Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Z. JENDLI*, J. FITOUSSI*, F. MERAGHNI** et D. BAPTISTE*. *LM3 UMR CNRS ENSAM Paris. **LMPF-JE ENSAM Châlons en Champagne. Micromechanical analysis of strain rate effect on damage evolution in discontinuous fibre reinforced composites

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 advantages of SMC composite material advantages of SMC composite material good strength/weight ratio manufacturing process devoted to large series high-energy dissipation with a diffuse damage. Passengers safety Structures lightweight Manufacturing and productivity. Interaction Manufacturing ↔ mechanical behaviour CONTEXT

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Industrial requests  Overall mechanical behaviour prediction under dynamicloading.  Overall mechanical behaviour prediction under dynamic loading. Inaptitude of current dynamic behaviour laws for the structures Inaptitude of current dynamic behaviour laws for the structures calculation in composite materials calculation in composite materials Insufficiency of the phenomenological approaches currently used. Insufficiency of the phenomenological approaches currently used. Development of multi-scale approaches. Material microstructure integration Material microstructure integration Physical description of damage mechanisms. Physical description of damage mechanisms. CONTEXT

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003  Behaviour stages  Damage threshold and accumulation Experimental analysis (at micro and macroscopic scales) Experimental analysis (at micro and macroscopic scales) AIMS  Visco-elastic  Visco-damage. Behaviour modelling using a multi-scale approach Behaviour modelling using a multi-scale approach Analysing effect in the SMC-R damage (  ). (  = – 400 s -1 )

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Interrupted high-speed tensile tests. Interrupted high-speed tensile tests. In-situ tensile tests. In-situ tensile tests. SEM observations. SEM observations.  Damage mechanisms experimental investigation MODELING COMPOSITE DYNAMIC BEHAVIOUR DYNAMIC BEHAVIOUR Introduction of s train rate effect in a micromechanical model Prediction of the dynamic m echanical behaviour and its interaction with behaviour and its interaction with the composite microstructure.

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 MATERIAL SMC-R26 composite Sheet Molding Compound-Random. glass E/polyester, discontinuous fibres 26%.  Hydraulic high speed tensile test machine 20 m/s, piezo-electric load cell 50 kN  Performed strain rates -4 s400s 10   TESTS PARAMETERS EXPERIMENTAL INVESTIGATIONS

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Strain rates effects on the SMC-R mechanical characteristics Strain rates effects on the SMC-R mechanical characteristics The composite macroscopic response is widely affected by  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. (  ).

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Strain rates effects on the SMC-R mechanical characteristics Strain rates effects on the SMC-R mechanical characteristics The first non-linearity Damage threshold The first non-linearity Damage threshold is considerably delayed in term of strain and stress.

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Strain rates effects on the SMC-R mechanical characteristics Strain rates effects on the SMC-R mechanical characteristics Behaviour accommodation when increases the strain rate  Behaviour accommodation when increases the strain rate :  Steady rise of the ultimate strain (38 %)  Maximum stress increases considerably.

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Damage analysis INTERRUPTED DYNAMIC TENSILE TEST TENSILE TEST Correspondence between ligament and force Ligament (mm) Force (N) , , , , The specimen geometry is a bar with dimension 36*6,5*2,7 mm3 The fuses material: PMMA (Poly Methyl Methacrylate) Fragile elastic behavior. Specimen/fuse Intermediate fixing Ligament SMC-R specimen Mechanical fuse 

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Macroscopic damage analysis Macroscopic damage vs. total strain for three strain rate values 0 1 E E D D  Damage initiation is considerably delayed in terms of strain thresholds Critical damage level : insensitive to the strain rate effect. D_critical

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003  Strain rate increase:  Delayed damage threshold  Decreased damage growth speed. Microscopic analysis results corroborate those obtained at the macroscopic level. Fibre-matrix interface damage description d_micro = fv_debonded / fv total. Overall behaviour accommodation  _ult Microscopic damage analysis

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Global d amage growth in term of micro-cracks length (including matrix and interface damage) Delayed damage threshold  Delayed damage threshold  Decreased damage growth speed. Microscopic damage analysis  l1l1 l2l2

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 CONCLUSION Strain rate effects : 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. Viscosity effects on the fibres-matrix interfaces damage. Viscosity effects on the fibres-matrix interfaces damage.

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003

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Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Experimental investigation Specimen optimised geometry : L 1 = 6 mm, L 2 = 80 mm, L 3 = 30 mm, R = 7 mm, e=3 mm Evolution of the longitudinal stress for a test calculated at 200/s  homogeneous  constant

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 Experimental Methodology Analysis on the two scales of material Macroscopic analysis Microscopic analysis

Composites Testing and Model Identification Châlons-en-Champagne, 28 January 2003 MULTI –SCALES MODELLING MatériauHomogèneEquivalent (  ). Matrix Polymer or Metallic FibresArchitecture,Geometry, quantity,... damage Micro discontinuities Mori Tanaka’s approach Eschelby inclusion theory Behaviour law Visco-elastic. PROCESS Probabilistic approaches Weibull, Monte Carlo,... Local criterion P r =1 - exp[(  0  2  0  2  m (  0   0  m) = f ( ).  Experimental investigation Interrupted dynamic tests SEM and ultrasonic tests Damage growth D=f ( Damage growth D=f ( 