This phenomenon was also observed
in experiments.
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The problem that we analyze in the present paper
is interfacial cracking of a symmetric sandwich specimen.
The specimen geometry is fairly general and
can represent a number of problems of great practical
importance, such as adhesive joints and sandwich
structures. Our analysis is performed within the framework
of linear elastic fracture mechanics and comprises
calculation of energy release rate and mode mixity. For
sufficiently long specimens the energy release rate and
the mode mixity attain steady-state values. We calculate
these steady-state values as function of loading,
the thickness and stiffness parameters. Emphasis will
be given to cases where the face sheets are much stiffer
than the core, since this is often the case in sandwich
structures. An analysis will quantify how long the specimen
must be before the mode mixity and energy release
rate attain steady-state values.
The present paper is organized as follows. First, we
will specify the problem. Next we give a short summary
of fracture mechanics for cracks along the interface
between two elastic dissimilar materials. Then,
we analyze the problem shown in Fig. 1, recasting it
as a reduced problem that is solved in terms of energy
release rate, G, and mode mixity, ψ. The solution is
analytical apart from one load-independent parameter,
ω, which is found by numerical means. Hereafter follows
an investigation regarding the influence of length
of the specimen on the validity of the presented equations.
Then follows an application example. Finally,
major results are summarized.
First, the problem is defined. We analyze a unit width
sandwich specimen of length L with and a crack of
length a starting at x1 = −a, x2 = 0(seeFig.1).
The face sheets have the same thickness, H, and the
thickness of the core is denoted h. The sandwich specimen
is loaded along its edges by moments pr. unit
width Mk, k = 1, 2, 3 and by forces pr. unit width Pk,
k = 1, 2, 3. In the first part of the analysis a/H ≫ 1
and L/(2H + h) ≫ 1 so that the crack tip is remote
from the ends of the specimen. Under these conditions,
the energy release rate and mode mixity attain steadystate
values.
2 Mechanics of interface cracks
Eachofthematerialsinthesandwichspecimenaretaken
to be isotropic and linear elastic. Let the elastic