Experimental And Numerical Study Of Static Penetration And Low-Velocity Impact Of Foam Sandwiched Composites With Different Ply Angles Face Sheets

Composite sandwiched structures have been widely used in the fields of aircrafts and high speed vehicles, etc. Investigations of the mechanical properties of sandwiched composite structures play a vital role in deciding their applicability in various engineering fields. After years of effort, along with several achievements, new difficulties have also been encountered with the emergence of many novel sandwich structures in the recent years. The quasi-static penetration behaviors and low-velocity impact responses of these structures have been investigated experimentally and numerically. The advances on the mechanical properties of foam sandwiched composites have been reviewed from several aspects, including the quasi-static, low-velocity impact, failure mode and the finite element model of composite sandwiched structures.The objectives of this work are to experimentally and numerically investigate the impact damage and failure mode of foam sandwiched composite with different ply angle face sheets. In order to evaluate the internal and external damage resulting under quasi-static and low velocity impact, the sandwiched composite samples were tested using an instrumented MTS (Material Test System) machine, and on an instrumental Instron Dynatup9250HV-Drop Weight Impact Testing Machine. From FEM (finite element method) model, the impact damage mechanisms were revealed to show the damage initiation, progression, and the failure of the composite panel. The combination of different stacks sequences of carbon fiber prepreg and foam structures, the impact behaviors of such kind of materials are not well investigated. Finally, the experimental results have been compared with the numerical results at several impact energies in terms of contact load histories, peak load, and absorbed energy of sandwiched structures.From the quasi-static penetration tests and low-velocity impact tests of foam sandwiched composite with different angles face sheets, the load-displacement curves were obtained to characterize the failure mechanisms of the face sheets and the core. Failure modes were studied by sectioning the samples at the impact location and observing under optical microscope. The results evaluated from static tests showed that the sandwiched composites with unidirectional face sheets have the highest peak load. On the contrary, the dynamic testing indicated that the foam sandwich with unidirectional face sheet have lowest peak load, lowest displacement at peak load and minimum energy absorption. It was also observed that largest damage size, highest penetration depth and shear cracking were experienced by unidirectional as compared to cross-ply, angle-ply, and quasi-isotropic face sheets. Mechanical behaviors of sandwiched structures were found to be strongly dependent on the loading rate. In the case of static loading, the structure had a ductile behavior. However, for impact loading, the sandwiched structure behaved in a brittle manner and failed catastrophically.Finite element analyses were conducted for analyzing the static responses of foam sandwiched composite with different ply angle face sheets. The FE (finite element) software, ABAQUS was employed to simulate static behaviour of foam sandwiched composite. A crushable foam model was used in order to explore core behaviors, while the Hashin criteria predicted extent of the failure of the face sheets. The load-displacement curves illustrated a good agreement between the experimental and numerical results in terms of overall trend. The finite element calculations were also used to obtain the failure modes including core damage, face sheet damage, and face-core interface damage. It was observed that the damages at the core can be classified as either core cracking or core crushing. However, damages to the face sheet were through matrix cracking and delamination, followed by fiber breakage, which lead to significant indentation. The face-core interface was typically induced by the cracks being initiated from the tensile side and propagates to the compressive side.Finally, the low-velocity impact properties and impact damage response of foam sandwiched composite with different ply angle face sheets were investigated numerically. The FE software, ABAQUS was also employed to simulate low-velocity impact properties of foam sandwiched composite. A crushable foam model was used in order to explore core behaviors, while the Hashin criteria gave an insight about the failure of the face sheets. The contact load histories, peak load, and energy absorption were obtained to compare the numerical and experimental results at several impact energy levels. The failure morphologies, damage size and damage shape were evaluated and compared with different types of sandwich structures. The comparisons illustrated a good agreement between the experimental and numerical results. From the investigation, it was found that the orientation angles and the stacking sequences of the face sheets will influence the impact behaviors, including the impact damage peak load and impact damage modes. From the appropriate combination of the different lamina sheets, the foam sandwiched laminated composites will have high impact damage tolerances and energy absorptions.We hope the conclusion of this thesis could be extended the designing of foam sandwiched composite structures, such as T-beams, shell, and plates.