* Corresponding author. Tel.: #1-313-593-5119; fax: #1-313-593-
5241.
E-mail address: pkm@umich.edu (P.K. Mallick).
0143-7496/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 1 4 3 - 7 4 9 6 (0 0 ) 0 0 0 4 7 - 6
International Journal of Adhesion & Adhesives 21 (2001) 145}159
Fatigue of hybrid (adhesive/bolted) joints in SRIM composites
Maofeng Fu, P.K. Mallick*
Manufacturing Systems Engineering, University of Michigan-Dearborn, 4901 Evergreen Road, Dearborn, MI48128, USA
Accepted 10 November 2000
Abstract
This paper presents a study on the static and fatigue performance of adhesive/bolted (hybrid) joints in a structural reaction injection
molded composite. It is shown that the hybrid joints have a higher static failure load and longer fatigue life than the adhesive joints.
However, the performance of hybrid joints depends on the washer design. Washers that provide full lateral clamping pressure over the
entire overlap area have a better performance than those that provide partial lateral clamping pressure. Finite element analysis of
adhesive joints shows that the presence of lateral clamping can signi"cantly reduce the maximum peel stress at the adhesive}substrate
interface and thus help in achieving improved joint performance. 2001 Elsevier Science Ltd. All rights reserved.
Keywords: C. Joint design; D. Fatigue; SRIM composites; Clamping area
1. Introduction
Joint design in composite materials has been a concern
and also, the focus of numerous studies in the past. Most
of the publications in this area have dealt with either
adhesive or bolted joints in laminated continuous "ber
composites [1,2]. Studies on hybrid joints combining
adhesive with bolts are very few. Sawa and his coworkers
[3,4] have considered hybrid butt joints in which two
hollow cylinders are fastened by a bolt and a nut with an
initial clamping force after being joined by an adhesive.
The external load was either tensile normal to the adhes-
ive layer [3] or torsional, and, in both cases, they
observed an increase in joint strength relative to the
adhesive joints alone. Recently, with increasing interest
in random "ber composites for automotive applications,
attention is being paid to joint design with sheet molding
compound (SMC) composites and structural reaction
injection molded (SRIM) composites. Mallick and his
co-authors have published several papers on bolted lap
joints [5,6] as well as adhesive lap joints [7}9] in SMC
and SRIM composites. In an attempt to improve the
joint strength of such composites, a hybrid of adhesive
and bolted joints has also been explored. In Ref. [10],
Satish Kumar and Mallick have reported the experi-
mental work on the static behavior of hybrid adhes-
ive/bolted joints (Fig. 1) in polyurethane}SRIM
composites. Static tests were conducted with 25.4-mm
wide/3.53-mm thick SRIM specimens, 6.35-mm diameter
steel bolts and 19-mm diameter round SAE Grade 8 steel
washers. An epoxy adhesive was used for these joints and
the adhesive thickness was 0.762 mm. The bolt was cen-
trally located in the 25.4-mm;25.4-mm overlap area. In
these tests, hybrid joints failed at a higher load than the
bolted joints and with the proper clamping torque
(between 3.39 and 5.08 N m) reached the same failure
load as the adhesive joints. Furthermore, unlike the ad-
hesive joints, hybrid joints failed in two steps, "rst by
initiation of "ber tear (akin to delamination in laminated
continuous "ber composites) at one of the lap ends and
then by tensile failure across the bolt hole. This led to
a slightly higher overall elongation at failure for speci-
mens with the hybrid joints. Later, static tests with 50-
mm wide specimens have shown similar, but perhaps less
dramatic results. In fatigue tests with 50-mm wide speci-
mens, the hybrid joints with 19-mm diameter round SAE
Grade 8 steel washers, on an average, survived longer
fatigue cycles than adhesive joints at high-load levels, but
they did not show any improved results at lower loads
(Fig. 2). Failure in fatigue also started by "ber tear and
when the "ber tear progressed to the bolted area, a com-
bination of half-net-tension failure and splitting (cleavage