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Fastener Failures

Take Away:

Although there are a variety of sources of fastener failures, the majority of failures can be linked to fastener installation (i.e. under or over torqueing), fastener environment (corrosion or thermal effects), or material defects (i.e. manufacturing defects). Appropriate analysis of the fracture surface and conditions allows for the source of these failures to be identified and corrective action to be taken.

Overview:

Fasteners are critical to modern industry and their failures can regularly result is significant damage and downtime to critical equipment. The causes of fastener failure are highly varied, but from a materials perspective fastener failure can be caused by overload, fatigue, corrosion, or embrittlement. Each of these failure mechanisms can lead to the same end result, a fastener failure and process shutdown. However, they have different origins and thus different solutions. The following three case studies show material failures from each of the causes, and discuss the likely origin of these and similar failures.

Case 1:

A client experienced failure of a large number of bolts that were responsible for holding a stiffening bar in place. Analysis of the bolts revealed both fatigue and overload failures (Figure 1). These failures were the result of the improper bolt installation. Specifically, these bolts were insufficiently torqued, which allowed for the loading of the stiffening bar to be larger than the pre-load on the bolt, resulting in the application of a cyclic stress to the bolts. This cyclic stress, when combined with the stress concentration of the thread roots, led to the initiation and propagation of the fatigue cracks seen in the first three images of Figure 1. The failure of the fatigued bolts in turn caused a significant increase in the load on the remaining bolts, causing them to fail in overload as seen in the final image of Figure 1. Insufficient pre-load (under-torquing) is the leading cause of fatigue failures in bolts. Conversely, over-torquing during installation can lead to fastener yielding and overload. This delicate balance between under and over-torquing is one of the highlights the need for appropriate fastener installation procedures.

Figure 1: Fatigue (Top) and Overload (Bottom)

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Case 2:

A client experienced 4 separate tie-rod failures on 3 separate occasions in an off-shore compressor. Examination of the fractured tie rods revealed that corrosion pitting in the thread roots led to stress concentrations large enough to initiate a fatigue crack, which then propagated through the material (Figure 2). Though the fracture occurred through fatigue, the failure occurred as a result of the corrosion pits. Thus, this was a corrosive failure of the tie rods. The source of the corrosion was determined to be Cl pitting, which is unsurprising considering the marine environment in which the tie rod was located. Notably, in performing this type of analysis, it is important to accurately model the tube pressure drop over the system from measured data. Also, the flow composition with pressure-temperature dependent properties, turbulence model applied, mesh refinement, boundary layer and specifically the wall Y+ must be accurately applied to ensure quality of results by CFD analysis. 3D flow effects from upstream elbows and inlet conditions must often be taken into account where applicable.

Figure 2: Photograph of Fatigued Fracture Surface (Top) and SEM Micrograph of Corrosion Pitting at the Fracture Initiation (Bottom)

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Case 3:

A client discovered two bolts that fractured at some point within the first 3 months of service. The bolts were high strength steel that had been electroplated and the client was concerned that the bolts may not have had an appropriate “bake out” treatment performed after the electroplating and could thus have been subject to Hydrogen embrittlement. Subsequent examination of the bolts revealed decreased fracture toughness and intergranular fracture, indicating that hydrogen embrittlement was indeed the cause of the bolt failures. The intergranular fracture was observed in the crack initiation region, indicating that the hydrogen embrittlement lead to an initial crack formation, which then led to a larger stress concentration and more rapid fracture of the remaining bolt material.

Figure 3: Photograph of Bolt Overload Failure (Top) and SEM Micrograph of Intergranular Cracking (Bottom)

 

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