Spring Wire Material Properties

[Dillinpolish]

I've been designing a spring form for an over-center latch.

Because the spring form is an abnormal shape, I've been using ANSYS to calculate the stresses and stay below the material yield strength I've found online (<78% tensile yield as a design criteria).

The music wire I'm using is 2 mm A228. Does anyone know the ultimate tensile strength for this material? I'm seeing this over-center latch fail in fatigue (I think there is an alternating load I'm not accounting for that is creating this failure) and I'd like to run some high-cycle fatigue calculations, but estimating the ultimate strength throws this calculation off.

Anything helps.

Thanks!

 

[goutam_freelance]

The ultimate tensile strength as quoted vary wildly. Refer below:

https://www.steelgr.com/Steel-Grades/Carbon-Steel/astm-a228.html

https://optimumspring.com/technical...carbon_steels/music_wire_228_spring_wire.aspx

https://materials.gelsonluz.com/2020/09/astm-a228-properties-chem-mech-rankings.html

You need to contact some suppliers and ask for test reports.


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[IRstuff]

Please refrain from double posting

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[desertfox]

Hi Dillinpolish

A rule of thumb I used to work by if you get the stress below 25% of the ultimate tensile strength (which I know you haven’t got at this point) you are unlikely to have a fatigue failure, now you could apply the same logic to the yield stress and that might cure the problem, the downside might be that the spring design won’t be at its most economical but it might save some fatigue analysis particularly if you don’t have any data on the fluctuating load. Ah wouldn’t the fluctuating load be that what it sees from opening and closing the over centre latch?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[btrueblood]

Expected "Min. Tensile Strength" for .080 Music Wire is about 260 ksi (Ref. Wahl, Mechanical Springs). As desertfox said, you will want the design stress down below about 1/3 to 1/4 of this ultimate to get infinite fatigue life. You might take a close look at how the bails are being formed, especially at the tighter corners, and see if any stress risers are being introduced there.

 

[Dillinpolish]

This over-center latch is used in a unique application where it has a high mean stress (from the normal over-center clamping that stretches the spring form) and an alternating stress due to the movement of the system it's holding together.

The spring form is seeing a high enough stress (mean+alternating) to fail in fatigue at 50k cycles. I'm modelling this spring in ANSYS to get the mean stress, and back-calculating the alternating stress from the fatigue calculations but it just doesn't make sense. The alternating load has to be extremely high for this spring to fail at 50k cycles even if we're at our maximum mean stress (max stretch of the spring possible in the mechanism).

I know standard spring-design rules such as maximum stresses for fully-alternating infinite fatigue life, but this isn't a fully-alternating (or fully-reversible) load. I'm thinking that there's just something I'm missing in the mechanical material properties. I'm using Goodman/Soderberg/Gerber for comparison.

The standard I'm using is JIS for the A228 wire. This standard says the Tensile Strength (Yield) of this material is 2000 MPa, but it does not show the Ultimate Strength which would help me more accurately back-calculate the alternating stress. What I'm hoping for is someone to know what the yield and ultimate strengths are of music wire like this. I haven't been able to find it on any material websites like MatWeb or Total Materia, and getting good tensile test data that can be input into ANSYS for something this high in strength is difficult to say the least.

I guess I don't fully understand why music wire data only provides a yield stress.

 

[desertfox]

Hi

Well this isn’t a designated reference but on this site it says 228 wire has a minimum tensile strength in psi of 230-399 x 10^3 which equates to the figure you have quoted but it doesn’t say it’s the yield stress. Why can you not use the 2000Mpa figure in light of know other information. By the way it mentions using 45% of the tensile strength so possibly redesign the spring to that figure, I notice you are using 78% according to your first post.

https://optimumspring.com/technical...carbon_steels/music_wire_228_spring_wire.aspx

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein