Composite Materials Research Progress

Damage Evaluation and Residual Strength Prediction of CFRP Laminates … 183
counts over all test (∼ 1.4⋅10 4 counts) is lower than in the case of QI1 and QI2. Considering
the AE energy diagram of QI3, figure 10B, it is possible to observe that events with a high
energy content happens also in the early-middle stage of the test, differently than in the case
of QI1 and QI2 where events with a high energy content appear only at the final test stage.
All these facts are related to the QI3 ply composition: the matrix volumes of each ply
influences the dominant material failure mode, the matrix cracking, as in the case of AP.
All the previous considerations can be effectively summarized and completed considering
the diagrams of the sentry functions f.
In the case of UD laminate the construction of the function f reveals the initial material
damage, figure 6C, that is highlighted by the sub-function PI,1 and PII,1. This initial damage
can be related to some material internal adjustment and to the onset of some internal cracks
but, as revealed by the following PI,2, such damages do not compromise the material
capability of storing strain energy. The following PIV,1 sub-function reveals that the internal
cracks propagate and that the material storing energy capability is progressively reduced until
a sequence of drops due to splitting (PII,2, PII,3 and PII4) mixed with some strain energy storing
phases (PI,3, PI,4, PI,5) that prelude to the first and most critical drop PII,5. This drop is mainly
related to the free edge delamination and after this a new but small strain energy storing phase
starts: PI,6. During this phase all the laminate plies work as springs in parallel and go on
storing strain energy. After this phase next drops, PII,6 and PII,7, mixed with a slowly
increasing part of f, PI,7, and two constant trends for f, PIII,1 and PIII,2, prelude the final crisis of
the laminate.
Considering the diagram in figure 7C of the AP laminate it can be observed that the
structure of the sentry function is simpler if compared to the UD case. In fact only three subdomains
of strain energy storing phases, PI,1, PI,2 and PII,6, and three drops, PI,1, PI,2 and PI,3,
characterize the structure of the sentry function for the AP laminate reported in figure 10C.
The smooth trend of the sub-function of type I reveal the modest capability of the material to
store the strain energy and this fact is due to the high matrix volume percentage in each ply
and to the fibre orientation.
The initial trend of the sentry function of QI1 is characterized by a first material crisis,
PIV,1, followed by a drop, PII,1. Such behaviour is due to an initial material adjustment and to
some inter-laminar cracks onset that will contribute to delamination process. The next phase
is characterized by two strain energy storing phases, PI,1 and PI,2, respectively followed by a
sudden drop, PI,2, and a decreasing sub-function, PIV,2. In particular the sudden drop and the
decreasing sub-function indicate the onset of the material crisis due to delamination and
transversal cracks. After the sub-function PIV,2 the sentry function is characterized by a
complex combination of sudden drops and constant sub-functions. This behaviour indicates
that the material integrity is compromised and that each ply is progressively damaged until
the final crisis. In this way it is interesting to notice that after the PIV,2 there are four sudden
drops indicating the important crisis of each basic ply type (0°, +45°, -45°, 90°) that
constitutes the original laminate QI1.
In the case of QI2 the sentry function has a different trend with respect to the case of QI1.
In fact at the test beginning damage is not appreciable and a sub-function of type I, PI,1,
indicates a strain energy storing phase. After the first drop, PII,1, a consistent material damage
indicates the reduced strain energy storing capability. Such material damage is mainly due to
a global laminate loss of strength: the absence of delamination contributes to the cracks
distribution and growth inside all the deformed material volume and this creates the condition