Fatigue Failure of a M16 Flanged Bolt
Failure of a M16 flanged fastener of strength grade 12.9.
This fastener failed as a result of fatigue failure. Following
a detailed investigation of the assembly, the torque specification
was too low. The torque specified was 180 Nm that resulted
in a preload of 71000 N. This preload was insufficient to
prevent the forces applied to the bolt opening the joint causing
high alternating stresses to be sustained by the bolt.
The problem was resolved by increasing the tightening torque applied to the bolt so that a higher proportion of its yield strength was used.
The strength of the fastener was not being utilised, reducing the preload that the fastener was capable of providing. A tightening torque of 300 Nm was subsequently specified that resulted in a preload of 119000 N that was sufficient to sustain the forces applied to the joint.
You may also be interested in an article on fatigue failure:
Fatigue Failure of Bolts Published in the Fastener and Fixing magazine in July 2004, this article provides some background to the fatigue failure of bolts. it used to be thought that metal fatigue was due, in some way to the metallurgical nature of the material changing. Somehow the metal got weaker, became tired (i.e. fatigued) and failed prematurely when subjected to alternating loading. For the last two hundred years it has been a known problem and has been the cause of numerous product failures including some involving the loss of life. Nowadays, following a great deal of research, it is known that fatigue is due to one or more cracks progressing through a material. The crack can start at some existing defect, such as an inclusion in the metal, or at point of high stress, such as a notch, and slowly grow in length at each loading. It may take millions of loadings and unloadings (known as load or stress cycles) before the crack is actually detectable. As the length of the crack increases, the material remaining is placed under increasing stress because there is less area to sustain the loading. When the crack actually reaches a critical length it progresses all the way through the material resulting in complete failure.In the vast majority of applications, the most effective way to ensure that the bolt is fatigue resistant is to ensure that it is tightened sufficiently. |
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