Impact Test

You're designing it, so you should already know!

I have to agree with AP#1. I'd look for a reasonable compromise of mass and velocity.

Hit the books, lad! You're in over your head.

No they are not the same. Consider the difference between a person getting shot and the kick the rifle butt gives the shooter.

Can't buy that. The OP has specified that the energy input into the sample will be the same.

You should buy that. The energy and momentum at impact can never be the same if the masses are different.

You are right of course I was being stupid : it's the momentum of the rifle that's equal to the momentum of the bullet. Ignoring direction if

mV = Mv

then

½mV² is very much greater than ½Mv²

Consider the difference between a person getting shot and the kick the rifle butt gives the shooter.
Guns don't kill people, bullets kill people.

I would think the answer lies in the question 'What sort of impact/eevent are you trying to simulate?'
Del

What we have here, is failure to communicate. _{(Strother Martin, the Warden, in Cool Hand Luke)}

Yup, it's one of those topics that we all think we understand, but the more we think about it the more confused we get.
At Wimbledon the tennis ball was being served faster than my regular bow shoots an arrow, the tennis ball is also heavier than the arrow, so obviously has more energy.
But when it comes to impact, which would you rather be hit by?
Del
(If you need a hint.... _{it's not the arrow} )

I get all that. The energy input into the test specimen is specified, not the duration.

My suggestion was to adjust the two forces to achieve the desired energy input over the desired period.

Never mind, I guess I don't understand.

_{I shall take a dip in yon pool, and relax.}

I've never really got to grips with the kinetic energy/momentum/impact thing at all. I get completely confused.
I can only go by gut feel and keep on shootin' my arrows, great way to let off steam!
Now you swim up and down and I'll take pot shots... I'll use the blunts honest.
Del

No input from the OP? Well perfect time for my 2 pence!

Velocity has a strong effect on impact. Take Del's arrow with a set energy value, hit a plank of wood clamped in a vice. At high velocity it will pierce the wood but at low velocity it will bend the wood and spring back.

Impact is a function of time, small time value stronger forces are exerted.

Drew K

_{<on a side note, what's with the new blue IHS ball that has acquired GS?>}

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At extremes it might matter. To the folks with the bullet and arrows analogy, you are talking about concentrating the force in a small area to obtain penetration of soft material, not exactly relevant. mass x velocity. I would think you'd want the velocity to be somewhere in the neighborhood of the impacts for which you are designing the test machine. If Newton's laws apply, then the much heavier rifle should possess the same kinetic energy as the much lighter bullet. Or are you trying to rewrite science?

I was referring to shear / tearing and bending forces. With some materials when force is applied rapidly they behave more as a rigid solid, others will absorb impact over a longer period of time and greatly reducing the force of the impact.

You must design the impactor for the specific purpose of the test.

Drew K

I think both give the same testing condition if the area and contact surface finish at the place of hitting is unchanged, because the intensity of hit depend on kinetic energy which is defined as mv**2/2.

Let me summarize what I have understood.

If the deforming factor is the energy, the mass and velocity combination do not matter, any combination will do. However, if the deforming factor is the force then this combination matters. Therefore which is the effective factor in deforming the object hit, energy or force? Assuming the hit area does not change.

Thanks

Hello Silver Ring,

Newtons Law should provide a simple answer to your question. Newton determined that F=MA, where the force (F) is equal to the mass (M) times the acceleration, or velocity, (A). This is an inverse equation. Therefore, twice the mass will require half the acceleration to achieve the same effect. Conversely half the mass will require twice the acceleration to achieve same effect. A problem may occur when an object of lower mass impacts an object of higher mass at a velocity high enough to "equalize" the mass times acceleration effect. Think meteors hitting a planet, or bullets hitting a concrete wall and you will get the idea. While many "theories" exist, Newtons formula is a law of physics.

Rega

The word "bouncing" written by Greg M reminded me of the law of conservation of energy.

If I use a large mass to minimize the bouncing, the whole energy will be transferred to the surface. If the mass is small and bounces after hitting, some of the energy will be lost, as it will be spent for bouncing. Therefore a larger mass will be more effective as there won't be any energy loss during testing.

Agreed?

From reading all these posts it seem obvious that you (and most of us) need to consult an authoritative text book on the subject.
Sill no response to #6 though, which surely is a pretty valid Q.
Del

Impact onto a plastic cover, in order to test its touhgness against possible impact loads during usage.

Ah, in that case, I'd look at the environment it is to be used in and take cues from there. If it's to be used in a kitchen area the impacts are likely to be different from thise in a loading bay or a bedroom.
Design the machine to simulate that which is most likely to impact the cover.
the KrisDel^TM FPIT (Feather Pillow Impact Tester) is available at a very reasonable price.
Del

This document will tell youASTM D4272 - 09 Standard Test Method for Total Energy Impact of ...

You should purchase a tester. If you build one, your data will have no validity at all.

As your customer, I would not accept any data from a home brew machine, unless it was certified by an independent, accredited test lab.

I wouldn't dismiss an individually designed testing apparatus. If you need your tests to be validated you can have your testing apparatus evaluated by a private engineering firm or often a local university will test it for you.

Drew K

A good point. It might be necessary to do impact testing of the finished assembly instead of just raw materials. As such a commercial off the shelf (COTS) impact tester may not be able to test the assembly properly. An individually designed tester must be certified by an independent group to have any validity.

From your original post, I was thinking you were wanting to design/build a tester for commercial production and sale. But this post sounds as if you want to test candidate materials for a plastic product you are developing. If this is the case, I recommend:

1. Review all ASTM (or other) standards relating to impact testing of plastics.

2. Select the standards which best parallel the conditions your product will experience.

3. Purchase a machine which performs that test to the applicable standard. Or contract a laboratory to perform the test, if that is more cost effective.

My only experience with impact test requirements is the Charpy test on metals (mostly steels) intended for use at low temperatures. The measurement there was based on energy applied - did the sample fracture at x lb-ft? If it fractured, I'd think there would be no rebound. If it rebounded, that would be good - it didn't fracture. I'm not sure how much of this is relevant to plastics, but keep in mind that the testing needs to be tested at the temperature extremes that your product will experience in service.

I assume (we all know what that means) that you are doing a drop weight test?

Why are you designing a test machine when commercial ones are available?

Memories from doing some Charpy testing in the 01970s: The tester was a pendulum, so it had a specific maximum energy that it would measure. We were looking for 15 ft-lb @ -20 degrees (I forget whether C or F). All specimens were machined to the same size. As I recall the Charpy test required a V-notch in the specimen. After the specimen broke, the energy absorbed was measured by how far the pendulum continued to swing. All this ought to be in the ASTM specs.

I just looked at the ASTM web site (astm.org.) They had at least 10 pages of different impact test descriptions. You may want to take a peek. The only other thing I can think of is, "To a hammer, everything looks like a nail."