Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

is the elastic foundation modulus parameter defined as
3
hf bE f
3K
and Ddebpnded and Dintact are the bending stiffness of the debonded and intact parts of the MMB
specimen (Quispitupa et al., 2009):
45
(3.10)
2 B
D
debonded 1 D
A
(3.11)
3 3
E f hf
2 E f hf
Echc
Dintact
hchf
2
6 12
(3.12)
The energy release rate can be expressed as
(3.13)
When the expressions for the compliance and the energy release rate are known, it is possible to
determine the crack length at a given load and deflection and fracture toughness as the crack
grows. However, to fully characterise the face/core interface the mode-mixity must be evaluated
as well. Since there is no analytical expression for the interface mode-mixity, it is usually
determined by use of the finite element method. For static characterisation, the fracture
toughness of the interface can be determined by Equation (3.13) and the mode-mixity is
evaluated using finite element modelling and the CSDE method as explained before. For fatigue
characterisation of the interface, Equation (3.1) is used to determine the crack length from the
compliance of the MMB specimens evaluated from the actual applied load and displacement of
the test specimen. Fracture testing of the MMB specimens was performed at a cross-head rate of
1 mm/min. Figure 3.3. shows typical load vs. displacement curves for representative loading
conditions and specimens. In Figure 3.3 the point where the crack starts to propagate is marked
with an open circle (“”). The load vs. displacement curves are fairly linear up to the point of
crack propagation. It is seen that the load drops due to a change in the specimen stiffness as the
crack propagates. The critical failure load was marked according to the ASTM D6671/D 6671M-
06 recommendation (the load at which the compliance has increased by 5%) and complemented
by visual inspection. These critical failure loads (Pc) were used in the determination of the
face/core interface fracture toughness (Gc), according to the procedure outlined by Quispitupa et
al., (2009).