Determining the Inspection Grid for Automated Ultrasonic Testing of Large Forgings

Large forgings for turbine and generator rotors are characterized by very low ultrasonic attenuation. Due to “phantom indications,” only low pulse repetition frequencies are often possible. Consequently, the spacing of the test points in the scan and index directions (test grid) significantly affects the inspection time—and thus the inspection costs—during automated ultrasonic testing of large forgings. The test grid must therefore be agreed upon between the manufacturer and the customer. Since the applicable standards and other regulations do not provide clear guidelines on this matter—or may even lead to gaps in coverage with insufficient inspection sensitivity—the Subcommittee on Automated Inspection Systems of the DGZfP Technical Committee on Ultrasonics has developed a guideline for defining the inspection grid for a complete (100%) volumetric inspection. The introduction of a standardized grid quality Rn establishes boundary conditions for gap-free and multiple-overlapping scanning. Furthermore, an average scan quality Rd is introduced, which indicates the average scanning with the -6 dB sound beam within the inspection volume. Based on this, criteria for optimizing the inspection effort can then be derived. To calculate the normalized grid quality Rn and the average grid quality Rd, the test areas must first be defined. From these, the sound paths are determined, for which the sound beam diameters required to meet the test sensitivity must be calculated. Depending on the incident sound situation, these can be calculated directly or through the projections of the sound beam diameters—as a function of the incident angle—for both flat and curved surfaces (e.g., the surface of a cylinder). The procedure for calculating the inspection parameters is explained using the example of a wheel disc inspection.