Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
1Low-cost and High-quality Drape Forming Technologyfor Complex Shape Composite Parts

YUSEI KONDO*1KAZUKI NOMA*1

HIROMICHI AKIYAMA*1 MASAYUKI KANEMASU*2

One of the forming methods for complex-shape composite parts such as spars and stringers
of aircraft wings is to form multi-layer prepreg at once (drape forming), which is promising as alow-cost forming method with a short flow time. However, this method has the problem that if thedeformation behavior of the prepreg is improper during forming into a part shape, manufacturing
defects such as fiber wrinkling occurs in shape change areas of the part, causing a decrease in thestrength of the composite part. Therefore, we have developed a low-cost, high-quality drape
forming technology using forming simulation that can predict the deformation behavior ofprepreg.

|1. Introduction
In the field of aircraft, it is required to improve fuel efficiency by reducing the weight ofaircraft. For this reason, composite materials with excellent specific strength and rigidity have been
increasingly applied to aircraft parts and such materials account for about 50% of the aircraft
weight in the case of the Boeing 787 and Airbus A350. Methods used for forming composite
materials into aircraft structural parts include a hand lay-up method, a method of using AFP(Automated Fiber Placement), both methods laminate and form one ply of prepreg at a time, and aforming method of multi-layer prepreg laminated on a flat plate at once (drape forming).
Figure 1 shows the flow of drape forming. Drape forming is promising as a low-cost
forming method because its flow time is short due to forming multi-layer prepreg at once.
However, this method is likely to cause wrinkling during forming into complex shapes such as
joggles. Since wrinkling of composite parts reduces the structural strength, reducing wrinkling is an
important issue.

Figure 1

Flow of drape forming

In drape forming, prepreg laminated in advance is placed on a mold, heated to reduce its resin viscosity andformed into the mold shape using vacuum, etc. When the mold has shape change areas, wrinkling is likely tooccur.

In order to solve this issue, we have been promoting the development of a new drape forming
technology using forming simulation with the aim of elucidating the mechanism of wrinkling
generation during forming and suppressing wrinkling(1). This report presents the results of*1

Manufacturing Technology Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Research Manager, Manufacturing Technology Research Department, Research & Innovation Center, Mitsubishi HeavyIndustries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
2

optimizing the prepreg forming process of C-shaped parts with joggles and verifying the wrinkling
suppression effect.

|2. Forming simulation of composite material
For the forming simulation, simulation software PAM-FORM (produced by ESI Group) ofcomposite material was used(2). This software enables simulation that considers the material
anisotropy and temperature/speed dependence of prepreg and can predict the wrinkling generation
process from the deformation behavior of prepreg during forming, so it is useful for optimizing
forming conditions and the mold shape.
Figure 2 illustrates the outline of forming simulation. First, input the mechanical properties
such as the tensile, bending, shearing strengths, etc., of the prepreg for each fiber orientation, thefriction coefficient/tackiness between the layers, etc. and then set the forming conditions such astemperature, pressure, speed, etc. As a result of this simulation, the deformation behavior, fiberorientation, stress, strain, etc., of the prepreg during forming can be obtained. By analyzing thesecharacteristics, the wrinkling generation process can be understood in detail.

Figure 2

Outline of forming simulation

This figure shows the outline of forming simulation. When inputting the material properties and forming
conditions and performing calculation, the resulting formed shape is output. By analyzing the deformation
behavior during forming, the wrinkling generation process can be understood.

|3. Verification of simulation accuracy
Figure 3 depicts mold shape and the simulation model. We verified a method of laminating
prepreg on a mold, sealing the entirety with a flexible rubber sheet and vacuuming the inside of therubber sheet to form the prepreg into the mold shape (vacuum bag drape forming method). The
upper surface of the C-shaped cross section of the formed shape is called the web and the sidesurface the flange. As shown in Figure 3, in this verification, we formed a symmetrical model by
8-ply laminating unidirectional carbon fiber/epoxy prepreg (size: 500 mm x 230 mm) in a region of
the mold containing one joggle. We also acquired the mechanical properties (tensile, bending andshear strength) of the prepreg for each fiber orientation and the friction coefficient between theprepreg layers at forming temperature and input them to the simulation. The mesh size of theprepreg was set to be finer in the order of at the corner R where the deformation was remarkable,on the flange and on the web as shown in Figure 4. The standoff block, which held the rubber sheet
during vacuum and applied tension to the prepreg, was placed parallel to the mold. Finally, a rubber
sheet was used to cover the entire surface and the same pressure as the vacuum pressure wasapplied in the direction perpendicular to the rubber sheet surface to simulate vacuum. The forming
temperature was set to a temperature at which the flexural rigidity of the prepreg was reduced andsufficient slipperiness could be ensured.
Figure 5 gives the results of the simulation and the test. The simulation results indicate thedistance from the surface of the mold to the outermost surface of the prepreg and the test results
indicate the same measurement made with a 3D scanner. The red areas indicate that the prepreg
deviates from the mold shape and wrinkling is generated. By comparing these results, it can be seenthat the positions of the wrinkling generated at the corner R and the flange near the joggle almost
agreed with each other. The simulation was also able to predict the height of each wrinkle to withinMitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
3

a margin of error of ±10%.
Based on the above results, this simulation method is determined to have a sufficientaccuracy to simulate actual phenomena.

