Designing Impact Force Test Rig for Material Strength Testing

[Jaydee]

TL;DR Summary

Help with calculation of impact force of a pendulum type rig.

Hi all,
I have just started work experience at an engineering company in the design department. I've be tasked to develop a test rig, Similar to a charpy rig. Intended to measure the impact strength of materials.

The design will consist of an A-frame structure with swing arm attached to the top axle, on the end of the swing arm is an anvil mass. The arm will swing and strike test pieces which are fixed to a solid, flat steel block fixed at the point of 0°

These numbers are just an example for the purpose of understanding how to do it and as accurately as possible.

Length of swing arm (inc anvil) =2m
Mass of anvil = 250kg
Size of anvil = 200x200x300mm
Mass of swing arm = 30kg
Starting position of swing arm = 90°
Strike point is 0°
Bearings on axle = 100mmOD.. 50mmID
Bearing friction = 0.002ų
Everything will be made from hardened
steel

My initial thought was the impact force wouldn't be overly difficult but after some research it seems to be much more complicated. I am beginning to think I've been setup with lack of information. Any help or advice on how to go about this problem/ what variables are missing would be much appreciated. Attached a rough sketch In case there's any confusion on the construction.

Many thanks

Jay

 

[Lnewqban]

Welcome, jaydee!
Congratulations on the new job.

I would anchor that A-frame to the ground very well, or it will swing around that mass of 250 kg attached to a 2-meter arm.
Second consideration would be reinforcing that bridge element between both halves of the A-frame, so it would be rigid enough under impact.
What do you want to calculate first?

Check this example out:

 

[jrmichler]

My initial thought was the impact force wouldn't be overly difficult but after some research it seems to be much more complicated.

That's why the Charpy test measures energy to fracture the specimen. Energy absorbed is easily measured using a pendulum. There are three ways to measure impact force.

1) A load cell at the point of impact. This is tricky because there are non-obvious error sources due to the frequency response of the load cell, getting the impact force centered on the load cell, and the mounting of the load cell.

2) Strain gauges on the structure supporting the specimen. Getting halfway usable results from this requires a deep knowledge of the relationship between impact duration, impact force vs time, and the natural frequencies of the structure.

3) Put an accelerometer on the backside of the hammer. Getting halfway usable results from this requires a deep knowledge of the relationship between impact duration, impact force vs time, and the natural frequencies of the hammer itself.

One of my more interesting projects back when was to measure the frequency response of a load cell that was integrated into an impact testing apparatus. We needed to find the frequency response of the test apparatus up to 10 kHz. Previous efforts with a hammer did not work because the impact duration was only about 1 msec, far too slow to excite 10 kHz. Previous efforts with blasting caps (one person owned half of a copper mine) did not work because, while the impact was fast enough, the force was too low for the 30,000 lb load cell. The solution was to shoot it with a bullet that was less than 0.5 inch long with a velocity about 1000 ft/sec. The bullet impact was both fast enough to excite the the necessary frequencies, and had enough force to get a good signal from the load cell. A 22 LR standard velocity was the solution, frustrating the person who wanted an excuse to bring in his 9 mm.

 

[Jaydee]

Welcome, jaydee!
Congratulations on the new job.

I would anchor that A-frame to the ground very well, or it will swing around that mass of 250 kg attached to a 2-meter arm.
Second consideration would be reinforcing that bridge element between both halves of the A-frame, so it would be rigid enough under impact.
What do you want to calculate first?

Check this example out:

Hi, many thanks for your reply :)

At this stage it is purely theoretical and in the design stage to see if this type of thing would be a feasible option for a specific application, or if the company should go for another testing route.
I haven't received the exact data yet so I've just input some made up numbers to get started and figure out the process of what I need to do.

I do know the impact force required will be pretty high so the structure will have to be extremely strong and possibly too large to be realistic, which I need to identify. The design is also not limited to the A-frame. A drop rig or vertical lever falling to the ground are other options I'm looking at.

As for what I'm trying to calculate, Essentially I'm looking how to find the maximum impact force the rig is capable of. This is to ensure the material in the test will be suitable for the final application.

