I was told that another problem is that traditional Non Destructive Testing Techniques don't work on composites so finding voids in the structure or identifying cracks is much more difficult. Any gurus know what techniques are being used nowadays?
Not true. As well as theremography mentioned by Arrakis, conventional ultrasonics are effective in finding delaminations and disbonds. Eddy currents are not effective, but then composites rarely exhibit the type of cracks that occur in metals unless they are associated with other damage which can be detected using ultrasonics. The old tap test is effective in finding delaminations close to the surface, but not so effective in thicker composite structure, and while it may be effective in finding disbonds and delaminations, it is important that other more accurate methods (ultrasonics or thermography) are used to determine the size of the defect prior to repair.
Remember that NDI is only capable of finding air gaps. It can not find weak bonds which are still in contact but which have a high potential for subsequent failure. I once asked an over-confident technician to tell me where a sample I had just made actually had good bonds. He easily found the deliberate void in the centre and then confidently told me the rest of the sample was well bonded. Pity it was joined together using double sided tape! NDI is effective at finding bad bonds. It can not give total assurance of good bonds.
Regards
Blakmax