Few manufacturing processes are so standardised, automated, and rigidly controlled that the product can be guaranteed perfect over large-scale mass production. If structures are to be constructed to meet design requirements and materials are to be used economically and efficiently, some form of testing of the finished product will almost certainly be necessary. Whenever production depends on human skills, human errors creep in and faulty products occasionally occur. With some small products, samples of production can be extracted and physically tested to destruction without great cost losses; proof tests can be done on a pressure vessel, or vibration testing can be carried out to simulate service conditions, but on many large structures such sampling or proof testing is virtually impossible. Also, if one postulates occasional human errors, sampling will not eliminate the defective items and on many critical components and structures 100 % inspection is often desirable. Non-destructive testing or inspection (NOT or NOI) are the terms used to describe a wide range of testing techniques designed to produce information about the condition of a specimen without doing any damage to it: i.e. after the testing the fitness of the specimen for use in service is unchanged.