Which of the following methods is primarily used for non-destructive testing?

The method that is primarily used for non-destructive testing is X-ray inspection. This technique utilizes X-rays to penetrate materials and create images that reveal internal features and defects without causing any damage to the item being examined. It is widely used in various industries to ensure the integrity and quality of components, especially in critical applications such as aerospace, manufacturing, and construction.

X-ray inspection provides valuable insights into the material's structure, such as weld integrity, voids, inclusions, and other anomalies, making it an essential tool for ensuring safety and performance without compromising the material's usefulness. Other methods, such as destructive testing, involve testing that intentionally causes damage to the specimen to understand its material properties, and thus are not classified as non-destructive. The remaining options, such as standards compliance audits and material fatigue testing, do not specifically pertain to non-destructive methods either, as they may not involve the direct examination of material integrity without the risk of damage.