Bolt Hole Growth in Graphite-Epoxy Laminates for Clearance and Interference Fits When Subjected to Fatigue Loads

This paper presents the results of an experimental program that was conducted to evaluate the damage to the bolt hole as related to the fit of the bolt to the hole in the graphite-epoxy laminate when the joint is subjected to a fatigue load spectrum. The experimental program was conducted using a double-lap bolted-joint test specimen. Three types of bolt fit were used in the experiment: interference fit, clearance fit, and clearance fit with wet sealant. The bolts were torqued to values representing standard installation torque. The graphite-epoxy laminate used for the test specimens was pseudoisotropic laminate of (0/90, ±45)_4S. The prepreg was T300/5208 and the laminate was cured in the standard Narmco 5208 cure cycle.

The test program consisted of applying cyclic loadings to produce bearing stress levels of 397 × 10⁶ N/m² (+57 600 psi) for 50 000, 100 000, 200 000, and 500 000 cycles. The bolted specimens were disassembled and the bolt hold diameters measured for hole growth. Preliminary results indicated that hole growth will occur when the bearing stresses are above 50 000 psi. Bolts installed in clearance holes with the PR1422 sealant had the same hole growth as bolts installed without the sealant.