Лако-красочные материалы — производство

Strain-based methods are effective SHM methods, because the presence of damage in the structure under operational loads can alter the local strain distribution due to the load path changing within the structure. Both electrical resistance strain gages as well as Fiber Bragg grating (FBG) optical sensors are usually used in the method to measure the strains. Due to the point nature of these sensors, strain monitoring is more suited to local structural ‘hotspots’ in large components rather than wide-area monitoring when individual or small no. of sensors are to be used. Nevertheless, thanks to the use of an array of sensors [27, 28, 29], as well as the multiplexing capability of FBG sensors, much wider area can be monitored, too.

The strain-based method can be performed by measuring the strain distribution of the intact structure in advance, which acts as the reference. Damage can be then detected when the strain measurement of the structure under loading condition significantly diverges from the reference strain.

FBG sensors provide a potential solution for strain based SHM techniques especially for composites and bonded structures. In composite materials, FBGs can be embedded inside the composite texture with a negligible effect on the mechanical properties [30, 31, 32, 33]. FBG strain sensors have been used in order to detect microscopic damages in composite laminates [34] and also for the investigation of delamination detection in CFRP laminates [35].

FBG sensors have been used to monitor crack growth in composite joints [36, 37]. Bernasconi et al., [27] investigated fatigue crack growth in adhesively bonded joints of thick composite laminates experimentally using an array of equally spaced FBG sensors. Sensors were applied to the side of a single lap tapered joint, in order to simulate its embedment into the composite laminate. They showed that this configuration of FBG array can detect and monitor a fatigue crack propagating in the adhesive joint.

Schizas et al., [38] used an array of five FBG sensors embedded between two unidirectional composite laminate plates near the adhesive bonding in a composite-Makrolon SLJ in order to investigate and monitor non­homogeneous strains. They concluded that FBGs can be used efficiently in non-homogeneous strain environments present in joints of dissimilar materials and delaminations and very useful in testing and monitoring durability.

One very efficient strain based technique for SHM is Backface strain (BFS) 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.