SAMPE Journal - September/October 2020 - 7

INTRODUCTION
Adhesively bonded joints have various advantages when compared with bolted
joints. They can significantly save the weight of mechanical fasteners and the
manufacturing cost on account of drilling and fastening. In addition, they can
eliminate stress concentrations around mechanical fasteners and provide efficient
load paths. However, because a fail-safe design is used in aircraft structures,
mechanical fastening is required even on adhesively bonded regions so that the
full load capacity can be sustained even after the bonded interface has fully failed.
At present, hundreds of thousands of bolts are used in the aircraft structure, and
the advantages of bonded structures are not fully utilized. Therefore, new design
features that can limit disbond to the uncritical size are required to achieve
certified bonded aircraft structures1.
Various crack-arrester features have been investigated for metallic
structures2-5. Many design features have also been studied for reinforcing
the interface of composite-metal joints[6-8]. For composite-composite joints,
Nogueira et al.9 evaluated the static and fatigue properties of single lap joints with
spiked metal inserts, termed a redundant high efficiency assembly. Tserpes et al.10
manufactured a corrugated bonded joint and investigated its effectiveness by the
double cantilever beam (DCB) and crack lap shear tests. Löbel et al.11 investigated
the hybrid bondline concept, which is a combination of high stiffness and ductile
adhesives. Tao et al.12 used a patterning strategy to induce adhesive ligament
bridging and prevent crack propagation. Stitching13-16 and z-pinning17-20 are
two of the most actively researched mechanisms. In stitching, the interface
is strengthened by sewing carbon fibers or aramid fibers in the out-of-plane
direction. In z-pinning, the bonded interface is reinforced by driving metal pins
or fiber bundles in the out-of-plane direction using an ultrasonic horn. Although
these methods can significantly improve the damage tolerance of composite
structures, their stiffness and initial failure strength are not satisfactory.
Our research group21-23 proposed a new crack-arrester concept based on
interlocking fibers. The concept is based on suppressing crack propagation via
massive fiber bridging in the adhesive layer. This interlocking-fiber-based crack
arrester can be prepared by introducing prepreg sheets into the adhesive layer and
does not affect the carbon fibers of the adherends. In addition, the crack arrester
itself can carry loads as part of the structure. Therefore, it is possible to minimize
the cost and weight and suppress the decrease in the initial failure strength.
Moreover, using automatic lamination technology, manufacture will be possible
in one step with the adherends. Previous studies have verified the superior
performance of this crack arrester under mode I, mode II, and fatigue loadings.
In particular, for mode-I failure, remarkable performance improvement has been
achieved by massive fiber bridging. In this study, the effect of the interlocking
fiber configuration on mode-I failure is evaluated using DCB tests. Three types
of arrester configurations with various fiber volumes are compared in terms of
the mechanical performance and failure patterns. Finite element analysis (FEA) is
performed to clarify the arresting mechanism of each configuration.
CONCEPT BEHIND INTERLOCKING-FIBER-BASED CRACK ARRESTER
Figure 1 depicts the basic structure of the interlocking-fiber-based crack arrester.
The crack arrester is composed of 0° layers and 90° layers. Carbon fibers are
oriented in the crack propagation direction in the 0° layers and in the orthogonal
direction to crack propagation in the 90° layers. The 90° layers are inserted between
the interlocked 0° layers. When a crack passes through the intersection of the 0°
layers, the 0° layers bridge the crack and suppress the crack opening (Figure 1a).
In addition, the 90° layers prevent the 0° layers from peeling off along the bonded
interface with the adherend (Figure 1a). Figure 2 displays a photograph of the
crack arrester under mode-I loading. It is observed that the 0° layers are bridging at

w w w. s a m p e . o r g

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SAMPE Journal - September/October 2020

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