Flexural Response of Syntactic Foam Core Sandwich Structures: Effects of Graded Face Sheets and Interpenetrating Phase Composite Foam Core

Abstract

In this thesis, flexural responses of sandwich structures with different syntactic foam core architectures are studied. The syntactic foam core is made by dispersing hollow glass microballoons in epoxy matrix at different volume fractions ranging from 20%-40%. The face sheets of the sandwich structures are made of thin AL6061 sheets. Three types of sandwich structures, first one with a regular syntactic foam core (identified as SFS), second one with an aluminum-syntactic foam interpenetrating core (identified as IPC), and third one with a syntactic foam core with graded face sheet (identified as SFS-b) are studied. Static three-point bend tests are carried out on all three types of sandwich structures and load-deflection responses are measured. The three different architectures are comparatively examined in terms of peak load, deflection at failure, nonlinearity of the flexural response, and strain energy absorbed. The global measurements are supplemented by digital image correlation measurements in the core to map 2D deformations and strains. The measured normal and shear strain fields and optical microscopy are used to discern failure mechanisms of the three architectures. The SFS sandwiches fail predominantly due to face-core debonding with a large scatter in the peak load and deflection at failure. They also show very limited nonlinearity in their load-deflection response. The IPC core sandwiches show a substantial improvement in the deflection at failure with a slight reduction in peak load and significant nonlinearity in load-deflection response. The nonlinearity in this architecture is primarily due to debonding between the phases of the interpenetrating core as well as face sheet yielding. The SFS-b sandwiches, on the other hand, show substantial improvement in both peak load and deflection at failure along with significant nonlinearity. This is attributed to a gradual shear strain variation at the graded face-core interface, unlike the SFS sandwiches, resulting in face sheet yielding before failure. The superior material under quasi-static loading based on load, deflection, and energy metrics was the syntactic foam core sandwiches with graded face sheets (SFS-b). The only time it might not be considered the optimal choice might be when considering the IPC core sandwiches for their ability to use the metallic foam network to hold the structure together longer. Low velocity impact tests were also conducted on select IPC and SFS-b architectures in order to further expound on the failure mechanisms in the quasi-static case, as well as initiate dynamic characterization research. Also, the feasibility of using DIC method in conjunction with high-speed digital photography to study impact behavior of sandwich structures is illustrated. Contrary to the quasi-static case, IPC foam core sandwiches appear to outperform the SFS-b sandwiches in terms of strain energy absorption. The presence of the interpenetrating phases seems to have a positive effect under dynamic conditions.