A vibration based damage identification method is investigated experimentally for a 2.5-dimensional composite structure. The dynamic response of an intact and a locally delaminated 16-layer unidirectional carbon fibre PEKK reinforced T-beam is considered. A force–vibration set-up, including a laser vibrometer system, is employed to measure the dynamic behaviour of the T-beam. The Modal Strain Energy Damage Index algorithm is applied using the bending and torsion modes. Special attention is paid to the effect of the number of measurement points required to detect and localize the delamination accurately.