TWI scientists have developed a method of incorporating ultrasonic data acquired from impact-damaged carbon fibre reinforced polymer (CFRP) laminates directly into simulations capable of predicting residual strength. The ground-breaking work was completed as part of a collaborative project known as IntACom - improving the inspectability of aerospace composite materials.
Increasing the usefulness of NDT data
Barely visible impact damage (BVID) from low-energy impacts can lead to a significant reduction in the residual strength of CFRP laminates.
There is currently no accepted method of incorporating ultrasonic scan data directly into predictive finite element models. The ability to relate non-destructive testing (NDT) data to mechanical performance is vital to allow damaged composites to be assessed rapidly and effectively prior to continued use, repair or replacement.
Seamless model generation
The methodology was developed around the widely used compression after impact (CAI) test. The simulations account for both inter- and intra-laminar damage onset and propagation.
The generation of the finite element models is fully automated, requiring only the ultrasonic scan data which is used to define delamination geometries in each ply interface as illustrated in Figure 1.
X-ray computed tomography (XCT) revealed that matrix cracking along the fibre direction always occurs in the intra-ply regions between delaminated interfaces, regardless of layup. This observation was idealised in simulations and is illustrated in Figure 1
A comparison was made between experimental and simulated CAI tests in 16-ply, quasi-isotropic (Q-iso) and cross-ply (X-ply) lay-up sequences. Impact energy levels of 14, 22, 30 and 38J were considered.
Achievements
The unique methodology of incorporating ultrasonic data into CAI simulations has provided:
- Accurate stiffness / CAI strength predictions within 10% and 15% of the average experimental results for Q-iso and X-ply layups respectively
- Good correlation between experimental and simulated delamination propagation prior to CAI failure
- Good correlation between experimental and simulated strain gauge results prior to CAI failure
- Buckling modes and propagation of delaminations, which induce final collapse, were well predicted (Figure 2)
The project validated the concept of incorporating ultrasonic data into predictive simulations. The methodology can be easily adapted to account for changes in lay-up, material, loading conditions and geometry, potentially allowing the simulation of whole components subjected to any desired loading conditions.
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The IntaCom project is a collaboration between TWI Technology Centre (Wales), Rolls-Royce, GKN,
Bombardier, and the Welsh Government.