Quality control and non-destructive testing

Appropriate test methods for the control of fabrication procedures and non-destructive testing (NDT) are basic requirements for the formation of structural adhesive joints. Such methods should be based upon relatively easily measurable parameters that have a close identity with the properties of the bonded assembly that need to be controlled. However the quality of bonded joints depends upon many factors, requiring a range of very different procedures. 

There are two essential but different quality aspects to be considered. The first is the adhesive strength of the bond between the polymer and the substrate, requiring control of the quality of that interface. The second is the quality of the adhesive strength of 

If the adhesive layer was always to represent the weakest link in the joint, the quality of the assembly would be limited to an assessment of the cohesive properties of that layer. Unfortunately it is the interface which tends to be of greater concern because it is here that joint failure is more commonly encountered, particularly after ageing. The most useful tests for indicating interfacial quality are in fact pre-bonding surface inspection procedures, which can nevertheless suggest potential joint performance. The tests used for assessing cohesive properties of the adhesive layer must necessarily be post-bonding activities. 

Most of the NDT techniques which have been developed are associated with determining cohesion quality. However those techniques currently available have been developed largely for the aircraft industry, and are not generally applicable to the scale, location or materials involved in civil engineering structures. It is hoped that this situation will change in response to demand, and a brief review of some promising methods is included where appropriate. 

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