Two parameters for bond strength evaluation are calculated. The first one is the velocity of the ultrasonic guided wave and the second is the difference in the characteristic frequency between the two recorded signals. The characteristic frequency is calculated as the weighted mean frequency and the velocity is calculated by the crosscorrelation and LSF methods. The averaged parameters that were calculated are summarized in Table 2. The parameters obtained in Table 2 were plotted in Fig. 6 with
Figure 6: Calculated velocity and characteristic frequency (a) 0 mm
spacer thickness (b) 0.1 mm spacer thickness.
a 
confidence interval. According to Fig. 6 the best bond quality is
obtained by A and AP type surface treatment, while the worst case is the
H type. Actually
all the bonds were found to have a reasonable strength, 2.8<V<3.1, which can
be explained by the fact that all the samples were prepared as well as
possible and no environmental degradation was applied. A perfect bond
corresponds to a velocity of V=3.2 and so, even A and AP type are
approaching this number but not really reaching it. The delamination case
correspond to V=2. All the other types of bonds are between these extremes.
| . | 0 mm spacer thickness | 0.1 mm spacer thickness | ||
| Bond type | Velocity | Characteristic frequency | Velocity | Characteristic frequency |
| A | 3.087 | 0.309 | 3.067 | 0.685 |
| AP | 3.09 | 0.363 | 3.067 | 0.735 |
| T | 2.871 | 0.035 | 2.821 | 0.342 |
| AZ | 2.942 | 0.117 | 2.842 | 0.388 |
| H | 2.797 | 0.004 | 2.795 | 0.23 |
| | UTonline | |