International Journal of Adhesion & Adhesives 29 (2009) 45–55
Crack-growth behavior of epoxy adhesives modiﬁed with liquid rubber
and cross-linked rubber particles under mode I loading
, Satoshi Motohashi
, Kazuaki Nishi
, Masaki Kimoto
Department of Technology Education, Osaka University of Education, Kashiwara, Osaka 582-8582, Japan
Department of Applied Chemistry, Osaka Institute of Technology, Asahi, Osaka 535-8582, Japan
Division of Material Technology, Technical Research Institute of Osaka Prefecture, Izumi, Osaka 594-1157, Japan
Accepted 5 November 2007
Available online 4 February 2008
The crack-growth resistance (R-curve) of bulk single-edge notch bend (SENB) and adhesively bonded double cantilever beam (DCB)
specimens was investigated under mode I loading conditions using two types of rubber-modiﬁed epoxy adhesive: one was a liquid rubber
(CTBN)-modiﬁed adhesive and the other was a cross-linked rubber particle (DCS)-modiﬁed adhesive. As a result, for both the SENB
and DCB specimens, the gradient of the R-curve for the DCS-modiﬁed adhesive was steeper than that for the CTBN-modiﬁed one,
however, the difference in fracture toughness between DCS- and CTBN-modiﬁed adhesives is smaller for DCB than for SENB
specimens. To elucidate such behavior, crack-growth simulation based on Gurson’s model was conducted, where the DCS- and CTBN-
modiﬁed adhesives were characterized by both the initial void fraction and nucleation. The difference in the behavior of R-curves was
also observed in simulations. Moreover, it was found that the difference in fracture surface roughness observed by SEM for both
adhesives correspond to the variation in R-curves.
r 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Epoxy; M. Fracture mechanics; M. Double cantilever beam specimen
Rubber-modiﬁed epoxy adhesives have attracted special
interest due to their property of satisfying static strength
and toughness simultaneously. Numerous studies have
been conducted on the fracture toughness of several
types of adhesive joints bonded by rubber-modiﬁed
There are two types of rubber-modiﬁed epoxy resin. One
is a liquid rubber-modiﬁed epoxy resin. The rubber
particles are precipitated by phase separation from the
homogeneous phase, and are well dispersed in the resin.
The other type has cross-linked or core–shell rubber
particles in the epoxy resin. As the rubber particles are
mixed with epoxy resin, aggregation of the particles occurs,
and the dispersion state is inferior to that of the
precipitation type. Recently, it was reported that the
toughening mechanisms for liquid rubber-modiﬁed epoxies
differ from those for core–shell rubber particle-modiﬁed
ones [4–8]. Some papers have reported that the fracture
toughness of core–shell rubber particle-modiﬁed epoxies is
higher than that of liquid rubber-modiﬁed ones [7,8].
However, there have been few studies on the fracture
toughness of adhesively bonded joints with cross-linked
rubber particle or core–shell rubber particle-modiﬁed
epoxy resins .
Fracture toughness of ductile materials is, in general,
characterized by the crack-growth resistance (R-curve). In
most studies on rubber-modiﬁed epoxies, however, crack
propagation occurs when the energy release rate reaches a
critical value. There are few studies on R-curve behavior in
rubber-modiﬁed epoxy resins. Most recently, the R-curve
behavior of liquid rubber-modiﬁed epoxy resin has been
studied using bulk double cantilever beam (DCB) speci-
mens  and adhesively bonded DCB specimens [11,12].
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Corresponding author. Tel./fax: +81 72 978 3444.
E-mail address: email@example.com (M. Imanaka).