Fracture surface in cases of proportional and non proportional stresses

[Znjmech]

Hi all

Does anyone know whether the fracture surface changes if the stresses applied in the cycles change from proportional (desired) to non proportional ?

 

[metengr]

from proportional (desired) to non proportional ?

If you want a response you need to better define your question. Non-proportional to me is variable amplitude loads but this could be different to what you intended.

 

[Znjmech]

You are right , sorry, I meant, when we apply a certain displacement, but in a way that might cause the resulting stresses in the component get non proportional

 

[Dhurjati Sen]

SFTmech
The fracture surface would change only if the nature of the fracture changes, i.e. ductile to brittle or vice versa. If the original stresses were producing fatigue leading to a ductile fracture, a change over to non-proportional stresses may well produce a fatigue leading to a brittle fracture!

 

[MagBen]

Yes,when the load was suddenly increased, the fracture structure COULD change from ductile to brittle. e.g. from spherical dimples to cleavage planes (transgranular or intergranular fracture).

 

[WKTaylor]

SFTMech.... in addition to others comments...

Flaw initiation/detection/characterization can be affected/driven by many complicated factors... but 'where-you-start' affects total crack-growth and fracture toughness.

For the X-material in question: Characterize the unique crack-growth [KI], critical crack length [fracture toughness, KIc] and stress corrosion cracking [SCC] parameters... bare minimum... using standard ASTM tests.

Geometry VS loading SS crack(s) orientations can affects crack growth dramatically.

The surface of the part can grossly affect fracture: rough/smooth [see ASME B46.1], porous [cast, pitted/corroded, scratched/scored/gouged], brittle [hard anodized, abusively plated, abusively machined, etc], shot-peened [properly or poorly], etc...

Service environment affects crack growth: Corrosives, chemical exposures, extreme heat, extreme cold

Regards, Wil Taylor

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I see many fracture modes mentioned here and possible being conflated, but I still have no clear idea about the question being asked.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."