1. Experimental and numerical studies on
the deformation behavior of polymer based
composite plates subjected to bending
F. Bosia, J. Botsis, T. Gmür, M.A.Mustafa & L. Humbert
Laboratory of Applied Mechanics & Reliability Analysis (LMAF)
Swiss Federal Institute of Technology
Lausanne Switzerland
Presented at: CompTest 2003
ENSAM Châlons en Champagne, France

2. outline background experimental results numerical simulations
m optical fiber sensors
m issues in laminated composites
experimental results
m strain measurements through the thickness using FBG
m strain measurements on the surface using strain
gauges and full field ESPI & strain gauges
numerical simulations
m 2-dimensional of equivalent single layer models
m 3-dimensional of the actual lay-up

3. use of FBG sensors 1. Basic experiments in mechanics of materials
m measure forces on fibers in a bridging zone
study the interface : matrix – fiber
study residual stresses in polymeric materials
measure deformation behavior through
the thickness on composite laminated plates
1. Basic experiments in mechanics of materials
2. On structural monitoring and control

4. fiber Bragg grating (FBG) sensors
silica doped with
germanium & hydrogen I R T  
core
FBG B
expose fiber to UV
over a certain length
change n0
FBG length : mm

5. FBG sensor - principles of operation
- compact
- lightweight
- minimally invasive (ø125 mm)
- immune to e-magnetic interference
- great resistance to corrosion
- high temperature capacity  3000C
FBG are:
UNIFORM STRAIN FIELD
Non-UNIFORM STRAIN FIELD

6. FBG sensors - principles of operation
opto-mechanical
relation
axial stress
transversal stresses

7. composite materials: relevant length scales@

8. CLASSICAL LAMINATE THEORY
equivalent 2D single layer theories
CLASSICAL LAMINATE THEORY
FIRST ORDER THEORY
HIGHER ORDER THEORY
(CLPT)
Analytical / numerical results
lack experimental validation
&

9. materials and specimens
4/1 Glass fibre / Polypropylene matrix prepregs
8-ply (90°/0°)2s , 250mmx300mm, thickness 4.9mm
FBG sensors (125m polyimide-coated monomode fibres)
λB 1530 nm; Grating length: 3 or 6 mm.
UPPER PLATE
Preparation:
1) Compression
moulding
2) Autoclave
PREPREG STACK
EMBEDDED
SENSOR(S)
LOWER PLATE
10 Bar, 190° C.

10. locations of FBG sensors
4ème couche (0°)
3ème couche (90°)
2ème couche (0°)
1ère couche (90°)
Fibres à 90° dans la 2ème couche (0°)
Fibres à 90° dans la 4ème couche (0°)

11. materials and specimens
4/1 Glass/PP Twintex (Vetrotex Int.)
PLATE
8-ply (90°/0°)2s , 250mm x 290mm, thickness 5mm
16-ply (90°/0°)4s , 250mm x 290mm, thickness 10mm
50 <Span/Depth <10 x O L b l
Fibre optique
b/2
L/2 y z h
FBG sensor
specimen quality checks
- void content
- thickness variation
mechanical characterization
- single layer and laminate properties
- influence of embedded FBG

12. experimental methods specimen “shift’’ y y z z O x O x s L L/2+D L/2b l
O. Fibre
b/2
L/2 y z h
FBG SG s x O
L/2+D y z
FBG SG D

13. locations of FBG sensors
length of FBG
= 6mm
FBG embedded centrally in the specimens
Location through the thickness
Through-the-thickness (8-ply specimens): a) b) c)
1/2
2/3
3/4
8-ply specimens: Z Y
16-ply specimens: a)
1/2, b)
2/3, c)
3/4, d)
4/5, e)
6/7

14. 8 finite element simulations
Plate specimen, 3-point bending configuration 8
Important problem is that of the boundary
conditions due to the variations in the
thickness of the plates.
ABAQUS
- CLPT and First Order laminate theories tested
- 3D simulations have been carried out

15. Electronic Speckle Pattern Interferometer

16. loading configuration
specimens
strain gauges
FBG sensor
loading arm

17. Electronic Speckle Pattern Interferometer
in-plane measurements in the presence
of large out-of-plane component
Discrepancies between ESPI-measured displacements (u,v,w)
and actual object displacements (u’,v’,w’) due to
Misalignment of the surface normal and the viewing direction;
Varying surface normal direction on a curved surface.
u = u’cosa +w’sina,

18. Three- point bending with line loads
experimental-numerical comparison
Three- point bending with line loads
(span/depth=10)
SHIFT : 0 mm
SHIFT : 10 mm

19. Three-point bending with concentrated loads
experimental-numerical comparison
Three-point bending with concentrated loads
(span/depth=10)
SHIFT : 0 mm
SHIFT : 10 mm
SHIFT 20 mm

20. SUMMARY e A non-linearity appears when s/d is about 10 and depends
on the type of load (line or point load) and the location
of the FBG with respect to the load.
e The single layer CLT and 1st order shear theories seem to work
for s/d more than 25.
e Optical Fibre Bragg Gratings offer an indispensable tool to study,
in a non invasive manner, deformation behaviour of layered
composites.

21. references additional results using FBR at LMAF/EPFL
- F. Bosia, J. Botsis, M. Facchini & P. Giaccari, Composite Science and Technology,
Vol. 62, 2002, p. 41.
- F. Bosia, T. Gmür, & J. Botsis, Composite Science and Technology, Vol. 62, 2002, p. 55.
- M. Studer, K. Peters & J. Botsis,, Composites B, to appear.
- K. Peters, P. Pattis & J. Botsis, Journal of Materials Science Letters, Vol. 21, 2002, p. 887.
- M. Studer, J. Pietrzyk, K. Peters, J. Botsis, & P. Giaccari, International Journal of Fracture,
Vol. 114, 2002, p. 379.
Sources used in the view graphs
@ From: I. M. Daniel & O. Ishai, ‘engineering mechanics of composite materials’, 1994
& From: J.N.Redy, ‘mechanics of laminated composites’, 1997
Work sponsored by the Swiss National Science Foundation