NG – Torque/Tension Testing of Bolts with Epoxy Primer

Project Description

• The MIL-PRF-23377 epoxy primer is used in aerospace applications to prevent corrosion
• The nut factor, K, is required to find the proper installation torque, T, given a bolt preload, F, and bolt diameter, D.

T = K*F*D

Design Requirements

• Aluminum and steel joints
• 1.25 safety factor above yielding
• Limited noise and reliable data
• Large sample size

Performance Review

• Calibration of load collars performed with Tinius Olsen Tensile Testing Machine
• Signal decay of output observed through repeated load cycling
• Calibration performed after steady behavior of loading, verified by loading following testing
• Significant misalignment of calibration results observed following testing
• Decaying signal could indicate plastic deformation of load collar

Design Description

• Setup a threaded plate with a load collar, washer compressed under bolt head
• Design load collar with strain gauges in full wheatstone-bridge configuration to measure compressive preload of bolts
• Plates are designed with enough holes to accommodate many tests for each bolt size and plate material
• Repeated testing will provide accurate epoxy nut factor

Conclusion

• Nut factor from empirical data is higher than would be expected for a thread lubricant/epoxy such as MIL-PRF-23377
• Testing should be repeated with tensile testing machine with verified calibration
• Epoxy cannot be considered a locking feature for bolts without further investigation
• Bolts larger than 1/4-in. diameter fail at interface with installation tool rather than in tensile stress

*Failure occured as stripping in bolt head, not in axial tension