Structural Health Monitoring Using Smart Sensors - ideals ...

damage is expected to be revealed. For example, the tension in the stay cables of cablestayed
bridges can be estimated by natural frequency measurement (Gardner-Morse &
Huston, 1993). The frequency changes, however, may have low sensitivity to damage as
mentioned earlier. Begg et al. (1976) pointed out that cracks have little influence on global
modes and suggested that local high-frequency bending modes of the individual members
provide a better indication of cracking. Srinivasan and Kot (1992) conducted experiments
on a shell structure and found that the resonant frequencies of the shell structure were
insensitive to damage. Also, frequency shifts depend on each mode. Salawu (1997)
mentioned that the stress induced by modal deformation is minimal at modal nodes, where
modal displacement is zero; a small change in a particular modal frequency could mean
that the defect may be close to the modal node. Hearn and Testa (1991) developed a
damage detection method that examines percentage changes in natural frequencies. To
relate observed frequency changes or percentage changes to structural damage, these
methods require theoretical structural models as well as a damage model or sensitivity
analysis; these model estimations and the analyses are not straightforward (Salawu, 1997).
Fluctuation in damping values is much larger than that in frequencies (Williams &
Salawu, 1997) and can potentially be a damage indicator. High damping would suggest
more energy dissipation mechanisms, indicating the possibility of cracks in the structure
(Morgan & Osterle, 1985). Changes in damping values up to 80 percent were reported by
Agardh (1991). Williams and Salawu (1997), however, pointed out that damping
properties are the most difficult to model analytically and can only be realistically
obtained through vibration tests. Nashif et al. (1985) and Sun and Lu (1995) give
comprehensive discussions. Relatively large estimation and modeling errors are against
the usage of damping for SHM.
Mode shape information has also been investigated. West (1984) compared mode
shapes of a space structure, using Modal Assurance Criterion (MAC), before and after
exposure to loading. The mode shapes were partitioned and changes in the local MAC
values along the mode shapes were used to locate the damage. Fox (1992) stated that
mode shape changes are insensitive to damage and “Node line MAC,” a MAC variant
based on measurement points close to a node point for a particular mode, is a more
sensitive indicator of damage. Pandey et al. (1991) demonstrates that absolute changes in
mode shape curvature, calculated through difference analysis of displacement mode
shapes, can be a good indicator of damage. Chance et al. (1994) and Nwosu et al. (1995)
used measured strains instead of the calculated curvature, reducing the numerical error
associated with difference analysis. Salawu and Williams (1994), however, pointed out
that the selection of which modes are used in the analysis is an important factor, and
curvature changes do not typically give a good indication of damage using experimental
data. Mode shape phase has also been reported to be a possible damage indicator. Mode
shapes of proportionally damped dynamic systems are theoretically purely real, i.e., they
are aligned within a plane, resulting in either 0 or radian phase. Structural damage,
especially damage with friction type mechanisms such as loose bolt connections, often
introduces nonproportional damping, which results in complex mode shapes, i.e., the
phases will differ from 0 and These phase changes can potentially indicate structural
damage, although structural damage is not the only cause to change the phase of mode
shapes (Nagayama et al., 2005). While some mode shape related indicators reflect
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