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Bending or Torsional Fatigue?

RE: Bending or Torsional Fatigue?

You have only one photograph of the fracture surface??? What does the side view of the failure reveal? Looks like reverse bending loads based on pre-existing cracks or defects on opposite corners. WAG with only one visual view.

Look closer at the fracture surface using an SEM.

RE: Bending or Torsional Fatigue?

2
Bending. Send the failed piece to a metallurgical lab for proper analysis with a complete description of the service or anticipated service conditions. No WAG about that.

RE: Bending or Torsional Fatigue?

Perhaps bending... doesn't look like a fatigue failure.

RE: Bending or Torsional Fatigue?

(OP)
Thanks Metengr

RE: Bending or Torsional Fatigue?

Looks like a symmetric kind of failure behaviour with two inition sites where material has been exposed to environment longer than the rest of the failed surface. however, macroscopic failed surface looks like a rapid fracture due to overload to me and not fatigue. Could it be a torsional overload since I would not expect the symmetric appearance in the case of bending? Can you give information on the loading case?

RE: Bending or Torsional Fatigue?

This really looks like torsional fatigue. The key is the fatigue zones are on opposite corners to each other, which is expected for rectangular geometry. Reversed bending would probably initiate fatigue midway between surfaces if applied to the long edges. Short edges would more likely see either fatigue zones in line with each other or in all 4 corners.

RE: Bending or Torsional Fatigue?

Not sure I would agree with you, mrfailure. Closer examination of the initial cracks at corners of the strap is required using an SEM. Corner cracks could indeed initiate from reverse bending loads. I don't see the torsional aspect of the fracture appearance.

RE: Bending or Torsional Fatigue?

The fact that they are corner cracks is not the reason for identifying torsion - it is the fact that they are initiating catty-corner from each other. That morphology is not typical for reverse bending loads.

RE: Bending or Torsional Fatigue?

Imo mrfailure has a very good point.
If compression or a lateral bending force is applied either to to long beams or beams with a section where one dimension is significantly larger than the other this will result in twisting. To the effect that torsion comes into play where nominally / seemingly only bending would apply.
Kind request to OP: Could you provide a sketch of the application, i. e. environment of the fractured item, position to support points, loading, operational characterisitics?
Thank you in advance...
Regards

Roland Heilmann
Lpz FRG

RE: Bending or Torsional Fatigue?

JHWC... is this a carbon or LA steel 'flat spring' from a vehicle?

1. Metengr "...Send the failed piece to a metallurgical lab for proper analysis with a complete description of the service or anticipated service conditions. No WAG about that." ... is exceptionally wise advice. However include a drawing and loading/environment data [etc] if available.

2. For 'recreational use'... IE: initial estimation purposes... You might find following documents useful...

ASM http://www.asminternational.org has several handbooks that detail metallography for various alloy families. However the 'bible' is latest version of...

ASM HANDBOOK, VOLUME 09 - METALLOGRAPHY AND MICROSTRUCTURES

Also... available on-line at degraded image quality is the monumental work...

WRDC-TR-89-4060 [ADA219747] FAILURE ANALYSIS HANDBOOK

NOTE.
I am lucky to have an original print-copy, in-which the photos/images are very clear and highly informative [procured in early 1990s]. High quality versions are likely available through libraries or Wright Labs [Wright Patterson AFB].

3. RE your second photo with the pieces rejoined: it looks like a 'narrow [~1/4] width sliver of material' is missing. This might alter or muddy the failure analysis somewhat

4. BTW... some serious advice for failure investigators. Fracture faces are highly valuable and must be carefully protected from further damage! Field handling and examination of damaged parts requires basic training [how-to] and carefully-restrained enthusiasm to avoid basic mistakes that could compromise further evaluation.

NEVER put fracture faces back-together! 'Close' is OK for graphical purposes... however even slight contact can alter the metallurgical evidence by incidental smearing.

ALWAYS Ensure that fracture faces are protected from further environmental deterioration by methods/means that will NOT transfer to the fracture faces. This advice extends to even touching the fracture faces with fingers and exposing the pieces to a 'dirty/polluted/corrosive atmosphere'.

Professionally photograph [very high quality/definition] the damage of each fractured piece from every conceivable perspective in both macro and close-up modes... including within the larger assy. These photos should be taken under direction/guidance by an experienced investigatorAS SOON AS POSSIBLE after the failure. It is amazing/humbling what good quality field photos can reveal when 'questions, concerns and puzzles' arise during lab examinations, days/weeks later.

NOTE.
Mishap/field-failure photography is an important part of field work... this is a hard lesson learned [HLL] from many field failure [aircraft mishap] investigations. Most pro-photographers are good at their craft… but utterly incompetent when it comes to understanding WHAT-TO-PHOTOGRAPH for general and specific forensic evidence [VHLL]! Great photographers will defer to technical competence and guidance to ‘take this image, of this area, at this angle/distance, with this lighting, with this definition, etc’... while not comprehending ‘exactly why’… and results are usually satisfying. CAUTION: non-cooperative/egotistical photographers are worse than useless [VHLL]!



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Bending or Torsional Fatigue?

May find this document useful...

SAE HS-788 SAE Manual on Design and Application of Leaf Springs

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

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