Composite Materials Research Progress

240
Maria Pia Cavatorta and Davide Salvatore Paolino
other but which represent different stages of the penetration process with the impactor
moving deeper and deeper into the specimen as the impact energy increases.
Penetration and perforation thresholds increase with thickness, so does the range of the
penetration process. In other words, while for thin laminates the difference between the
penetration and the perforation thresholds can be negligible, for thick laminates the same can
become quite significant. For cross-ply glass-epoxy composite laminates, Liu [17] found:
Pn
= 0.
8t
P
r
0.
0247
where t is the laminate thickness. Equation (4) indicates that for the investigated glass/epoxy
laminates, the penetration threshold is about 80% of the perforation threshold. In case of 3mm
thin laminates, the range of penetration process (Pr–Pn) is less than 2 J. For 6-mm
laminates, a difference of 15J is found, while for 12-mm thick laminates, (Pr –Pn) exceeds
100J and by far can not be neglected.
In addition to identification of the laminate Pn, Pr and range of penetration process, the
energy profile was used by Liu to define a coefficient η, named the Efficiency of Energy
Absorption. The coefficient is defined as the ratio between the area bounded by the
polynomial regression line of equation (2) up to Pn and the horizontal axis and the area of the
rectangular triangle having for hypotenuse the bisector from zero to Pn. The bisector of the
energy profile represents the equal energy between impact and absorption; therefore, the
triangular zone corresponds to the highest energy-area the material can possibly have. As all
materials have an energy absorption capability less than 100%, the regression curve is always
below the bisector. However, the closer the regression curve to the bisector, the higher the
energy absorption capability of the laminate.
An interesting analysis of the energy profile was provided in [19]. By normalizing the
impact energy and the absorbed energy by the laminate Pn, Mian and Quaresimin were able to
obtain a single master curve which proved to work very well when thin laminates were
investigated. A direct consequence of the existence of a master curve is that, when normalized
by the laminate Pn, the efficiency of energy absorption is basically constant for all laminates,
i.e. η varies linearly with the laminate Pn.
The range of penetration process yet remained to be investigated. To this aim, a new
variable, named the Damage Index (DI), was recently introduced by the Authors [30-32]. The
DI definition aroused considering that in the range of the penetration process, the impactor
moves deeper and deeper into the specimen as the impact energy increases. On the contrary,
pure energy variables as the DD by definition saturates to one over the entire penetration
process.
s
≈
0.
8
(4)
MAX
DI = DD
(5)
sQS
The value sMAX in equation (5) refers to the displacement value recorded at the instant
when the force approximately reaches a constant value, in case of impact tests that cause