Blast Dynamics: Overpressure, Fragmentation, Deflagration vs Detonation

When a condensed high explosive detonates, the detonation front propagates through the explosive at velocities between 6,000 and 9,000 metres per second. The explosive converts from solid or liquid to gas products at vastly greater volume, driving a hemispherical (or spherical, if suspended) shock wave outward into the surrounding air. At the shock front, ambient pressure rises from atmospheric (approximately 101 kPa) to the peak static overpressure in a time measured in microseconds, then decays exponentially back to ambient. This decay is the positive-phase duration. After the positive phase passes, the pressure drops below ambient, creating the negative phase, a partial vacuum that draws air and loose material back toward the seat of blast.

The idealised shape of this time-pressure history is the Friedlander waveform. The peak overpressure (Ps) at any given range from a given explosive charge is the primary determinant of structural damage in the near field. For a charge of 1 kg TNT equivalent at 3 m range, Ps is typically around 1,000 to 5,000 kPa, sufficient to destroy a timber-framed internal wall. At 10 m, Ps falls to roughly 50 to 200 kPa, capable of shattering glazing and damaging masonry facades. These values scale with the cube root of charge mass, the Hopkinson-Cranz scaling law, widely applied by the Forensic Explosives Laboratory (FEL) at Porton Down in the UK and the Energetic Materials Branch of the FBI Laboratory in Quantico, US.

The negative-phase underpressure (which rarely exceeds 100 kPa below ambient because a perfect vacuum is its physical limit) is less obvious but forensically significant. Inward-facing cracks in masonry, window frames pulled outward from their surrounds, and internal panels that collapsed away from the blast direction all suggest that the structure survived the positive phase, then failed in the negative phase. In the 1993 Bishopsgate bombing in London (Provisional IRA, ANFO-based vehicle bomb), surveyors documented negative-phase collapse signatures in several Georgian buildings on the periphery of the immediate damage zone.

The Mach stem forms when the direct shock wave from the blast reflects off a flat surface (a road, a building floor, a wall) and the reflected wave catches up with the direct wave, merging into a single reinforced front. The Mach stem is taller near the surface than the original wave, and the resulting pressure enhancement explains why damage is sometimes paradoxically greater at ground level than at height, a pattern observed in the aftermath of the 1998 Omagh bombing in Northern Ireland and analysed in ATF post-blast reconstruction reports.

Static overpressure describes the instantaneous pressure increase at the shock front. Dynamic pressure is a separate quantity, proportional to the square of the particle velocity of the gas behind the shock front. Where static overpressure acts uniformly in all directions (like submersion under water), dynamic pressure acts in the direction of gas flow, producing a wind that can reach hundreds of metres per second immediately behind the shock front and falls off with range and time.

The impulse delivered by dynamic pressure, the integrated force over time, determines how far fragments travel and how deeply they embed in target surfaces. Primary fragments are pieces of the device casing or container that were in direct contact with the explosive charge. They are propelled at the highest velocities, often 1,000 to 3,000 m/s in the immediate detonation zone, and may travel hundreds of metres from the seat of blast while retaining enough energy to embed in structural materials. Secondary fragments are objects that were not part of the device but were accelerated by dynamic pressure: glass shards, paving stones, vehicle components, steel reinforcing bars.

The Mott formula, developed by Nevill Francis Mott during the Second World War in research on bomb casings for the UK Ministry of Supply, predicts the distribution of fragment masses from a fragmenting metal cylinder. The anatomy of device types whose casings generate these fragment distributions is covered in the topic on improvised explosive device anatomy and triage. The formula relates fragment mass to casing thickness, casing diameter, and an empirically derived fragmentation parameter for the specific metal. Its practical forensic use is in confirming, from the size distribution of recovered casing fragments at a scene, whether the device was consistent with a thin-walled pipe (pipe bomb), a heavy military case, or a pressure-cooker. US Army Research Laboratory studies and UK FEL validation work have confirmed the formula's utility in casing characterisation for devices from Northern Ireland to Afghanistan.

Fragment orientation at the point of impact also carries information. A fragment that struck a wall face-on will leave a circular cavity; one that struck at oblique velocity will leave an elongated or ovoid crater. Depth-to-diameter ratios in fragment impact craters can be used, in combination with recovered fragment mass and the material properties of the substrate, to estimate fragment velocity at impact, and hence to back-calculate the standoff distance from device to target.

Brisance is the shattering effect of an explosive on objects in immediate contact or very close proximity. It is conceptually distinct from the heaving power of an explosive (its ability to move large masses of earth) and from its total energy output. Brisance is a function of the rate at which pressure rises in the detonation zone, specifically the rate-of-pressure-rise (dP/dt) rather than the peak pressure alone. An explosive with an extremely fast pressure rise shatters metal casings, reinforced concrete beams, and ceramic body armour through shear and tensile fracture before the material has time to yield plastically. An explosive with a slower rise (such as black powder, a propellant, or an ANFO mixture under certain conditions) may produce a large total gas volume without the brisance to shatter rather than heave.

