An abstract of the thesis of

2.1.1 Imperfect Interface 
An analysis for the role of composite interfaces in composite stiffness was 
proposed by Hashin (Hashin 1991). He introduced a set of interfacial stiffness constants 
that quantify the degree of interfacial imperfection. Nairn (2007) has illustrated how to 
identify such interface parameters from experimental data in static loading. Bogren et al 
(2008) used a dynamic mechanical analyzer to measure Young’s modulus and damping 
of wood-fiber reinforced polyactide. They claimed that the mismatch between the 
predicted and experimental values of Young’s modulus may be attributed to imperfect 
interfaces with reduced stress transfer. 
“Imperfect Interface” 
“Perfect 
Interface” 
“Slippage” 
a)
b) 
Figure 2.1. Glue line mechanics for perfect interface and imperfect interface. (Adapted 
from Nairn, 2007) 
Most analyses of stress transfer, from a matrix to a fiber or from glue to wood, 
assume a perfect interface (Nairn 2007, Huang et al 2005). A definition of a perfect 
interface is that there is zero discontinuity along the interface. For an imperfect interface 
there is a slippage along the interface (see Figure 2.1). Figure 2.1 illustrates glue-line 
Real 
Model

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17 
mechanics for perfect and imperfect interfaces and shows a model for the glue line. 
Figure 2.1a shows complications in “real” glue-line because it includes the glue-line 
region and an interfacial zone. Figure 2.1a also shows a schematic drawing of simpler 
model representing the “real” interface as an “imperfect interface”. The model allows 
displacement discontinuities to develop between adherends. The magnitude of the 
discontinuities is assumed to be proportional to the normal and shear stresses at the 
interface. The proportionality constants define the effective normal and shear compliance 
of the interface. The model uses zero compliance for a perfect interface (infinite stiffness 
or zero discontinuity) to infinite compliance for a debonded interface (or zero interfacial 
stress). More detailed discussion on the model is below. 
Although the glue line in wood composites is not a 2D interface but rather a finite 
interphase region where glue may penetrate into the wood cell, an “imperfect interface” 
approach can avoid that complexity. “Imperfect interface” theory seeks to approximate a 
3D interphase to a 2D interface by lumping all mechanical properties of the interphase 
into a small set of interface parameters (Hashin 1990) and all deformation into slippages. 
Hashin’s model assumes the slippage is proportional to the interfacial stress. For shear 
loading (see Figure 2.1), the tangential slippage is:
(Eq. 2.1) 
where [u
t
] is the displacement jump 
along the interface, τ is the 
interfacial shear stress 
and 
D
t
is an interfacial stiffness property. At 
D
t

∞, [u
t
] 

0 and the interface is 
perfect. When 
D
t

0, τ 

0 and the interface is debonded. All other values of 
D
t
characterize an imperfect interface. 
D
t
may also be negative (1/
D
t
< 0) and correspond to 
negative slippage due to an interphase that is stiffer than either adherend (Nairn 2007). 
The goal of this work was to use DLS specimens to load glue-lines in shear and to 
determine 
D
t
for the glue bond.
t
t
D
u
τ
∝
]
[

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18 
2.2 Experimental