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I'm watching a movie, The Hurt Locker, and the first scene shows an IED explosion which kills a soldier. Of course movies don't depict explosions with maximum realism, but I noticed the debris and smoke / flame didn't reach him, and it made me curious about whether invisible aspects of an explosion - heat or concussive blast can be lethal (without carrying shrapnel).

How strong are the unseen forces from an explosion such as a road side bomb? Strong enough to be lethal?

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closed as off-topic by ACuriousMind, Kyle Kanos, John Rennie, Ryan Unger, Neuneck Aug 12 '15 at 9:50

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    $\begingroup$ Shrapnel isn't all that visible - you'll mostly only see it when it actually hits you (or a wall or something). In anti-personnel explosives, the shrapnel is usually the fragmented casing plus some wire - you can't really see it, it's too small and fast. And it's quite deadly, unlike concussion explosives (which seem to always be used in Hollywood movies) - those are great for taking down buildings, but not people. $\endgroup$ – Luaan Aug 11 '15 at 8:03
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    $\begingroup$ In 2006 in Lebanon a mortar landed just about a meter or two away from me. I was wearing a helmet, combat vest, and very light (shrapnel) armor at the time. There was no shrapnel from the (possibly improvised) mortar, but the blast blew me back into the wall behind me and I was out for a few seconds. I'm still here to tell about it! $\endgroup$ – dotancohen Aug 11 '15 at 8:07
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    $\begingroup$ Mythbusters once tested this, underwater. Water propagates the shockwave better than air and you can be killed by a bomb more easily and at a much bigger range inside water than outside of it (while in a lot of movies they show the opposite. ) $\endgroup$ – Bakuriu Aug 11 '15 at 8:13
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    $\begingroup$ I'm voting to close this question as off-topic because it is about lethality of explosions, and not physical concepts. $\endgroup$ – ACuriousMind Aug 11 '15 at 13:49
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    $\begingroup$ @ACuriousMind and you somehow don't see the link between physical concepts and the concussive blast of an explosion? Who was your physics teacher? $\endgroup$ – Viziionary Aug 11 '15 at 14:59
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Blast can definitely kill you, although it is only lethal at much shorter ranges compared to shrapnel. A building can be destroyed by 5psi overpressure while a Human can withstand up to 45psi and live. Some data here:

A 5 psi blast overpressure will rupture eardrums in about 1% of subjects, and a 45 psi overpressure will cause eardrum rupture in about 99% of all subjects. The threshold for lung damage occurs at about 15 psi blast overpressure. A 35-45 psi overpressure may cause 1% fatalities, and 55 to 65 psi overpressure may cause 99% fatalities. (Glasstone and Dolan, 1977; TM 5-1300, 1990)

BTW, damage in Humans mainly occurs at the interface of areas of different density eg lungs and eardrums. It is essentially a spallation effect like Newton's Cradle in tissue. At much higher pressures the shock wave tends to tear tissue.

Here is a FEMA report on TNT equivalent blast overpressures and distances.

However, there is the question of "impulse". For example, high explosives (HE) typically create very high over pressures for very short duration. That's why a Human can easily survive 5psi overpressure. This is equivalent to around a tonne of pressure on the body. Obviously, if that duration was in seconds instead of milliseconds the person would die. HE creates a shattering effect called brisance, which is more damaging to hard and rigid materials than soft ones. Thermobaric explosions, OTOH, create lower overpressures but for much longer duration.

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    $\begingroup$ Good answer - if you could add some respective examples for typical explosives/distances it would be great. Because I can't imagine what 45 psi is... A handgranate in 5 meters distance? 10kg TNT on your chest? $\endgroup$ – Falco Aug 11 '15 at 8:23
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    $\begingroup$ In my grandfather's memoirs of WW1 he mentioned several times observing people just disappearing with no remains at all if directly under the blast. $\endgroup$ – OldCurmudgeon Aug 11 '15 at 10:57
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    $\begingroup$ A large enough unsymmetrical overpressure will convert you into a fine mist of tissue and bone fragments. $\endgroup$ – user56903 Aug 11 '15 at 11:42
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    $\begingroup$ Dirk, a nuclear bomb--or any above-ground blast, propagates a shock wave outwards, so there is nearly zero energy which could enter a foxhole (think Babinet's principle). But at the same time, anyone standing within several km will die shortly thereafter from the radiation blast. The only saving grace there is that the radiation is gone in a few microseconds (other than radioactive material left over near ground zero). $\endgroup$ – Carl Witthoft Aug 11 '15 at 12:11
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    $\begingroup$ A concussion grenade is not so effective out in the open. But the concussion is much worse in confined spaces, as the pressure wave reflects around. So they are effective for clearing bunkers, even if they dont kill it incapatates and disorients. That is enough for the soildiers comming after the bang. $\endgroup$ – joojaa Aug 11 '15 at 18:32
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This is a fun question and has about 4 or 5 different factors at play:

