ADDITIVE MANUFACTURING HOT BONDED INSERTS IN SANDWICH STRUCTURES

Marta García-Cosío (1)

, Juan Miguel González (1)

, Jeroen Vermeulen(1)

, Christian Rossmann (2)

, Jannis Kranz (2)

(1)

Atos Spain S.A., C/ Albarracín, 25 28037 Madrid Spain, +34 91 2064151,

marta.garcia-cosio@atos.net, juan.gonzalezh@atos.net, jeroen.vermeulen2@atos.net (2)

Materialise NV, Technologielaan 15 3001 Leuven Belgium, +32 16660406,

christian.Rossmann@materialise.be, jannis.Kranz@materialise.de

ABSTRACT

Hot bonded inserts in sandwich configurations are widely applied in spacecraft structures. These

inserts are co-cured with the composite structure sandwich panels and are commonly used for

highly loaded interfaces. The large capability to transmit the load is directly related to the bonded

area which connects the insert and the skins of the sandwich structure.

The inserts are commonly made of aluminium alloy since the mass of this interface solution is an

important design parameter. On the other hand, the thermo-elastic stresses which occur during the

curing process can affect the insert´s strength capability. That is why for Carbon Fibre Reinforced

Plastic (CFRP) skin sandwiches, titanium hot bonded inserts are chosen to reduce this negative

thermo-elastic effect.

Additive Manufacturing (AM) provides the opportunity to improve this kind of structural elements

in three main areas: mass, thermo-elastic behaviour and bonding capability. AM opens a new way

of thinking and defining design solutions; facing new material properties directly dependent of the

manufacturing process, topology optimization of the part as the main design driver and different

manufacturing limitations depending on the corresponding technology.

The result of the design optimization can be introduced into part manufacturing with relatively few

manufacturing limitations. In this case, topology optimization is used to reduce weight in

combination with maintaining the strength of the part. Different lattice structures are also studied to

improve the design.

This study shows a complete design loop with the subsequent additive manufacturing of an

optimized aerospace part, combining the optimization and FEM analysis with the manufacturing

requirements inherent to the relatively new fabrication method.

1. INTRODUCTION

1.1 Background

Hot bonded inserts in sandwich spacecraft structures are currently limited in their geometrical shape

mainly because of traditional manufacturing procedures. Aluminium or titanium alloy inserts

generally have block-shapes as they are manufactured by machining. They are usually completely

solid, which increases the mass and has a higher stiffness than strictly necessary. The bonding area

is one of the main design-drivers of the insert, since a larger bonding surface will increase the load

transfer capability.

1.2 Study definition

A traditional hot bonded insert, located at the edge of the satellite panel is represented in Fig. 1. It is

surrounded by the aluminium core and at the top and bottom sides by the CFRP skin. The

connection between the insert and the core is established by bonding foam. The skin is connected to

the insert and the core by means of an adhesive layer. The load enters the insert via the interface

bots and is transferred to the sandwich panel through the bonded area. The adhesive connection

directly determines the insert size due to the minimum adhesive surface necessary to withstand the

loads and moments.

Fig. 1 Representation of a traditional insert configuration in a sandwich panel

1.3 Traditional method for structural analysis

The current hot bonded insert preliminary design is traditionally determined by an analytical

calculation. In a first approach, the inserts are designed to withstand the failure of the skin, the core

and the adhesive, neglecting any other failure source (such as the insert itself).

In-plane loads are the main cause of skin de-bonding, which is usually the predominant failure

mode and the first being analysed. Together with in-plane forces and moments, drilling moments

are also considered. Bending moments (in-plane) are relevant as they are reacting on both faces of

the insert as a shear load. The bonding surface is determined (neglecting adhesive peeling failure),

resulting in the overall size of the insert.

Based on the previous dimensions derived from the bonding analysis, skin and core rupture are

analysed. In general, out of plane loads are resisted by the

core and in-plane loads are absorbed by the skins. Bending

moments and pull out forces applied on the insert edge can

result in the necessity of core reinforcements or an overall

size increase of the insert.

The analyses of the skin take into account both in-plane

moments and forces. Even when these forces and moments

could be absorbed by the skin as shear stresses, this

contribution is considered as marginal and only failure due

to tension and compression of the skin is taken into account.

