# Comparing materials abality to absorb impact

## Main Question or Discussion Point

Hi,
I'm doing this investigation for school, and i came up with the idea of comparing the effectivenss of different matrials at absorbing impact.
I have an idea about using a pendulum, where i release a bob of known mass at known starting height then let it hit the material. Thus, afterwards if i can record the rebound height then i can find the energy absorbed. (change in Ep= mgh(before) - mgh(after)). Not entirely sure if this is the best method to do this.
I would REALLY appreciate any comments on his, or alternative methods.

Also, I need to write a 2500 word report on it afterewards, so i really need to make it more complex. -i thought about linking with SHM (cos a swinging pendulum and its kinda like damping), or maybe circular motion (i.e find angular momentum or something, but would need w)

Any ideas??

Thanks.

David

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brewnog
Gold Member
Your idea about using a pendulum weight to measure energy absorbed is fantastic; so much so that I'd suspect you've seen that this is how industrial machines accomplish the same task!

I'm not so sure about your ideas regarding SHM or circular motion (I don't really see what you'd gain from adding this), but I suppose it depends what you're trying to get out of the project.

Gokul43201
Staff Emeritus
Gold Member
To the OP: Look up the keywords Izod, Charpy Impact Test. For more on the subject, try to understand how the impact toughness of a metarial can be estimated from its stress-strain curve.

Last edited:
Further

Thanks for your comments. The Izod, Charpy techniques were of great help.
I have decided to go ahead with this investigation, but as i said i am trying to increase the complexity of it also. SHM and Circular motion would help in the actual analysis of the motion and impact. I'm just not sure if they would strictly apply to a pendulum striking an object (i.e. is this a special case of damping??) ???

Also, I am planning on investigating many materials including: Rubber, foam, wood (different kinds), metals, paper, building material (brick, stone, etc) plastics, different cloth materials, bone. However, in the investigation i will need all these materials in the same dimensions. So, I am at a complete loss as to how i am going to aquire all these materials with the same size and thickness. Some are of course easier then others... but im not sure a hardware shop would have most.
- Or alternatively would it be possible to state the energy absorbed in terms of energy absorbed per unit width of the specimen. So wouldnt need same dimensions. I'm not sure if that would be possible. ????

Again, i would really appreciate any comments of help..

Thanks

David

Homework Helper
Thanks for your comments. The Izod, Charpy techniques were of great help.
I have decided to go ahead with this investigation, but as i said i am trying to increase the complexity of it also. SHM and Circular motion would help in the actual analysis of the motion and impact. I'm just not sure if they would strictly apply to a pendulum striking an object (i.e. is this a special case of damping??) ???
As brewnog mentioned before, I don't see where circular motion would fit in, but I guess you can always add some additional information to make things look nicer.

Also, I am planning on investigating many materials including: Rubber, foam, wood (different kinds), metals, paper, building material (brick, stone, etc) plastics, different cloth materials, bone.
Stick to wood and metal first, eventually some kind of brick/stone, as you mentioned.

- Or alternatively would it be possible to state the energy absorbed in terms of energy absorbed per unit width of the specimen. So wouldnt need same dimensions. I'm not sure if that would be possible. ????
Usually, in a classical Charpy test, the impact strength of a material sample is defined in units [J / m^2], i.e. you have to divide your energy with the area of the cross section at the point of impact.

PerennialII
Gold Member
.....
- Or alternatively would it be possible to state the energy absorbed in terms of energy absorbed per unit width of the specimen. So wouldnt need same dimensions. I'm not sure if that would be possible. ????
....it is naturally possible to normalize the energies with the fracture area. However, to be accurate these tests do not produce specimen size independent results (well what does but anyways ), so I'd recommend you keep the dimensions "similar". With the accuracy you're seeking I don't think this is much of an issue, but I'd keep the specimen sizes, say, within 'tens of percents' of each other if possible (with respect to width) and then normalize the results with the appropriate dimension that changes. If you've as radically different materials as it seems this will likely be very much a non-issue but thought I'd mention (if you were comparing similar materials, like 2 different kinds of steels it would definitely be an issue).

brewnog