This invention relates to permanent, ultrasonic, flexible, dry-coupled, linear arrays for the inspection of pipelines, process equipment, and the like. The permanent, ultrasonic, flexible, dry-coupled, linear arrays detect and/or measure corrosion wall loss, stress corrosion cracking, and/or internal initiated pipeline cracking. The apparatus for ultrasonically testing materials includes a linear array of ultrasonic sensors, and a flexible, acoustically transmissive, dry-coupling surrounding at least a portion of each of the ultrasonic sensors.