In plane forces transferred through the adhesive to the skin

also induce a tension-compression state and the maximum stress in the skin extends over the insert

edge, see Fig. 2, where α is the opening angle of the shear stress contribution (typically 20 degrees).

This analysis is meant to verify the skin integrity or to define the need for redesign by means of skin

reinforcements around the hot bonded insert or a change in the overall dimensions of the insert.

The described traditional methodology neglects several failure modes (of which experience shows

as not critical) and it is limited to a typical insert configuration. New complex designs will require

new analysis methods in order to assure the robustness of the product.

2. REQUIREMENTS

The objective of this study is to design and manufacture a new insert with three improved

characteristics: less weight, better thermo-elastic behaviour and an improved bonding capability.

The design will be focussed on metal AM, which offers a high geometrical freedom. In contrast to

many other manufacturing processes the part costs are primarily depending on the printed volume

and not on the part complexity. The new design will pursue the reduction of the thermo-elastic

deformations. In parallel, lattice structures, which can be manufactured by additive techniques, can

contribute to improve the insert weight and the part stability during the manufacturing process.

The loads and moments applied in this study are derived from a real-life insert application in the

hoisting points of a heavy satellite. During the study, fatigue is not taken into account since the

Fig. 2 Pull-out forces stress

distribution in the skin

insert is loaded only during Assembly, Integration and Test (AIT) operations which will take place

just a very limited amount of times [1]. The two most critical load cases are selected for the

mechanical analysis, which cover the entire load envelope (see Table 1).

Table 1. Load cases used in the mechanical analysis Load Case Fz (N) Fy (N) Fx (N) Mx (Nm) My (Nm) Mz (Nm)

LC-1 5112 52697 128095 1327 0 734

LC-2 1889 37774 85488 3386 0 557

Due to different thermal expansion coefficients of the materials, during the curing process, stresses

are generated between the parts and could affect the load capability. The curing process of the insert

in the panel is simulated by a thermo-elastic analysis applying a uniform temperature decrease from

120ºC to 20ºC. The scope of this analysis is to determine the relative improvement with respect to

the original design, not to obtain quantitative data about the stresses during the curing process.

The applied material properties are shown in Table 2.

Table 2. Material properties of the parts involved in the analysis Part Material E (GPa) G (GPa) Υ α (1/ºC) at 20ºC ρ (kg/m

3)

Insert Titanium 110.5 42.5 0.31 8.78E-06 4410

Core Aluminium 0.6 0.375 0.29 2.38E-05 91.3

Skin CFRP 110.43 30.28 0.247 -1.01E-07 1650

The detailed Abaqus FEM models used during the virtual validation process contain the full ply

representation of the CFRP skin and an incorporated failure mode for the adhesive material.

3. DESIGN

3.1 Topology optimization

Based on the original insert design (square box shape,Fig. 1), a topological optimization has been

performed as per [2] and [3]. The insert design space during the optimization is based on the

original volume to maintain the bonding surfaces. The optimization can be divided in two main

strategies; one is to establish the maximum part stiffness with a corresponding user-defined volume

and the other is a volume reduction respecting the minimum user-defined displacement. The

optimization objectives and design constraints are shown in Table 3.

Table 3. Optimization strategy based on the square-box insert Model version V01 V02 V03 V04 V05 V06

Objective Min.

compliance

Min.

compliance

Min.

compliance Min. volume Min. volume Min. volume

Design constraint 20% of

volume

30% of

volume

40% of

volume

Max.

displacement

at ref. point

0.3mm

Max.

displacement

at ref. point

0.4mm

Max.

displacement

at ref. point

0.5mm

The topological optimization, which is based on the loads envelope, will determine the load paths

and in this way the design can be adjusted to obtain a lightweight structure fulfilling the structural

requirements. The following graphs show the results of the topology optimization loops.

Fig. 3 Results of topology optimization

Fig. 4 Results of topology optimization

This kind of graphs usually shows a typical distribution; some design solutions have a significantly

reduced mass but their reduced stiffness will result in structural failure (results in the upper left area

of Fig. 4). The design options indicated by the red oval in Fig. 3 and Fig. 4 respect the minimum

structural requirements in combination with a significant mass reduction. Within these most

interesting options, design version V02 is considered the optimum topology optimized insert design

showing a theoretical mass reduction of 70% and respecting the structural requirements.

