Uncertainty in Aerospace Composites An Overview for ITT3 Richard Butler 26th January 2016 Royal Academy of Engineering – GKN Aerospace Research Chair in Composites Analysis

Overview Aerospace structure Composites Multiscale Testing Modelling Research challenges

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”

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)

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…

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

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)

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)

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)