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.