Date of Award

2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering and Applied Mechanics

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Arun Shukla

Abstract

An experimental study has been conducted to investigate the blast resistance and mitigation behaviors of novel composites and sandwich structures. Understanding the overall behaviors and failure mechanisms will aid in the development of optimally designed light-weight structures that can mitigate energy and maintain structural integrity when subjected to blast loadings. Due to the increased threat of damage to civilian and defense structures in the form of terrorist attacks and blast loading, a comprehensive understanding on blast mitigation of composites and sandwich structures, as well as an optimal design, is essential.

The dynamic behavior of various sandwich composites made of E-glass Vinyl-Ester (EVE) facesheets and Corecell TM A-series styrene acrylonitrile (SAN) foam core was studied using a shock tube apparatus. The overall specimen dimensions were held constant for all core configurations studied, more specifically the foam core thickness. Prior to shock tube testing, the quasi-static and dynamic constitutive behavior of the facesheets (tensile/compressive) and foam (compressive) was evaluated. During the shock tube testing, a high-speed photography system was utilized to capture the real-time deformation process, as well as mechanisms of failure. In the later studies, high-speed photography was coupled with the optical technique of 3-D Digital Image Correlation (DIC) to obtain the real-time, full-field deformation process, including the out-of-plane deflection and velocity, as well as in-plane strain. Post-mortem analysis was also carried out to evaluate the overall blast performance of these configurations.

First, shock tube experiments were performed to study the dynamic response of sandwich panels with E-glass Vinyl-Ester (EVE) composite facesheets and stepwise graded styrene acrylonitrile (SAN) foam cores. Two types of core configurations, with identical areal density, were subjected to the shock wave loading. The core layers were arranged according to the density of the respective foam; configuration 1 consisted of low / middle / high density foams and configuration 2 consisted of middle / low / high density foams. The method to calculate the incident and reflected energies of the shock wave, as well as the deformation energy of the specimen, were proposed based on the shock wave pressure profiles and the high-speed deflection images that were obtained. The experimental results showed that configuration 1 outperformed configuration 2 in regards to their blast resistance. Significant core material compression was observed in configuration 1, while in configuration 2 the core layers disintegrated and the front skin (blast side) fractured into two pieces along the midsection. The foam core compression in configuration 1 reduced the dynamic pressures seen on the back facesheet, and thus limited the total amount of damage imparted on the specimen. The estimated energies were then calculated for both configurations. The total energy difference between the incident and reflected energies was almost identical, even though the deformation energy for configuration 2 was larger.

Since it was observed that a stepwise graded foam core allows for more compression in the core, thus reducing dynamic pressures seen on the back facesheet, and limiting the total amount of damage imparted on the specimen, the study was then continued to investigate the influence of the number of foam core layers, as well as material interfaces, on the dynamic response of sandwich structures. Four types of core configurations were subjected to the shock wave loading. The foam core was monotonically graded based on increasing acoustic wave impedance, with the foam core layer of lowest wave impedance facing the blast. The specimen dimensions were held constant for all core configurations, while the number of core layers varied, resulting in specimens with one layer, two layer, three layer, and four layers of foam core gradation. The results indicated that even though each configuration allowed for a stepwise compression of the core, the number of core layers has an influence on the dynamic response of the structure under blast loading. More specifically, by increasing the number of monotonically graded layers, the acoustic wave impedance mismatch between successive layers is reduced. Therefore, the strength of the initial shock wave (stress wave) can be weakened by the time it reaches the back facesheet, resulting in lower back face deflection, in-plane strain, and velocity. More importantly, the overall damage imparted on the structure can be reduced and structural integrity can be maintained.

Due to the fact that higher levels of core gradation helped maintain structural integrity and improved the overall blast performance of sandwich structures, the study was then continued to investigate the blast response of sandwich structures with a functionally graded core and polyurea (PU) interlayer, and more importantly how the location of this polyurea interlayer affects the overall behavior and blast performance. Two types of core configurations were subjected to shock wave loading. The materials, as well as the core layer arrangements, were identical, with the only difference arising in the location of the polyurea interlayer. The foam core itself was layered with monotonically increasing wave impedance of the core layers, with the lowest wave impedance facing the shock loading. For configuration 1, the polyurea interlayer was placed behind the front facesheet, in front of the foam core, while in configuration 2 it was placed behind the foam core, in front of the back facesheet. The results indicated that applying polyurea behind the foam core and in front of the back facesheet will reduce the back face deflection, particle velocity, and in-plane strain, thus improving the overall blast performance and maintaining structural integrity.

Since an optimized core configuration was determined, the study was continued to investigate the relationship between the weight of the panel and its overall blast performance. Two types of core configuration were subjected to shock wave loading. The materials, as well as the core layer arrangements, and overall specimen dimensions were identical, with the only difference appearing in the core layers; one configuration utilized equivalent core layer thickness, while the other configuration utilized equivalent core layer mass. The foam core itself was layered based on monotonically increasing the acoustic wave impedance of the core layers, with the lowest wave impedance facing the shock loading. The results indicated that with a decrease in areal density of ~ 1 kg/m2 (5%) from the sandwich composites with equivalent core layer thickness to the sandwich composites with equivalent core layer mass, an increase in deflection (20%), in-plain strain (8%) and velocity (8%) was observed.

Finally, since an optimal core configuration was developed to better mitigate blast loadings, and an in-depth study was performed on the relationship between the weight of the panel and its overall blast performance, the research was continued with composite facesheet designs to better mitigate impact and blast loadings. Two types of core configurations were subjected to shock wave loading. The core material and thickness, as well as overall specimen dimensions were held constant, with the only difference arising in the resin system used during the infusion. The non-core-shell rubber toughened resin system (Non-CSR) consisted of a Vinyl-Ester resin only; while the CSR toughened resin consisted of the same Vinyl-Ester resin, but with Kane Ace MX-153 nano-scale core-shell rubber particles added to the mixture. Results indicated that adding nano-scale core-shell rubber (CSR) particles to sandwich composites, aids in dispersing the initial shock wave loading, thus reducing the overall deflection, strain, and velocity and improving the overall blast resistance of the structure.

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