Thanks again checking out the video now :)

 

[Jaydee]

That's why the Charpy test measures energy to fracture the specimen. Energy absorbed is easily measured using a pendulum. There are three ways to measure impact force.

1) A load cell at the point of impact. This is tricky because there are non-obvious error sources due to the frequency response of the load cell, getting the impact force centered on the load cell, and the mounting of the load cell.

2) Strain gauges on the structure supporting the specimen. Getting halfway usable results from this requires a deep knowledge of the relationship between impact duration, impact force vs time, and the natural frequencies of the structure.

3) Put an accelerometer on the backside of the hammer. Getting halfway usable results from this requires a deep knowledge of the relationship between impact duration, impact force vs time, and the natural frequencies of the hammer itself.

One of my more interesting projects back when was to measure the frequency response of a load cell that was integrated into an impact testing apparatus. We needed to find the frequency response of the test apparatus up to 10 kHz. Previous efforts with a hammer did not work because the impact duration was only about 1 msec, far too slow to excite 10 kHz. Previous efforts with blasting caps (one person owned half of a copper mine) did not work because, while the impact was fast enough, the force was too low for the 30,000 lb load cell. The solution was to shoot it with a bullet that was less than 0.5 inch long with a velocity about 1000 ft/sec. The bullet impact was both fast enough to excite the the necessary frequencies, and had enough force to get a good signal from the load cell. A 22 LR standard velocity was the solution, frustrating the person who wanted an excuse to bring in his 9 mm.

Hi,
Thank you for your reply, it does seem increasely likely that this type of rig is not feasible. The task is really to identify what impact force the rig would be capable of in theory, using the math to design a rig of a specific impact force to a given material. Obviously I still need the data of the test material for this but from your answer and what I've found elsewhere it seems like practical tests is the only way to get a definitive answer. I'm wondering if I've been tasked this purely to test me instead of the material. :)

 

[jrmichler]

You can estimate the maximum impact force by making some assumptions:

1) The test specimen does not fracture or yield.
2) Assume the largest, stiffest, test specimen.
3) The test fixture is anchored to a "large" concrete block. Think a couple tons.
4) The anchors do not yield or fracture.

Calculate the stiffness of the test specimen and supporting structure, assuming that the support structure is fastened to a rigid body (the concrete block). You will probably need FEA. The 250 kg hammer (and its arm) has a maximum starting height. From that, calculate the maximum potential energy. That potential energy is transformed to kinetic energy at the bottom of its swing. When a mass with kinetic energy hits a spring, the kinetic energy is transformed to potential energy in the spring. At the instant that the mass comes to a stop, all of its kinetic energy is potential energy in the spring. The equation is 0.5mv^2 = 0.5kx^2. Knowing the maximum spring deflection and the spring constant, calculate the maximum force.

The calculated maximum force is directly transmitted to the concrete block, so the specimen attachment (vise?), its mounting structure, and the attachment to the concrete block all need to be designed for that force. They need to be designed for repeated application of that force.

I'm wondering if I've been tasked this purely to test me instead of the material.

It is normal practice to assign a challenging problem to a new engineer for the purpose of finding out how they deal with a challenge. A legitimate response is to take it as far as you can, then ask for help from a more experienced engineer.

 

[russ_watters]

I'm wondering if I've been tasked this purely to test me instead of the material. :)

Maybe. Are you an engineer? More specifically, did you take a materials science/engineering course in college? It would be shocking to me that an engineer and his/her supervisors at an engineering company wouldn't know what a Charpy test is actually for. So, what is it that you actually need -- why do you or your managers think you need a Charpy-like test rig? Did they really say they wanted a Charpy test rig to measure force or was it you who assumed that that's what it was for? In the thesis statement OP, you said "impact strength", and only later (and in the summary) said "force". So which did they actually task you with? Did they say "impact strength" and you assumed they meant force...?

