High strength composites, such as carbon fiber composites are known to have higher strength to weight ratios than other traditional engineering materials. Making a sandwich structure out of composites plates with foam material in the middle can lower the weight of the structure even more. One of the drawbacks in this type of structures is that any damage within the foam core may not be readily detectable. It is the ultimate goal of this proposed, long-term research to be able to design composites structures in such a way that interior damages can be controlled and directed to predesignated location on the structure where inspection can be conducted easily as part of regular maintenance program.

With an increase use of composites in many industrial sectors, some of which are consumer oriented, there will be a need to accelerate the speed and decrease frequency of inspection. The ability to control and direct damages in a composite structure will make both goals easier to meet. Inspection can be concentrated on predesignated locations in the structure.

Funding for this research is anticipated to come from automotive and aeronautical/aerospace industrial sources. Both industrial sectors are increasingly turning to composites to build higher performance products.

In the aeronautical sector, there has always been a need for strict maintenance and inspection programs. Traditional metal-on-frame planes have been in service long enough to have developed a robust inspection routine where potential failure points such as riveted joints can be inspected more closely. Composite parts, on the other hand, may consist of one single body where potential failure points may not be obvious, thus requiring more effort to inspect. The ability to design a structure where damages can be directed to non-critical, easily inspected areas will be beneficial.

In the automotive sector, where the applications of composites will be more consumer oriented, the robustness of a structure is important because of a possible lack of the continual inspection process that is common in the aeronautical sector. Composite structures used in automotive application must be tough and damage tolerant. One way of accomplish these goals is to contain damages to easily accessible, non-critical areas.

Composites sandwich panels are sheet materials; usually with high performance composites face sheets with low cost, lower strength, lighter weight foam at its core. Since the strength of the foam core is much lower than the face sheets, it is reasonable to assume that failure will originate in the core layer or the interface between the core and the face sheets. However, with the foam being much weaker than even the weakest adhesive used in bonding of various components, it is likely that damage will occur in the foam layer.

Primary mode of loading of a composite structure is likely to be bending. Flexure will put certain portion of the structure in compression while some others in tension. Compressive damage may occur frequently in sandwich structure, yet its effect will not be as detrimental as tensile damage. Compressed foam layer will result in reduced flexure strength of the sandwich structure. Tensile-type damage can be in a form of fracture in the foam layer. In this case, there will be a dramatic drop in strength of the entire structure. If damage is assumed to happen eventually as assumed in a
damage tolerant design
paradigm, the best course of action is to be able to anticipate and control the fracture that will eventually form.

One of the advantages of laminated composites is the ability to manipulate the properties of the composites via layering and fiber orientations. When used in a sandwich structure, the ability to vary the mechanical properties within a composite lay up can provide a mechanism to direct the damage within the sandwich structure. The entire premise of this proposed research is that the properties of the face layers in a composites sandwich structure influence how a fracture in the core layer propagates.

This is a long-term project that has been planned to have multiple parts that are workable by many graduate students at MS and PhD levels either sequentially or concurrently.