The goal of this paper is Finite Element (FE) stress analysis of stitched glassfibre composites. The stress distribution gives an idea of laminate fatigue behaviour and potential alarm zone locations. The research includes a Finite Element (FE) simulation and an experimental study for laminates with different stitching parameters. Analysis is performed on meso-level: fiber bundles and matrix are considered, but not separate fibers. Bundle waviness, variable thickness and volume factor are taken into consideration.

1. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
SIMULATION AND TESTING OF STITCHED
GLASSFIBRE LAMINATES FATIGUE BEHAVIOUR
Novozhilov Y.V.a, Mikhaluk D.S. a, Borovkov A.I. a, Gilyov E.E. a;
Kemppinen M.b, Dufva K. b, Karttunen T. b, Koulu J. b
a CompMechLab of St. Petersburg State Polytechnic University
Politekhnicheskaya 29, Bld. 1, Aud. 433, 195251 St.Petersburg, Russia
research@compmechlab.com
b Research Centre YTI of Mikkeli University of Applied Sciences,
Patteristonkatu 3, room E118, P.O. Box 181, 50101 Mikkeli, Finland
martti.kemppinen@mamk.fi
July 27, 2009
Edinburgh, Scotland

2. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Laminate Structure and Stitching Parameters
Y
X
Vf
2= 60 %
Vf
3= 63 %
Vf
1= 57 %
Glassfiber bundle
Stitching yarns
L = 150 mm
1 mm
Young's modulus(E) Poisson ratio (υ) Ultimate stress (ϭu)
Glass 76.00 GPa 0.3 1.40 GPa
Epoxy 3.50 GPa 0.3 0.06 GPa
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3. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Goals and Methods
Finite Element (FE) analysis and study of different stitching factors:
• presence of stitching (bundle waviness)
• non-uniform stitching yarn tension (variable bundle thickness and fibre volume factor)
The stress distribution gives an idea of:
• potential alarm zone locations
• laminate fatigue behavior
Experimental study for laminates:
• experimental fatigue tests
• FE model validation
Analysis is performed on meso-level: fiber bundles and matrix are considered.
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4. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Composite dimensions
Symbol Size, mm
А 3.09
В 15.00
С 1.51
D 3.44
E 0.60
0.25 mm
8.25 mm
А
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5. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Z
Y
Effective elastic moduli of the bundle
Young's modulus Shear modulus Poisson's ratio
EX = 44.13 GPa GYZ = 3.81 GPa υYZ = 0.28
EY = 10.33 GPa GXZ = 4.10 GPa υXZ = 0.07
EZ = 10.33 GPa GXY = 4.10 GPa υXY = 0.07
Chaotic packing of the fibers
SEM photo
Hexagonal packing of the fibers
Geometrical model
¼ periodicity cell
FEM
Direct FE Homogenization
( ) ( )
( ) ( )
, ,
( ) *,
, ,
f f
e e
C r V fibre
C r C r V
C r V epoxy
 
  

Vf = 56%
* Borovkov A.I. Effective Physical and Mechanic Properties of Fiber Composites – VINITI 1985
Ø fiber ≈ 16 μm
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6. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
UX = <εx>L
3D FE Model and Boundary conditions
3D FEM
Quadratic 20-node 3D FE
Number of elements 391 400
Number of nodes 1 617 262
Number of degrees of freedom 4 851 786
X
Y
Z
Y
Z
0| 0
0,0005
| 4,125
X
X L
U
U L m

 



  
 
X
Y
UX = 0
L
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7. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Transformation of Lamina Stiffness Matrix
X
Y
1
2
θ
Local coordinate system rotation
θmax = 14 º
      
11 12 1611 12
1
21 22 21 22 26
66 61 62 66
0
0 ;
0 0 2 2
Q Q QQ Q
Q Q Q Q T Q T Q Q Q
Q Q Q Q

  
          
      
 
2 2
2 2
cos sin sin 2
sin cos sin 2
sin 2 sin 2 cos2
2 2
T
  
  
  
 
 
  
 
  
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8. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
FE results
15 25211917 23
0 7654321
X
Y
Z
σX, MPa
σi, MPa
Bundles Epoxy
σmax, MPa 21.6 6.4
σu, MPa 1400.0 60.0
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9. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Local Elastic Coefficients Computation
Y
Z
Y
X
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10. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Local Elastic Coefficients Computation algorithm
Vf
2= 60 %
Vf
3= 63 %
Vf
1= 57 %
2
2
2
X x x f x f
Y y y f y f
Z z z f z f
E a b V c V
E a b V c V
E a b V c V
  
  
  
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11. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Variable effective elastic moduli of the medium
X
Y
Z
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40.0
41.0
42.0
43.0
44.0
45.0
46.0
0 1.5 3 4.5 6 7.5
Ex,GPa
X, mm
Bundle № 1
Bundle № 2
Bundle № 3
Bundle № 4
Bundle № 5
Bundle Vf=56%

12. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
3D FE model with variable bundle thickness
X
Z
Y
Quadratic 20-node 3D FE
Number of elements 381 320
Number of nodes 1 577 628
Number of degrees of freedom 4 732 884
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13. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
X
Y
Z
X
Y
Z
FE results
15 25211917 23
σX, MPa
σi, MPa
0 7654321
Bundles Epoxy
σmax, MPa 24.3 6.8
σu, MPa 1400.0 60.0
X
Y
Z
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14. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
0 765432115 25211917 23
Regular geometry
Influence of irregular geometry
Irregular geometry
σX, MPa σi, MPa
X
Y
Z
σi max = 6.4 MPa
σi max = 6.8 MPa
σx max = 21.6 MPa
σx max = 24.3 MPa
X
Y
Z
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15. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Other patterns of stitching
Y
Z
X
Y
Z
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16. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
15 25211917 23
FE results for other patterns of stitching
0 7654321
σX, MPa
σi, MPa
X
Y
Z
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σi max = 6.7 MPa
σx max = 22.1 MPa σx max = 25.2 MPa
σi max = 4.8 MPa

17. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
FE model validation and experimental fatigue tests
Data from EX, GPa
Regular bundles (FEM) 33.2
Irregular bundles (FEM) 30.3
Experimental data 35.0 ÷ 40.0
Damaged test specimen after 3.8·106 cycle
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18. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Conclusions
 FE stress analysis of stitched glassfiber composites is made
 Model with regular bundles geometry
• Analysis was performed at a meso-level
• In-plane bundles waviness was considered
• Transformation of lamina stiffness matrix
• 95% correlation with experimental data
 Influence of non-uniform value of the chopped strand tension during laminate
manufacturing
• 13% growth of maximal stress in bundles
Results for other type of stitching are obtained
• ϭx max in bundles from 21.6 MPa to 25.2 MPa
• ϭi max in matrix from 4.8 MPa to 6.7 MPa
• Significant influence of stitching type
 Experimental static and fatigue test are carried out
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19. St.Petersburg State Polytechnic University
Computational Mechanics Laboratory
Mikkeli University of Applied Sciences
Research Centre YTI
Thank you for attention!