How does the notch test measure fracture energy in materials?

[richnfg]

For my AS Physics course, I will need to learn about the notch test (it can measure the energy to break a material using a pendulum, apparently). Anyway, it says I need to learn about it yet it does not really provide much information on how it works...Anyone know?

Thanks

 

[russ_watters]

Welcome to PF!

This is known by engineers as the http://www2.latech.edu/~jordan/courses/me215/ManualF03/04CharpyImpact.htm test.

Basically, the energy required to break a piece of metal depends on both the force and the amount the material flexes when it breaks. Ie, iron is hard, but it doesn't bend, so it breaks with very little energy. Aluminum is soft, so it does bend, and absorbs more energy as it breaks. Using the pendulum, you can measure the energy by measuring the height of the pendulum swing after impact.

 

[richnfg]

Thanks for the quick reply and taking the time to help. :)

I understand that the energy can be seen by the different heights, like a higher outcome than another (if they both have the same starting height) would mean it took less energy to break? But can the amount of energy used be measured on the dial? Or does that just represent the height?

Also, I'd just like to clarify the difference between tough and strong and weak and brittle if that it is ok. It's just that we need to know the exact definitions for the testing materials topic.

A brittle material cannot withstand much tensile forces acting upon it and it will break apart quite easily (cracks form easier) and tough is the opposite of this, right? But I don't really know how to explain strong and weak (sounds stupid really, but I'm sure there is a physics answer as opposed to an ordinary answer )

Even though the exams not until June, I just want to get clear on these things. Sorry about the amount of writing. :P

 

[PerennialII]

I understand that the energy can be seen by the different heights, like a higher outcome than another (if they both have the same starting height) would mean it took less energy to break? But can the amount of energy used be measured on the dial? Or does that just represent the height?

You can get the energy the breaking takes from the potential energy difference between the starting and end heights, the dial is then simply calibrated accordingly to make the test intepretation simple and straightforward (usually assuming negligible friction and so on (=machine is calibrated regularly etc.)).

A brittle material cannot withstand much tensile forces acting upon it and it will break apart quite easily (cracks form easier) and tough is the opposite of this, right? But I don't really know how to explain strong and weak (sounds stupid really, but I'm sure there is a physics answer as opposed to an ordinary answer )

What you get from an impact test is essentially a measure of ductility, which is only indirectly related to strength. Russ supplied that link and the Fig. 9.23 there was from Herzberg which is an example of a load-time curve measured from an instrumented Charpy test, the area under it is comparable to the energy we're talking about, and its value is the basis of classifying a material as "brittle" or "ductile" ductility wise.

 

[russ_watters]

A brittle material cannot withstand much tensile forces acting upon it and it will break apart quite easily (cracks form easier) and tough is the opposite of this, right?

Actually, no - a brittle material is often extremely strong (some bridges are being made of glass). But the fact that it doesn't bend means it doesn't take much energy to break it: since it doesn't bend, the impact has a high force for a short distance and time.

Impact energy is toughness which is a combination of strength and ductility.

 

[richnfg]

Actually, no - a brittle material is often extremely strong (some bridges are being made of glass). But the fact that it doesn't bend means it doesn't take much energy to break it: since it doesn't bend, the impact has a high force for a short distance and time.

Impact energy is toughness which is a combination of strength and ductility.

yeah, that's what I said wasn't it? What about a material being strong?

Simple words yet so hard to get the exact definition.

Right, so it is the difference in potential energy I understand that. THanks!

You guys are very helpful!

 

[PerennialII]

Yeah, the definitions associated with these things aren't really straightforward, and in some respects even a bit incomplete. In general "the school" I belong to has a tendency to classify strength and ductility issues to three "sets" :

1) the stress-strain curve related measures, such as yield and tensile strength (fracture strength, fracture stress), fracture strain etc.
2) ductility understood as energy, like in the impact test such as the Charpy. The relationship of 2) and 1) is basically such that measures like 2) equal to 1) integrated over time, deformation etc. -> ductility is affected both by how "strong" the material is and how much it can deform without breaking
3) toughness, which I'd reserve for fracture mechanical parameters, and as such the fracture toughness, which are measures of material toughness when a defect initiates and progresses in a material (what happens in 2) and also in 1)). And 3) is then naturally related to both 1) and 2).

I separate 2) from 3) 'cos 2) is more like an engineering measure ... you can't really tell that much quantitatively about material behavior e.g. in a structure simply on the basis of energy measured in a Charpy test (if I was to change a specific of the test the result would change, so it's an indirect observation of material behavior, a bit qualitative measure), whilst using 3) this can be achieved quantitatively.

Hope this helps and does not confuse further ...

 

[russ_watters]

yeah, that's what I said wasn't it? What about a material being strong?

Simple words yet so hard to get the exact definition.

You said "tensile strength" which is the constant force it can handle when in tension. That's completely independent of how brittle something is. Yeah, definitions are important here.

"Ductility" is basically just the elastic modulus: how much it stretches with a given force.

Stress is force in psi.

Strain is elongation.

 

[richnfg]

You said "tensile strength" which is the constant force it can handle when in tension. That's completely independent of how brittle something is. Yeah, definitions are important here.

"Ductility" is basically just the elastic modulus: how much it stretches with a given force.

Stress is force in psi.

Strain is elongation.

Yeah, I knew tensile strength is a constant force related to the material. I think I have just completely lost myself in these definitions. At least I undertstand the notch test!

I think I might just visit the exam board website or ask a teacher for definitions that, if I was to right down in an exam, they would definitely be right. You've been a big help. Thanks guys.