Editing Nondestructive testing (section)

===Structural mechanics===
Structure can be complex systems that undergo different loads during their lifetime, e.g. [[Lithium-ion battery|Lithium-ion batteries]].<ref>{{cite journal
|author = Waldmann, T.
|year = 2014
|title = A Mechanical Aging Mechanism in Lithium-Ion Batteries
|journal = Journal of the Electrochemical Society
|volume = 161
|issue = 10
|pages = A1742–A1747
|doi = 10.1149/2.1001410jes
}}</ref> Some complex structures, such as the [[turbo machinery]] in a [[liquid-fuel rocket]], can also cost millions of dollars. Engineers will commonly model these structures as coupled second-order systems, approximating dynamic structure components with [[spring (device)|springs]], [[mass]]es, and [[dashpot|dampers]]. The resulting sets of differential equations are then used to derive a transfer function that models the behavior of the system.

In NDT, the structure undergoes a dynamic input, such as the tap of a hammer or a controlled impulse. Key properties, such as [[Displacement (vector)|displacement]] or [[acceleration]] at different points of the structure, are measured as the corresponding output. This output is recorded and compared to the corresponding output given by the transfer function and the known input. Differences may indicate an inappropriate model (which may alert engineers to unpredicted instabilities or performance outside of tolerances), failed components, or an inadequate [[control system]].

Reference standards, which are structures that intentionally flawed in order to be compared with components intended for use in the field, are often used in NDT. Reference standards can be with many NDT techniques, such as UT,<ref>{{cite web|url=https://customers.phtool.com/custom-reference-standards/edm-notch-reference-standards-gallery/|title=EDM Notch Reference Standards » PH Tool|website=customers.phtool.com}}</ref> RT<ref>{{cite web|url=https://customers.phtool.com/custom-reference-standards/radiography-rt-reference-standards/|title=Radiography (RT) Reference Standards » PH Tool|website=customers.phtool.com}}</ref> and VT.