 

[hutchphd]

I am not an engineer. But I recall (*somewhere? the Charpy test does not directly measure Force but in fact is a prescribed test to measure materials relative strength under impact. I think the pendulum rig is the most reliably repeatable way to do this an you can easily change the mass and lengths involved.You can instrument the heck out of it pretty cheap these days

 

[Jaydee]

Maybe. Are you an engineer? More specifically, did you take a materials science/engineering course in college? It would be shocking to me that an engineer and his/her supervisors at an engineering company wouldn't know what a Charpy test is actually for. So, what is it that you actually need -- why do you or your managers think you need a Charpy-like test rig? Did they really say they wanted a Charpy test rig to measure force or was it you who assumed that that's what it was for? In the thesis statement OP, you said "impact strength", and only later (and in the summary) said "force". So which did they actually task you with? Did they say "impact strength" and you assumed they meant force...?

I'm not an Engineer but hope to be one. It's an unpaid placement to get workplace experience. The design of the rig was specifically sketched out to me in three ways. They seemed keen on the design I recreated in my crude sketch. From what I understood it's a one off test for a heavy tracked vehicle component. (If it's not just an exercise) but that's what I was told.

Perhaps I'm getting a little confused but charpy wasn't mentioned. It's just the closest thing I've come across to what they asked for. I'm aware it wouldn't produce the same results. The brief wasn't for the rig to measure force but to design the rig it so it exerts a set force on the material, ensuring it will handle the impact.

I was only given the task friday afternoon so I will get more details next week. Very possible I have missed something.

 

[hutchphd]

I haave seen a lot of military specs written that the product has to stand up o a rig a particular way. I'd bet this is similar.

 

[Jaydee]

I haave seen a lot of military specs written that the product has to stand up test rig a particular way. I'd bet this is similar.

I have a feeling you're right and I've made a mistake thinking it was impact force when intact it is strength ^^

 

[Jaydee]

You can estimate the maximum impact force by making some assumptions:

1) The test specimen does not fracture or yield.
2) Assume the largest, stiffest, test specimen.
3) The test fixture is anchored to a "large" concrete block. Think a couple tons.
4) The anchors do not yield or fracture.

Calculate the stiffness of the test specimen and supporting structure, assuming that the support structure is fastened to a rigid body (the concrete block). You will probably need FEA. The 250 kg hammer (and its arm) has a maximum starting height. From that, calculate the maximum potential energy. That potential energy is transformed to kinetic energy at the bottom of its swing. When a mass with kinetic energy hits a spring, the kinetic energy is transformed to potential energy in the spring. At the instant that the mass comes to a stop, all of its kinetic energy is potential energy in the spring. The equation is 0.5mv^2 = 0.5kx^2. Knowing the maximum spring deflection and the spring constant, calculate the maximum force.

The calculated maximum force is directly transmitted to the concrete block, so the specimen attachment (vise?), its mounting structure, and the attachment to the concrete block all need to be designed for that force. They need to be designed for repeated application of that force. It is normal practice to assign a challenging problem to a new engineer for the purpose of finding out how they deal with a challenge. A legitimate response is to take it as far as you can, then ask for help from a more experienced engineer.

Thanks man, I really appreciate your help. I'm going to get more detailed info next week so hopefully I can figure out what I must have missed. The way it was 3xplained sounded like it would be definitive. Perhaps I've mistakenly assumed force when it was strength. Makes more sense.

 

[russ_watters]

I am not an engineer. But I recall (*somewhere? the Charpy test does not directly measure Force but in fact is a prescribed test to measure materials relative strength under impact. I think the pendulum rig is the most reliably repeatable way to do this an you can easily change the mass and lengths involved.You can instrument the heck out of it pretty cheap these days

Yes, specifically the purpose is to measure fracture energy (as @jrmichler said), not force.

 

[russ_watters]

I'm not an Engineer but hope to be one. It's an unpaid placement to get workplace experience. The design of the rig was specifically sketched out to me in three ways. They seemed keen on the design I recreated in my crude sketch. From what I understood it's a one off test for a heavy tracked vehicle component. (If it's not just an exercise) but that's what I was told.

