NHML Resources - Burn Pattern in Vehicle Fires: Forensic Analysis

Introduction

The hottest spot in a vehicle fire is often different from the point of origin. Determining the point of origin requires the interpretation of burn patterns, often progressing backwards from the hottest spot to the point of origin. Burn pattern interpretation requires both understanding of the underlying science and knowledge of vehicle materials, fuel and electrical systems. This paper reviews the science which underlies the interpretation of burn patterns in vehicles.

Burn patterns result from the oxidation of the steel ash and pyrolysis residues from materials which were in contact with the steel, and condensation deposits. Copper wire has metallurgical properties which make available additional information.

Oxidization of Steel

When steel is exposed to a fire the oxide which forms depends on the temperature. Fe2O3 (hematite) forms at lower temperatures, Fe3O4 (magnetite) in a higher range. The low temperature form tends to be brown-reddish and turns into common rust as it picks up moisture from the atmosphere. When the thickness increases the rust tends to form flakes. X-ray diffraction analysis shows it to be a hydrated iron hydroxide, FeO(OH) x H2O.

The high temperature oxides of most steels is the familiar black oxide which forms on a wood stove. A flake or grain of it will respond to a magnet. Because it is more adherent and chemically stable than the low temperature form it does not pick up moisture from the atmosphere. Moisture diffusing through the oxide allows the underlying steel to rust and undercut the high temperature oxide layer. With time the entire surface acquires the red, powdery appearance of common rust.

Most bodies and frames are low carbon steels. A few panels are a new type called "duplex". The oxidation of these duplex steels is indistinguishable from the carbon steels.

Another family of steels is called "HSLA" which stands for "High Strength, Low Alloy". HSLA steels offer reduced weight for the same strength but their use is restricted to body panels which undergo limited deformation during forming. Door stiffeners, station wagon tail gates and highway tractor frame rails are examples. Many of the HSLA steels have markedly different low temperature oxidation and rusting behavior than the low carbon steels. The low temperature oxide on these HSLA steels is darker and is tightly adherent. Rust builds up more slowly than on the rest of the vehicle and the rust does not become flaky. The darkness of the HSLA low temperature oxide and its tighter adherence can mislead an investigator into believing it is the high temperature form. However, the HSLA high temperature oxide is much the same as that of low carbon steels. An example of rust on an HSLA steel is a large highway light pole which has been allowed to rust without benefit of paint.

Iron oxidizes at low oxygen levels so different degrees of access to air seem to make little difference in the appearance of the oxides.

The removal of oxygen atoms from compounds is termed "reduction". In marked contrast to the copper alloys, the iron oxides are difficult to reduce so that one rarely observes deoxidation patterns on steels.

Oxidization of Copper

Copper has several metallurgical properties which work together to provide information for the investigator. Examination in a metallurgical laboratory is often required to "nail down" a conclusion which might have been tentatively drawn in the field.

At elevated temperatures, solid copper oxidizes very easily to form two different oxides depending on oxygen availability. The lower oxygen form is Cu2O which is somewhat red. The higher oxide CuO is brown. If the windows of a vehicle are closed, the oxygen supply is limited, and as a dashboard and upholstery fire progresses the available oxygen is depleted. If, however, the windows subsequently shatter, the oxygen supply will be restored as air rushes in. Thus it is not uncommon to find a varying pattern of oxidation inside the passenger compartment from high to low to high concentrations, progressing away from the initiation point. If the windows in a vehicle are open at the beginning of the event the oxygen levels will be continuously high.

Burning materials in a vehicle produce carbon dioxide (CO2) when ample oxygen is present and carbon monoxide (CO) when oxygen is limited. At elevated temperatures the copper oxides mentioned above are easily reduced to pure copper by CO. The reduced copper has the same color as the original wire surface but has a finely granular surface which is markedly different from the original surface's wire drawing marks. This information is useful in tracking both thermal history and the sequence of oxygen access during a fire.

In the presence of hydrogen the copper alloy used for wiring develops internal gas porosity. The extent ranges from intergranular pores which can only be observed in the metallurgical laboratory to massive blistering which is easily seen during field examinations. Certain hydrocarbons provide hydrogen when they are heated. Since these hydrocarbons are not usually present in vehicles the presence of hydrogen blistering may point to certain modes of assisted fire initiation.