Two standard tests quantify brisance. The Hess test (also known as the plate-dent test or Kast test variant) presses a defined quantity of the explosive against a standardised lead cylinder and measures the permanent deformation (dent depth) after detonation; depth in millimetres per 10 grams of explosive gives a comparative brisance number (PETN approximately 20 mm; TNT approximately 16 mm; ANFO approximately 8 to 10 mm). The sand-crush test (Trauzl lead block test or sand compression test) measures the volume of sand crushed by 10 grams of explosive detonated inside a standardised steel tube embedded in a sand column. Both tests are used in EU explosive classification under the STNA framework and in licensing under the Explosives Rules 2008 (India).

For the blast investigator, brisance manifests at the scene as the radius within which materials are pulverised or shattered rather than merely displaced. High-brisance explosives (RDX, PETN, C-4) produce sharply defined damage at the seat: reinforcing bars severed cleanly, concrete pulverised rather than cracked, casing fragments showing intense hot-gas erosion on inner surfaces. Low-brisance, high-heaving-power explosives (ANFO, black powder, commercial blasting agents) produce larger craters and displaced heavy material but less shattered peripheral debris.

Deflagration is combustion that propagates at subsonic velocity relative to the unreacted material ahead of the flame front. The governing mechanism is thermal conduction and mass diffusion: the flame heats the adjacent unburned material above its ignition temperature, which then ignites and heats the next layer. Deflagration velocities range from a few millimetres per second (a slow smouldering fuel-rich mixture) to a few hundred metres per second in rapidly burning propellants or fuel-air mixtures approaching the detonation regime. Black powder deflagrates; thermite deflagrates; fuel-oxygen premixed gases deflagrate under most conditions.

Detonation propagates supersonically via a shock wave coupled to the reaction zone. The shock front compresses the explosive or fuel-air mixture to conditions at which it reacts in nanoseconds, releasing energy that sustains the shock. This coupled shock-reaction mechanism is self-sustaining above a minimum charge diameter (the critical diameter) that is characteristic of each explosive. Below the critical diameter, a detonating charge can fail to detonation (dead-pressing) and transit back to a slower deflagration. The Chapman-Jouguet (CJ) condition defines the detonation velocity (VoD) and the CJ pressure (PCJ) at which the shock and reaction zone are in steady state.

The deflagration-to-detonation transition (DDT) describes the mechanism by which a deflagrating system accelerates to detonation, through turbulent flame acceleration, autoignition in hot-spot regions, or confinement-induced pressure run-up. DDT is a key design concern in industrial explosive storage and is the mechanism by which several confined gas explosions in India, Russia, and the United States became detonation events rather than fires. Forensically, DDT is important because a device that was designed to deflagrate (a petrol bomb or a pipe packed with black powder) may, under sufficient confinement and fuel-oxidiser homogeneity, transit to detonation, producing a scene with mixed signatures.

Scene signatures of deflagration: scorching and thermal damage propagating from an ignition point; objects displaced by gas pressure but not shattered; soot deposits on surfaces ahead of the flame path; hydrocarbon or oxidiser deposits identifiable by gas chromatography-mass spectrometry (GC-MS) residue analysis. Scene signatures of detonation: cratering at the seat; shattering of material in immediate contact; pressure-wave damage in concentric annular pattern; high-velocity fragment impacts at standoff; thermally discoloured but not sooted near-field surfaces.

The blast damage pattern around a detonation is not uniform. It organises into concentric zones that grade from total destruction at the seat outward to glass breakage, then audible report, then nothing. Each zone boundary is a function of peak overpressure at that distance for the given charge mass and explosive type. The investigator's task is to identify the innermost zone and work inward to the seat.

Near-field signatures (within roughly 1 to 3 charge radii) include: complete disruption and inverted ejecta (material thrown outward and sometimes upward before raining back); a crater in soil, asphalt, or concrete whose diameter and depth index the charge mass and depth of burial; brisance effects on hard materials (severed steel, pulverised concrete); inner surfaces of metal containers showing gross plastic deformation or petalling (outward-flared fracture of the casing wall). Far-field signatures (at ranges of 10 to 100 or more charge radii) include: glazing fracture patterns (radially oriented cracks from the blast-facing corner of each window pane, consistent with the overpressure load direction); displacement of lightweight objects (ceiling tiles, furniture, standing people) in the direction of travel; aerosol or residue deposition on surfaces in the downwind direction from the seat.