Fig. 5 Topology optimized insert design

Fig. 6 Topology optimized insert design

The topology optimized design is shown in Fig. 5. Fig. 6 shows the conversion of the optimization

result (light blue) into a parametrised CATIA model (orange). The material distribution respects an

improved load path within the insert. Mass is concentrated around the mounting holes and the upper

and lower bonding areas, since load intends to pass through the insert to the skin via the shortest

distance. Although the remaining area of the insert is necessary to respect the required bonding

area, it can be completely hollow to reach an optimum mass reduction.

3.2 Shape optimization

The topology optimization is focussed on the internal volume of the insert. The subsequent shape

optimization is performed to improve the stress distribution in the joint with the sandwich panel.

The shapes have been selected based on the stress concentration areas indicated by the FEM

analyses and focus on the reduction of the critical areas with high stresses.

Five different insert shapes have been analysed in detail;

Fig. 7 Original; the original insert design.

Fig. 8 V02-OPT01, the optimum topology optimized design.

Fig. 9 Circle V05-02; an angular solid shape.

Fig. 10 Gothic V06-02; an additional circle on top of the angular solid shape to reduce

rotation.

Fig. 11 Butterfly V04-05; a double angular solid shape.

Fig. 12 Butterfly V04-03; a double angular hollow shape.

Fig. 7 Original

Fig. 8 V02-OPT01

Fig. 9 Circle V05-02

Fig. 10 Gothic V06-02

Fig. 11 Butterfly V04-05

Fig. 12 Butterfly V04-03

Each insert shape design option is analysed within the complete assembly by means of a detailed

Abaqus FEM model. The curing process of the insert within the assembly is simulated by a thermal

analysis applying a uniform temperature decrease from 120ºC to 20ºC. Fig. 13 contains the micro-

strain levels which occur in the CFRP skin, which clearly shows the improvements obtained in each

design option.

Fig. 13 Elastic strain level in skin (thermal analysis)

Besides the thermal behaviour also the structural requirements are analysed applying the before

mentioned load cases. It can be concluded that design option “Butterfly V04-03” is the optimum

shape optimization solution based on its thermal and structural behaviour. The butterfly shaped

design results in a significant improvement in the micro-strain level in the skin, which is a good

indicator of the effectiveness of the design.

3.3 Design and Manufacturability

Once the topology and shape optimization process is finished, the final insert design needs to be

adapted according to mechanical requirements which are not covered in the optimization process.

For the AM process adapted design as shown in Fig. 14, additional changes in the design are

implemented since the process restrictions differ from conventional manufacturing processes due to

the layer-wise build-up of the parts. A part design adapted to the production process is a necessity in

order to assure a good manufacturability [4].

One of the biggest differences between AM and conventional manufacturing processes is the need

for support structures. The goal for the design of the insert was to keep the post machining efforts at

a low level in order to minimize manufacturing costs and assure a high repeatability in the

manufacturing process outcome. The defined build up strategy is shown in Fig. 15 and Fig. 16.

Fig. 14 AM-prepared and optimized

design

Fig. 15 Build orientation

Fig. 16 Build

orientation

An avoidance of supports in the inner structure is achieved by using a triangular shape of the upper

part section that considers the critical overhang angle of 45° to the build plate, as shown in Fig. 17

and Fig. 18. The down facing area as well as the bonded side walls are modelled with an offset in

order to assure later post machining according to the part demands on accuracy. These three

surfaces and the threads in the bore are the only interfaces that need to be post machined.

Fig. 17 Critical

overhang

Fig. 18 Internal support to

prevent overhang problems

Fig. 19 Powder removal holes

Powder removal is achieved by holes in the lower surface that assure the accessibility to the part´s

hollow section when built up and still connected to the building platform, as shown in Fig. 19.

In order to reduce the possibility of a part failure or the appearance of cracks due to thermally

induced stresses in the manufacturing process, the part is moreover designed in such a way that

sharp corners are mostly avoided. The final design for manufacturing will be referred to as INSERT

1 and its weight data are shown in Table 4. The mass reduction with respect to the original insert

design amounts 62% taking into account all manufacturing related constraints, which only adds 45

grams to the design without manufacturing constraints.

