Elongation relationship to fatigue life

[BrianE22]

I'm comparing two aluminum bronze cast materials, 953 vs 955. We currently use 953 for our worm gear blanks. A vendor has quoted doing them in 955. I like the higher strength of the 955 (we're trying to optimize the shape, less material, higher stresses) but it comes with decreased elongation. It's been a long time but I believe decreased elongation means higher notch sensitivity, correct? Also, the impact resistance could be less, correct?

 

[EdStainless]

Maybe you are correct, if it was same alloy heat treated to a higher strength then you would almost surely be correct.
But with a slightly different alloy you cannot always make those same assumptions.
Even if the notch sensitivity is up and impact toughness is down that doesn't mean that you have less fatigue life. The higher strength will at least in part offset that. It would mean that once fatigue started you would likely have more rapid propagation.

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P.E. Metallurgy, consulting work welcomed

 

[redpicker]

Remember, 90%+ of a component's fatigue life is realized before a detectable crack is formed. Increased notch sensitivity certainly reduces the time from when a detectable crack forms and final failure, notch sensitivity does not seem to have a direct effect on the 90%.

 

[BrianE22]

O.k., it's slowly coming back to me. Looks like the higher strength alloy gets me to the crack initialization (with higher stress) and then there's the crack growth. Is "notch sensitivity" the correct term to be using? A review of my machine design handbook states that notch sensitivity is only related to geometry. Does decreased elongation even figure in the endurance limit? Maybe endurance limit only means to crack initialization?

Edit - now I see where notch sensitivity is brought in. I was missing the part where "q", notch sensitivity, affects Kt, the stress concentration factor.

 

[blacksmith37]

Aluminum bronze is heat-treatable :Copper Development Assn, 1978, lists as cast 9D ( 355) as 90,000 and heat-treated as 110.000 Tensile . I had written notes in the reference for Charpy V of 10 to 16 ft.lb , for various temperatures ( no brittle transition for copper alloys). Aluminum bronze was used for waterflood well heads but I do not recall any particulars.

 

[Guest102023]

You are making the very common error of conflating/confusing notch sensitivity, fatigue, and impact toughness.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[BrianE22]

Yes, I am definitely conflating the terms. I tend to jump ahead a few steps as I get older. Another question: Does elongation relate linearly to notch sensitivity, i.e. does twice the elongation equal 1/2 the notch sensitivity?

 

[WKTaylor]

Observation only.

For optimum/highest strength toughness, wrought material tends to give superior service relative to castings. I guess this is not a practical alternative?

Regards, Wil Taylor
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[EdStainless]

No on notch sensitivity, it tends to be material related.
Though in some materials it is only an issue at higher strength/lower elong but in some materials it is always an issue.

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P.E. Metallurgy, consulting work welcomed

 

Notch sensitivity and fatigue are two different (but sometimes related) problems. Your fatigue limit will increase with higher tensile strength, but notch sensitivity not necessarily so, especially for two similar alloys.

Anyway, for gearing applications, once you get to the point where you encounter notch sensitivity issues, the game is probably already over.

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

 

[BrianE22]

Want to thank all the contributors - I'm feeling more comfortable with the terms and the relationship to fatigue.

Yes, I do consider the life to be to the start of the crack.