13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- also conveniently evaluate the evidence in the Bayesian model class selection. However, the proposal distribution of the TMCMC is the random walk of Gaussian distribution, which cannot explore the local properties of the posterior PDF well. So the slice sampling is used as the algorithm of the candidate value for TMCMC, which is named as TMCMC-slice. The improved TMCMC-slice algorithm, which has integrated the advantages of the TMCMC and slice sampling method, is used to generate the sample of the unknown parameters. Based on slice sampling, the generation of the candidate value can automatically adapt to the local features of the post PDF with fewer user-adjusted parameters. Though the gradually transitioning from the prior PDF to the post PDF, TMCMC can easier sample from the intermediate PDFs than other methods. And the ratio of Metropolis-Hasting in each iteration step could make the sample concentrate the region of high probability, which improves the convergence properties of the TMCMC-slice algorithm. 3. Experimental verification 3.1. Experimental apparatus and procedure A 304 stainless steel plate, which is attached by four bolts on a support, was used to simulate one part of the mechanical structure. The geometric parameters of the plate are 500 mm by 500 mm and 3 mm thick. Twenty-nine gratings were bonded on the surface of the plate according to the symmetry of the structure. These gratings were assigned to four individual fibers in order to facilitate wiring, and then connected with the four channels of the optical demodulator produced by the Micron Optics Inc. The sampling rate of the FBGs was set to 2 kHz. There were also 15 accelerometers to monitor the modal parameters of the structure. The sampling rate of the accelerometer was also set to 2 kHz. The arrangement of the FBGs and accelerometers sensor network is shown in Fig. 2. Providing the excitation for the plate structure, a vibration exciter was attached to the center of the plate using a screw with a 3mm diameter. This vibration exciter was controlled by the LMS modal testing system, which produced the 0~8 kHz broad-band stochastic excitation. Figure 2. Layout of the FBG and accelerometer sensor network The structural defect usually appears in most mechanical structures because of stress concentration,
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