1. School of Engineering
University of Surrey
Guildford, Surrey
GU2 7XH, UK
A Physically-Based Model for the Notched Strength of Woven Quasi-Isotropic CFRP Laminates
H.M.S. Belmonte a, S.L. Ogin a, P.A. Smith a, R. Lewin b
a School of Engineering, University of Surrey
b Rolls Royce plc.
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2. Introduction
In previous work, it was shown that in woven GFRP laminates with circular central holes there was matrix cracking and fibre tow fractures at the hole edge prior to ultimate load and catastrophic failure.
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3. Introduction
A physically based model (CDG Model) was shown to give good predictions of notched strength with out reliance of fitted parameters.
Present work investigates the ability of the CDG model to predict the notched strength of quasi-isotropic woven CFRP laminates.
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4. Experimental Material: Six different Quasi-isotropic CFRP Laminates.
Reinforcement: Plain Weave (PW) and Five Harness Satin Weave (5HS).
Thickness: 3 thicknesses investigated for each weave.
Fibre : Toray T300 high strength carbon fibre.
Matrix : Vantico MY750 epoxy resin system.
Preparation: C-scanning, sectioning, end tabbing & notching
Testing: Volume fraction, Mechanical testing and damage investigations.
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5. Experimental Testing Mechanical Testing: Fracture Mechanics:
Un-notched - Young's Modulus, Poisson’s Ratio and Un-notched Strength.
Notched – 2.5, 5 and 10 mm Circular Centre Holes in mm Wide Coupons and 20 mm Circular Centre Holes in 120 mm Wide Coupons.
Fracture Mechanics:
SEN Calibration & SEN Fracture.
Damage Studies – Loading notched coupons to just below ultimate load.
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6. Material Properties
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8. Analysis
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9. Fracture Mechanics Properties
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10. Damage Investigations
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11. Layer 1
Layer 2
Layer 4
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12. Layer 4
Layer 8
Layer 7
Layer 10
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13. Notched Strength Modelling
Whitney Nuismer (WN) failure criteria (Whitney Nuismer 1974)
Semi-empirical
Require elastic properties, unnotched and notched strength
Parameter must be calculated for each laminate investigated
Critical Damage Growth (CDG) fracture model (Soutis and Fleck 1990)
Uses physically meaningful parameters
Considers the growth of a notch edge crack

14. Whitney Nuismer
Point Stress Criterion (PSC) - failure occurs when the stress reaches the unnotched laminate strength at a characteristic distance, d0, ahead of the notch.
Average Stress Criterion (ASC) - failure occurs when the average stress over a distance from the notch edge, a0, reaches the unnotched laminate strength.

15. WN Criteria Strength Predictions
WN Criteria Strength Predictions
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17. Critical Damage Growth
Analysis is based upon the stable growth and subsequent catastrophic failure of a damage zone at the notch edge
Crack Growth - defined using the ASC of WN model
Failure predicted when stress required for crack propagation is equal to stress required for crack growth.
Crack Propagation - given by a fracture mechanics approach

18. CDG Strength Predictions
CDG Strength Predictions
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20. CDG Parametric Study
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22. Conclusions
Range of mechanical property has been obtained for quasi-isotropic CFRP laminates
Fracture from circular holes studied showing tow fracture occurring at hole edge prior to catastrophic failure
WN models result in accurate predictions (ASC more accurate than PSC), however fitted parameters need to be re-evaluated for each new case
CDG model produced good results for quasi-isotropic CFRP laminates as it did for quasi-isotropic GFRP laminates
Parametric study showed that varying both the CDG input parameters by 10% only gives a variation of 10% in the notched strength predictions.
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