Traditionally scanning is conducted using manual techniques or using linear actuator stages. Manual techniques are susceptible to human factors – steadiness of the hand; mood; difference in opinion or interpretation. Linear stages are limited in their movement capability – perhaps only 1D or 2D scanning will be possible – but have many advantages over manual methods in terms of automation leading to consistency and cost-effectiveness. It is becoming increasingly common to deploy robots or collaborative robots (cobots) to extend the capability into 3D and beyond (e.g. ‘6-axis’ scanning) and to deploy more than one robot for large or complex structures.

Theta develops linear-stage and cobot scanning systems [link to or videos/images of RASCAL and 4G rig, 3G rig]. These are high-precision, fully automated systems which provide detailed NDT analysis of structures without human intervention.

We explain below the techniques that we use for scanning NLA.

1. Pulse Inversion

Pulse Inversion (PI) uses two pulses of ultrasonic energy. The first is transmitted into the structure, and the response of the structure is captured. After the structure has returned to equilibrium, a second pulse is transmitted. The latter is a phase-inverted (180 degrees out-of-phase) replica of the first pulse. The response of the structure is captured for a second time. These two responses are then summed to provide the residual response of the structure. If the structure is linear, the residual will be zero. If the structure is nonlinear, the residual will be non-zero, containing harmonics of the original pulse frequency.