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238 Chapter 8. Identification of damage in cracked beamsx 10 −62.5x 10 −521.6damage index1.51damage index1.20.80.50.405 10 15 20 25 30DOF05 10 15 20 25 30DOF(a)(b)Figure 8.16: Results from the CMC method – input-output experimental setup: damagelevels 3 (a) and 4 (b)x 10 −52x 10 −521.501−2damage index0.50damage index−4−6−0.5−8−1−10−1.55 10 15 20 25 30DOF−125 10 15 20 25 30DOF(a)(b)Figure 8.17: Results from the CMF – input-output experimental setup: damage levels3 (a) and 4 (b)The unfavorable influence of using the input-output setup for the early detectionand identification of cracks is confirmed by the results of the CSE, CMC and CMFmethod. Similar to the sensitivity-based techniques, the CSE and CMC methodare only able to correctly identify the last 2 levels of damage (Fig. 8.15–8.16). TheCMF method only succeeds in correctly identifying the last and most severe levelof damage (Fig. 8.17).
8.5. Conclusions 2398.5 ConclusionsIn this Chapter, the results were presented from damage identification experimentsperformed on a lightweight aluminum beam specimen, suffering from fatigueinduced crack propagation. Four increasing levels of damage were obtainedby subsequent dynamic loading of the beam in its previous (damage) condition.Special attention was paid to the effect of changes in boundary conditions, thatoccurred due to a repeated assembly/disassembly of the diagnostic measurementsetup, in between the damage stages. For this purpose, two different experimentalsetups were compared: a classic input-output setup, that included a mechanicalshaker, and an output-only setup were the excitation was provided acoustically. Aseries of repeated measurements were performed, involving the reproducibility ofthe experimental setups. Based on this information, together with vibration measurementson the beam in each of its damage cases, the presence of damage wasdetected on a statistical basis. The results showed a higher detection resolution(all 4 levels of damage) for the output-only setup than for the classic input-outputsetup (2 most severe levels of damage). This result could be expected due to thepresence of the shaker connection in the input-output setup.In addition, a number of damage localization techniques (discussed in Chapter 7)were applied to both the input-output and output-only damage identification experimentsperformed on the cracked test specimen. The sensitivity-based damagelocalization results were in agreement with the statistically-based damage detectionverdicts: the use of the output-only setup enabled a correct identification ofall considered damage levels, whereas the input-output setup only allowed a successfulidentification of the two most severe damage cases. In order to increasethe objectivity of the experimental findings, other damage localization techniques(that require similar information: natural frequency and mode shape estimates)were applied to the experiments as well. Although not all damage cases couldbe correctly identified on a basis of all of these methods (i.e., changes in modalflexibility method for the output-only setup), the deteriorating effect of additionalboundary changes – caused by the input-output setup – on the damage identificationperformance, was confirmed by all of the methods.
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288 BibliographyPintelon, R. and Sc
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290 BibliographyVerboven, P. (2002)
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