Composite Materials Research Progress

D
Damage Evaluation and Residual Strength Prediction of CFRP Laminates … 191
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014
Strain
Figure 20. Damage for QI3 laminate types.
Case study 2: materials and method
Eighteen graphite/epoxy composite square laminate plates 250x250 mm 2 were studied.
Their thickness was 1.6 mm. They have been made in autoclave from pre-pregs by stacking
eight unidirectional plies with quasi-isotropic orientations [0,90,45,-45]s.
The specimens were placed in a circular clamping fixture with an internal diameter of
200 mm and they were loaded at the centre: an hemispherical hardened steel ball with a radius
of 7 mm was indented in the top centre point of the laminate by means of a servo-hydraulic
Instron 8033 testing machine controlled by an MTS Teststar II system and equipped by a
25kN load cell.
The specimens were loaded monotonically out-of-plane in control of displacement and
the head speed was 0.05 mm/s.
The tests were stopped at three different damage levels, one (Low) corresponding to the
load value of 2 kN, the second (Medium) corresponding to the first load drop in the loaddisplacement
curve and the third (High) to the complete perforation of the plate.
During the test, the AE has been monitored by a Physical Acoustic Corporation (PAC)
PCI-DSP4 device with four transducers PAC R15 setting up the amplitude threshold at 40 dB.
In figure 21a it is possible to see the fixture system equipped with AE piezoelectric
sensors and in figure 21b the complete experimental setup.
After each quasi-static test, the damaged plate was sliced by a diamond saw to obtain
tensile specimens with the same geometry suggested by ASTM D 5766 for open hole testing
of CFRP, a width of 40 mm and a length of 250 mm. The indented zone was in the centre of
these tensile specimens and the whole damaged zone was included in the specimen width.
The damaged zones size was previously identified by means of the localization tool of the
AE system as it is shown in figure 22 for the High damage level.
QI3