Figure 3

Forming simulation model

Forming simulation model for vacuum bag drape forming.

Figure 4

Mesh size of prepreg

The mesh size of prepreg was set according to mold shape and quality assessment priority.

Figure 5Comparison of simulation and test results

The upper shows the forming simulation results and the lower the test results of the 3D scan image. Bothindicate the distance from the mold surface to the outermost surface of the prepreg.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
4|4. Development of wrinkling suppression forming process
4.1

Wrinkling generation mechanism

Based on the simulation results described in Chapter 3, the wrinkling generation mechanism
was clarified. Figure 6 is a cross-sectional view of the position where the wrinkling at the corner R
is maximized. The prepreg during forming easily deformed in the out-of-plane direction and cameinto contact with the mold at the flange before the prepreg at the corner R was completely formed.
This is because the rigid carbon fiber is difficult to fit into the corner R shape of the joggle, while
the prepreg at the flange is easily deformed by the pressure from the rubber sheet due to vacuum.
When the prepreg at the flange comes into contact with the mold first, a clearance is createdbetween the prepreg and the corner R of the mold. It is clarified that since the prepreg at the flange
is restrained by the mold due to the subsequent vacuum, fiber is left behind at the corner R andremains as wrinkling when the forming is completed.
In addition, as shown in Figure 7, in the prepreg at the flange, the compression force isgenerated in the red line direction due to the joggle shape, and the prepreg buckles out in theout-of-plane direction. It is considered that remaining fiber generated at the flange is concentrated
in the center direction of the compression force by the progress of vacuum, resulting in wrinkling.

Figure 6

Wrinkling generation process at the corner R

The cross-sectional view shows the cutting position in the above figure viewed from Arrow A. Since theprepreg at the flange comes into contact with the mold first, remaining fiber was left at the corner R.

Figure 7

Wrinkling generation process at the flange

The prepreg at the flange deforms in the out-of-plane direction due to the compressive force due tothe joggle shape, resulting in wrinkling generation.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
5

4.2

Optimization of forming process

The results described in 4.1 indicate that in order to suppress wrinkling it is necessary to
suppress out-of-plane deformation of the prepreg at the corner R during forming and to optimize
the forming process so that the prepreg at the corner R is formed first and then the prepreg at theflange is formed. Therefore, in order to restrain the out-of-plane deformation of the prepreg, wedevised a method of forming with the prepreg attached with an out-of-plane deformation control
plate. This method can be realized simply by adding the procedures of manufacturing, attaching
and removing the out-of-plane deformation control plate to the general vacuum bag drape forming
method, and the wrinkling suppression effect can be expected without impairing the inherent
low-cost advantage of a drape forming method.
Figure 8 is the simulation model. The out-of-plane deformation control plate was divided
into three parts according to the joggle shape and attached to the prepreg at the flange.

Figure 8Application of out-of-plane deformation control plate

This figure shows a simulation model to which the out-of-plane deformation control
plate is applied. The out-of-plane deformation control plate was divided into threeparts according to the joggle shape and fixed to the prepreg.

Figure 9 gives the simulation results. The simulation indicates that the wrinkling height ofthe corner R is reduced by 30% or more by applying the out-of-plane deformation control plate andthat the wrinkling of the flange disappears due to the suppression of the out-of-plane deformationof the flange. The cross-sectional view shown in Figure 10 indicates that the prepreg is formed in
the ideal order from the corner R to the flange. It is considered that remaining fiber at the corner R
moves to the flange and the wrinkling at the corner R is reduced.
From the results described above, it was clarified that the developed method is effective forsuppressing wrinkling, and we obtained the prospect of realizing a low-cost, high-quality drape
forming technique for complex-shaped composite parts at the completion of the forming.

Figure 9

Simulation results

Applying the out-of-plane deformation control plate reduces the wrinkling at the corner R by 31% andeliminates the wrinkling at the flange.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
6

Figure 10 Suppression of wrinkling at the corner R due to out-of-plane deformation controlplate
The cross-sectional view shows the cutting position in the above figure viewed from Arrow A. Since theprepreg at the corner R is formed first and then the prepreg at the flange is formed, the wrinkling at the corner
R is reduced.

|5. Conclusion
This report presented an elucidation of the wrinkling generation mechanism during drape
forming of complex-shaped composite parts using forming simulation, and indicated that awrinkling suppression effect was obtained by the forming process using an out-of-plane
deformation control plate. As shown in Figure 11, by incorporating this simulation into thedevelopment flow, repeated forming tests using trial and error will be unnecessary, and test costs
can be significantly reduced. Furthermore, since the simulation can be used to optimize the moldshape, it is expected that repeated forming tests and mold manufacturing will become unnecessary.
Moving forward, in order to make this forming process effective even for thicker and longer
parts, we will optimize the shape, properties, application position, etc., of the out-of-plane
deformation control plate to further improve the wrinkling suppression effect.

Figure 11Development flow before and after applying the forming simulation

Applying the forming simulation to the development flow contributes to the elimination of prototyping andreduces the test cost significantly.

References(1)(2)

Kondo, Y., et al., Development of Drape Forming Process for Composite Structure Using Forming
Simulation, SE Conference 20 Amsterdam, (2020)
ESI Group, https://jp.esi-group.com/