Energy absorption in sandwich laminate structures

Robert White IM 2005

INTRODUCTION• Sandwich laminate boat hulls• Maximum survivable impact speed at sea?• Due to the nature of composite structures defining energy absorption

abilities for composites is different from structures made from traditional homogenous materials. (Saarela, O. Airasmaa, I. Kokko, J. Skrifvars, M. Komppa, V. 2003. Komposiittirakenteet. Muoviyhdistys, Helsinki)

GOAL

• Energy absorption properties of sandwich laminates.• Develop simplified spring force model to predict

behaviour of laminate during slow, non-cutting, non-bending penetration.

• Test the model against results of tests made on 7 different Divinycell - glass fibre sandwich laminates.

• Test procedure: non-cutting, non-bending penetration test.

MATHEMATICAL MODEL

• Kinetic energy of a moving body:• When the boat collides with the object,

momentum is converted to velocity (v1) through the following function:

• The amount of energy absorbed by the composite can be found by integrating the force over distance:

(I.S. Grant, W.R. Phillips 2001)

MATHEMATICAL MODEL

• Since the change in energy is known the equation can be seen as:

• Or when solving for the displacement of the collision into the foam:

• Assuming the laminates behave like a spring during a collision

FOAM CORES

• DIAB DIVINYCELL foam coresMaterial Tensile strength Compressive strength

H250 9,2 MPa 6,2 MPa

H100 3,5 MPa 2,0 MPa

H80GS (grid-scored) 2,5 MPa 1,4 MPa

P150 2,45 MPa 2,30 MPa

H60 1,8 MPa 0,9 MPa

H45GS (grid-scored) 1,4 MPa 0,6 MPa

P60 1,10 MPa 0,55 MPa

H45GS

H60

P60

P150

MAKING THE LAMINATES

• Bi-axial -45/45 E-glass fibre 430g, • Atlac 580 AC 300 vinyl ester• One shot lamination

SANDWICH PANEL

• Skin of fibres laminated on both sides of a foam core

TESTING LAMINATES

• Non-cutting• Non-bending• 13,5mm sphere• Destructive test

TESTING LAMINATESH60

H100

TESTING LAMINATES

0 10 20 300

1000

2000

3000

4000

5000

6000 P_150

P_60

H_60

H_100

H_250

H45GS

H80GS

(mm)

Force N

TESTING LAMINATES

TESTING LAMINATES

• Straight line at beginning of graph.• y-value can be related as spring constant k.• reasonable to assume that a simple spring model

to determine performance of foam cores in a collision situation.

3 4 5 6 7 8 9 100

500

1000

1500

2000

2500

3000

f(x) = 460.177216382612 x − 1886.9740237301R² = 0.983788119342153

P60

P60

mm

Force [N]

3 4 5 6 7 8 9 100.0

1000.0

2000.0

3000.0

4000.0

5000.0

6000.0

f(x) = 725.678923982124 x − 2264.47220720528R² = 0.994839870058627

H250

H250

mm

Force [N]

CORRELATION OF MODEL TO CORES

• Relationship between the manufacturers stated compression and shear modulus values and the k –value derived from actual tensile tests exists for compressive modulus at a rate of 90,5%.

• In the case of shear modulus the rate of occurrence is 88,34%.

CORRELATION OF MODEL TO CORES

362 412 462 512 562 612 662 712 7620

50

100

150

200

250

300

350

f(x) = 0.262342668592761 x − 97.943257701162R² = 0.883386816621197

f(x) = 0.755592701294769 x − 266.82572585298R² = 0.905012311121268

Comp mod

Linear (Comp mod)

Shear mod

Linear (Shear mod)

k-value

Mpa

MODEL AT WORK

• The tests imply that the cores exhibit performance that can be modelled in a simple way.

• Practical application of results to find out the max. allowable speed:

• z value is mass-ratio:

MODEL AT WORK

• BUSTER X COLLIDING WITH SEA CONTAINER

H250 P150 H100 H80GS P60 H60 H45GSk 725,68 547,5 530,24 507,15 460,18 446,62 362,31

k*(x*100)*1000 725 680,00 547 500,00 530 240,00 507 150,00 460 180,00 446 620,00 362 310,00∆E 36,28 27,38 26,51 25,36 23,01 22,33 18,12

m/s 0,26 0,22 0,22 0,21 0,2 0,2 0,18

MODEL AT WORK

• NAUTOR SWAN 60 COLLISION WITH SEA CONTAINER

H250 P150 H100 H80GS P60 H60 H45GS

k 725,68 547,50 530,24 507,15 460,18 446,62 362,31

k*(x*100)*1000 725 680,00 547 500,00 530 240,00 507 150,00 460 180,00 446 620,00 362 310,00

∆E 36,28 27,38 26,51 25,36 23,01 22,33 18,12

m/s 0,08 0,07 0,07 0,07 0,07 0,07 0,06

MODEL AT WORK

H250 P150 H100 H80GS P60 H60 H45GS

k 725,68 547,5 530,24 507,15 460,18 446,62 362,31

k*(x*100)*1000 2 902 720,00 2 190 000,00 2 120 960,00 2 028 600,00 1 840 720,00 1 786 480,00 1 449 240,00

∆E 2 322,18 1 752,00 1 696,77 1 622,88 1 472,58 1 429,18 1 159,39

m/s 2,82 2,45 2,41 2,36 2,25 2,21 1,99

H250 P150 H100 H80GS P60 H60 H45GS

k 725,68 547,5 530,24 507,15 460,18 446,62 362,31

k*(x*100)*1000 2 902 720,00 2 190 000,00 2 120 960,00 2 028 600,00 1 840 720,00 1 786 480,00 1 449 240,00

∆E 2 322,18 1 752,00 1 696,77 1 622,88 1 472,58 1 429,18 1 159,39

m/s 2,06 1,79 1,76 1,72 1,64 1,61 1,45

Buster X colliding with Buster X

Nautor Swan colliding with Buster X

DISCUSSION

• Performance of the laminate in a collision situation will change depending on the type of reinforcement, the resin and the way the laminate has been manufactured.

• Age, wear and conditions affect laminates.• Laminates studied in this thesis had four layers of

glass fibre on each side of the foam core.• How do sandwich laminates behave with

different reinforcement and matrix materials?

DISCUSSION

• When a cut or groove extends through the foam connecting the top and bottom skin the compression properties of the laminate change.

• Tests that simulate a cutting, glancing collision could be advisable to produce information on how the foam and fibre laminates behave.

DISCUSSION

• So far there are few reliable ways to pre-determine the final strength of a composite laminate in a way that would allow for estimates between varying amounts of fibre layers.

• Future research into developing systems that could be fibre and resin specific in determining the resulting strength of the structure could be useful.

CONCLUSIONS

• Relationship between the datasheet properties of compressive and shear modulus of most of the foam core materials tested and their actual energy absorbing capabilities shown to exist.

• Sandwich laminates display a reasonably straight force over distance line before the end of elastic deformation occurs.

• This means that these laminates contain a section of their behaviour under load that can, within reason be modelled using an applied spring model equation.

QUESTIONS