Fracture Surface Failure mode identification

[NJAus1]

Hi,

I am looking into a failure where the component has two parts (cylinder rod and rod eye) welded together and is subject to tension only. Refer to the attached image.

The outer edges (shear lip?) have progression marks indicating that the was a crack present and grew over time. This would indicate that the crack has progressed from the outside toward the center.

Fracture surface:

Shear lip 1:

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1568091374/tips/TS-20190909132300868_kz53wy.tiff[/url]

Shear lip 2:

https://www.eng-tips.com/

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1568091450/tips/TS-20190909134001624_n0ayzr.tiff[/url]

A few questions:

1)Is there a scenario where the crack could initiate from the center and radiate outward based on the fracture surface? There is no indication of beach marks toward the center. I was initially looking at a fracture from the center outward, however the beach marks on the outer surface would rule that out?

2) Does a shear lip typically indicate the final part of the fracture?

Thanks for the help.

 

[Metalero]

Answer 1: Yes. The pattern of circumferential lines along the fracture surface is a pattern of beach marks that extends outward. River lines indicate propagation from the internal diameter.
The beach marks seen in the detailed pictures may correspond to secondary starting points.

Answer 2. Yes, it does.

 

[mrfailure]

So why would this initiate on the ID if the only load is tension? Is this seeing cyclic internal pressurization? What are you not telling us?

 

[NJAus1]

The cylinder has spherical bearings on the rod eye and on the clevis. The cylinders push and pull only during typically use. The spherical bearings prevent bending loads.

The hole you are seeing is a locating hole on the cylinder eye, there is a spigot on the rod which locates on this hole and is welded together. Once welded this is solid.

I think for the crack to emanate from the inner diameter, there needs to be a defect possibly in the weld, see below an image at the ID, which appears to show "ratchet" marks.

As Metalero indicated, the benchmarks on the shear lip may correspond to secondary fatigue cracks at the outer diameter.

 

[Metalero]

It is correct, it corresponds to ratchet marks (fatigue), which can be generated even by the roughness of the surface, without the presence of a "significant" defect. I can´t detect the weld in your photos.

In the first photo there is an area between 11:30HT and 2:30HT that looks different, have you searched there?

"The cylinders push and pull only during typically use" sounds like cycclic loads.

 

[NJAus1]

Yes you are correct there will be cyclic tensile loads applied.

In the photos, the weld is not observable however the fracture surface is a weld, it has fractured right through the weld.

In the above photo, the shiny silver region is the parent metal (of the rod eye) and the darked surface is the weld. As you can see the weld has not penetrated completely.

 

[Metalero]

To my mind: The shiny silver region that you mentioned is a mechanical damage post fracture, however welds are fatigue susceptible too and don't have fatigue limit (10^6) as the base metal, and more discontinuities are possible.

 

[mrfailure]

You can see the machining marks in the image NJAus1 presented, which are consistent with lack of fusion here as opposed to post-fracture damage. Since this is believed to be the origin, the next step should be to prepare a radial mount at this location through both sides of the fracture and through-wall. This will let you see to weld and penetration, and provide a sample to assess weld quality, microstructure, and microhardness.

 

[btrueblood]

"The spherical bearings prevent bending loads."

As long as they are lubricated.

 

[Metalero]

Share your results (if possible).