The medical consequences of the overpressure and fragment pattern, primary, secondary, tertiary, and quaternary blast injuries, are detailed in the forensic medicine topic on blast injuries: primary, secondary, tertiary and quaternary. Witness marks, fragments embedded in surrounding structural elements, are directional indicators of particular forensic value. A fragment that has penetrated a concrete column from one face to a measurable depth, and is recovered at a measurable angle below horizontal, provides two pieces of information: the horizontal azimuth back toward the seat and the elevation angle, which (combined with the range to the column) constrains the height above grade at which the device was detonated. Multiple witness marks from different structural elements overdetermine the problem and allow least-squares reconstruction of the seat position. This method, formalised in ATF post-blast investigation manuals and used by the Australian Federal Police Scientific Services after the 2002 Bali bombings, is now standard in all jurisdictions that train specialist post-blast investigators.

Crater geometry independently constrains charge type and mass. For a surface-laid charge detonated on flat ground, empirical relationships (validated against controlled test data from the US Army ERDC and the UK FEL) allow estimation of TNT-equivalent charge mass from measured crater diameter and depth. Unburied devices on hard surfaces (reinforced concrete floor slabs, road surfaces) produce shallow, wide craters and characteristic spalling on the underside of a slab above the device. Contact or embedded devices (vehicle underside bombs, under-floor devices) produce deeper, narrower craters with more pronounced upward spall. The 2005 London Underground bombings (7 July) showed surface-on-seat signatures consistent with rucksack devices carried by individuals in a standing position in the carriage, a reconstruction confirmed by CCTV geometry and confirmed by FEL fragment analysis.

The forensic classification of the explosive used at a scene begins with the physical signatures described above and is confirmed by laboratory analysis of recovered residues. Post-blast residues are trace quantities (micrograms per gram of debris, or less) of unreacted explosive, explosive decomposition products, and additives that survive the detonation. Gas chromatography coupled with mass spectrometry (GC-MS), high-performance liquid chromatography (HPLC), and ion chromatography (IC) are the core analytical tools, used by the FEL Porton Down (UK), the FBI Laboratory Explosives Unit (US), the CFSL New Delhi (India), and the Australian Federal Police Forensic and Data Centres (AFP FDC). The full analytical workflow, destructive versus non-destructive method sequencing, and split-sample protocols are set out in the topic on laboratory explosives analysis: LC-MS, GC-MS, IC, XRF and SEM-EDX. The six-element device model that frames how blast signatures map back to specific device components is covered in the topic on IED anatomy and triage.

Triacetone triperoxide (TATP) produces distinctive residue signatures including acetone, hydrogen peroxide, and the parent compound, identifiable by mass spectral library matching (m/z 222 for the cyclic trimer). Its low melting point and moderate vapour pressure make it detectable by ion mobility spectrometry (IMS) portal scanners in airports, widely deployed across EU member states under the performance standards for explosive detection equipment set by Commission Regulation (EU) 2015/1998 on civil aviation security. Ammonium nitrate fuel oil (ANFO), the most widely used commercial blasting agent globally, leaves ammonium nitrate residue identifiable by IC as the nitrate and ammonium ion pair. In India, ANFO or its emulsion equivalent is the dominant commercial explosive under the Mines Act 1952 and Explosives Rules 2008; post-blast ammonium nitrate residue has featured in reconstruction of improvised device incidents in the central Indian corridor.

Military explosives (RDX, PETN, Composition C-4, Semtex) leave characteristic residues that, when recovered in post-blast debris, narrow the supply-chain investigation to military or professional commercial sources. Semtex, the Czech PETN-RDX blend manufactured by Explosia a.s., carries a detection taggant (ethylene glycol dinitrate, EGDN, or dimethyldinitrobutane, DMDNB) mandated under the ICAO Convention on the Marking of Plastic Explosives 1991, to which 152 states are parties. Recovering DMDNB taggant residue from post-blast debris places the explosive as Semtex or another ICAO-marked plastic explosive, constraining the source.

1. Scene safety and cordon
   Establish inner and outer cordons. Render-safe sweep by disposal team (UK EOD, US EOD/ATF, India BDDS). No investigative access until the scene is cleared of secondary devices.
2. Initial overview photography and video
   Aerial or elevated photography of full damage zone before any material is disturbed. Documents the concentric damage gradient and any directional asymmetry pointing toward the seat.
3. Seat identification from damage gradient
   Map the innermost damage zone (crater, inverted ejecta, near-field brisance effects). Cross-reference with witness-mark vectors from surrounding structures.
4. Swab sampling for residue
   Swab sealed crack surfaces, concrete undersides, and any protected microenvironments at and near the seat. Use IMS screening on-scene; retain samples for GC-MS / HPLC / IC at the lab.
5. Fragment collection and documentation
   Recover all metal, ceramic, and electronic fragments with grid coordinates and orientation. Sieve soil and debris from the crater for microfragments and wire remnants.
6. Comparative analysis and charge estimation
   Apply Hopkinson-Cranz scaling to crater dimensions to estimate TNT equivalent. Cross-reference with brisance signatures and residue identity. Document chain of custody for all exhibits.