Standard DTI FAQs
Can surface coatings be used on the faying surfaces when DTI's are used to control bolt pretension?
Yes. For "pre-tensioned" joints (that is, non-"slip-critical"), the faying surfaces can be coated with whatever you want. You don't have to hold the coating back in the area of the connection so you can save money on fabrication and get very good corrosion protection as well.
It is advantageous to use DTI's in this situation because for very thick paints and for hot dipped galvanized surfaces, since the coating is compressible, a part-turn procedure is inadvisable because it merely compresses the material and doesn't stretch the bolt. DTI's will work whatever the coating on the faying surface since the snug pass (defined as partial compression of the DTI) exerts a force about half or more of final tension on the joint and will pre-compress the coating.
Also the DTI will read tension no matter what is occurring within the "clamp". The final pass on the bolts fully compresses the DTI's and as the South Carolina work shows (see paper entitled "DTI's Mean Less Bolt Relaxation, Creep" available for download from this web site) even when the coating is extremely thick the presence of a DTI minimizes creep in the joint.
