SLM Components in CFRP Composite Assemblies

Presented by Lyle Campbell

Supervised by Professor Tim Sercombe

Project Purpose

• Develop SLM cores

• Out-perform current

methods

• Construct and test

FRP assemblies

with SLM

components

SLM core assembly of upright developed in this work

Sandwich Structure Response

• Bending

dominant over

large spans

• Shear significant

over short spans

Sandwich panel deflection modes. Adapted from Hexcel (2000)

Cores and SLM

• Layer by layer

additive

manufacturing

• Geometric freedom

• Al12Si

Selective Laser Melting process schematic (Mumtaz and Hopkinson, 2009)

‘Shear Lattice’ Core

• Align material along

principal directions

• High shear, low

axial properties

• Low solid fraction

applications, <10%Illustration of shear lattice unit cell with two major shear planes depicted

‘Web’ Core

• Closed cell structure

• Even load distribution

• Higher mechanical

properties

• High stiffness

applications

Schematic of web core section

Failure Modes

• Shear overloading

• Buckling

– Struts

– Webs

– Skins

• Core crush

• De-bonding

Upper; Honeycomb shear wrinkling. Adapted from Bertrand (2006)

Lower; Adhesive filleting at web core-skin interface.

Modelling and Testing

• ASTM 273 not

possible

• Short span 3

point bend

• HyperMesh 12.0

Upper; 3 point bend test set-up

Lower; Half symmetry 3 point bend FE model

Results – 3 Point Bend

0

5

10

15

20

25

30

35

40

45

10% 12% 14% 16% 18% 20% 22% 24%

Sa

mp

le S

tiff

ne

ss

(k

N/m

m)

Solid Fraction (Percentage)

Lattice Measured

Web Measured

Lattice FEM

Web FEM

All error bars show one standard deviation

Results – Shear Modulus

0

1

2

3

4

5

6

7

10% 12% 14% 16% 18% 20% 22% 24%

Sh

ea

r M

od

ulu

s (

GP

a)

Solid Fraction (Percentage)

Web Ideal

Lattice Ideal

Web Short Span

Lattice Short Span

Ideal = 2.8 x short span

Results –Normalised Specific Shear Modulus

0.0

0.5

1.0

1.5

2.0

2.5

Honeycomb 18% Web 21% Lattice Foam

*Normalised w.r.t. honeycomb

Joining Cores

• Bolted joint

• Tapered

interference fit

• Mechanical

interlocking with

adhesive

– Grid of rectangular

tongue and slot Illustration of similar joint.Adapted from Décor Arts Now (2014)

Testing

• Lap shear test

• Sandblast & acetone bath

• Adhesive failure

Sandwich Panel Inserts

• Introduce concentrated

loads into panel

– Bending into skins

– Shear into core

Typical ‘cotton reel’ panel insert. Adapted from Shurlock (n.d.)

Insert Design

• Avoid large

modulus step

changes

• Smooth deflection

profile

• Tapered density

SLM inserts

Upper; UWAM high load insert design. Adapted from Bertrand (2006)

Lower; Illustration of shear deflection profile around insert.

Testing, Model Verification

Left; Single shear insert test in foam core sandwich panel.

Right; Insert pull-out test in foam core sandwich panel.

Insert Density Study

• Lattice properties varied

and effects examined

• Design requires balance

depending on design

constraints

– Insert mass, diameter

– Skin peak stress

– Bulk core peak stress

Plot of major principal stress illustrating stress concentrations at insert-core junctions

Final Insert Design

• Cubic profile for tapered

stiffness profile on ‘top hat’

• Female thread

• Core properties reduce

radially

• Lattice stiffness compromise

• 25g, 45% of equivalent CF

stack

Cross section view of panel insert designed for pull-out and single shear. 60 mm OD.

FSAE Vehicle Upright

• Connects wheel to

suspension system

• High stiffness

requirement

Schematic of UWAM 2014 rear axle unsprung assembly

Design and Modelling

Left; Complete upright core assembly Right; exploded view of upright core assembly

Testing and Results

• Modelled in HyperMesh

• Physical testing and

stiffness comparison

• 20% Discrepancy

– Manufacturing errors

– Simplified FEM

geometry

– Tetrahedral elements &

mesh density

• 0.05 degrees/g, 800 grams,

first design iteration

Cross section through upright testing assembly

Outcomes and Conclusions

• Developed core structures that outperform current core materials

used by UWAM

• Developed methods for assembling core structures into larger

components

• Developed sandwich panel inserts which improve on current

UWAM design

• Developed composite upright suitable for further testing on a

vehicle

• Developed requisite manufacturing techniques and modelling

methods for the above components

Future Work

• Small feature build quality

• Surface finish improvement

• Strength and failure modes,

axial loading

• Fatigue

• Compare wider variety of

cores

• Thermal modelling of inserts

• Energy absorption

• Modelling of adhesive layer

Presentation References and Sources• Hexcel Composites, 2000. Honeycomb Sandwich Design Technology, s.l.:

Hexcel Composites.

• Hopkinson, N and Mumtz, K, 2009 Selective laser melting of thin wall parts

using pulse shaping. Journal of Materials Processing Technology, Volume 210,

pp. 279-287.

• Bertrand, A., 2007. Composite Chassis Construction for the 2006 UWAM

FSAE Car, s.l.: University of Western Australia.

• Shur-Lok, n.d. Fasteners for Sandwich Structure Catalog. [Online]

[Accessed May 2014].

• European Corporation for Space Standardisation, 2011. Space Engineering;

Insert Design Handbook. Noordwijk: European Space Agency.

• Cote, F., Deshpande, V. & Fleck, N., 2006. The Shear Response of Metallic

Square Honeycombs. Journal of Mechanics of Materials and Structures, 1(7),

pp. 1281-1299.

Questions

Mesh Independence - Hex

0.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

0 5 10 15 20 25 30

De

fle

cti

on

(m

m)

Element Count (Thousands)

Mesh Independence - Tet

0.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

0 100 200 300 400 500

De

fle

cti

on

(m

m)

Element Count (Thousands)

Tet Deflection

Material Property Testing, Composite FE Model Validation

Results – Shear Strength

0

5

10

15

20

25

30

35

40

10% 12% 14% 16% 18% 20% 22% 24%

Sh

ea

r S

tre

ng

th (

MP

a)

Solid Fraction

Web Measured

Lattice FEM

Lattice Measured

Web FEM

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Honeycomb 14% Web 21% Lattice Foam

Results –Normalised Specific Shear Strength

Results – Compressive Modulus

0

100

200

300

400

500

600

8% 10% 12% 14% 16% 18% 20% 22%

Yo

un

g's

Mo

du

lus (

MP

a)

Solid Fraction

Web

Lattice

Results – Normalised Specific Compressive Modulus

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Honeycomb 10% Web 11% Lattice Foam

Results – Compressive Strength

0

10

20

30

40

50

60

70

8% 10% 12% 14% 16% 18% 20% 22%

Co

mp

ressiv

e S

tren

gth

(M

Pa)

Solid Fraction

Web

Lattice

Results – Normalised Specific Compressive Strength

0.00

0.50

1.00

1.50

2.00

2.50

Honeycomb 18% Web 21% Lattice Foam

SLM Settings, Material Properties

Scan Speed (mm/s) Laser Power (W) Laser Focus (mm)

400 200 4

σy (MPa) σUTS (MPa) Density Elastic Modulus (GPa)

223 ± 11 355 ± 8 97.5 ±0.3 68