Types of forces involved: Pressure vs. Inertia Types of resistance involved: Rigidity vs. Plasticity Objects involved: Soldiers vs. Buildings

Scenario: Damage from a bomb blast (energy wave) vs. shrapnel impact/penetration (kinetic collision).

How damage is applied with:

  1. Blast - the PSI of a blast is uniform over the face of it's wave-form (sphere). All material objects are subject to this energy being propagated over their surface and structure.
  2. Shrapnel/Bullet - the kinetic energy (m*V) is imparted during a collision. There is very high relative force at the point of impact, at which point the total energy is propagated throughout the object.

The damage of either occurs when the strength of the medium is unable to absorb or deflect the energy, and thus the material bonds are broken. In buildings this results in cracks or holes in lesser cases, or structural collapse in greater ones. For soldiers there are additional physiological factors that relate to perforations of internal organs, or limbs torn off, or the "fine-red-mist" scenario in higher energy exchanges.

If the tensile strength of a surface is greater than the force of a collision it will be mostly reflected - so buildings hit by shrapnel are much less likely to be damaged as the actual force is relatively low.

While most shrapnel is very small, it travels very fast. It is easily able to penetrate the surface resistance and impart its energy on the structure. At this point we can see that soldiers cannot resist nearly as much total force as a building.

Now for some figures: A 9mm/.40 cal bullet has about 350 to 400 ft lbs. of energy. This converts to 2.4 - 2.7 PSI, however this is applied over a very small point (probably not even a square inch). At 5 PSI the force is approximately equivalent to .357 magnum, or .45 ACP (standard officer's side arm). You won't be knocking down a building with one of those, yet they do a good job of stopping a soldier.

Why does a building not withstand a 5 psi blast then? Find the surface area of the building in square inches (an 8' x 12' wall is 1152 square inches), multiply by 5 and then do the same for the human (approximately 250 square inches).

Optionally divide the 250 / 1152 and see that the human is absorbing only 20% of the damage. Factor in the reduction due to deformation of the surface (I have no idea what figures would be in play here.), and humans suddenly look like super-men vs. blast damage.

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  • $\begingroup$ As an interesting addition to your answer, since no other answers mention this as I left it out of my question, is the scenario of a soldier on a disarming team wearing a full body blast resistant suit. In the scene I spoke about in my question, the solider was actually 25+ meters from the blast running away with a heavy blast resistant suit yet even from that distance the soldiers helmet fills with blood as the concussive wave hits. I have to think the directors of this movie focused on IED realism since the movie was only about IEDs, and they focus on the distance in the scene. $\endgroup$ – Viziionary Aug 12 '15 at 2:02
  • $\begingroup$ bbc.co.uk/news/world-asia-33900268 The Chinese authorities said this was caused by 27 tonnes of HE. I think it was much more than that following the data presented here. $\endgroup$ – user56903 Aug 13 '15 at 12:17
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The blast overpressure of the explosion is a very strong shock wave which can kill humans. There are a number of ways an explosion without shrapnel can do harm to people:

  1. Rupturing of the hollow organs due to rapid compression and expansion by the shock wave.
  2. The body can get thrown through the air if a strong detonation occurs nearby. Impact of the body with the head can cause brain injury.
  3. Burns to the body due to heat or chemicals from the detonation.
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The other answers already mention pressure and heat.

A bomb sets nearby bodies in motion with a speed depending on the strength of the explosion, the distance to the body, and how much surface area of the body was facing the bomb. While - as explained in the other answers - being set in motion is rarely lethal, being smashed against a wall can easily lead to lethal internal bleeding. It gets even worse if there are pointy objects between the body and the wall. Explosions indoors are much more unpleasant than explosions outdoors.

Additionally, there's the chance of objects/debris in the surroundings acting as a random substitute for building shrapnel into the bomb.

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