Shaft Design - Ductility/Toughness

[mechowl]

This may be a silly question, however in the consulting industry I see it over and over again so I thought I would reach out to the mechanical engineering world for advice/suggestions.

In the design of rotating shafts, parameters such as external loads, service life, geometrical features, etc. are used to select a material with strengths suitable to prevent failure with some appropriate safety factor in mind. Deflection is also critical and based on items such as geometrical size, modulus of elasticity, etc.).

Most detailed drawings (and performance specifications) used for fabrication of rotating shafts note the type of material, complete with yield strength and ultimate strength, but never (or from what I've come across) outline the required measure of ductility or toughness. My understanding is that this can be specified by means of listing a corresponding Percent Elongation and/or Percent Reduction of Area, or with Minimum Charpy Values (at some temperature).


Two main questions:

1. In others experience, is this typically done? (and if so can anyone elaborate or share a sample drawing or specification)

2. How do you define this via calculations? (i.e. Minimum Strengths are calculated based on forces, geometrical features, geometrical sizes, etc. - What calculation method is used for measuring minimum ductility or toughness?) - Can somebody share a sample calculation or reference to one within a text?

 

[desertfox]

Hi
shafts are designed as you stated ie minimum strengths of materials,taking into account geometry, service conditions etc.
When we look at the various materials we could possibly use they will conform to some British,American or European standard, that standard will normally state the ductility,hardness, strength etc for that material and for different sizes of round bar, ingot or whatever.
If the material properties for that particular section that we want to use are satisfactory, then the designer will just call up the section size, material and the relevant standard which controls the ductility,hardness etc.
If the material doesnn't meet the required hardness, ductility, strength etc the designer will either specify a heat treatment to achieve those goals or find another material, in specifying a heat treatment though the designer will sometimes need to quote hardness, strength, elongation to get the required properties.
Measuring techniques for hardness are :- diamond pyramid, brinell,Rockwell measuring scales.
Ductility is normally measured when materials are tensile tested.
Impact strength is normally measured on a pendulum recording the energy required to fail the material.

 

[mechowl]

Thanks desertfox,

One of my questions was how does one define the amount of ductility or toughness by calculations. Is there a method that I can reference in a textbook? I have yet to find anything like that.

 

[rb1957]

why do you need, sorry Need, to define elongation ? isn't this defined by your material spec ? select a material at suits your installation ... no?

Quando Omni Flunkus Moritati

 

[Jboggs]

I've been designing special production machines for over 30 years and I don't think I have ever seen a callout for ductility or toughness on a drawing. I don't think I even know what the units would be. Except for VERY special circumstances (turbine blades, etc.) all I have ever seen specified is the material, the treatment, and the final hardness. That pretty well locks it all in, right?

 

[redpicker]

I have been told by mechanical engineers that 13% Elongation is sometimes used to define the difference between a brittle material and a ductile one. That is, they use different design rules when dealing with a brittle material than with a ductile one, and if the material exhibits over 13% Elongation, then it can be considered ductile.

IMO, this is not a very good method, but in the lack of any other, it does have some sense. For rotating shafts, the big concern, apart from torsional strength, is fatigue strength, not ductility, so it may have some merit for this application. That is, if a shaft material is too brittle, then a very small fatigue crack can initiate a brittle fracture, but if the material has at least 13% Elongation, then it probably has sufficient ductility to allow the crack to grow to a size large enough to be detected by normal NDT methods before fracture. Whether or not this needs to be specificially qualified for each lot of material thorugh testing, or can just be demostrated by the average properties of the material and heat treat condition is a question that should be considered by the designer based on (among other things) the material, its variation of properties in the chosen heat treat condition, and the consequence of failure.

rp

 

[Tmoose]

Depending on which organization's "specificationZ" are enlisted an elongation and reduction in area will likely be in the tables right along with BHN, UTS and Yield strength. When talking to big name steel suppliers and well known heat treat shops about buying materials with particular requirements, especially for 1045 and 4140 shafts >6 inch or so, the standard response we've been hearing, at least from 2008 is "pick any 2, but for the rest you get what you get."

 

[desertfox]

Hi me howl
To answer your last post directly, I know of no method which would enable you to calculate ductility or toughness by formula, to me those are material properties that are measured by experiment.
Yesterday ironically I was checking a design of a shaft, I checked it for strength and hardness, hardness because it is running on a brass nut, but the main criteria was its strength, so once I was happy with the strength the rest of the properties
were governed by the material specification.

