Browse Publications Technical Papers 2008-01-2311
2008-09-16

Effects of Fastener Coating and Shear Strength on Joint Lap Shear Strength 2008-01-2311

In aerospace fastener industry, all materials used or being considered for fastener applications must meet specified minimum shear strength values tested via single-shear test and double-shear test. However, shear testing results are not as precise as tension and compression testing due to the introduction of friction and bending forces in the testing process, resulting in the difficulty in obtaining accurate test data. Previous study showed the various effects of coating tribology on fastener single and double shear strength. Experimental results, statistical analysis and finite element analysis all confirmed that surface tribology could significantly affect the shear strength test results. Comparing to un-coated or bare fasteners, confirmed that fasteners from the same lot exhibited significantly lower shear strength values after coating. This effect is well known in the Aerospace fastener industry.
First, in aerospace fastener industry, to meet specified minimum shear strength values all fasteners had to be heat treated to higher strength level to compensate for the effects of coating tribology on the shear strength test results. This leads to higher fastener material intrinsic bare shear strength with a corresponding drop of material ductility which could potentially affect overall fasteners lap shear joint strength. Thus, how the surface tribology and shear strength effect on overall structural performance.
Therefore, the objective of this study was to achieve a better understanding of the relationship between lap shear performance and the fastener shear strength. Three different fastener groups were used to investigate the relationship between the shear strength values and lap shear strength. Three different Ti-6Al-4V fasteners groups were chosen. Group H is the fasteners compensated shear strength drop due to surface coating, resulting high bare strength but low ductility. While Group L contains fastener having low bare strength but high ductility without compensating shear strength drop. Furthermore, Group R was introduced to have different strength and ductility combination values from fastener head to thread portion. Testing results demonstrated that different fastener mechanical properties lead to different lap shear performance. Group L fasteners demonstrated higher lap shear strength than group H, even with lower shear strength. In addition, the effects of materials properties optimization on the joint performances were further discussed.

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