Backface strain

There are a number of techniques in the field of SHM for analyzing structural integrity of bonded composite joints. Most of the conventional monitoring systems, such as microscopic failme analysis, ultrasonic. X-ray, thermography and eddy current method are off-line techniques [10, 11], i.e., it s not possible to monitor real time during the service life. But for adhesively bonded joints, real time in-situ monitoring is required. SHM techniques which enable the possibility of in-situ monitoring include acoustic emission, carbon nanotubes network, active vibration method and backface strain based technique. 

One very effieient strain based technique for SHM is Backface strain (BPS) measurement in order to monitor crack initiation and propagation under both static and fatigue loading in adhesive joints. In this method, sensors are placed onto the exposed surface i.e., backface of the material to monitor the strain. This technique and sensors associated with this method is described in detail in the following section. 

Backface strain (BFS) measurement is a technique that can be used for the in-situ monitoring for SHM of crack initiation and propagation in adhesively bonded composite joints. Zhang et al., [39] assessed the use of the BFS technique in bonded joints. Their work with single lap joint (SLJ) allowed for finding out that by detecting the switch in the direction of the backface strain crack initiation can be monitored. 

Case Studies on Backface Strain Technique FOR Fatigue Crack Growth Monitoring... 

Bemasconi A, Comolli L. Proc. SPIE. 8421, 2012, 84214Y. An investigation of the crack propagation in a carbon-fiber bonded joint using backface strain measurements with FBG sensors. 

Solana AG, Crocombe AD, Ashcroft lA. Int J Adhe Adhes, 2010,30, 36-42. Fatigue life and backface strain predictions in adhesively bonded joints. 

In this case, two types of sensors were applied in order to reconstruct the strain distribution along the backface of aluminum substrate of the joint. Both electrical strain gauges (SG) and FBG sensors were used on different specimens. Strain gauge arr s were used for two specimens, one without any defect and another with an artificial of 5.7 mm. FBG arrays were used for other two specimens, one without any crack and the other with a pre-crack of 10 mm. 

From the two studies discussed above, it is clear that, in order to measure the entire profile of BFS along the backface of the bonded region, the use of an array of strain sensors is a feasible solution. 

---