Noise-limited thermal/infrared nondestructive testing

• Detection limits may be determined by introducing noise parameters into numerical modeling.
• The contribution of multiplicative and additive noise components to the inspection process is discussed.
• The Thermal NDT signal noise emanating from a material being tested is evaluated.
• The use of noise evaluation in the specific applications of Thermal NDT is discussed.

In thermal/infrared nondestructive testing (T/I NDT), the presence of noise in the infrared signal is an inconvenience in the method. It is convenient to introduce two extreme types of T/I NDT noise: (1) this type is purely additive and is defined by background reflections and the IR detector and (2) this is purely multiplicative and is defined by the material's absorptivity/emissivity variations. Multiple T/I NDT tests performed on various materials have shown that none of materials reveal a ‘pure′ additive or multiplicative type of noise. However, in the case of optical heating, many composite and black-painted materials exhibit multiplicative noise with a noise contrast of Cn=2−5%, and this determines the defect detection limits. The Cn concept has been applied to a graphite epoxy composite to demonstrate that the maximum depth of detected defects in a one-sided procedure is about 4 mm. Also, in 1–10 mm-thick black-painted steel, the minimum detectable material loss is from 3% to 9%.