Which of the following is a method of non-destructive testing?

Ultrasonic testing is a widely recognized method of non-destructive testing (NDT). This technique utilizes high-frequency sound waves to detect flaws or irregularities within a material. By sending ultrasonic waves into the material being tested, the technique allows operators to characterize internal structures without causing any damage. Reflected waves are analyzed to assess the integrity of the material, making it a valuable method for ensuring safety and reliability in various applications, particularly in industries such as aerospace, automotive, and manufacturing.

In contrast, the other methods mentioned involve destructive testing or practices. For instance, the bending test and material fatigue testing assess the mechanical properties and durability of materials by applying stress until failure occurs, which inherently damages the material. Therefore, these processes would not be classified as non-destructive methods, as they compromise the specimen's integrity to gather data.