SAMPE Journal - November/December 2023 - 33

1. INTRODUCTION
Advanced composite materials continue to become
more frequently used to construct large and
complex aerospace structures. Composites create
a stronger, yet lighter and more efficient structure
compared to traditional counterparts. One of the
leading composite manufacturing methods is
Automated Fiber Placement (AFP) which utilizes
the robotic layup of strips of material to additively
manufacture a laminate. Currently, major aircraft
manufacturers, such as Boeing and Airbus, are
utilizing AFP for manufacturing substructures
such as fuselage and wing skins. More recently AFP
is used in the space industry, with companies such
as Firefly Aerospace and Rocket Lab using AFP to
construct composite rockets.
The AFP process consists of a robotic or gantrybased
system coupled with a fiber placement end
effector to additively create a given structure. The
method typically uses strips of pre-impregnated
composite material called tows. These tows are
laid on a tool surface in groups, called courses,
until the desired shape is reached, constituting a
single ply. The placement of material is assisted
by applying heat and compaction pressure to
achieve appropriate tackiness and adherence to
the substrate. This process is repeated until the full
laminate is created completing the green state of
the structure. Due to interactions between the axial
stiffness of fibers within the tows, underlying tool
surface geometry for the parts being created with
AFP, and limitations of the process, a variety of fiber
defects are likely to occur during the placement
of material. The main defects are gaps, overlaps,
wrinkles, puckers, bridging, angle deviation,
steering, folds, twists, wandering tows, loose tows,
missing tows, position errors, and foreign objects.
Harik et al. describes the cause, anticipation,
existence, significance, and progression of these
defects while summarizing some of their effects
on individual plies and the overall laminate1
. The
prediction of these defects exists in the essential
process planning phase of AFP. Process planning
develops optimal inputs and efficient machine
processes based upon the working material,
composite design, and manufacturing resources.
1.1 Literature
Gaps and overlaps are the most common AFP
defects and their effect on the internal geometry
and mechanical properties of a laminate has
been studied at length. Harik et al. attributes the
cause of gaps and overlaps to purposeful steering
during the AFP process and laying up on complex,
doubly curved tool surfaces1
. Sawicki and Minguet
found that laminates containing overlaps and gaps
saw compression strength reductions upwards
of 27% and recommended accounting for this
effect by reducing design allowables2
. Depending
on the size of the defect, out-of-plane waviness
occurs in adjacent plies which promotes damage
initiation and causes the reduction in strength.
It was shown that while any gap size induced a
strength decrease, the rate of this decrease was
more prominent for smaller gap sizes (0.030 " ) and
comparatively consistent for larger gaps. Turoski
conducted an experimental and numerical analysis
on the impact of the quantity of gap defects on
the mechanical properties of a laminate3
. It was
concluded that the number of defects present in
unnotched specimens was related to a decrease
in mechanical performance, with failure strength
reductions of 2%, 3%, and 11% for laminates with
1, 2, and 3 stacked gaps respectively.
Woigk et al. built on these works by
experimentally analyzing the impact of different
configurations of stacked gaps and overlaps4
.
This work verified the general impact of gaps and
overlaps on mechanical properties while also
highlighting some of the complexity of crossdefect
interactions and through-thickness defect
distribution. They concluded that if the defect
Figure 1. Gaps and overlaps specimen showing a) ply book, b) CT scan after consolidation, c) initial geometry for
finite element (FE) analysis, and d) internal ply geometry of the FE model after forming6
.
www. sampe.org
NOVEMBER DECEMBER 2023
|
SAMPE JOURNAL |
33

http://www.sampe.org

SAMPE Journal - November/December 2023

Table of Contents for the Digital Edition of SAMPE Journal - November/December 2023