Date of Award

2003

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Advisor

Vincent Caccese

Second Committee Member

Donald A. Grant

Third Committee Member

Richard C. Hill

Abstract

Stress relaxation and creep are major concerns when loading composite materials. Due to the viscoelactic nature of the matrix material, composite materials tend to lose initial loads at a decreasing rate. This is especially true through the thickness of the material, where the behavior of the material is dominated by the matrix. Of particular interest to the current study presented in this thesis is the investigation of stress relaxation in bolted composite/metal hybrid connections. It is ultimately desired to be able to use bolted composite/metal hybrid connections for naval applications, where it is important to maintain as much of the initial preload in the connection as possible in order to maintain watertight integrity. In order to quantify the stress relaxation in bolted hybrid connections, it was decided to study the connections at a sub-component level. Several different effects were studied in the connections, including reloading effects, possible advantages of using tapered head bolts, and environmental effects. All tests were run for a time period of at least 3-month in order to get an estimate of long-term stress relaxation effects in the bolted connections. Test results showed that the load curves could be fit to a power law equation using the method of least squares. Reloading tests showed that some of the preload in the connections could be maintained with periodic retightening of the bolts. The tests also showed a large temperature dependence in the connections that were reloaded multiple times. Connections that are reloaded can maintain more of their initial preload, but are extremely sensitive to temperature shift, even small shifts of only 5 degrees Fahrenheit. Temperature shifts cause the connections to move at a much greater stress relaxation rate. In general, reloading the connections will help the connections to maintain their initial preloads, but great care has to be taken to avoid any temperature changes in the connections. Tapered head bolts were tested in some connections and compared to results obtained from non-tapered head bolts. Little to no advantage was seen when using tapered head bolts over non-tapered head bolts. The connections using tapered head bolts had roughly the same stress relaxation rate as the connections using non-tapered head bolts. Environmental testing has recently been started on the hybrid connections, and results are not yet available. Pilot tests were inconclusive, as they were run at 150 degrees Fahrenheit, and thus thermal expansion effects are indistinguishable from stress relaxation effects. Moisture also may have gotten into the gaged bolts and effected the pilot test results. The environmental tests should provide an accurate description of what happens to the bolted hybrid connections in naval applications. Once completed, environmental test results will be presented to the project sponsor.

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