Session: 03-05-01: 8th Symposium on Fastening and Joining Research and Advanced Technology

Paper Number: 116967

116967 - Vibration Loosening Performance of Additively-Manufactured Bolted Joints 

In this study, the vibration loosening performance of additively-manufactured threaded joints is investigated, using a Junker testing apparatus. The under-head frictional characteristics of the bolted joint are replicated using square cutouts (square washers) fitted in the Junker testing equipment.

The vibration loosening performance of Additively Manufactured (AM) aluminum AlSi10Mg alloy clamped parts is compared to cast (AlSi10Mg alloy) and to AM machined components. The AM parts are produced using Direct Metal Laser Sintering (DMLS), followed by shot peening. Steel shims are employed to guarantee parallel clamping surfaces. M8 flange head bolts and conventional hex nuts are used to clamp the joint. Fresh fasteners are used in each experiment.

A transverse load is applied at a 10 Hz frequency, and the clamp load is monitored at 57 Hz throughout the duration of the test, via the integrated load cell. An eccentric mechanism cyclically translates the top cutout with an amplitude of 0.01 in. (0.254 mm). The transverse load is measured via a strain-gauged pin, and is a function of the vibration amplitude and of the clamp load. Inertial effects are minimal and can be neglected.

All bolts are laser-scanned before being mounted in the Junker testing apparatus, to verify dimensional tolerances and hole clearance. The under-head bearing surfaces are analyzed using an optical profiler, and vertical scanning white light interferometry. The frictional characteristics of the threaded joints are first analyzed using a computer-controlled RS Torque Tension system, up to the clamp load used in the Junker testing apparatus. Measurements of under-head friction coefficient obtained with the torque Tension research heads for AM and non-AM parts are then correlated to the vibration loosening performance of the respective joints. According to models published in the literature, the under-head friction coefficient affects significantly the rate of the clamp load decay, when joints are subjected to transverse excitations. However, the tribology and topography of the mating surfaces is continuously altered by the clamping and fretting actions of the bolt, with the latter being caused by the micro-slips in the under-head bearing region typical of applications with large amounts of vibrations. Moreover, small variations in the integrity of the bolt-joint interface can generate a large clamp load reduction, even without causing any fastener rotation.

Additionally, the load relaxation performance of AM, AM-machined, and cast parts is studied: after tightening to full preload, but before the application of the transverse load, the clamp load is monitored for 48 hours, using the load cell of the Junker system.

Data is analyzed by means of statistical tools (ANOVA) in order to evaluate the effect of the material on the output variables. Results and conclusions are provided.

Presenting Author: Marco Gerini-Romagnoli Oakland University

Presenting Author Biography: Marco Gerini-Romagnoli is an assistant professor at Oakland University, in the Department of Mechanical Engineering, and he serves as the Site Manager for the NSF IUCRC for Composite and Hybrid Materials Interfacing. His research interests include material joining (adhesive bonding and threaded fastening), composites, and polymeric materials.

Authors:

Marco Gerini-Romagnoli Oakland University
Massimiliano De Agostinis Università di Bologna
Sayed Nassar Oakland University
Khushboo Tedlapu Oakland University