Fatigue and Tightening Performance of Plasma Surface Hardening Treated Titanium Fastener for Composite Aircraft Structure

Recently, a number of structures in aircraft have been dramatically replaced by carbon fiber reinforced plastics (CFRP) composite materials. Titanium fastener is one of the most appropriate selections for fastening and joining CFRP materials because of galvanic and thermal expansion compatibility with composites. For nearly 50 years, titanium threaded fasteners has been used in solution treated and subsequent aging condition with minimum tensile strength of 1100 MPa (160 KSI). On the other hand, various surface treatments such as insulating coating have been often applied on titanium fasteners to prevent galvanic corrosion on aluminum materials and to obtain an adequate clamping force because titanium fasteners tend to gall due to its high friction characteristic and low hardness.

A newly developed conductive surface modification technique using plasma-carburizing was applied on externally threaded titanium fasteners for joining of CFRP composite structures and its mechanical properties including fatigue performance were evaluated. The fatigue strength of the modified fastener was higher than that of the unmodified fastener. The tightening performance of the modified fastener was significantly improved by the hardness increase on the thread surface. The modified fastener not only satisfied requirements of aerospace industry enough but also had a more efficient performance.