New Technique Accurately Measures Strength of Composite Interfaces

NEW ORLEANS -- A University at Buffalo professor has developed the first accurate, nondestructive way to evaluate the strength of a bond between two materials.

The new method, called electromechanical pull-out testing, can be used throughout the construction industry and in any composite application where neither of the interface materials is completely electrically insulating.

"This technique takes the guesswork out of determining small, but important, differences in bond strength," said Deborah D.L. Chung, Ph.D., Niagara Mohawk Professor of Materials Science at UB, professor of mechanical and aerospace engineering and principal investigator.

Chung will describe the method on Tuesday, July 23, at the Third International Conference on Composites Engineering. Xuli Fu, a doctoral candidate in the UB Department of Mechanical and Aerospace Engineering, is co-investigator on the project.

When two materials are bonded together, the new composite will prove only as strong as the connection, or interface, that holds them together.

When steel and concrete are connected, for example, the interface is the potential weak link in the system.

"If the interface is bad, then the steel's strength cannot be utilized," Chung explained.

Whether the composite is part of a new building or a new aircraft, determining the strength of interfaces has been problematic.

The new technique overcomes a major difficulty in characterizing interfaces, even detecting the variability among interfaces that are supposedly identical.

"The bond strength of interfaces that are identical in material components and preparation procedure can vary greatly from one sample to another because of the difficulty of controlling interfacial cleanliness or contamination," said Chung.

"That makes it very hard to study the effect of a surface treatment on an interface."

The new technique makes use of a direct correlation Chung discovered between the strength of the bond between materials and their contact electrical resistivity, that is, the electrical resistance associated with the interface.

"The interface itself represents resistance and the amount of resistance relates to the quality of the interface," Chung noted.

She pointed out that the nature of that correlation depends on the chemistry of the interface, so that for some interfaces, contact resistivity increases with bond strength, whereas for others, it decreases.

"But in all cases, there is a clear correlation," she said.

The traditional method of measuring bond strength, called single fiber pull-out testing, involves embedding a single fiber of a material, such as steel or carbon, in the matrix, such as cement, and measuring the amount of force it takes to pull it out. The bond is the connection between the fiber and the matrix.

With the new technique, after the fiber is embedded in the matrix, electrical contacts are applied to the fiber and to the matrix near the fiber. An electric current is passed through a contact on the fiber and a contact on the matrix. The electrical resistivity is calculated based on these voltmeter readings.

"We can then plot the contact resistivity versus bond strength," explained Chung. "The curve of the plotted points represents that correlation. A different formulation or surface treatment would cause the whole curve to shift. From that shift, one can tell clearly what the change in bond strength is."

Steel/concrete and carbon fiber/cement interfaces were measured.

The trend of the correlation between contact electrical resistivity and bond strength also provides information on the structure of the interface.