Acoustic Emission Monitoring of CFRP Laminated Composites ...

18 Chapter 2. Carbon Fibre-Reinforced Polymer Composites
reach saturation point and the rate of new damage start to decline. When
saturation occurs the matrix cracks start to form a regularly spaced pattern.
This pattern was discovered by Reifsnider, who called it the "Characteristic
Damage State" (CDS). 106
The formation of CDS is located at the
end of stage I, as shown in Fig. 2.4. Reifsnider determined that the saturation
state is a property of the laminate and does not depend on the load
history, environment, or residual stresses. The properties of the laminate
include the material system, geometry, and both the properties and the
orientation of the individual layers. Reifsnider explained that although the
CDS does not depend on the loading history, it does depend on the maximum
loading, i.e. if the maximum load level is not increased, the pattern
will remain roughly unchanged during the cyclic life. Changes in material
properties, i.e. corrosion and any damage which changes the load distribution,
will also change the CDS. The micro-matrix cracks do not cause
catastrophic failure when their number is low. However, as the number of
cracks increases they can start to coalesce and form regions of high stress
concentration. Stress concentrations can cause the development of critical
damage mechanisms, e.g. delamination and bre breakage. For this reason,
the state when the number of micro cracks reaches saturation point
(CDS) is generally considered to be the starting point for the formation of
critical damage mechanisms.
Stage II begins after the CDS stage has been reached. In this stage
micro cracks start to coalesce and delamination begins. 8
This stage is characterized
by a gradual, steady damage growth rate. 104
Both the stiness
reduction and damage accumulation become almost linear with the number
of cycles, but at dierent rates. The gradient depends on the stress amplitude.
103
This stage accounts for approximately 80% of the fatigue life.
Occasional deviations from linearity are often due to local delamination
initiation which occurs when transverse cracks link up. 8
In the third and nal stage the rate of damage accumulation increases
rapidly 104 and the stiness decreases signicantly. 99 The damage accumulation
is mainly due to irregular delamination growth, 2
e.g. coalescence
of delamination cracks. 92
Damage accumulation in this stage ends in a
catastrophic failure. The nal failure depends on several factors, such as
the geometry, the loading conditions and the lamina sequence. In some
composites failure can be in the form of a delamination, but in others the
damage growth can change into bre breakage.