Tensile/Impact strength relationship???

[Ugly]

I have been asked to find a relationship between an impact strength and a tensile strength.

I have a welded part which must meet an impact test specification and I looking for a way to emulate that test with a simple tensile test.

Is there an equation or a conversion or any direct relationship between the two?

 

[insideman]

I think you are out of luck. Tensile strength can be vey high in steels that are very brittle so have low impact strength. NO correlation!

 

[Maui]

I agree. There is no general correlation between Charpy impact toughness and ultimate tensile strength. That's why they each have to be measured experimentally. If you have the ability to perform the tensile test already, why can't you perform the Charpy test as well? If you don't have the equipment on site, you could send out a sample to a local lab for testing.



Maui

 

[JimMetalsCeramics]

Impact strength is a function of tensile strength. But since it is also a function of fracture toughness, you cannot correlate the two.

 

[Ugly]

The problem is that I am not looking for a test on a material.

I am trying to emulate an impact test with a tensile test for its simplicity.

I build MacPherson struts and all the welded components that are attached to the strut are required to pass a specified load impact test. There are so very many different configurations of these welded brackets throughout the many different part numbers, that to build a test fixute with a dynamic loader (as I do for other simpler welded parts) would be a nightmare.

A brainstorming session turned up a suggestion that we build a tensile testing jig (easier to reconfigure) but I told them I didn't have any way to compare a tensile result to the impact specification. So the typical response was, "Your an engineer figure it out!!".

So I thought I might get a second opinion.

 

[CoryPad]

Ugly,

Your additional information raises other questions. What do you mean by "impact strength" and "impact test"? The previous responders assumed impact strength to mean a material's inherent impact strength as measured by a Charpy impact test. But, there are many "impact tests" for component level testing that may be closer to your needs. If you provide some additional details, we may be able to help you.


Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Ugly]

The spec. requires a ft-lb test.

The suggestion was to enact a lbf test and make it equivelant.


CoryPad, I apologize, this is my first use of the forum and I am not exactly e-text savvy, so if I am violating the rules I apologize. I did review this FAQ you mentioned and believed I was following them? No?

 

[CoryPad]

Ugly,

A "ft-lb" test is not descriptive. Is the goal to ensure that welded components of your strut can absorb a certain energy level (X ft-lb_f) without damage/failure? Do you have rate, temperature, or other factors to consider?

Regarding the FAQ, that is a standard part of my signature and appears on every post I make. You will see similar signatures from other users. It isn't meant to tell you that you posted something inappropriate - if you did, a user will "Red Flag" it and it will be handled by the site management.


Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[JimMetalsCeramics]

The closest you can get would be to perform the tensile test under extremely rapid strain. This is just an indicator of toughness.

 

[Ugly]

Jim, that was my thought.

That the only way to mimic this would be to force a dynamic factor by attempting to speed up a strain test. But then we are back to the point CoryPad made, we are wanting to know how much energy it will absorb and a rapid strain will be a poor way of showing that.


Thank you all for your assistance. My materials 101 isn't as rusty as I thought but I was having trouble with the techical descriptors.

All were a big help and the promptness is unreal!!!

 

[JimMetalsCeramics]

You plot stress vs. strain. Area under the plot is energy.

 

[Maui]

Have you considered performing a bend frature test with your tensile testing set-up? There should be a very good correlation with the Charpy impact toughness, and by designing a suitable set of fixtures, I believe that you could accomplish your goal.


Maui

 

[brad]

Jim's suggestion directly above is the closest approximation--the area under the stress-strain curve is an energy variable, which should somewhat correlate to the impact strength. You must have the curve, however; the tensile strength in itself is not enough information to determine the impact strength. Even with this, it is still ballpark as this will not account for strain-dependent effects.

FYI--for steels, the tensile strength can vary from 200-2000 MPa, while the Fracture toughness (the material variable most dictating impact strength, previously stated by Jim) varies likewise by an order of magnitude (source text: Materials Selection in Mechanical Design, Michael F. Ashby). There also tends to be an inverse correlation between these variables in steels--the higher the tensile strength, the lower the energy (due to brittle vs. ductile fracture). Somebody suggesting to you that it is easy to correlate these two variables does not appreciate that they are not directly interrelated.

Brad