Identifier

etd-0708103-114809

Degree

Master of Science (MS)

Department

Engineering Science (Interdepartmental Program)

Document Type

Thesis

Abstract

The vast differences in strength, ultimate strain and modulus during high strain rate (HSR) deformation of materials have been a very long-standing subject of engineering interest. This thesis deals with characterization of mechanical properties of two composite materials, balanced angle-ply graphite epoxy laminates (fibrous composite) and syntactic foams (particulate composite). The focus of this study is to compare the mechanical properties of these composite materials at high strain rates and quasi-static conditions and to find out the effects of failure modes on HSR mechanical properties of these materials. Split Hopkinson Pressure Bar (SHPB) apparatus is used for the HSR testing of balanced angle-ply IM7/8551-7 graphite/epoxy laminates and syntactic foams at varying strain rates, ranging from 500 s-1 to 1700 s-1. Graphite/epoxy laminates with seven different fiber orientations including longitudinal and transverse are used in this study. Syntactic foams of four different densities are used in order to observe the density effect on the HSR properties. The aspect ratio (L/D) of all the specimens is kept equal to one. Failed specimens are consequently observed under optical and scanning electron microscope in order to understand the fracture modes of these materials. The results of the tests on both materials demonstrate considerable increase in peak strength and the elastic modulus under HSR. It is also observed that the failure strain values vary considerably with increasing strain rate. Fiber orientation, in case of balanced angle-ply graphite epoxy laminates and density in case of syntactic foams are found to influence the HSR mechanical properties and strain rate sensitivity of peak stress. Delamination caused by edge effects is the prominent mechanism of failure for graphite/epoxy specimens whereas vertical cracking through cenospheres is recognized to be the dominant mode of failure for syntactic foam specimens under HSR testing conditions. These results are essential for conducting realistic numerical simulations for safe design of structures.

Date

2003

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Eyassu Woldesenbet

DOI

10.31390/gradschool_theses.2475

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