Damage Tolerance of Impacted Composite Sandwich Structures

Sammanfattning: This thesis deals with damage tolerance of impacted sandwich structures for load bearing applications. Composite sandwich structures find wide application as lightweight solutions in aerospace components, since weight reduction and less fuel emissions are primary concerns for aircraft manufactures. Sandwich structures are built of stiff face-sheet materials bonded to a low-density core material. In this thesis, the face-sheets are composite materials reinforced with carbon fibre non-crimp fabrics whereas the core consists of a closed cell foam material. Sandwich structures are susceptible to impact damage and even a small amount of damage can reduce the residual strength of components significantly. Therefore, damage tolerance assessment of such structures is essential and needs to be taken into account in the design process.Main objective of this thesis is assessment of test methodologies for estimation of compressive properties of foam core materials. An extensive experimental study of different densities of closed cell foam materials is presented and existing test standards are evaluated in this regard. Two different test methods were investigated for strain measurements of the foam material during compression testing assisted by a digital image correlation technique. A parametric study was also performed to investigate the effect of in-plane specimen size on the compressive modulus measurements. Both homogenized and stochastic finite element models are used to back the experimental observations. Different types of boundary conditions were used to simulate the effects of in-plane specimen size and prediction of compressive modulus. The findings were also used as basis for recommendations for updating current test standards.A part of the thesis work concerns the design and construction of a new drop-weight impact rig for low-velocity impact testing of sandwich structures. A test setup was designed to capture the true impact response without adulteration by oscillations. A novel catch mechanism was designed and implemented for preventing secondary impact. A detailed experimental evaluation and uncertainty analysis was also performed to evaluate the drop-weight rig in terms of repeatability and precision.The developed drop-weight rig was used to perform low-velocity impact characterization of sandwich structure with different face-sheet thicknesses. A range of impact energies were investigated for the identification of low level damage (LLD), barely visible impact damage (BVID) and visible impact damage (VID). A thorough fractography study was performed to understand the damage mechanisms at different energy levels and for different face thicknesses. A finite element model was developed to simulate the impact response and delamination extent, including both inter-laminar and intra-laminar damage modes.Finally, the impact damaged specimens were tested for damage tolerance assessment. Both symmetric and asymmetric specimen configurations with different face-sheet thicknesses were investigated. The effect of face-sheet thickness on the residual strength of sandwich structures was studied. Three different test methodologies for damage tolerance testing were investigated and the results were compared. A finite element model was developed for simulation of the edgewise compression test methods and the residual strength predictions were compared with the experimental results.