An Investigation of Thrust and Torsional Force Profiles and Their Effects on Hybrid Aerospace Structure Joining

Christopher M. Miskell

doi:10.7273/000007427

Aerospace manufacturers are seeking to make their processes fully autonomous to enhance productivity while reducing costs. The final assembly of an aircraft requires the mechanical fastening process, which is complex and time-consuming. This level of complexity stems from the wide variety of fasteners, an array of installation tooling, and the need for operators to interpret and make in-situ decisions about the installation process, all while maintaining a high level of accuracy and precision. In the pursuit of developing an autonomous robot-based mechanical fastening operation of aircraft structures, it is critical to comprehend the mechanisms associated with the mechanical fastening process. With these goals in mind, this study examines the mechanisms of pin insertion and collar tightening during the mechanical fastening process when joining a composite/metal hybrid stack. The hybrid stacks evaluated in this study consist of Carbon-Fiber Reinforced Polymer composite (CFRP) and 2024-T351 Aluminum coupons in a single shear application. The fastener system used is a commercially available aerospace-grade 4.76mm (0.188in) diameter cadmium-coated 4340 alloy-steel Hi-Lite™ pin system, consisting of a pin and a 7075-T6 Aluminum collar. The experimental process considered the angularity effects of both the stack hole and the pin insertion as the sole input process variation. This resulted in a total of five configurations chosen for both the fastener being inserted, as well as the hole bore’s angle variation, and resulted in the following combinations: 0°-0°, 0°-1°, 1°-1°, 0°-2°, and 2°-2°. These conditions also allowed for the derivation of a hypothesized interference fit percentage between the pin being inserted and the hole bore. The fastening experiments consist of two sequences: pin insertion and collar tightening. During the insertion experiments, a Hi-Lite™ pin was inserted into the hybrid stack using a quasi-static velocity of 25.4mm/min (1in/min), while a 3-axis load cell was used to collect both in-plane, and out-of-plane forces. A total of ten trials per condition were conducted, resulting in a total of fifty independent insertion runs. Following the insertion experiments, each of the pins inserted had a Hi-Lite™ collar installed. During the installation of each collar, a commercially available pneumatic tool was used to apply a constant torsional loading until each collar reached its manufacturers-specified torque-off value. A torque sensor attached to the fixture holding each coupon record torque profiles, and a 3-axis load cell simultaneously collected the associated in-plane and out-of-plane force profiles. Lastly, after completing the tightening process, one coupon set for each configuration was disassembled and inspected for any related damage in either the CFRP or Al. This evaluation was performed to capture any resultant influence the angularity condition imparted on the hybrid stack during either of the installation processes. The experimental results of the insertion process showed that the pin insertion force profiles were close in the three configurations (0°-0°, 1°-1°, and 2°-2°), which are considered to be clearance-fit conditions. When the hole bore and pin are concentric/coaxial, the effects of the angularity are negligible. However, in the case of the interference conditions (0°-1°, and 0°-2°), these two configurations resulted in distinct trends and mainly increased resultant force magnitudes up to 165 N. A sudden change in force magnitudes as the pin passes across material boundaries is potentially due to the variation of friction coefficients of the two dissimilar materials or issues associated with hole bore concentricity. In both the 0°-1°, and 0°-2° it was seen that as the pin being inserted was met with increasing resistance, that it would align itself to the hole bore rather than create the hypothesized localized compressive damage in the CFRP upper layers, or at the exit of the 2024-T351 aluminum (i.e., burr formation). This self-aligning mechanism was also observed to be influenced by the Maximum Material Condition (MMC) of a hole as well as the angularity condition, showing an increase in the aligning behavior as both the hole bore angle and the MMC of a given experiment increased. No localized CFRP surface damage or aluminum exit-burrs were found, concluding that the angularity tolerance of +/- 2° would not result in any material damage during the pin installation. The collar tightening experiments resulted in the required increase in the applied axial loading to the pneumatic fastening tool to ensure the collar fastening completion. Given the single-sided nature of installing a Hi-Lite™ pin and collar system, as the angularity of an installed pin increased, so did the off-angled interference of the pin, hex key, collar, and socket, all of which must line up during the installation. As a result, this increasing interference of the tooling required additional force to ensure the socket and collar remained engaged until the collar reached its shear-off torque value. It was noted that regardless of the external load variation or angularity condition, neither one has any significant impact on the measured torsional profile, and in almost all cases, the torque profile was observed to remain unchanged. The assembly mechanisms altered by the hole and pin angularity will assist in the design and development of the end-effector intended to carry out these tasks in an autonomous fashion, as well as the development of the process monitoring sensor specifications.

An Investigation of Thrust and Torsional Force Profiles and Their Effects on Hybrid Aerospace Structure Joining

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