Composite Materials Research Progress

Damage Evaluation and Residual Strength Prediction of CFRP Laminates … 175
The AE transducers were placed in a linear configuration located at a distance of 110
mm, as shown in figure 4A. The Elastic modulus was measured by means of HBM LY41-
6/350 strain-gauges.
Case study 1: results and discussion
The experimental results analysis and discussion is here organized in two phases: the first
one in which the only classical mechanical information (stress and strain) are considered, and
the second one in which the AE information is analyzed and related to the material
mechanical response.
The stress-strain curves in Figure 5 show the effect of the fibre orientation and volume
percentage and also the small, but not negligible, influence of the plies sequence in the
laminates.
In particular the AP diagram was qualitatively different from the other laminate ones
because of the different failure mechanism that was fibre-dominated for UD and QI and
matrix-dominated for AP.
In fact, the mechanical response of AP laminates with respect to in plane tensile load was
strongly nonlinear and the ultimate stress value was about 60 MPa. The two aspects that
determined the behaviour of AP laminate were the fibre direction (±45°) and the low volume
percentage of fibres. Visual inspection revealed also a marked necking due to the high
percentage of matrix forming the composite and the sliding of fibres in it.
Stress (MPa)
2500
2000
1500
1000
500
UD
QI2
QI1
QI3
AP
0
0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090
Strain
Figure 5. Examples of stress-strain diagram for the different types of laminates.
From the stress-strain diagram it was possible to determine some of the most significant
information regarding the mechanical response of laminates. In particular the elastic modulus
(EX) and the ultimate strength were estimated considering all tested specimens. In table 2 are
summarized the theoretical Elastic modulus, estimated by means of the lamination theory, and