Large-Format CFRP Membrane Inflation Testing

An instrumented inflation test campaign on glass fibre/epoxy composite membranes, fabricated in-house in two formats — circular specimens of approximately 80 cm diameter and large square panels reaching 2,500 × 2,500 mm. The contribution to this research programme was hands-on and end-to-end: from managing the cure of the membranes themselves through to running the measurement systems during the inflation tests.

Three independent measurement systems — strain gauges, a 2D digital image correlation setup, and a pressure gauge — were operated simultaneously and cross-validated against each other, giving both local point measurements and full-field surface strain data at every pressure step.

The membranes were fabricated specifically for this test programme. Curing was carried out under heat blankets with the temperature cycle monitored and logged via an Arduino-based acquisition system, giving a controlled and traceable thermal history for each specimen before it reached the test rig — particularly important for large-format panels where temperature uniformity across the surface matters for the resulting laminate properties.

Strain gauges were bonded directly to the membrane surface, with full calibration carried out before each test: zero offset correction, Wheatstone bridge balancing, and shunt calibration to verify the measurement chain from gauge through to data logger. This calibration step is often treated as routine, but on flexible composite membranes where surface preparation and adhesive cure affect gauge behaviour, it is a meaningful source of measurement uncertainty if not done carefully. Wiring was routed to avoid constraining the membrane's out-of-plane displacement during inflation, so the gauges could track the developing strain field without mechanically influencing it.

The 2D DIC system was set up to measure the full strain field across the membrane surface simultaneously with the gauge readings. Having both systems operating on the same surface at the same time made it possible to cross-check local gauge readings against the DIC-derived strains at the same locations — a direct validation of both measurement systems against each other rather than relying on either one in isolation. Where the two agreed, confidence in the measurement was high; any discrepancy pointed immediately to either a gauge installation issue or a DIC calibration or speckle pattern problem that could be investigated and resolved before the data were used further.

Pressure was recorded synchronously with the strain and DIC data via a dedicated gauge integrated into the acquisition unit, so every strain map from the DIC could be unambiguously tied to a specific pressure level throughout the inflation sequence. Managing the timing and synchronisation of three independent measurement channels across a large membrane test — where data rates, trigger timing, and coordinate system alignment all need to be consistent — was a core part of the experimental responsibility on this programme.