SAMPE Journal - July/August 2022 - 12

FEATURE / THREE-DIMENSIONAL STITCH PATHS
INTRODUCTION
Composite materials are now commonly used
in high-performance, light-weight aerospace
structures.
Typical composite laminates
exhibit very good in-plane properties, but their
performance can be limited by their low throughthe-thickness
properties, such as interlaminar
shear and interlaminar tensile strengths. These
reduced through-the-thickness properties make
such laminates susceptible to the formation of
delaminations that are often caused by impacts
that occur during part assembly or service
operations, or by bending loads, or by the presence
of imperfections. As a result, laminated composite
parts
typically
undergo
costly
nondestructive
inspections and evaluations prior to insertion
into service and during service. These inspections
are necessary to evaluate the delamination sizes
and locations to determine if a part needs to
be repaired, scrapped, or replaced. Therefore,
the need to improve the through-the-thickness
properties of composite laminates has led to the
development of various methodologies to increase
these properties.
Through-the-thickness reinforcement methods
have been developed in order to increase the resistance
of laminated composites to delamination
and other forms of impact damage. These methods
are grouped into two main categories, namely matrix
augmentation or architecture augmentation1
.
Matrix augmentation, or toughening, includes
rubber toughening, nanoparticle toughening, and
carbon nanotube treatments. Architectural augmentation
includes through-thickness approaches
such as stitching, tufting, z-pinning, three-dimensional
(3D) weaving/braiding, and needling. Other
architectural augmentation includes approaches
such as the use of a combination of thin and thick
plies in a hybrid laminate2
, however, herein the focus
is on the through-the-thickness reinforcement
of stitching. Stitching typically consists of one or
two threads or yarns that penetrate through the
thickness and connect the through-the-thickness
reinforcement across the surfaces of the laminate.
Stitches are usually through the entire thickness of
the laminate, but blind stitching that penetrates
through most of the laminate thickness and only
connects the through-the-thickness reinforcement
across a single surface is possible. Tufting is similar
to stitching except that it uses a single thread or
yarn that may or may not connect the through-thethickness
reinforcement across one of the laminate
surfaces. Z-pinning consists of inserting precured
pins through the thickness of the laminate prior
to curing. 3D architecture such as 3D weaving
and braiding provides through-the-thickness reinforcement
incorporated into the composite architecture,
and, depending on the architecture, can
remove some or most of the unidirectionality associated
with typical composite constructions. Lastly,
needling is a process whereby barbed needles are
pressed through a preform, thereby pushing fibers
from one layer down through the layer below, and
in the process can cause significant damage to
the preform fibers. Stitching was chosen by NASA
Langley Research Center (LaRC) as a means to address
through-the-thickness strength limitations
and has been extensively studied.
To achieve the goal of reducing the cost of air
travel, NASA has pursued the development of
technologies needed for future low-cost, lightweight
composite structures for commercial
transport
aircraft. Through-the-thickness
reinforcement was selected for study because
the use of traditional
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12 | SAMPE JOURNAL | JULY AUGUST 2022
material systems requires fasteners to suppress
delaminations and to join structural elements,
ultimately leading to fastener pull-through
problems or heavy pad-ups in the fastener regions.
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SAMPE Journal - July/August 2022

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