Structural Health Monitoring Using Smart Sensors - ideals ...

Table 8.2. Mass Normalization Constant Estimates from Imote2 Data
Without Mass Perturbation
Natural Frequency (Hz)
With Mass Perturbation
Mass Normalization
Constant
20.4680 20.3722 0.004776
32.7488 32.1563 0.001474
41.2410 40.9012 0.007941
64.5915 64.0984 0.009026
69.3455 69.0262 0.005865
95.5995 95.1737 0.011405
1
Normalized accumulated stress
0.8
0.6
0.4
0.2
0
3 4 5 6 7 8 9 1011 7 8 9 10 111213 1415 1112 1314 151617 1819
Element ID
Figure 8.7. Normalized accumulated stress in sensor community 1, 2, and 3 (the mass
perturbation DLV method).
potentially damaged elements. Figure 8.7 shows the normalized accumulated stress
calculated for the three sensor communities. Element 8 is identified as the element with a
normalized accumulated stress smaller than the threshold value of 0.3. Note that the stress
in element 18 is also small. The DLV method identifies potential damage sites as elements
with small stress. The method does not exclude undamaged elements from a set of
elements with small stress. Gao (2005) pointed out that such false-positives in damage
detection do not appear repeatedly; repetitive measurements and damage detection can
distinguish damaged elements as elements which consistently show small stress. The
results in Figure 8.7 support that the mass perturbation DLV method on Imote2s can
detect damage.
The SDLV method is then employed as the damage detection method. This method
does not require estimation of mass normalization constants illustrated in Figure 7.24. The
same set of sensor communities, 1, 2, and 3 again measures acceleration responses before
and after element 8 is replaced with an element of reduced cross section; subsequently, the
SDLV method is applied. Using the conversion matrix, the normalized accumulated
stresses are calculated from DLVs. Figure 8.8 shows the accumulated stresses estimated
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