Bird poop and the exploding windshield

The Show
1

Can bird poop explode a windshield? It seems that might actually be the case. A few weeks back, Tom and Ray heard from Manette in Glenwood Springs, Colorado, whose windshield seems to have been shattered by-- get this-- a gigantic, high-velocity bird poop. (You can hear Manette's call right here.)

What IS the terminal velocity of a bird poop? Could such a thing really happen? Fortunately for us, this last week our esteemed Car Talk staff physicist, Wolfgang Ruckner, called in with his analysis -- take a listen and let us know if you think Wolfgang's numbers add up.

And, whatever you think about this weighty question -- keep a wary eye out on the skies above!

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Many birds eat small stones to aid in digestion. The stones help grind up food in the crop. Occasionally, these stones make their way through the digestive system and, well…they come out the other end. So it’s entirely possible there was a stone mixed in that cracked the windshield. At that height, it wouldn’t be any different than a stone getting kicked up by a tire on the highway.

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An interesting field guide that would assist in Manette’s inquiry is “What Bird Did That?” a drivers guide to some common birds of North Amerca by Peter Hansard and Burton Silver. This guide allows you to identify many birds by the splatter of their droppings on the windshield

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And, the bigger the bird, the bigger the stone(s).

Even my little pet parakeets were supplied with a type of sand for them to aid digestion.

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Is the caller sure that it was bird dropping? Could this falling flotsam be a bit of “blue ice”?

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Since she said she was parked and not in the car, I’m thinking someone may have shot her car with a paintball. What do you think?

7

Consider this scenario: A bird is perched on or near the windshield; someone fires a pellet gun or hurls an object at the bird. They miss, but they are close enough to scare the poop out of the bird, which ends up on the windshield which has been cracked by their attempt.

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Actually, it had very little to do with the bird and much more to do with the properties of glass. Any little bump or tap can create a weak spot. Then it only takes the sightest touch in the right spot and it can crack or even shatter. So what probably happened was that a small rock or something hit the windshield some time in the past and the bird poop just happened to hit the right spot.

9

Maybe the poor bird was passing a kidney stone.

10

I vote thermal stress. Nice hot windshield (yes, I know it was a 75 degree day, but that was the air temp, not the glass that is seeing that summer sun). Big blob o ballistic birdpoop dropped by a goose at altitude, evaporative cooling of the plummeting mass, sudden cold spot on hot windshield. Local material contraction causes tensile stress. Glass is not so good under tensile loads. Add the mechanical shock and it might have been enough to crack the windshield.

However, I really like sueg’s paintball idea. I especially like the idea of birdpoop colored paintballs! What a perfect way to get even with that bozo that keeps blocking the passing lane! Maybe they really exist, and that is what happened here. Either way, I am going shopping right now!

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Not sure what caused the damage, but Manette sure does make Glenwood Springs sound like a great place to live, work and play!

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We had geese at work that occasionally ate 1/4-20 nuts or other little metal bits on the ground. We had a pond next to the machine shop and we used to watch them at lunch. I could imagine a 1/4-20 nut being enough to break the glass, even embedded in the soft chewy outer coating.

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I dont believe it for a second. The only bird turd that would break glass would be from an Andean Condor with a Roots type blower stuck in its rear

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I wonder if it wasn’t an actual bird strike, not just the poop. Many times I’ve seen “stains” left behind by birds who ran into windows. Bird could have delivered a hard blow and careened off the windshield never to be seen again.

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Thought you guys would like to see this-- just posted!

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I could never prove him right or wrong, but the math seems to be full of crap! :wink:

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After hearing the show where in a woman questioned the ability of a bird splat to crack her windshield I laughed and thought that she was crazy. Then I saw a piece on wired.com by Rhett Allain saying it was possible based on some hair brained physics math and I thought he is nuts. I am a engineer so I never trust a physicist. I did the hair brained math myself and turns out I am a whacko too! The pressure of the poop could crack the glass. However Rhett was right only because he got lucky which is the usually case with physicists. He compared an impact pressure to the compressive strength of the glass. This is crazy, as any mild mannered mechanics professor would tell you, the actual mode of failure of glass would be cracking in the glass due to tensile stress in plate bending. Utilizing some numbers and math I cite below I found that the tensile stress in the glass under the impact pressure of descending dove detritus was 132 MPa, greater than the estimated stress capacity of the glass at 105 MPa. Now his estimate for the impact pressure may be bo o o o gus, but at least his bogus conclusion is supported by my bogus calculations as well.

The Glass has a tensile strength according to some germans as 105 MPa. Lets assume the impact area functions as a fixed plate giving us established formulas to work from.

Uniform Pressure on Flat Circular Plate
(Reference: Roark’s Formulas for Stress and Strain, 7th Edition, Table 11.2 Case 10)

Given:

Fix Supported
Radius of plate: a = 50 mm (The size of the poop)

Thickness of plate: t = 6.72 mm (Properties of Auto Glass According to Germans)

Poisson’s ratio: u = 0.22 (Properties of Auto Glass According to Germans)

Modulus of elasticity: Es = 70000 MPa (Properties of Auto Glass According to Germans)

Specified uniform pressure: q = 3180 kPa (Rhett’s Impact Pressure)

Results:
D = Es. t3 / [12(1 - u2)] = 70,000 x 6.73 / [12(1 - 0.222)] =1.86e+06 N.mm

Deflection at the center: yc = -q . a4 / (64 D) = -1 x 3,180.0 x 10-3 x 504 / (64 x 1.86e+06) =-0.2 mm

Bending moment at the center: Mc = q . a2(1 + u)/16 = 3,180.0 x 10-3 x 502 x (1 + 0.22) / 16 =606.2 N.mm/mm

Normal stress at the center: (≤ 0.6 Fy): s = 6 Mc / t2 = 6 x 606.2 / 6.72 =80.5 MPa

Bending moment at the support: Mra = -q . a2/8 = -1 x 3,180.0 x 10-3 x 502 / 8 =-993.8 N.mm/mm

Normal stress at the support: (≤ 0.6 Fy): s = 6 Mra / t2 = 6 x -993.8 / 6.72 =-132.0 MPa

THIS IS THE BIG ONE 132 MPa AT THE EDGE OF THE PLATE > 105 MPa STRENGTH THUS THE CRACK!

Maximum shear: Qa = -q . a / 2 = -1 x 3,180.0 x 10-3 x 50 / 2 =-79.5 N/mm
Maximum shear stress: t = 1.5 Qa / t = 1.5 x -79.5 / 6.7 =-17.7 MPa

The GERMANS:
M. Timmel, S. Kolling, P. Osterrieder, P.A. Du Bois, A finite element model for impact simulation with laminated glass, International Journal of Impact Engineering, Volume 34, Issue 8, August 2007, Pages 1465-1478, ISSN 0734-743X, 10.1016/j.ijimpeng.2006.07.008.

The Wired Piece

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Maybe I missed it, but shouldn’t the terminal velocity have appeared in the above calculation?

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I was thinking “temperature differential,” similar to yrogerg’s hypothesis, but hearing the “birds eat stones” factoid, I now think it might have been a combination of the two.

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Please keep in mind that while birds do ingest stones for the purpose of grinding/chewing their food these stones are actually small enough that you cannot even call them stones or pebbles. They are more grit than anything else. I’m talking about the typical songbird from the size of a Junco to a Robin which is the size of most birds seen on an everyday basis.