Comparison of modal frequencies

A general decrease in modal frequencies is seen very similar to the earlier case of 0^o plies. However, the general decrease in modal frequencies in this case is less prominent compared to 0^o plies. Healthy laminate has first modal frequency at 25 Hz. It decreases marginally to 23.8 Hz for both the cases i.e. Delam Type-I and Delam Type-II. Similar marginal decrease is observed in other three modal frequencies. These changes are shown in Figures 39.3 (a), (b) and (c). Similar to the earlier case of 0^o plies, additional local peaks are present in both types of delaminations. In Delam Type-I, three local modes are present at 823 Hz, 902.5 Hz and 961.3 Hz. Peaks present in the healthy laminate are flattened out in the delaminated specimen at 1876.5 Hz and 1976 Hz probably due to increase in damping. Three local modes are present at 823 Hz, 902.5 Hz and 961.3 Hz in Delam Type-II. Peaks present in the healthy laminate are flattened at 1700 Hz, 1876.5 Hz and 1976 Hz in the delaminated specimen, again possibly due to increased damping.

Figure 39.3 (a) Shift in natural frequencies corresponding to B1 and T1 modes due to delamination in 90 0 plies

Figure 39.3 (b) Shift in natural frequencies corresponding to the B2 mode due to delamination in 90^o plies

Figure 39.3 (c) Shift in natural frequencies corresponding to the B32 mode due to delamination in 90⁰ plies