EMATs for on orbit wall remaining thickness measurement after an impact--feasibility study

Space nondestructive testing (NDT) looks likely to grow if important projects such as the International Space Station are achieved. These kind of orbital facilities may collide with fragments coming from the 11000 previous satellites launched since Sputnik in 1957. An estimated three thousand tons of fragments are orbiting the Earth with an average satellite speed of 11 km/s (6.85 mi/s) and a density of about 2.8 g/cm3 (175 lb/ft3). The European Space Agency (ESA) wants to have practical NDT tools for diagnose and repair of collision damage. This is the only way to ensure a lifetime of 15 years for the European ISS module, the Columbus Orbital Facility (COF). Hurling projectiles onto aluminum plates demonstrated that a large variety of defects are created after a high-speed impact. In particular local thinning down was observed at the center of craters (see Figure 2). Conventional UT with piezoelectric transducers is not applicable for thickness measurement in space environment because the vacuum and wide variations in temperature do not permit the use of any liquid couplant. For this reason electromagnetic acoustic transducers (EMATs) are a good alternative. The objective of this study was to prove their efficiency under space conditions. It was concluded that temperature variations are not a problem if EMATs are used for wall thickness measurement purpose on the COF, in space environment. Results demonstrate that a probe generating 0 deg shear wave provides values with relative errors lower than plus /minus 5% between -150 (-238 deg F) and +170 deg C (+338 deg F), for a thickness 1-5 mm (0.04-0.2 in.). Development should be continued to optimize the probe, particularly regarding the lift-off effect, and to adapt the whole system to space standards and procedures. EMATs might also be used for crack detection and sizing, for example with probes generating surface or guided waves. But other NDT tools such as eddy currents could prove to be more efficient.