Module 5: Experimental Modal Analysis for SHM
  Lecture 39: Composite plates with 90o plies
 

Comparison of modal damping

The damping values (in percentage) for the representative modes B1, B2, B32 and T1 are determined using 3dB method for all the laminates with 900 plies. These are detailed in Table 39.1. The damping changes are quite prominent for first and second bending modes but no fixed pattern of increase in damping is observed in this case as observed in the case of laminates with 0o plies. This change is probably due to transverse orientation of the fibers.

Table 39.1 summary of modal parameters for 900 laminate

S.No.

Property

Damage Type

Mode shape

B1
T1
B2
B32

1

Modal frequency,Hz

Healthy

Delam type-I

Delam type-II

25.0

23.8

23.8

45.1

45.0

43.8

81.3

80.2

67.8

238.3

217.5

182.5

2

Modal Damping, %

Healthy

Delam type-I

Delam type-II

 

11.65

20.13

11.49

5.84

5.96

4.44

16.60

16.60

22.43

3.21

2.71

3.32

3

Displacement, µm

Healthy

Delam type-I

Delam type-II

 

5.742

5.499

6.944

0.994

0.865

0.952

0.980

0.963

1.271

0.337

0.398

0.558

4

Velocity, µm/s

Healthy

Delam type-I

Delam type-II

 

850.7

1040.3

860.1

281.1

244.5

276.9

500.3

284.4

519.1

506.4

620.2

649.3

 

Comparison of displacement and velocity from frequency response plots

Magnitudes of displacement and velocity for the representative modes obtained from the frequency response plots are summarized in Table 39.1. A change in velocity amplitude pattern is observed in case of Delam Type-I. In this case, where a decrease in velocity is observed for torsion and second bending mode which is opposite to what is observed in the case of 0o plies.