Table 4 Weight comparison

Original

Design without manufacturing

constraints

Design with manufacturing

constraints

Weight (gr) 1464 517 562

Weight Reduction (gr) -- 947 902

Percentage of reduction -- 65% 62%

4. LATTICE STRUCTURE DESIGN

Based on the manufactured INSERT 1, an additional design loop is performed to improve the

manufacturing process and cost. The goal of this phase is to reduce the post-process machining

operation of the bonding area. Lattice structures are used to stabilize the surfaces during the

manufacturing process, with an additional improvement in thermo-elastic distortion of the outer

surfaces.

4.1 Lattice structure design

In this new design, the insert outer shape is maintained; the external shape corresponds to the

butterfly geometry obtained during the shape optimization.

The lattice structures refer to a specific type of meso-structures that consists of small beam

elements. The versatility of AM allows the fabrication of these complex unit cell lattice structures

which can be used as “basic design blocks” to generate macro-scale geometries [5].

The design concept is based on the hollow butterfly shaped insert, with walls at the adhesive

surfaces as thin as possible, and with material around the screw threads. The insert´s interior

volume is filled with a uniform cellular lattice structure.

The lattice cell selection is based on the manufacturing method Selective Laser Melting [6] [7].

From the three basic cellular designs shown in Fig. 20 to Fig. 22, the X-shaped cell design 1 (Fig.

20) is selected since it provides the optimum combination of structural behaviour and mass

reduction.

Fig. 20 Cell design 1

Fig. 21 Cell design 2

Fig. 22 Cell design 3

Based on the chosen X-shaped cell, the adequate cell size and the bar diameter need to be

determined [8]. These parameters are obtained by FEM analyses changing the cell size in

combination with different bar diameters and applying the mechanical load cases. Apart from the

theoretical analysis, practical manufacturing requirements and powder removal also need to be

respected. The lattice structure needs to be able to support the mechanical stresses once assembled

and the thermo-elastic stresses during the curing process. The mechanical stresses are used to

dimension the cell. The results of the mechanical analysis stresses can be seen in Fig. 23.

Fig. 23 Stress level lattice structure

Fig. 24 Detailed lattice structure (CAD)

The further away from this bolt area, the lower the stress level is in the lattice bars. Taking into

account this stress distribution, the lattice bars are provided with a variable diameter to assure the

structural integrity. Thus a variable bar diameter between 2.3 mm in the area of the bolt holes and

1.0 mm at the rear is applied, as can also be seen in Fig. 24.

Fig. 25 shows the variation of weight in each lattice configuration versus the maximum

displacement of the insert in the assembly. Fig. 26 shows the maximum bar stresses versus the

lattice weight. It indicates that it is more effective to increase the bar diameter than to increase the

cell dimensions. The most effective combination is a lattice cell with size 10x8x8 mm, as indicated

by the dashed green oval in Fig. 25 and Fig. 26.

Fig. 25 Relation lattice weight - displacement

Fig. 26 Relation lattice weight – stress level

In the FEM analysis the lattice structure is represented by bar elements. As a direct consequence,

the possible stress peaks at the connection nodes of the bars are not taken into account. The bar

connection nodes contain the highest stress levels, as referred in [9]. To determine these critical

stresses, a detailed non-linear FEM analysis is performed to define the adequate fillet radius to

respect the yield strength of the lattice material Ti6Al4V. Furthermore the weight influence needs to

be taken into account, since an increased bar diameter results in additional mass.

The study is focussed on the most critical lattice connection node. This node is modelled in detail

and the loads from the general FEM model are applied. The range of the connection node radius is

between 0.0 mm and 1.2 mm. The results of all the analysed fillet radii are shown in Fig. 27. In the

final lattice design a fillet radius of 0.6 mm is applied since it shows allowable peak stress values in

combination with an acceptable corresponding weight. The typical Von Mises stress distribution in

the connection node with a fillet radius of 0.6 mm can be seen in Fig. 28.

