But attaching a strain gauge to the surface of a concrete dam does not guarantee that the integrity of the dam's interior is being monitored. These coupons are particularly sensitive to crack initiation at points of high stress concentration such as at the apex of an acute angle junction. By subjecting the fuse to the same cyclic stresses as the structure, the onset of cracks can be spotted before the structure fails. Results from finite element analysis of fatigue fuse geometries. The philosophy is that the fuse need not necessarily be attached to the structure but can be cyclically stressed at a remote location where it can be optically monitored.
Cyclic fatigue is often the nemesis.
These periodic stresses can cause atomic vacancies, small voids, and dislocations to merge and form high stress regions that initiate crack movement. The fatigue fuse can be subjected to millions of cycles and give warning of the potential origins of failure. Photoelastic measurements permit visualization of the strains. Most commonly, optical fiber sensors have been employed for more than a decade. They are attached to a structure and monitor strains by measuring the optical polarization.