Composite Materials Research Progress

166
Giangiacomo Minak and Andrea Zucchelli
In particular the prevision of the performances (e.g. stiffness, damping) and the strength
limits (e.g. tensile, compressive and fatigue) of this kind of material is an important task for
the material scientists and engineers.
Numerical models for the composite laminate progressive failure are currently developed
by the researchers for different applications [1-3] .
These models require an experimental validation by means of tests in which damage
progression is monitored in a suitable way.
Nowadays, different techniques are proposed such as electrical resistance [4], fibre Bragg
grating sensors [5], photo-elasticity [6] and acoustic emission [7-9].
On the other hand, residual strength evaluation after fatigue or impact loading is
important for the determination of composite components reliability. In fact, laminate
composite materials have a wide application in light-weight structural members. In particular
fibre-reinforced plastics are increasingly used in airborne structures and the long range
passenger airplanes of the future may include many important parts of the fuselage and
components made with Carbon Fibre Reinforced Plastic (CFRP). This class of materials is
characterized by outstanding strength-to-weight and stiffness-to-weight ratios. Nevertheless
their resistance to accidental damage is an important issue for the designer. In particular,
CFRPs are very susceptible to internal damage caused by transverse loads such as indentation
and impact, while the probability of such loadings occurring during the manufacture, service
or maintenance of composite structures is very high [10]. This lack of resistance to low
velocity and low energy impact damage [11-13] is one of the main obstacles to a more
widespread application of these composite materials, especially in the case of a thermo-set
matrix like epoxy.
A threshold, conventionally located at 20 m/s, divides the impact problems into two
fields, high and low velocity, due to the different types of induced damage [14].
In the low-velocity impact field, a quasi-static loading can simulate the actual behaviour,
since the vibrational effects are negligible [15, 16]. In fact, many researchers [17-23] use
load-displacement histories to compare structural responses from impact and quasi-static tests
and they find that both the dynamic and static responses have corresponding load drops due to
failures in the laminates.
Low velocity and low energy impact damage usually consists of matrix cracking [24, 25]
and delamination [23, 26, 27], while debonding and fibre breaking occur for higher impact
energy values [28, 29].
As said, besides the behaviour of the material during an impact, an issue of great interest
is the evaluation of the post-impact resistance characteristics of CFRP. In fact, damage due to
impact often can be present in the component before it is put into service and loaded.
To detect the damage level present in the laminate and the damaged zone area, several
techniques are used, such as simple visual inspection , C-scan and X-ray. AE event counts are
also utilized to predict the residual tensile strength (RTS) after impact [30].
Due to the importance of delamination, which decreases locally the buckling load, much
effort has been spent in researching the compression after impact (CAI) performances of
composites [31, 32]. Nevertheless tensile [30, 33, 35] and fatigue properties [36, 37] are also
important to predict the component failure.