Models and methods to simulate low-energy impact damage on composite aerospace structures

This Thesis describes the research activities, performed during the PhD course in Aerospace Engineering, whose objective is to develop and to validate numerical models and methods to evaluate the performances and the damage state of composite aeronautical structures subjected to low-velocity impacts. A three-dimensional damage model has been developed to simulate the progressive failure of thin composite structures when subjected to static and dynamic loads. Both intralaminar and interlaminar damage mechanisms have been considered and the constitutive model has been developed on the basis of the thermomechanics of the nonlinear irreversible physical process. The intralaminar damage mode has been analysed in the context of the Continuum Damage Mechanics, whereas the Cohesive Zone Model has been used to simulate the interlaminar damage mechanism. The objectivity of the numerical discretisation has been assured using the smeared crack formulation. The developed damage model has been implemented in a commercial finite element code by means of dedicated user-defined subroutines and subsequently subjected to a first validation by means of a selection of test cases from the Literature; the objective in this phase is to reproduce the main damage mechanisms, i.e. fibre breakage, fibre kinking, matrix cracking, matrix crushing and delaminations. Moreover, a dedicated experimental campaign has been carried out in order to obtain an extended material database of mechanical properties of a thin carbon fibre-reinforced plastic to be used for the validation of the numerical simulations. The experimental campaign consisted of tensile and compressive static tests in the longitudinal and transverse directions, shear tests, Three-Point Bending tests and low-velocity impact tests. In the last part of the Thesis, numerical simulations of some experimental tests are described, particularly focusing on the low-velocity impacts. The effective capability of the developed numerical tools to predict the performances of the structure in terms of contact forces and intralaminar and interlaminar damages is evaluated. The numerical damage models can be very helpful to better understand the several damage mechanisms of the composite structures and, despite the considerable computational costs and numerical difficulties, they represents a powerful tool to assist the design of composite structures.