1. Uncertainty in Aerospace Composites An Overview for ITT Richard Butler th January 2016 Royal Academy of Engineering – GKN Aerospace Research Chair in Composites Analysis

2. Overview Aerospace structure Composites Multiscale Testing Modelling
Research challenges

3. Aero structures & composites
Wing skin (stiffened cover)
Wing backbone (spars and ribs)
Laminated composites
Probability of catastrophic failure: 1 in 109 flying hours (= 1 in 5,000 aircraft x 40,000 flights x 5 hours)
Highly regulated manufacture, in-service operation, inspection & maintenance
Most composite structures are stiffened shells with multiple (redundant) load paths: “fail safe”

4. Manufacturing: defects & quality control
Example: wrinkling defects in curved laminates
Section of defect (from physical cut or X-ray CT)
Ultrasonic image
Design requirement: allow for strength reduction due to manufacturing defects by testing (small) curved elements containing representative defects
B-basis criterion applied (at least 90% of the population will exceed strength with 95% confidence)
Assume this strength for all (large) curved laminates
Damage growth can be difficult to detect (below surface) hence not permitted (unlike metals)

5. Multi-scale performance: testing
Pyramid of testing
Are manufacturing defects & in-service damage representative?
Are test boundary conditions representative of component?
Expensive
Constrains design
Need for a model-test based approach…

6. Multiscale Modelling of Aerospace Composites
Finite element analysis: Homogenization of fibre-resin layers (& laminates) with mean elastic properties – without defects

7. Finite Element Analysis (FEA): Edge effect in test
Roller
L-shaped specimen
Applied load, P
Qualification of aerospace structures is validated with a programme of testing. Out-of-plane loading can be assessed by 4-point bend test on small specimens
Along free edges of composite laminates, severe stress concentrations (singularities) occur
Strength of narrow specimen is influenced by the edge effect. Wide aerospace parts are not.
(F)

8. Edge effect assessment by FEA
Reduction of edge effects
X-ray CT of test specimen with resin edges Detailed FEA
Test & detailed FEA confirms resin edge treatment in test coupon increases component strength by up to 20%
Within 10% of wide part strength
Fine scale FEA (5m elements) within 5% of test (with little or no defect)

9. Research Challenges Use of reduced set of test data
Monte Carlo defect modelling (on m scale) in large-scale structures
Prediction of rare events – simultaneous failure in redundant structure
Defect/damage characterisation
Allow slow damage growth (in-situ inspection)