Perhaps I'm getting a little confused but charpy wasn't mentioned. It's just the closest thing I've come across to what they asked for. I'm aware it wouldn't produce the same results. The brief wasn't for the rig to measure force but to design the rig it so it exerts a set force on the material, ensuring it will handle the impact.

I was only given the task friday afternoon so I will get more details next week. Very possible I have missed something.

...well, wait -- are the people you are working for engineers? You are saying they sketched-out a device and you figured out it looks a lot like a Charpy test rig?

Here's the most interesting thing I learned from using one in college: Very strong materials may also be very brittle, which means that when hit in an impact they break easily. Softer (more ductile) materials will bend and may absorb more energy before fracturing. Think iron (very strong in tensile strength but snaps with little energy) vs aluminum (can bend far without breaking).

 

[Jaydee]

...well, wait -- are the people you are working for engineers? You are saying they sketched-out a device and you figured out it looks a lot like a Charpy test rig?

Here's the most interesting thing I learned from using one in college: Very strong materials may also be very brittle, which means that when hit in an impact they break easily. Softer (more ductile) materials will bend and may absorb more energy before fracturing. Think iron (very strong in tensile strength but snaps with little energy) vs aluminum (can bend far without breaking).

Highly skilled engineers, world leading for military applications. Yes their sketch was identical to the one I posted. Along with another one that's the same but in a horizontal orientation and a final one which was a vertical drop.

That's pretty much what I figured, certainly looks like I've made an error with my understanding. If you don't mind I will give an update when I find out for sure what silly mistake I've made. Glad I started this thread it has helped me to atleast get a list of questions made up. Great forum.

 

[Lnewqban]

Hi, many thanks for your reply :)

At this stage it is purely theoretical and in the design stage to see if this type of thing would be a feasible option for a specific application, or if the company should go for another testing route.
I haven't received the exact data yet so I've just input some made up numbers to get started and figure out the process of what I need to do.

I do know the impact force required will be pretty high so the structure will have to be extremely strong and possibly too large to be realistic, which I need to identify. The design is also not limited to the A-frame. A drop rig or vertical lever falling to the ground are other options I'm looking at.

As for what I'm trying to calculate, Essentially I'm looking how to find the maximum impact force the rig is capable of. This is to ensure the material in the test will be suitable for the final application.

Thanks again checking out the video now :)

You are welcome, Jay.
We will be happy to help you with this challenge.

Just a few more issues that came to mind:
One thing to consider would be how to manipulate or lift the impacting mass weighting 250 pounds.
That is one of the reasons I post the vertical impact rig for testing motorcycle helmets against a rounded anvil, vertically lifting heavy weights seems to be easier to me.

The noise and vibrations of the stopping impact is another issue in my mind.
Many years ago, I worked in a foundry where huge industrial parts were cast.
There was this huge mass of steel that was slowly lifted within a tall metal tower (around 20 meters or 65 feet tall), just to be released at the top and free fall.
That contraption was used to fragment big heavy parts made of cast iron, aluminum, etc.
The noise and ground shake were huge.

The material of the mass and anvil must be hard enough to impact many times, but ductile enough not to eventually shatter.
Some safety mechanism should be devised, so there is no chance for the operator to be hurt by accidental release of the mass.
Something should also keep the tested part solidly in place, while preventing it from becoming a dangerous projectile after impact (off-center, for example).

 

[some bloke]

One thing which has not been suggested yet (that I could see) was whether you are bound to a specific test piece or if it is possible to produce a test piece from the part you wish to test.

For example, if you are only trying to test a homogeneous metal to ensure that the heat treatment has given it the correct properties, you could cut a sample out which is thinner than the original part and test that, allowing you to have a slightly less dangerous test rig! This is called Excised Sampling and is also used for tensile tests.

If the samples can be made to move cleanly out of the path of the pendulum, then the energy lost to break it can be worked out by measuring how far up it swings after breaking the sample. This is usually achieved with a dial which is moved with the pendulum, and is on a ratchet system so doesn't swing back, and is as light and unobtrusive as possible so that it doesn't have an effect on the function of the rig.

Do you have an expected value for the test piece to achieve? You don't want to swing a quarter of a ton if 50kg would do the job.