Percussive Riveting

Percussive Riveting of Advanced Aerospace structures

Fasteners such as countersunk head aluminum rivets are routinely used in aircraft to assemble fuselage structures. Although manufacturers use automated processes to assemble new aircraft, manual rivet installation is still prevalent because of automation machine rigidity and accessibility issues. Various experimental and numerical studies have been performed to understand and judge the performance of riveted lap joints present in aircraft fuselages. Experimentally, combinations of Riveting Quality Indices (RQIs) like air-pressure level, pneumatic rivet gun impact duration and impact accelerations have been proposed to evaluate installed rivets on Go/No Go basis. In numerical simulations, studies have been published extensively on rivets installed using quasi-static hydraulic squeeze equipment.

Simulation of rivet forming under dynamic installation loads on both the bucking bar and the rivet gun side has not been pursued actively. This is especially important because the rivet gun and bucking bar exert loads on fuselage panels over a broad frequency range around fastener holes, which are areas with high geometric stress concentrations. Also, consistency and repeatability of rivet material flow is crucial to providing good interference fit joints. This depends on the strain rate induced by the loads. Fatigue crack initiation is tied to presence rivet gaps, especially under the countersunk rivet heads. Fuselage structural integrity is threatened by phenomena like Multiple-Site Damage (MSD). MSD is characterized by the simultaneous fatigue cracking in an aircraft structure. These cracks propagate rapidly, coalesce with each other or other lead cracks, and reduce the residual strength to unacceptable levels.

The residual stresses and strains set up during the rivet installation process ensure extended fatigue performance of the lap joints. Issues like the following  can cause significant variations in material flow and residual stress/strain fields.

1) Gun not being held normal to rivet head,
2) Removal of bucking bar before riveting process is completed,
3) Miscommunication between technicians working in a noisy environment and
4) Slipping of bucking bar by bucking technician at instance of impact.

Also, the fuselage structure may suffer damage during installation. Hence, lap joint fatigue strength may be negatively affected.

A thorough numerical study that implements an accurate constitutive model of the rivet material behavior and incorporates realistic boundary conditions and dynamic contact conditions is motivated to analyze the rivet forming phenomenon under manual installation conditions. Results generated from this study could be used to formulate good manual riveting practices that may lead to consistent lap joint fatigue lives.