Shear Load Transfer for Corrosion Coated Clamped Joints

Rotorcraft transmission housings contain multiple joints that are typically held together by threaded fasteners. For many of these installations, the material surfaces are now coated with corrosion inhibitors that can change the strength of the joint. To identify these changes, Bell Helicopter has conducted coupon testing of coated materials in shear. Historically, the faying surfaces have been left bare with the perception that coatings would degrade the ability to transfer shear loads. Fastener and joint design has been based on assumed joint slip coefficients. A test effort to identify the shear load transfer characteristics of a clamped joint was undertaken. A double shear coupon test was used to measure the shear load capability of the clamped joint with bare material surfaces as well as with corrosion protection coated surfaces. Tests were run with calibrated bolt preloads to reduce variability. To simulate the gearbox temperature environment, the test coupons and mechanical grips were placed in an oven to maintain a temperature of 180°F. Testing showed that the addition of coating reduced the joint slip coefficient when compared to baseline bare materials.