Krishnan Kanny, Ph.D. (2003) 

Dissertation Topic:
   Effects of Viscoelasticity on the Flexural Fatigue Performance of Foam Core Sandwich Structures
Major Professor: Dr. Hassan Mahfuz, Professor of Mechanical Engineering (currently @ Florida Atlantic University)
M.S: Mechanical Engineering, University of Natal, South Africa
Employment:  Professor, Durban Institute of Technology, Durban, KZ Natal, South Africa

Dissertation Abstract:

In many applications sandwich structures are subjected to repetitive cyclic loading, which means that fatigue life is of concern. Sandwich constructions have gained a reputation for being a very good concept in avoiding fatigue failures. One reason may be that the core is designed with a high margin of safety due to lack of knowledge about its fatigue properties. The constituents in a sandwich are subjected to different kinds of loading; the faces exhibit almost entirely membrane tension/compression and the core, pure shear. Fatigue of the faces could then easily be included in the design process since fatigue properties are fairly well known, and the loading situation is simple. The core, on the other hand, exhibits a more complex loading situation and fatigue data are almost non-existent. The fact that the cellular foam cores are viscoelastic makes the problem of fatigue even more complex.

Researchers have put tremendous efforts in designing superior face materials while the behavior of the core material is relatively unexplored. It is clear therefore, that as the need for more advanced sandwich structures continues to grow, research needs to be focused on the behavior of the core. Moreover, if the core is viscoelastic then factors such as time, temperature and frequency become important criteria to be considered and makes the research challenging.

In a polymer foam core sandwich structure, the yore typically is the weakest part and it is expected that it may be the first to fail under cyclic loading. Like all engineering materials, failure of the polymer foam often ensues as a consequence of accumulated irreversible damage or growth of a flaw to a critical dimension. In general, polymer foams are more sensitive to the testing environment than their metal or ceramic counterparts. The fatigue life of a polymeric component is controlled by several factors, which include the stress and strain amplitudes of the 
loading, the mean stress and the frequency of loading and the temperature. These factors are of considerable interest and practicality for the safe design of polymeric sandwich structures. There is  a great need to improve our understanding of relationships between loads, material properties, damage, and long-term performance of these materials behavior. The level of understanding must advance so that the complex damage processes that occurs in sandwich structures can accurately be included in predictive models of long term behavior, including strength and life of sandwich structures and the components materials.

This study shall focus on the viscoelastic effects on the fatigue performance of foam core sandwich structures. Since the PVC core is a predominantly viscoelastic component in the sandwich structure, emphasis will also be placed on the treatment of the core.