18 декабря, 2015 
Pokraskin 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.
The work of Crocombe et al., [40] regarding BFS technique concluded that the BFS response was highly dependent on the location of the sensor and the greatest change in backface strain with damage occurs in the overlap zone. Therefore, the position of the strain sensor should be inside the overlap because here it will produce the largest change in BFS with damage. Shenoy et al., [41] investigated the crack initiation and propagation behavior in SLJ using BFS technique and showed that fatigue failure in adhesively bonded joints goes through various stages, starting from an initiation period, then a slow crack growth period followed by a faster growth period before a rapid quasi-static fracture.
Solana et al., [42] investigated the fatigue life of adhesively bonded joints using backface strain technique. They used six electrical strain gauges (SGs) placed along the overlap, 3 on each end of the substrates to monitor fatigue initiation and propagation within the adhesive layer. This configuration was able of discriminate the damage profiles at both ends, showing where it initiated and which side was damaged as a consequence.
As BFS technique is based on sensing the pattern of strain of the backface, strain sensors are needed for the sensing. Both conventional electrical resistance strain gages as well as FBG sensors can be used for BFS techniques.
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