Fastener, Bolted Joint & Lug Failures

Joint and Fastener Failure Modes

Bolted joints and lugs are among the most common locations for structural failure. The joint introduces stress concentrations, load transfer interfaces, contact conditions and potential preload issues — all of which can contribute to failure. Understanding the failure modes and their mechanisms is essential for both design and investigation.

A FAILED FASTENER MAY BE THE INITIATING FAILURE — OR THE FINAL MEMBER LEFT CARRYING A REDISTRIBUTED LOAD. The investigator must determine whether a broken bolt caused the failure or was a consequence of it.

Fastener Failure Modes

Individual fasteners can fail in several ways. Each mode has a different mechanism, different evidence and different implications for the investigation.

Joint Failure Modes

Beyond the fastener itself, the joint — the connected plates, the hole, the surrounding material — can fail. These modes involve the local structural behaviour around the fastener hole.

• Bearing failure — the fastener bears against the hole edge, crushing or elongating the material
• Net section failure — tension across the reduced section through the fastener hole exceeds ultimate
• Shear-out / tear-out — the material ahead of the fastener shears out in the load direction
• Hole elongation — progressive bearing damage under cyclic load, producing an elongated hole
• Joint slip — the clamped plates slide relative to each other, producing fretting and load redistribution
• Fastener pull-through — the fastener head pulls through the connected sheet (particularly in composites)

Clamp Load and Preload

Preload — the tension in the bolt created during assembly — is central to joint behaviour. The clamp load compresses the joint interfaces, providing friction that resists slip and maintaining contact that distributes bearing load. Preload also affects fatigue: a properly preloaded bolt carries a smaller fraction of the external cyclic load, because the joint stiffness absorbs much of the load through compression change. Preload loss — from relaxation, embedment, vibration or thermal cycling — can dramatically change the joint load path and fatigue behaviour.

F_clamp  =  T / (K · d)         [torque-preload relationship]

where:
F_clamp  =  clamp force (preload)
T        =  applied torque
K        =  torque coefficient (friction-dependent, typically 0.15–0.20)
d        =  nominal bolt diameter

External load sharing:
F_bolt  =  F_preload  +  C_bolt/(C_bolt + C_joint) · F_external
F_joint  =  F_preload  −  C_joint/(C_bolt + C_joint) · F_external

where C_bolt and C_joint are the stiffnesses of the bolt and clamped material

External Load Sharing

When an external load is applied to a preloaded joint, the load is shared between the bolt and the clamped material according to their relative stiffness. A stiff joint (thick, metallic plates) absorbs most of the external load — the bolt sees only a small fraction. A compliant joint (thin sheets, composite) absorbs less — the bolt sees a larger fraction. This load sharing is fundamental to joint fatigue behaviour. If preload is lost, the joint separates and the bolt carries the full external load — dramatically increasing the cyclic stress in the bolt.

ANALYSIS CONSIDERATION: The bolt stiffness ratio C_bolt/(C_bolt + C_joint) determines how much of the external cyclic load the bolt sees. If preload is lost and the joint separates, the bolt carries 100% of the external load — a dramatic increase in fatigue stress.

Load Redistribution in Multi-Fastener Joints

In a multi-fastener joint, load is not shared equally. The fasteners at the load entry point typically carry the highest load. If one fastener loses preload or fails, the load redistributes to the remaining fasteners. This redistribution can overload adjacent fasteners, initiating a progressive failure that cascades through the joint. The investigator must consider whether a failed fastener was the initiator or a consequence of load redistribution.

1. One fastener loses preload — perhaps from vibration, embedment or relaxation
2. Joint slips at that fastener — load transfers to adjacent fasteners
3. Adjacent fasteners carry increased load — higher bearing, higher fatigue stress
4. An adjacent fastener fails — by fatigue, bearing or overload
5. Load redistributes again — remaining fasteners carry even more load
6. Progressive failure cascades through the joint — until final structural failure

A FAILED FASTENER MAY BE THE FINAL MEMBER LEFT CARRYING A REDISTRIBUTED LOAD — NOT THE INITIATING FAILURE. The investigator must reconstruct the sequence, not simply identify the most damaged fastener.

Lug Failures

A lug (or clevis) is a fitting with a single pin or bolt hole that transfers load through a pin. Lugs are common in aerospace and mechanical structures — control surface hinges, actuator attachments, strut fittings. The lug introduces a high stress concentration at the hole bore, making it susceptible to fatigue, bearing and shear-out failures.

• Net tension — failure across the minimum section through the hole
• Bearing — crushing of the lug bore against the pin
• Shear-out — material ahead of the hole shearing out in the load direction
• Fatigue — crack initiation at the hole bore, particularly at the stress concentration
• Hole wear — progressive enlargement of the bore under cyclic bearing load
• Pin wear — fretting and wear between pin and bore, accelerating fatigue

Cross-Link to Joints

This article addresses fastener and joint failures from a failure investigation perspective. For detailed treatment of bolt preload, joint stiffness, bearing-bypass behaviour, joint fatigue and joint verification, see the relevant articles in the Structural Analysis category.

• Bolt Preload & Clamp Load — preload calculation, loss mechanisms and effects on joint behaviour
• Joint Stiffness & External Load Sharing — how external load is distributed between bolt and joint
• Bearing, Bypass & Net-Section Behaviour — load transfer in multi-fastener joints
• Joint Fatigue & Fretting — fatigue at fastener interfaces and fretting-induced initiation
• Joint Verification, Test & Sensitivity — testing and validating joint analysis

Key Takeaways

• Fasteners can fail in tension, shear, combined loading, fatigue or thread stripping
• Joints can fail in bearing, net section, shear-out, hole elongation or slip
• Preload is central to joint behaviour — preload loss dramatically changes the load path and fatigue stress
• In multi-fastener joints, load redistribution from a failed or loosened fastener can cascade through the joint
• A broken fastener may be the initiating failure or the final consequence of load redistribution
• Lugs introduce high stress concentrations at the bore — susceptible to fatigue, bearing and shear-out