Full-Scale Helicopter Blade Torsion-Tension Testing

A research collaboration conducted at CNRS-LMA to support the experimental validation of a FEM progressive damage model in fatigue, using notched composite coupons representative of helicopter blade material. The coupons — woven carbon/PEEK quasi-isotropic laminates with a central straight notch — were tested under a combined tension and alternating torsion load cycle, replicating the multiaxial fatigue environment that blade structural elements experience in service.

The central contribution to this programme was in the test methodology itself: defining how the experiment should be instrumented and conducted, and establishing the full-field strain measurement approach using a 3D digital image correlation system to feed directly into the FEM model validation workflow.

Validating a progressive damage model against real fatigue behaviour requires more than measuring load and displacement at the machine crosshead — you need the full strain field across the specimen, including around the stress concentrator where damage initiates. The notched coupon geometry was chosen precisely because the central straight notch creates a well-defined strain concentration zone that challenges the model to predict both the elastic strain distribution and the evolution of damage as cycling progresses.

Defining the test procedure meant working through how the combined loading should be applied — tension and alternating torsion simultaneously, representative of the multiaxial fatigue the blade material experiences in flight — and how the DIC measurement system should be positioned, calibrated, and synchronised with the load cycle to capture meaningful strain data at the relevant phases of loading. Getting this right before the test begins is critical: a DIC measurement that isn't properly set up for the load path and the speckle pattern on a carbon/PEEK surface will miss the strain gradients around the notch that the model needs to be compared against.

The 3D DIC correlation itself produced full-field surface strain maps at each measurement step, resolving the strain distribution across the notch zone with a spatial resolution that strain gauges alone could not provide. These maps — capturing both the in-plane strain components and the shear strains that the torsion load introduces — were the direct experimental input to the FEM damage model comparison: rather than comparing only global stiffness or failure load, the model's predicted strain field could be checked against the measured one field-by-field, giving a far more demanding and informative validation basis.