Z. JENDLI *, J. FITOUSSI*, F. MERAGHNI** et D. BAPTISTE*. *LM3 UMR CNRS 8006. ENSAM Paris.

1. Micromechanical analysis of strain rate effect on damage evolution in discontinuous fibre reinforced composites Z. JENDLI*, J. FITOUSSI*, F. MERAGHNI** et D. BAPTISTE*. *LM3 UMR CNRS 8006. ENSAM Paris. **LMPF-JE 2381. ENSAM Châlons en Champagne.

2. CONTEXT Passengers safety Structures lightweight Interaction Manufacturing ↔ mechanical behaviour Manufacturing and productivity. • advantages of SMCcomposite material • good strength/weightratio • manufacturing process devoted to large series • high-energy dissipation with a diffuse damage.

3. Development of multi-scaleapproaches. CONTEXT • Industrial requests • Overall mechanical behaviour prediction under dynamicloading. • Inaptitude of current dynamic behaviour laws for the structures calculation in composite materials • Insufficiency of the phenomenological approaches currently used. • Material microstructure integration • Physical description of damage mechanisms.

4. . Analysing effect in the SMC-R damage ( e ) ( e = 10-4 – 400 s-1 ) AIMS • Experimental analysis (at micro and macroscopic scales) • Behaviour stages • Damage threshold and accumulation • Behaviour modelling using a multi-scale approach • Visco-elastic • Visco-damage.

5. Introduction of strain rate effect in a micromechanical model Prediction of the dynamic mechanical behaviour and its interaction with the composite microstructure. MODELING COMPOSITE DYNAMIC BEHAVIOUR • Damage mechanisms experimental investigation • Interrupted high-speed tensile tests. • In-situ tensile tests. • SEM observations.

6. -4 & £ e £ -1 -1 10 s 400 s EXPERIMENTAL INVESTIGATIONS MATERIAL SMC-R26 composite Sheet Molding Compound-Random. glass E/polyester, discontinuous fibres 26%. TESTS PARAMETERS • Hydraulic high speed tensile test machine • 20 m/s, piezo-electric load cell 50 kN • Performed strain rates

7. . ( e ) Strain rates effects on the SMC-R mechanical characteristics • The composite macroscopic response is widely affected by • Minor variation of the anelastic slope as a function of the strain rate • Insensitivity of the Young’s modulus to strain rate increase.

8. The first non-linearity Damage threshold is considerably delayed in term of strain and stress. Strain rates effects on the SMC-R mechanical characteristics

9. Strain rates effects on the SMC-R mechanical characteristics • Behaviour accommodation when increases the strain rate : • Steady rise of the ultimate strain (38 %) • Maximum stress increases considerably.

10. SMC-R specimen Specimen/fuse Intermediate fixing Ligament Mechanical fuse å Damage analysis INTERRUPTED DYNAMIC TENSILE TEST The specimen geometry is a bar with dimension 36*6,5*2,7 mm3 The fuses material: PMMA (Poly Methyl Methacrylate) Fragile elastic behavior.

11. D_critical D E = - D 1 0 E Macroscopic damage analysis Macroscopic damage vs. total strain for three strain rate values Damage initiation is considerably delayed in terms of strain thresholds Critical damage level : insensitive to the strain rate effect.

12. e_ult Microscopic damage analysis Fibre-matrix interface damage description • Strain rate increase: • Delayed damage threshold • Decreased damage growth speed. d_micro = fv_debonded / fv total. Overall behaviour accommodation Microscopic analysis results corroborate those obtained at the macroscopic level.

13. l1 q l2 Microscopic damage analysis Global damage growth in term of micro-cracks length (including matrix and interface damage) • Delayed damage threshold • Decreased damage growth speed.

14. Viscosity effects on the fibres-matrix interfaces damage. CONCLUSION • Strain rate effects : • Insensitivity of the material elastic properties. • Delayed damage threshold. • Decreased damage growth speed. • Accommodation of the overall behaviour leading • to an increase of the ultimate characteristics. Integration of the experimental findings to set up physical damage modelling. Prediction of elastic visco-damaged behaviour.

16. THE END☺

17. homogeneous • e constant Experimental investigation Specimen optimised geometry : L1= 6 mm, L2 = 80 mm, L3 = 30 mm, R = 7 mm, e=3 mm Evolution of the longitudinal stress for a test calculated at 200/s

18. Experimental Methodology Analysis on the two scales of material Macroscopic analysis Microscopic analysis

19. Probabilistic approaches Weibull, Monte Carlo, ... Local criterion PROCESS Pr =1 - exp[(0202m (0 0 m) = f ( ) Matrix Polymer or Metallic Mori Tanaka’s approach Eschelby inclusion theory Matériau Homogène Equivalent . Fibres Architecture, Geometry, quantity, ... Behaviour law Visco-elastic damage Micro discontinuities . Experimental investigation Interrupted dynamic tests SEM and ultrasonic tests Damage growth D=f (e)  . ( e ) MULTI –SCALES MODELLING