Fig. 27 σ in connection node as function of Rfillet

Fig. 28 Detailed FEM model of lattice node

The final design for INSERT 2 is shown in Fig. 29 and Fig. 30. It has an external shape equal to

INSERT 1 as mentioned before, but with thinner surrounding walls and an internal X-shape graded

cellular structure with a cell size of 11.3x7.5x8.5mm. Fig. 31 shows the detail of the smooth lattice

integration into the outer insert volume.

Fig. 29 Final design INSERT 2

Fig. 30 Final design INSERT 2

Fig. 31 Detail design

INSERT 2

4.2 Design and manufacturability

IINSERT 2 shares the same basic manufacturing orientation

and strategy as INSERT 1. The down facing interface which

has the four bolt holes is orientated downwards towards the

build plate. The up-facing structure segments are designed

in such a way that no support structures are necessary. All

surfaces show an orientation of 45° with the build plate and

do not need any support. The wall thickness was set to the

minimum recommended value for manufacturing in order to

assure a stable part build up. In contrast to INSERT 1, the

internal volume of INSERT 2 is filled with a lattice

structure. Fig. 32 shows the lattice structure as a printed trial

version.

An easy powder removal after the building process is

guaranteed by top and down facing bores that allow access

to the inner volume of the insert.

Table 5 shows the weight comparison between INSERT 1

and INSERT 2.

Table 5 Weight comparison

Weight comparison* INSERT 1 INSERT 2

Weight (gr) 562 508

Weight Reduction (gr) -- 54

Percentage of reduction -- 10%

*Reference weight of original insert amounts 1464 gr.

Fig. 32 Manufactured lattice structure

5. PART MANUFACTURING

5.1 Manufacturing process

Fig. 33 describes the process chain for the insert’s manufacturing by metal AM. The manufacturing

process strongly depends on a digital part model. During the part data creation phase a STL-file

(standard triangulation language) is generated on the basis of the parametric CAD model. The STL-

file format describes the part surfaces by a tessellation of its surface. During the manufacturing pre-

process phase the part is orientated with respect to the build plate.

Based on the part orientation, the support structures are defined. The part and the corresponding

manufacturing support structures are sliced for the manufacturing process according to the material

and the machine dependent layer thickness. The data containing the layer information of the part

and the support structures is then transferred to the manufacturing machine. Typically, the process

parameter definition is then defined on the machine before the actual manufacturing process starts.

Fig. 33 Process chain for metal AM

During manufacturing, phenomena like melting of the powder particles, melt pool solidification and

thermal shrinkage during cooling leads to thermally induced stresses in the part. After fabrication a

heat treatment is performed as part of the post-processing campaign in order to achieve a stress

relief. In order to meet part demands on accuracy as well as surface roughness a post machining was

necessary. Typical accuracies as built are 0.2 mm as a lower boundary. Therefore, after part

removal from the machine build plate, shape cutting and support removal is performed according to

the parts technical requirements. Both insert design options are manufactured, the successfully

manufactured and post machined INSERT 1 is shown in Fig. 34 to Fig. 36.

Fig. 34 Manufactured INSERT 1

Fig. 35 Manufactured INSERT 1

Fig. 36 Detail

INSERT 1

6. VALIDATION AND VERIFICATION

6.1 Dimensional validation

Functional parts must fit into the system environment they are designed for. In order to assure that

the result of post processing INSERT 1 is in accordance with the part requirements, a 3D scan of

the part is conducted. The equipment used is a GOM ATOS IIe with a measurement accuracy of

0.02 mm. The critical feature was the planarity of 0.05 mm and the overall maximum interface

thickness deviation of -0.2 to -0.3 mm of the part. Fig. 37 shows the drawing with the dimensional

requirements and Fig. 38 contains the results of the 3D scan of INSERT 1.

Fig. 37 Drawing INSERT 1

Fig. 38 3D scan results INSERT 1

The result of the measurements shows that the post-machined part fulfils the accuracy demands

according to the specified part requirements.

Table 6 shows the relation between the theoretical weight according to the CAD info and the final

manufactured part weight.

Table 6 weight data INSERT 1

Insert

Original Insert Type 1 (CAD)

Insert Type 1

(manufactured)

Weight (gr) 1464 562 560

Weight Reduction (gr) -- 902 904

Percentage of reduction -- 62% 62%

6.2 Structural validation

A virtual structural validation has been performed by means of a detailed FEM analysis in Abaqus

version 2016 for the thermomechanical and mechanical load cases. The final design options with

and without lattice structure are compared with the original design [10], [11].

