To detect internal damage in structures you can use Impact Echo Method.

Ultra pulse velocity or ground penetrating radar

.Some of these methods are discussed in these articles

It is greatly depends on the type of internal damages. You may use the followings:

a) Electromagnetic waves - able to detect the damage that be detected based on its dielectric changes before and after the damage

b) Mechanical waves - able to detect the damage based on impedance changes

c) Nuclear radiation - able to detect the damage based on energy of damage

image processing is one of the methods that can be used for internal damage

Lumped Damage Mechanics is a method that permit to evaluate RC framed structures and steel. Dual System (walls and frames)

image morphology and X Ray scanning is the useful method.

Take a look at NDE web manual designed by FHWA

https://fhwaapps.fhwa.dot.gov/ndep/TechniqueLocatorNew.aspx

Article An overview of 38 least squares-based frameworks for structu...

Georadar .

Tomography:

http://www.ein.org.pl/sites/default/files/2018-03-11.pdf