Knowledge: Failure analysis

Abstract. Vibration can be desirable, such as when it is used to make music. More often, however, vibration is undesirable. It can introduce stress into mechanical systems and create fatigue that decreases service life. It can also loosen fasteners. This article discusses the root causes of the self-loosening of fasteners, then two practical examples illustrate the benefits and possible applications of vibration testing of fasteners, according to either standardised testing protocols or customised testing strategies simulating real-life conditions of the assembled parts. The requirements and protocols of the international standard ISO 16130, the USA national aerospace standard NASM 1312-7, the German national standard DIN 25201-4 B and its predecessor DIN 65151 are compared, with insights on their most suitable applications. Finally, the article describes a lean methodology to design a meaningful testing protocol.

When a fastener or bolted joint fails, the results can be catastrophic and also sometimes tragic. The impact of the failure might be an isolated component malfunction, or an aircraft dropping from the sky. Whatever the impact, any fastener or bolted joint failure puts the pressure on the fastener manufacturer, the original equipment manufacturer (OEM), the operator and their insurers to determine the root cause and, ultimately, liability. Underpinning this process is the work of forensic engineers who apply fastener and bolted joint tests to find out what went wrong, so a solution can be developed to ensure it never happens again. Vibrationmaster’s Morten Schiff and Bolt Science’s Bill Eccles explain how forensic fastener and bolted joint testing methodologies can be used to determine failure modes.