The detailed FEM model (Fig. 39) represents the skin

with composite shell elements containing the laminar

plies. The adhesive layers are represented with solid

cohesive elements taking into account a failure criterion

which respects the maximum allowed stress according

to manufacturer´s material data.

In case of INSERT 2, the lattice structure is modelled

with solid tetrahedral elements in the most critical area

and the remaining lattice structure is modelled by beam

elements.

Fig. 39 Abaqus detailed FEM model

With respect to the thermal analysis, which represents the adhesive curing process by a uniform

temperature decrease from 120ºC to 20ªC, the micro strain values in the skin clearly represent the

obtained thermal improvements as shown in Fig. 40. The adhesive shows similar improvement

results and the insert itself reveals higher stress levels in the modified lattice version, but far below

the yield strength level of the applied titanium alloy Ti6Al4V (860MPa).

Fig. 40 Elastic strain level in skin (thermal load)

The mechanical analysis, with the application of forces and moments on the assembly, also shows

improvements in the behaviour of the skin despite the significant mass reduction. In Fig. 41 can be

seen that the logarithmic strain peak values in the skin as well as the critical area are reduced.

Fig. 41 Logarithmic strain level in skin (mechanical load)

Generally the optimization process pushes the part design more in the direction of the allowable

stress, reducing the safety margin. The results of this study are a clear example of this statement,

since the achieved mass reductions of INSERT 1 and 2 are related with an increased level of Von

Mises stress, as can be seen in Fig. 42.

Fig. 42 Von Mises stress level in insert (mechanical load)

According to the adhesive material data sheet provided by the manufacturer, the stress resistance up

to failure amounts 20 MPa. This value is implemented in the FEM model and Fig. 43 shows the

results of the corresponding quadratic nominal stress damage initiation criterion (QUADSCRT). In

case the QUADSCRT output value exceeds the dimensionless limit of 1.0, the element is supposed

to fail. It can be observed that both modified design options show a clear failure improvement for

the adhesive, as well in maximum value as failure area. The insert version with lattice shows a

slightly increased failure area compared to the version without lattice due to slightly higher insert

flexibility.

Fig. 43 Nominal stress damage in adhesive (mechanical load)

7. CONCLUSIONS AND NEXT STEPS

Additive Manufacturing provides the opportunity of improving the hot bonded insert design in

several areas. The first one is a mass reduction of 65%, the new insert saves more than 0.9 kg which

is a significant improvement for a spacecraft device. The thermo-elastic behaviour is improving as

well due to a more flexible insert. The microstrain levels in the CFRP skin are significantly

decreased compared to the original design.

The strength capability of the panel is improved by obtaining a better stress distribution therefore

the adhesive damage area is drastically reduced.

In terms of costs, the mass reduction is a direct production material cost saver, and additionally it

reduces the total weight of the satellite with the related transport cost improvements.

Although the improvements and opportunities achieved with optimization and subsequent metal

AM are huge, some challenges need to be solved. Since the insert contains an internal structure,

proper inspection and verification procedures (e.g. tomography) need to be used to check geometry

and defects. Besides, mechanical testing needs to be performed to validate and qualify the design

concepts. Finally, to assure repeatability in production, manufacturing process parameters will be

defined and process monitoring systems need to be implemented.

As a further development, new strategies can be developed to improve the existing joints between

the insert and the CFRP panels. In particular, advanced hybrid joints, which incorporate a specially

designed array of macro-scale pins over the metallic part, have shown promising results in

increasing the strength of interfaces [12]. One of the most relevant advantages of this technique is

the fact that the pins can be inserted in the CFRP material before the curing process. Moreover the

SLM technique used in this study can be used for the in-situ fabrication of pins on the outer surfaces

of the insert, due to high accuracy and dimensional tolerance. This can replace the conventional

welding operations used to assemble the pins. The insert provided with pins can be produced in a

single operation and subsequently assembled with the skin by co-curing. Besides the manufacturing

and assembly improvements, this concept changes the design philosophy drastically by offering the

possibility to reduce the adhesive surfaces and thus, the complete insert design reducing insert

volume, and as a consequence, further reduction in weight and costs.

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