Composite Materials Research Progress

In: Composite Materials Research Progress ISBN: 1-60021-994-2
Editor: Lucas P. Durand, pp. 237-256 © 2008 Nova Science Publishers, Inc.
Chapter 7
DAMAGE VARIABLES IN IMPACT TESTING
OF COMPOSITE LAMINATES
Maria Pia Cavatorta and Davide Salvatore Paolino
Mechanical Engineering Department – Politecnico di Torino,
Corso Duca degli Abruzzi, 24 – 10129 Torino (Italy)
Abstract
The Chapter presents an overview of the damage variables proposed in the literature over
the years, including a new variable recently introduced by the Authors to specifically address
the problem of thick laminates subject to repeated impacts. Numerous impact data are used as
a basis to address and comment potentials and limitations of the different variables. Impact
data refer to single impact events as well as repeated impact tests performed on laminates with
different fiber and matrix combinations and various lay-ups. Laminates of different thickness
are considered, ranging from tenths to tens of millimeters.
The analysis shows that some of the variables can indeed be used for assessing the
damage tolerance of the laminate. In single impact tests, results point out the existence of an
energy threshold at about 40-50% of the penetration energy, below which the damage threat is
quite negligible. Other variables are not directly related to the amount of damage induced in
the laminate but rather give an indication of the laminate efficiency of energy absorption.
Introduction
Composite laminates are expected to absorb low velocity impacts either during assembling or
use. Even when the impact damage is barely visible, the incurred micro-damage may have a
significant effect on the laminate strength and durability. The impact energy can be absorbed
at any point of the laminate, well away from the point of impact, and by means of various
laminate level failure mechanisms including front face indentation (indicative of local matrix
crushing and local fiber breakage), interlaminar delamination, back face splitting and fiber
peeling. In the literature [1-11], it is acknowledged that matrix cracking is the first type of
damage introduced during impact; however, the presence of matrix cracks per se does not
significantly change the overall laminate stiffness. Rather, the matrix crack tips may act as