Tonnia Thomas, Ph.D. (2003)  

Dissertation Topic: 
Effects of Temperature and Strain Rate on Impact Response of Foam Core Sandwich Structures

Major Professor:  Dr. Hassan Mahfuz, Professor of Mechanical Engineering (currently @ Florida Atlantic University)

M.S.:  Mechanical Engineering, Tuskegee University

B.S.: Mechanical Engineering, Tuskegee University

Employment MeadWestvaco Corp., Center for Packaging and Innovation, Raleigh, NC 

Dissertation Abstract:

The growing need for optimal designs, where lightweight in combination with mechanical strength is demanded, has increased the use of foam core sandwich structures in marine and aerospace applications.  As primary load bearing components in marine and aerospace structures, sandwich composites are often subjected to thermal loading due to the environment and significant dynamic loads due to impact by foreign objects, tool drops, mine blast, and slamming waves.  The stressing conditions and the environments to which a composite is subjected plays a key role in determining its impact failure process especially when using polymer foam core sandwich composites.  Foam core sandwich composites in these applications often consist of facings and cores, which are both made from polymeric materials.  Many factors exist that can alter the mechanical properties of polymers.  Because of the viscoelastic nature of polymeric foam core sandwich composites, their mechanical properties are dependent on strain rate as well as temperature.  These two factors are of major concern because they control the vulnerability of polymeric sandwich composites to impact loading.  It is obvious from the literature that polymers undergo a significant change of impact energy with increasing temperature especially around the glass transition region, whereas, FCC metals show high impact values regardless of temperature.  Therefore, studies regarding the effects of temperature and strain rate on the impact of viscoelastic foam core sandwich composites are very much needed.

The effects of environment on a composite material are largely determined by its effect on the individual components.  For foam core sandwich composites, the components are the facing, the core, and the interface between them.  Many works are found in the literature regarding the effects of temperature on the impact response of polymer matrix composites, which may be used as facing materials.  However, no works have been found concerning the effects of temperature on the impact response of core materials or polymer sandwich composites as a whole.

When using closed-cell polymeric foams as core material, the effect of strain rate and temperature is further complicated by the presence of the gases within the cell.  Therefore, in order to give a complete description of the dynamic performance of sandwich structures a through understanding of the response of the foam core and the gas within the closed cell to high strain rate loading and elevated temperatures is essential for tailoring and using these materials effectively in such applications.  The core material is perhaps the most important component in a sandwich structure.  It is also the material component in which the engineer has the least knowledge.  This lack of knowledge makes it difficult to develop finite element models for foam core sandwich composites.  There are many issues that must be taken into consideration when modeling closed-cell foams.  First, one needs to know the effects of various parameters, which may affect the compressive behavior, such as temperature, frequency, and perhaps cell-gas compression.  Secondly, the model must include the deformation mechanisms that occur in practice.  Thirdly, the model must consider the polymer mechanical properties.