Mechanical Properties Of Composites Sandwich Structures

Since composite sandwich structures have excellent properties, they are widely used in the fields of high technology such as aeronautics and astronautics, etc. It is very important to investigate the mechanical properties of composite sandwich structures, which decide whether composite sandwich structures can be applied in engineering fields. After years of effort, a plenty of results have been obtained by scholars. However, new difficulties have been encountered with the emergence of a lot of novel sandwich structures in resent years. The quasi-static indentation response, low-velocity impact response, residual strength after impact and hypervelocity impact response of composite sandwich structures have been investigated by theoretical, numerical and experimental methods in this dissertation.The process of composite sandwich structures is introduced in chapter one and the advances on the mechanical properties of three-dimensional braided composites are reviewed from several aspects, including the quasi-static indentation response, low-velocity impact response, residual strength after impact and hypervelocity impact response of composite sandwich structures. In addition, the background and significance of the present project are demonstrated in this chapter.In chapter two, the quasi-static indentation response of foam sandwich beams and plates reinforced by fiber columns have been investigated theoretically and experimentally. Based on the superposition principle, a new model is established for predicting the indentation response of sandwich beams with and without reinforced fiber columns. The analytical predictions on indentation behaviors are in good agreement with experimental data. Furthermore, the analytical solution of indentation response of foam sandwich plate reinforced by fiber columns is derived by the principle of minimum energy and is compared well with experimental results. According to analytical and experimental results, advantages in the indentation resistance of foam sandwich structures reinforced by fiber columns are obvious compared with traditional composite foam sandwich structures.In chapter three, the low-velocity impact response of carbon fiber composite lattice structures is investigated by experimental and numerical methods. Impact tests on composite plates are performed using an instrumented drop-weight machine (Instron 9250HV) and two kinds of new damage modes are observed. Different impact energies and impact locations are considered. A three-dimensional finite element model is built by ABAQUS/Explicit and user subroutine (VUMAT) to predict the peak loading and simulate the complicated damage problem. It can be found that numerical predictions coincide well with experimental results.Low-velocity impact characteristics and residual tensile strengths of carbon fiber composite laminates are firstly investigated experimentally and numerically in chapter four. The quasi-static tension response of carbon fiber composite laminates are simulated by the explicit finite element method and its user subroutine (VUMAT) in order to improve precision and efficiency of damage simulation. Two different tensile damage modes after different impact energies are observed. Using this calculation method, low-velocity impact characteristics and residual tensile strengths of carbon fiber composite lattice core sandwich structures are investigated. The critical impact energy which can only obtain from the experiment is obtained. The degradation of residual tensile strengths can be divided to three stages for different impact energies, and amplitudes of degradation are affected by stacking sequences. These results are very important to the design and service of carbon fiber composite lattice core sandwich structures.In chapter five, hypervelocity impact experiments of carbon fiber composite laminates and carbon fiber composite lattice core sandwich structures are conducted. Energy absorption efficiency of metal monolithic and carbon fiber laminates is compared under the same ballistic velocity and energy. It can be seen from experimental results that the energy absorption efficiency of metal monolithic is higher than one of carbon fiber laminates; Instead, the energy absorption efficiency of metal monolithic is smaller than one of carbon fiber laminates. Analogous conclusions are gained for the energy absorption efficiency of metal and carbon fiber composite lattice core sandwich structures. Next, the numerical method is employed to simulate hypervelocity impact behavior of carbon fiber laminates and carbon fiber composite lattice core sandwich structures. Numerical results are compared well with experimental results. Optimized resistance concept is proposed by computing energy absorption efficiency of different thickness laminates. Total mass of resistance structures can be declined under this optimized resistance concept.