Impact load calculation 3

[krithika19]

I am arriving at a corrective action for my company. We need to do drop testing of a component to check the weld. The following are the details:

Weight of the compact - W kg

Height from which it is dropped - X inches

Of course, gravity is known. It is not pounded on to the ground, but is just dropped.

I need to calculate the impact load from the above. Can anyone help me out with this? Please let me know if you need anything else.

 

[drawoh]

krithika19,

We need to know what happens when it hits the ground.

If we know the distance within which it will decelerate to a stop, we can figure out the acceleration. This is all assuming your structure remains intact. Knowing the acceleration, we know the force being exerted on your part.

Impact loads are analyzed using strain energy. There should be a chapter on this in your mechanics of materials textbook.

JHG

 

[desertfox]

Hi krithika

This is not an easy subject as the time over which the impact occurs is infinitesimal and cannot easily be evaluated. The conservation of linear momentum cannot be applied in your case as the fixed object the floor exerts an external force on the component but the floor doesn't under go a change of momentum.Best I can think of to give
a very rough estimate of the impulse is to assume that falling from X height to the floor there is no bounce (in practice there will be some)of the component :-


m*g*h=o.5*m*V^2 transposing to find V = [m*g*h/(0.5*m)]

where m = mass of component
g = gravity
h = fallen height
V = velocity of component as it hits ground

now mass*velocity = impulse = m*V= Ns

Ns = Newton seconds


regards desertfox

 

[IRstuff]

This subject has been discussed before:

thread404-74498

You need to know something about the ground you're impacting. The ground physical characteristics will uniquely determine the impact time and hence, the impact force.

TTFN

 

[bwge]

This comes from Steinberg's Vibration book for electronic equipment. If you assume that the ground is very stiff relative to your structure you can use the following formula.

G=sqrt((2*h*k)/W)

Where,

h is drop height in inches
k is the stiffness of your structure in lb/in
W is the weight of the structure in lbs
G is the (a/g), where a is the acceleration and g is the acceleration of gravity.

Depending upon how complex the device you are dropping you will have to figure out a stiffness for it. You can do this with FEA doing a simple linear static analysis.

bwge

 

[J1D]

krithika19,
Theorm of Equilibrium of Mechanical Energy might help you find out the impact load. From State 1 to State 2
m*g*h1+0.5*m*v1^2+0.5*K*x1^2=m*g*h2+0.5*m*v2^2+0.5*K*x2^2
where
h1,h2=positions of the component at State 1 & State 2
v1,v2=speeds
x1,x2=displacements of the structure on which the impact load applies
K=structure stiffness

 

[arto]

When they require drop testing to check the weld - they don't mean Drop Weight Testing per ASTM E-208, do they? Or Charpy Impact testing of the welds [ASTM A-370]?

Is this to a spec? [inches & kg do not seem to be consistent units] Also, did they specify a target to land on? {i.e., concrete block?}

 

[MASSEY]

I'm sorry but this test seems riduculous.

There are other ways to 'check' a weld. In fact you will have to perform these other tests to determine if the weld has failed internally after you drop your component.

Furthermore as above posts point out, much has to be known that is very hard to determine such as the hardness of the 'ground' and the amount of time the momentum is arrested, and much more, such impact angles etc.

Good Luck!

 

[patprimmer]

I agree with Massey

A free fall produces very erratic results as the impact point is uncontrolled and can be very influential on the outcome.

Also the nature of the ground can have a profound effect, due to its stiffness, hardness and surface geometry. For instance, if it were concrete, does the test pice land on a flat surface, or on the tip of a pointy piece of aggregate.

The only reliable method I know for drop type tests is to place a straight pipe of specified length and dia, in a vertical position, against a specified point on the test specimen, then drop a specified ball through the pipe.

Regards
pat

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[CanEngJohn]

Instead of dropping it onto the ground why not drop a specified weight a specified distance on a specified point on the part.

This would allow you to control all the test parameters a lot more effectively and create more repeatable results.

Might be a bit easier to do.

 

[patprimmer]

I think I just said that. And to drop it through a pipe so it hits exactly were you want from an accurately controlled height

Regards
pat

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[arto]

If you need an "official" shock test standard, see MIL-S-901D "SHOCK TESTS, H.I. (HIGH-IMPACT) SHIPBOARD MACHINERY, EQUIPMENT, AND SYSTEMS, REQUIREMENTS FOR "

 

[CanEngJohn]

Yes you did. I just didn't read it that way and was skimming by the time I got to your post. My apologies for the redundancy.

 

[IRstuff]

A barge test might be a wee bit extreme for this application, but it would be a blast to see ;-)

TTFN

 

[ccw]

In our non-linear, post-yield, impact computer models we simply declare the ground "infinitely rigid". This typically results in a conservative answer for what we are interested in, which is container or "component" survivability. The container or "component" absorbs all of the impact energy. In the actual drop tests we use steel plates backed by many tons of concrete for the "infinitely rigid" ground.

As was pointed out, initial strike attitude is problematic. One interesting attitude, particularly for flanged lid containers is the "slap down". Consider a 2-D rectangular component of constant density. It falls such that initial impact is on one corner with the center of gravity CG overhanging the one corner. The rebound energy, energy not absorbed by local post-yield failure, may impart a rotational velocity about the CG. This can cause the second corner to strike with a local velocity greater than the CG velocity. Local forces at the second strike corner can be greater than the first strike corner. There will be a "sweet angle" that maximizes this effect. This attitude and many others (end on, side, corner with CG over the corner, drop on penetrator, etc.) are simulated and results evaluated to determine maximum damage attitudes.

For actual drop tests, the predicted worst case attitudes are attempted. As pointed out, it is difficult to manage the initial strike attitude. Internal accelerometers and strain gages, high speed cameras, etc., help to validate the model predictions. Also, as pointed out, post-impact assessment of damage is not trivial.

 

[prohelper]

krithika19,,

I suspect this test is for ASTM or UL type product verification?

If so, it is best to use a chrome plated steel ball of specific weight and Rockwell hardness...which are readily available.

You'll need to drop onto wood, which should be supported by concrete (such as floor). This will severely limit "absorption". Drop from a specified height - - you may want to make a release tube which will control repeatability of the test.

I agree with CamEngJohn, the key is controlling the parameters. The rest of the information is available on the web, as this is a real world test performed daily

 

[funnelguy]

The originator of the thread hasn't responded in any way.

A military contract I worked on required us to drop the first article from a height of 6 feet and then drag it behind a Jeep at not less than 15 MPH! Our structure was robustly overdesigned and bounced end over end like a ping pong ball about 4 or 5 times before stopping. Lots of fun! I don't recall distance being specified, surface was assumed to be concrete or tarmac. Permanent deformation or loss of function were reasons for failure.

Perhaps krithika19 finds him/herself boxed into a similar situation.