Acoustic Emission Monitoring of CFRP Laminated Composites ...

20 Chapter 2. Carbon Fibre-Reinforced Polymer Composites
sures for evaluating composites. Such measures include non-linearity and
energy loss, also known as hysteresis. Huang et al. estimated the dissipated
energy from the load-displacement curves for predicting failure in
composites. 112
Although it is an interesting approach, their model did not
treat laminates with delamination and was only tested using static load.
Dzenis does not believe that hysteresis will be useful for monitoring
damage development. 87
He believes that the cumulative damage increments
will be too small to have a signicant eect on the stress-strain diagram.
There are, however, other ways of working with hysteresis. Thompson
et al. analyzed the hysteresis loops of three dierent wood-based panels:
OSB, chipboard, and MDF. 113
Four parameters were extracted from
the loops and monitored. The rst parameter, the loop area, represents
the energy dissipated during one loop. The second parameter, the dynamic
modulus, is the gradient of the line drawn from the two extreme points of
each loop. The third parameter, the fatigue modulus, is the gradient of the
line from the origin to the upper extreme point of each loop. This parameter
combines the eects of both fatigue and creep. The last parameter,
the microstrains, represents the deection. Thompson et al. demonstrated
how the information provided by these parameters could be interpreted.
Kim and Matthews suggested that the Specic Damping Capacity (SDC)
would be an interesting way of presenting the variations in loop area. 114
SDC is dened as the ratio of the energy dissipated in one cycle to the
total energy stored in the same cycle. They also pointed out that since the
area of one loop represents energy, the SDC can be computed as the ratio
of one loop area to the next one. Using this technique Guild reported that
SDC allowed him to detect cracks in unidirectional composites, which were
not detectable by visual inspection. 115
The technique also allowed them to
detect cracks which were not discernable using an infrared camera.
2.4 Discussion and Summary
This chapter introduced carbon bre-reinforced polymer composites. A
brief introduction was given on the properties of the constituent materials,
the advantageous material properties of composites and also how one can
design the properties of an composite. Emphasis was put on discussing
what faults and defects can be found in composites, their formation and