Coupling adhesion and adherence to contact and friction for studying debonding and damage in compositesLaboratoire de mécanique appliquée et d'analyse de fiabilité (LMAF) Ecole Polytechnique Fédérale de Lausanne (EPFL)
OUTLINE • Problem definition and motivation • Theoretical background • Specimen design • Specimen fabrication • Experimental procedure • Preliminary results • Conclusion
1COMPTEST 2006 - Porto
Composite fracture mechanisms:
Debonding: irreversible loss of adhesion between two materials in contact, followed by their interfacial friction and eventually their separation.
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3. Fibre pull-out and fracture
Problem definition and motivation
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Although debonding between fibers and matrix in composites is identified as a key mechanism which governs damage in these materials, a satisfactory model for studying and preventing this phenomenon is still lacking.
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This research belongs to solid, contact and fracture mechanics. Its general philosophy is to use the tribology of pairs of materials to explain the rheology of composite materials.
Theoretical background
However: These adhesive properties are difficult to measure accurately because the decohesion process is often unstable!
Two adhesive properties:
• Debonding occurs over finite γn.
Contact-adhesion law
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KI = 2P/t (m)1/2
• KI stress intensity factor • P applied load • t sheet thickness • m = (3a2/h3+1/h) • h distance of specimen edge
from fracture plan • a crack length
P
P
Two geometries improve stability:
Computational tool : : TACT (A. Curnier) - Contact Analysis Program Finite Element Method (Galerkin) for spatial discretization Linearization Method (Newton) for non-linearities Finite Difference Method (Newmark) for time integration
2D linear elastic axisymmetric element, 2D/3D selective integration linear elastic element Small-slip contact-friction-adhesion node-on-node element
penalty augmented Lagrangian
Plane-strain numerical simulations
a) Cross-section with tension envelope
Axisymmetric concave epoxy-stamp on glass-disc specimen
b) Perspective with cut sector
Specimen fabrication
Glass disc (mechanical glass) composition:: o Silicon Oxide (silica) SiO2 – 60 %, o Aluminum Oxide (alumina) Al2O3 – 25 %, o Calcium Oxide (lime) CaO – 7 %, o Magnesium Oxide MgO – 6 % and o Boron Oxide B2O3 – 2 %
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Glass drying mode:: o 2 hours in oven at T=1000C o 24 hours in desiccator at T=250C
Epoxy stamp solution:: o 70 % – DER 330 o 30 % – DER 732 o 13 % – DEH 26
Silane solution:: o 96 % – pure Ethanol o 4 % – distilled water o 1.7 % – Silane A-1100
Mold for production Epoxy-stamp on glass-disc specimen
Experimental procedure
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Crack propagation sequence • r – initial internal radius • a – current internal radii • R – final external radius
min mm1
Force histographs for differently dried silane coated glass discs
• 1,2 – glass dried for 2 hours in the oven at T = 100 oC • 3,4 – glass dried for 24 hours in the desiccator
b) Glass disc and epoxy stamp after rupture
a) Force histograph
( ) nn n ra πππ
ωγ 122 22 − Ω==
Conclusions
The epoxy-stamp on glass-disc specimen is the seat of stable debonding.
It allows for an identification of the adhesive properties: adhesion peak stress πn and rupture gap γn.
Actual elastic unloading is expected to improve further the accuracy.
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Debonding stability is achieved by combining axisymmetry with concavity of the epoxy stamp.
The good agreement between numerical and experimental results shows that our simple adhesion-decohesion law is feasible.
QUESTIONS ?
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