Evaluating the bonding condition of NASA spray on foam insulation using audio frequency sound absorption measurements
The bonding condition of the spray on foam insulation (SOFI) used to insulate the external tank of the NASA space shuttle can be found by using the audio frequency sound absorption coefficient. The ASTM El050 standard method for sound absorption measurements was used with an open ended one inch diameter cast acrylic impedance tube sealed to the SOFI with closed cell PVC foam. Two artificially disbonded locations, measuring 1.0 inch by 5.5 inches by 0.0625 inches and 2.0 inches by 8.0 inches by 0.0625 inches, were detected by peaks in the sound absorption coefficient spectrum. The peaks in the sound absorption spectrum between 1000 Hz and 4000 Hz were 25% to 50% higher over disbonded areas when compared to bonded locations. The maximum and minimum sound absorption levels for the foam ranged between approximately 0.1 and 0.3. The entire sample was scanned using the sound absorption peaks as indicators. Samples of 2 inch thick polystyrene foam were used with different sized defects at different locations in the foam to relate defect size and location to peaks in absorption coefficient spectrum.