Molten copper rapidly picks up oxygen and when the copper freezes the microstructure reflects the oxygen level. There are five, metallurgically distinct bench marks with reliable gradations in between. Arcing is responsible for melting the copper. A high oxygen microstructure is consistent with arcing before the fire reduced the available oxygen while a low oxygen microstructure shows that the fire was already established when that particular arc occurred.

Copper oxy chlorides have spectacular blue and blue-green colors. The chlorides can come from PVC wiring insulation.

Unlike most oxides, the copper oxides are reasonably good conductors, although they have much higher resistivity than metals. In a joint, or bridging between a conductor and ground, the oxides can carry enough current to cause overheating, Since these oxide bridges are unsteady conductors interviewers should inquire into an increase in background noise when occupants were listening to AM radio stations.

Plastics and Insulation

There are two types of plastics, or polymers, commonly found in automotive applications: thermosets and thermoplastics. They are discussed below. Pyrolysis is defined as thermal decomposition without combustion. Combustion always involves oxidation.

Thermosets and Thermoplastics

Thermosetting plastics do not melt when they are heated but retain their form until they either pyrolyze or burn. They tend to be more expensive than thermoplastics but offer superior high temperature strength. They are used for distributor caps, valve covers, oil filler caps, and are just beginning to be used for intake manifolds.

Thermosoftening plastics are also called thermoplastics. When they are heated they soften and usually melt before burning. Thermoplastics are used for all wire insulation in vehicles and for most non-structural applications. If heated in the absence of oxygen they will pyrolyze.

Fillers and Plasticizers

Both thermoplastics and thermosets commonly incorporate fillers which include glass fibers, mica flakes, wood flour, calcium carbonate, titanium dioxide, talc, and carbon black. After the plastics have burned, glass fibers remain as a white, silky mat. Mica flakes and titanium dioxide remain as fine particles. Calcium carbonate and talc tend to form a fused, slag-like deposit. If the calcium carbonate is heated above about 1400 °F the slag becomes foamy. Wood flour and carbon black are completely oxidized to CO 2 if the fire is hot enough and leave black residues following cooler fires.

Most fillers in wire insulation are lost when the insulation burns. The exceptions seem to be the high temperature wires to individual fuel injection solenoids, spark plugs and exhaust gas sensors which often leave glass fibers after they burn.

Most thermoplastics incorporate plasticizers. A plasticizer is a light molecule which acts as a lubricant or spacer between the plastic's molecules to prevent crystallization. Because plasticizers are light, they tend to evaporate over time. They form the residue on the windshield of your new car and are responsible for much of the "new car smell". When the plasticizers evaporate the plastics shrink, become brittle and crack. This can give important clues as to whether a plastic was hot for an extended period.

After a cool fire the thermoplastics form sagged, lumpy deposits.

After a tire burns there may be a residue of white powder which is the zinc oxide used as a strengthening agent.

Practical Aspects

The oxidation on steel panels and chromed bumpers as well as the amount and type of plastic/paint residue indicate the temperatures which were reached.

In reconstructing the flame spread one must keep in mind that plastic heater/air conditioner housings and inner fender panels represent a considerable mass of combustible material and generate considerable heat.

The pattern often focuses on particular locations. It may take considerable effort and understanding of both materials and vehicles for the investigator to establish for each location whether it was a hot spot or the point of origin.

Vehicles should be stored under cover until the investigators have finished their examinations. Oxidation patterns are important in identifying the point of origin, but they are temporal. Exposure to the elements rapidly degrades the information they have to offer. Covered storage is available at "self-storage" areas for about half the cost of the open air storage at most commercial insurance storage lots.

Conditions at the time of the fire, or just prior, are often very important to investigators. Was the vehicle running smoothly or not? Were there any prior problems? Was the vehicle running when the fire started? How recently had the vehicle been run if it was off? Had it received any recent repair work? What color was the smoke, if any? The answers to these and other relevant questions can make the difference as to whether or not the fire initiation can be reconstructed.

Frederick Hochgraf, Wade D. Bartlett, 3/91

See our Industry Definitions for further insight.

 

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