Deformation and damage evolution of a full-scale adhesive joint between a steel bracket and a sandwich panel for naval application

Abstract

The increasing interest for the application of adhesive joints in naval superstructures motivates researchers to gain an in-depth understanding of the mechanical behaviour and failure mechanisms of these joints. This work reports on an experimental study of the deformation behaviour and damage evolution of a full-scale multi-material joint using different instrumentation techniques. Adhesively bonded joints of steel to sandwich panel components have been subjected to quasi-static tensile tests during which the global deformation of the joint and local strain distributions were monitored using digital image correlation (DIC). During one particular tensile test, fibre optic Bragg sensors (FBG) were also applied to the specimen’s surface at different locations in order to quantify the evolution of local strains. Additionally, acoustic emission (AE) sensors were installed in order to monitor damage initiation and evolution with increasing levels of imposed deformation. This test showcased adhesive failure at the interface of the steel adherend and the adhesive, while cohesive failure was observed within the adhesive and skin failure at the interface between adhesive and the composite skin of the sandwich panel. The post-mortem observed failures modes were compared to the acoustic events that originated during the test due to damage initiation and propagation within the joint. The evolution of the different sensor signals, i.e. the damage expressed as cumulative AE energy and local strains measured with Bragg sensors and DIC, are mutually compared and acceptable correlation is found.

Adhesive Joint Integrity Monitoring Using the Full Spectral Response of Fiber Bragg Grating Sensors

“…Instead of directly looking for defects, there are relatively more economical techniques suitable for real-time monitoring of the integrity degradation of adhesive joints. These include strain/stiffness monitoring using back face strain gages [21][22][23], resistance monitoring of adhesive joints made conductive by adding carbon nanotubes [24,25] and optical fiber sensors signal surveillance [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Strain gages can only be applied to the outer surface, but they will disrupt an otherwise smooth surface and are susceptible to environmental degradation.…”

“…There are different kinds of optical fiber sensors. In adhesive joints, both the distributed sensing [29][30][31] and the discrete fiber Bragg grating (FBG) sensors have been used [8,[32][33][34][35][36][37][38][39][40][41][42][43]. The distributed sensors provide a spatially continuous strain measurement along the whole fiber and can reveal the location of any perturbation in strain caused by damages in the bond.…”

“…The distributed sensors provide a spatially continuous strain measurement along the whole fiber and can reveal the location of any perturbation in strain caused by damages in the bond. With the discrete single peak FBG sensors, the peak wavelengths were often logged and converted to strain [32][33][34][35][36][37][38]. In this way they behave as strain gages but have advantages such as embeddability, multiple sensors on the same fiber and immunity from electromagnetic interference.…”

Hygrothermal Damage Monitoring of Composite Adhesive Joint Using the Full Spectral Response of Fiber Bragg Grating Sensors

“…Instead of monitoring moisture content or directly looking for defects, there are techniques suitable for real time monitoring of the integrity degradation of adhesive joints. These include strain/stiffness monitoring using back face strain gages [ 49 , 50 , 51 ], resistance monitoring of adhesive joints that were made conductive by adding carbon nanotubes [ 52 , 53 ] and optical fiber sensors signal surveillance [ 43 , 44 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 ]. Strain gages can only be applied to the outer surface, they will disrupt an otherwise smooth surface and are susceptible to environmental degradation.…”

“…There are different kinds of optical fiber sensors. In adhesive joints, both the distributed sensing [ 57 , 58 , 59 ] and the discrete fiber Bragg grating (FBG) sensors have been used [ 30 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 ]. The distributed sensors measured strain along the fiber and is equivalent to a train of strain gages with the best available spatial resolution going down to the millimeter range.…”