Sandwich structures for high temperature applications: Microstructural development and mechanical performance

There has been a great deal of interest in the development of sandwich structures as viable engineering constructions. It is due to their unique structural, physical, and mechanical characteristics such as light weight, high strength to weight ratio, high bending stiffness, excellent thermal and acoustical insulation. Although the service temperature for most applications of sandwich structures is low, the outstanding properties make sandwich constructions attractive for use in high temperature service conditions as well. The overall performance of sandwich constructions depends on the material properties of skin and core, and their geometrical characteristics.;An investigation of the characteristics of sandwich structures suitable for high temperature application is presented. The objective of the investigation was to develop a process for producing sandwich structures by depositing alloy 625 skins on Ni foam cores using air plasma spraying (APS), and to characterize the structure and mechanical behavior of the constituents and the sandwich structure. The experimental investigation consisted of optimization of deposition process parameters, microstructural and mechanical characterization of sandwich constituents and testing of mechanical performance of sandwich structures under flexural loading conditions. The responses of the as-received foam, as-sprayed coating, and as-fabricated sandwich structure to heat treatment were investigated. Available analytical models and numerical simulations linking the mechanical properties of the Ni alloy foam, the alloy 625 coating, and the sandwich structure to their physical properties and microstructures were compared with the results of mechanical testing to understand the behavior of the sandwich structure in terms of the properties of the constituents.;An empirical model was developed to predict deposition parameters which simultaneously minimize the oxide content and the porosity in the alloy 625 coating. The optimum spraying conditions produced a coating with less than 4% oxide and less than 3% porosity. The elastic modulus of the as-sprayed coating increased from ∼10% of the value of conventionally processed alloy 625 material to ∼25% of that value after a 5 hour heat treatment at 1100°C. The ductility of the as-sprayed samples was as low as 1% compared to 15--20% for conventionally processed materials. The elastic modulus and yield stress of the as-received Ni alloy foam samples was also improved significantly by heat treatment.;The mechanical behavior of sandwich structures with skin thickness of 0.5 and 0.1 mm were examined under four point bending test. The flexural rigidity of the sandwich structures increased after heat treatment for both skin thicknesses, however the rigidity was found to be lower in all cases than the rigidity predicted by analytical models based on the measured properties of the constituents. The dominant failure mode observed for the sandwich samples with skin thicknesses of 0.1 mm and 0.5 mm were core yielding and skin indentation, respectively.