The part is a clevis and was insterted in a steering rack . Part was in Tension, torsion and bending. It failed at the end of the threaded bolt part. Is it a common problem for bolted joints ? How can it be resolved ?

At first a bend was observed in the threaded part in the direction in which it is kept in the picture before failure.

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  • $\begingroup$ Looks like the nut may have loosened, causing the stress point to move down the bolt, stressing it there until it sheared off. $\endgroup$ – Ron Beyer Dec 22 '17 at 23:58
  • $\begingroup$ Yeah, the part was getting a torque in the direction to loose it ( anti-clockwise sense ). But how is the loosening related to stress point moving down the bolt ? $\endgroup$ – Anshuman Sinha Dec 23 '17 at 0:10
  • $\begingroup$ it looks like the nut was tightened too much weakening the threaded section which later failed under the "normal" loading. But it does not look like a clevis pin to me... $\endgroup$ – Solar Mike Dec 23 '17 at 0:57
  • $\begingroup$ @SolarMike , I have updated the picture. At first a bend was observed in the threaded part in the direction in which it is kept in the picture before failure. $\endgroup$ – Anshuman Sinha Dec 23 '17 at 1:33

this is a classic fatigue fracture, caused by a cyclic/reversed bending load superimposed on a static tension load. The cyclic load is caused by the reversed bending; the neutral axis shows clearly as a straight line running diagonally across the center of the part. the fact that it is a straight line demonstrates that the torsion component was not the dominant load.

the initiator of the fracture was the sharp tip of the cut thread in the part and the reversed cyclic loads caused the fatigue fracture crack to develop on both sides of the bending neutral axis. Finally, note that the cross-section of the final failure caused by overload (the area defined by the shear failure zone along the diagonal line as defined above) is small compared to the unfractured cross-section of the part; this demonstrates that the loads originally imposed on the part were small compared to the tensile strength of the part itself as designed. The conclusion therefore is that this part was not designed for fatigue service; it should have had the threads pressed into it with a set of roller die rather than having them cut on a lathe or with a threading die.

  • $\begingroup$ The conclusion may be that the part was over-tightened causing the initial fracture, probably because it kept working loose in service. $\endgroup$ – Solar Mike Dec 23 '17 at 9:09
  • $\begingroup$ But part was not moving in any periodic motion however the loading was variable , and analysing the part through dynamics implies that the part was in compression, however the moment of the compressive force was doing this bending. I have added a new photo which is showing the forces and their torques. Please have a look if this helps. $\endgroup$ – Anshuman Sinha Dec 23 '17 at 15:38
  • $\begingroup$ the part does not have to move in order to experience cyclic loading. in cases like this, the failure morphology can be used to infer the presence of unanticipated loads. $\endgroup$ – niels nielsen Dec 23 '17 at 19:01

I agree , reversed bending fatigue, no torsion, likely sharp thread roots, maybe a loose nut. But the surface looks granular, not smooth as typical of fatigue. That suggests very low strength steel. So, radius the thread roots and be sure the nut is properly torqued and check if the hardness meets spec.

  • $\begingroup$ A larger diameter thread section would help ( to lower stresses ). $\endgroup$ – blacksmith37 Dec 23 '17 at 17:48
  • $\begingroup$ The material used is AISI 1018 ( Mild Steel ). And the radius can't be increased further because it's a male part which is to be inserted in a steering rack . What will be the role of hardness ? since the thread may already act as a good site for fatigue . $\endgroup$ – Anshuman Sinha Dec 23 '17 at 18:27
  • $\begingroup$ Why did you choose mild steel - because it was cheap? Or that is what is available? What is the full purpose of the part as you don’t seem to have provided full information but do expect us to solve it for you anyway. $\endgroup$ – Solar Mike Dec 23 '17 at 22:29
  • $\begingroup$ Hardness will give a very good indication of tensile strength, tensile strength gives a very good indication of the endurance limit- the stress below which fatigue life it very ,very long ( I hate to say infinite ).Steel is the only metal with an endurance/fatigue limit. AISI 1018 defines chemistry but not strength, A ASTM spec like A-108 will give some indication of strength. $\endgroup$ – blacksmith37 Dec 24 '17 at 4:18

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