 

[rb1957]

again, why do you think you need to specify toughness ? toughness is pretty much predetermined by strength (inversely, high strength tends to be brittle)

are you designing a fracture critical shaft ?? even so, the toughness is dictated by the spec ... you'll need to do fatigue/crack growth/residual strength calcs based on the spec value.

Quando Omni Flunkus Moritati

 

[chumet1]

i will agree with desertfox that you cant calculate ductility and toughness, these values along with UTS and yield strength are found from extensive experiments. my method for shaft design is rigidity and strength, if the material fits the recommended strength and rigidity for application then i am good to go.... ductility and toughness issued will be encompassed within strength and rigidity

 

[geesamand]

My company has many horizontal shafts where ductility is important for the notch sensitivity of the material and overall life and robustness.

That said, we simply choose a ductile material (1018 c/s, 304L s/s, 316L s/s) and excessively hard material simply never occurs.

The one place where I do spec a limit is cast irons where I limit hardness to 220 Bhn for machinability. That's not really the same situation at all though.

David

 

[metengr]

Material callout for shafts should specify in order; material specification, and Grade or Type. IF there are specific Classes of Grades, normally the Class will be called out in addition to the Grade because this would further define strength, or hardness and heat treatment requirements in the material specification.

For example on a print, shaft material of construction could be listed as, ASTM AXXX, Grade 1, Class 1 or simply Grade 1. All of the minimum tensile requirements for design purposes are provided. There is absolutely no need to specify any ductility requirements because by virtue of the material specification, a minimum ductility value is provided. If it is too low, the designer selects another Grade or Class or even other material.

Toughness is a different matter. For low temperature service, the designer may require minimum notch toughness in terms of Charpy V-notch impact value. This could be a special call out if design conditions require it.

 

[mechowl]

I will change the angle of my inquiry:

1. We often talk about minimum strength requirements, however how would I know what is considered too high of an ultimate strength that I may be concerned of brittle fracture?
(I have seen a shaft previously rejected due to "overshooting" the ultimate strength in the heat treatment process causing the Charpy values to fall short of what was listed in the European standard for the material)


2. When only the minimum yield and ultimate strengths are specified, what should I watch for if approving a Material Certification from a steel supplier?


3. To tie both questions listed above together, is there a standard for materials (say 4140) by SAE or AISI that defines what other mechanical properties should be expected with corresponding strengths? I know it also depends on diameter.


Thank to all for the responses so far.

 

[desertfox]

Hi

To answer the first part you wouldn't know what strength would be considered to high without comparing it with the specification or standard and even then without knowing what heat treatment had been carried out,or what the final microstructure of the steel was like you could still be in the dark.
Impact testing is carried out usually by the supplier if there is a requirement of low temperature service and or very high impact loads induced on the component in service and these impact test results would be shown on a steel suppliers certification along with ultimate strength etc.
To my knowledge I know of no separate standards listing various properties with varying strength it's all usually in the particular material standard, if you have access to it look in bs en10083 part 2, it gives various heat treated conditions for supply with corresponding Charpy values, ultimate strength etc for various sizes.

Unless you know the service conditions and the design intent,then should you receive a drawing with only yield and ultimate strengths specified that's all you would be looking for.

 

[metengr]

1. We often talk about minimum strength requirements, however how would I know what is considered too high of an ultimate strength that I may be concerned of brittle fracture?
(I have seen a shaft previously rejected due to "overshooting" the ultimate strength in the heat treatment process causing the Charpy values to fall short of what was listed in the European standard for the material)

You have to understand that for a material specification, minimum tensile strength and MINIMUM elongation values are provided. So, why would you be worried about higher than minimum tensile strength as long as a MINIMUM elongation requirement is met. Most engineers know that strength and ductility are inversely proportional to one another so if by chance a material strength is higher than minimum specified because of chemical formulation from alloying yet it possesses higher than minimum elongation, so what?

2. When only the minimum yield and ultimate strengths are specified, what should I watch for if approving a Material Certification from a steel supplier?

UTS minimum, YS minimum, % Elongation minimum and or reduction of area minimum.

 

[rb1957]

you pick a material that meets all of your requirements ...
it has to be strong enough and it has to be durable enough, it has to be able to perform at your service temperature, etc ...
you have static requirements and fatigue requirements (you may also have fracture requirements).

a good starting place is "what do other people use for similar apps ?"

Quando Omni Flunkus Moritati

 

[GregLocock]

SAE 4140 is better known as axle steel, and has the following mechanical properties

http://www.interlloy.com.au/data_sh...igh_pdf/Interlloy_4140_High_Tensile_Steel.pdf

It is ductile and tough

Cheers

Greg Locock


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