SAMPE Journal - January/February 2017 - 7

Feature Article
a

S. E. Mouringa, L. A. Loucab, and R. Bramblebyb
Department of Naval Architecture and Ocean Engineering, United States Naval Academy, Annapolis, MD
b
Department of Civil & Environmental Engineering, Imperial College London, UK

Experimental and Numerical Study of Hybrid
Steel-to-Fiber Reinforced Polymer
Joints Under Tensile Loading
Abstract

Hybrid metal-to-fiber reinforced polymer (FRP) joints are being used more commonly for load bearing applications. However,
these hybrid joints usually entail geometry and material discontinuities which can induce stiffness mismatch and cause local
stress concentrations. The shock impedance mismatch caused by the different wave propagation characteristics can also be
crucial to the structural response of the hybrid joints under impulsive loads due to sources such as an air blast or underwater
explosion (UNDEX). Recent research at Imperial College London (ICL) and the U.S. Naval Academy (USNA) has focused on
characterizing the behaviour and ultimate load capacity of metal-to-composite hybrid joints with different configurations under
various loading conditions. This paper presents results from tensile strength testing of steel-to-vinyl ester GRP double lap joints,
comparing pseudo-static strength with dynamic strength and comparing joints that exploit perforated steel plates with those
manufactured with non-perforated steel plates. An intentional manufacturing flaw also was incorporated into half of the joints,
both perforated and non-perforated joints, in order to assess the effect of this flaw type on joint strength. These experimental
results are compared to Finite Element Analysis (FEA) results for both perforated and non-perforated joints.

Introduction

FRP materials such as glass
reinforced polymers (GRPs) are
being more widely used as primary
structural members due to the high
demand of advanced naval vessels
with enhanced performance in terms
of payload, range, stability, corrosion
resistance,
damage
tolerance,
stealth, and at the same time, a
reduction in lifetime costs. Such
materials have demonstrated that
their material properties fulfil the
higher performance requirements
of these naval applications. A
comprehensive review of the
potential use of composite materials
for future vessels has been provided
by Mouritz et al1. Practice shows
that vessels made of FRP can have a
greater survivability; however, FRP
materials are not typically applied
in isolation due to their insufficient
stiffness and ductility compared
to metallic materials. This has led
quickly to growing interests in metal
and composite combined structures.
However, the susceptibility to failure
at the joints in composite structures
when subjected to high rate loading
from an impulsive load source is one
weakness often observed. Thus, one

of the major structural challenges
appears to be the design of metalto-composite hybrid joints. Hybrid
joints usually entail geometry and
material
discontinuities
which
can induce stiffness mismatch and
cause local stress concentrations.
In addition, the shock impedance
mismatch in hybrid joints caused
by the different wave propagation
characteristics can be critical to the
structural response under impulsive
loads from air blasts or UNDEX.
Recent research at ICL and USNA
has focused on characterizing the
behaviour and ultimate load capacity
of metal-to-composite hybrid joints
fabricated with different joining
methods under various loading
conditions. Initial tensile test results
of steel-to-vinyl ester GRP double
lap joints are presented in this paper,
comparing joints with perforated
steel plates with those manufactured
with non-perforated steel plates and
comparing pseudo-static strength
with dynamic strength.

SAMPE Journal, Volume 53, No. 1, January/February 2017

Background

Unden2 was the first to consider
applying perforations to the metal
part of hybrid joints. The original
purpose of the perforations was
to increase the cohesion between
metal plates and adjacent layers of
polymeric material, hence improving
the transfer of load between the steel
and FRP parts. Also the perforated
steel plate was believed to decrease
the elastic mismatch between the
stiff steel part and the relatively
compliant FRP part. This design was
further investigated and improved by
Melograna and Grenestedt3 and Cao
and Grenestedt4. They found that the
joint had the highest strength when
it was comprised of seven to nine
rows of circular holes (see Figure 1).
The best performing geometry that
they tested yielded 25-30% higher
tension strength than their nonperforated counterparts. This agrees
with Hart-Smith's5 conclusion that
the capacity of hybrid joints can be
increased when the joint is detailed
such that stiffness imbalance is
reduced. It should be noted that
most of the recent work has been
focused on perforated joints tested
under pseudo static loads.
7

Table of Contents for the Digital Edition of SAMPE Journal - January/February 2017