Indentation and impact of sandwich structures
School of Mechanical and Aerospace Engineering
High specific strength and stiffness dominate the choice of material selection in the fields of weight sensitive industries such as aerospace, automotive, wind turbine and ship building. To this end, sandwich constructions form a better choice compared to the conventional bare metal plates of same thickness. For practical load-bearing applications, it is often an important task to assess the sandwich structures for its safe working as well as its operational life under the given working conditions. In the present thesis, quasi-static indentation and low velocity impact loadings on the foam cored sandwich plates is investigated. Initially, finite element (FE) models are developed for predicting the low velocity impact response of foam cored sandwich plates. The sandwich plates used for the present work have a core made of commercial aluminum alloy foam (Alporas) with faceplates made of either ductile aluminum (Al) or elastic carbon fiber reinforced plastic (CFRP). A spherical ended impactor of 2.65 kg mass with 6.7 m/s velocity is impacted on to a clamped sandwich plates. All the FE simulations are performed using 3D finite element models in the commercial FE code LSDYNA. Selection of suitable constitutive models and erosion criterion for the failure analysis were investigated. Predicted load versus displacement curves were compared against experimental measurements. Then, the relative performance of graded metal and polymeric foam cored sandwich plates is studied under low velocity impact loading. The metal foam sandwich plates are constructed using aluminum alloy foam (Alporas) core and polymeric foam sandwich plates are constructed using polyvinyl chloride (Divinycell H80 and H250) foam. A core of 40 mm thickness (with two layers of 20 mm each) and aluminum faceplates of 0.5 mm and 1.0 mm were used. Impact experiments were conducted with a hemispherical punch of mass 8.7 kg at a nominal velocity of 5.8 m/s. The effect of stepwise core grading on the maximum dynamic penetration force as well as energy absorption capacity is studied. To maximize the energy absorption or to minimize the mass of the sandwich plate for a given penetration force, alternatives to Alporas foam are chosen based on either equivalent density (viz., H250) or through-thickness compressive yield strength (viz., H80). The second major contribution of this work is in the development of analytical models for the indentation failure of composite sandwich plates under bending and also explores the failure mode map. The analytical models are developed for estimating the indentation behavior of circular composite sandwich plates with a rigid flat ended circular punch. Initially, core was treated as an elastic foundation to derive the design loads for the indentation failure. Additionally, analytical models are extended to elastic-perfectly plastic foundation. Conventional indentation analogy and radial compression analogy of top faceplate are used to derive the load displacement curves using small deformation theory. All the small deformation cases are derived by solving the differential equations exactly. Finally, large deformation of a plate subjected to indentation on rigid-perfectly plastic foundation is derived using Galerkin’s weighted residual method using indentation analogy. Axisymmetric finite element (FE) models are used to validate the proposed analytical models. Finally, competing failure modes are investigated for circular sandwich plates comprising quasi-isotropic E-glass/epoxy composite faceplates (with [-60/0/60]ns configuration) and Polyvinyl chloride (PVC) foam core under bending. Clamped sandwich plates were loaded using flat ended punch at the center of the plate. Three competing failure modes, viz., core indentation, core shear and face failure/microbuckling, are considered. Analytical estimates for the elastic response (stiffness) and initial failure load are proposed, and these are verified with experimental measurements and FE predictions. Analytical estimates for the failure modes are used to plot the failure mode map in non-dimensional plate radius versus faceplate thickness for a given material system. The failure mode map thus constructed is assessed by considering a few sandwich plate geometries. Normalized sandwich mass and failure load contours are superimposed onto the failure mode map to identify the locus of minimum weight design by numerical search. Effect of geometrical parameters of the sandwich plate on failure load and mode is also investigated.
DRNTU::Engineering::Mechanical engineering::Mechanics and dynamics