When considering catastrophic failures in structures, one often imagines the worst-case scenario, in which a large force can cause an immediate destruction. However, events of these types are rare in nature. More often, a structure succumbs to fatigue failure long before any major destructive event can befall it. This fatigue can be an insidious threat to structures, as it is often imperceptible until the damage has occurred, resulting in a need to repair the structure and costing time and money. Thus, in the case of the aircraft industry, as more airframes see an extended life cycle, monitoring the strain and fatigue experienced by the aircraft has become increasingly important.

CURRENT METHODS OF MEASURING STRAIN

Strain Gages

Relatively inexpensive and reliable, strain gages have been used for decades to provide effective point-based measurement of strain in a material. As shown in Figure 1, the strain gage is made of a long, thin wire embedded in a thin material that is affixed, usually by adhesives, to the surface of whatever structure is being observed. When force is applied in a strain gage, it causes a geometric change in the gage, resulting in a change in the resistance of the gage as well. Applying Ohm’s law allows one to solve for the current, which can be translated into strain.

One downside to strain gages is that they must be individually wired and emplaced on a structure and connected to a data acquisition system. Accordingly, these limitations make large-scale wide-area monitoring by strain gages impractical.

Fiber Bragg Grating

A newer development for widearea monitoring is the Fiber Bragg Grating (FBG) sensor. As Yolken and Matzkanin describe in a paper for the Nondestructive Testing Information Analysis Center (NTIAC) [2]: Since the advent of photo-induced Bragg gratings in optical fibers in 1978, Bragg fiber gratings have found many applications in telecommunications and sensing. Bragg gratings have emerged as elegant in-fiber sensors particularly suitable for multiplexed and distributed applications. The growing interest and rapid progress made in the area of strain sensing using Bragg grating based sensor systems indicate recognition of the fact within the sensor community that fiber Bragg grating based sensors provide powerful sensing techniques which can be uniquely applied to a range of structural sensing applications. read more