The Application of Forensic Engineering to Criminal Cases
Written by Colin R. Gagg   

An Excerpt from Forensic Engineering: The Art and Craft of a Failure Detective

EVIDENCE, IN ANY CRIMINAL CASE, is usually sent for analysis by government-recognized forensic science laboratories. These laboratories are usually staffed by scientists and various specialists (such as handwriting experts) who, although generally quite experienced and skilled in their respective fields, seldom have experience in manufacturing and general engineering. Occasionally, investigating police officers may instruct independent experts in relevant engineering fields. However, it is more common for the independent expert to be appointed (given instruction in the matter) in order to challenge or expand upon the evidence given to the court by scientists. In addition, insurance companies may also have an interest in the outcome of a case. So, they too may wish to have material evidence examined by an independent expert and, if necessary, to challenge the interpretation given to the court. The criminal cases that follow are selected from the authors’ experience in this field and, not surprisingly, have a strong metallurgical bias.

Case Study: Deliberate ‘Damage’
A Mercedes 300D had allegedly hit a white people carrier when losing control on a greasy road surface (Figure 1A). The third-party vehicle had suffered only minor damage, whereas the Mercedes had the appearance of having collided with a solid upstanding such as a bollard or fire hydrant. The vehicle had been recovered and transported to a storage facility where a local independent engineer inspected it. His report concluded that the extent of damage sustained had made repair of the vehicle an uneconomic proposition—the vehicle should be written off, with the owner being compensated for its full market value.

However, when inspected by an assessor, the general interior condition of the vehicle—steering wheel shiny, seats worn, peddle rubbers worn, seat belts frayed, leather stitching coming undone, etc.—along with only 46,000 miles recorded on its odometer, aroused his suspicion. An examination by a forensic engineer then followed, where regular vertical score marks at a 30-cm pitch (Figure 1B), and yellow paint (no sign of white paint) on damaged areas (Figure 1C), was found.

On investigation, a JCB (back-hoe) was noticed at the rear of the compound. Measurement of its bucket tooth pitch (Figure 1D) was found to be 30 cm, and furthermore, the JCB was yellow! It transpired that both parties were working with the recovery storage company and deliberately inflicting damage to old vehicles in order to fraudulently recover an inflated insurance value. Furthermore, there were at least four other old vehicles in the facility, perhaps waiting their turn to “sustain damage”.


Figure 1. (A) Frontal damage to Mercedes. (B) Vertical score marks at 30-cm pitch. (C) Evidence of yellow paint at damage sites. (D) JCB (back-hoe) bucket with a 30-cm tooth pitch.

Case Study: Metal Theft
Nonferrous metals are of considerable value, and many small enterprises make a living by collecting scrapped items at source and selling them to scrap merchants who sort and re-grade them before sending them to refineries. At the very bottom of this pyramid are individuals who ask no questions about the source of their scrap and who make their living by selling it to the highest bidder. This case concerns one such individual who made a serious mistake.

An electroplating firm ordered 9 tons of copper cathode from a copper refinery (Figure 2A). This is a pure form of copper that is produced only by electrolysis; the cathodes are somewhat irregular, 5 to 10 mm thick, with a smooth surface on one side and numerous tiny nodules standing proud on the other, as illustrated in Figure 2B. This sample is a piece of cathode of similar size to those stolen and is much larger than the guillotined pieces that it was to have been cut into for the electroplating process. It is included here to show the characteristic nodules on the surface as well as along the edge at the right-hand side. A freshly produced cathode is a bright salmon pink color when it leaves the refinery but oxidizes to a dull brown after a few weeks.


Figure 2. (A) A stack of copper cathodes similar to those stolen. Notice the side lug hooks. (B) Surface of the copper cathode. Notice nodules and surface wrinkling.

For use as anode in barrel electroplating, the cathode has to be reduced to small pieces about 25 mm2, so the electroplating firm needed to send its metal to another firm that had a powerful guillotine. The electroplating firm arranged to have this done on a Monday but, because the consignment arrived late on Friday afternoon, the truck was driven inside the plating firm’s yard and the gates locked. It was not visible from the outside. On Monday morning when the gates were opened up the truck was gone, and with it 9 tons of copper worth more than £42,500 ($56,000) in terms of today’s money.

The nonferrous metal trade has a system of notifying all dealers and metal merchants of a theft and the nature of the material stolen. Some four to five days after the theft, a small van drove to a scrap dealer 250 miles away from where the truck was taken. The driver offered to sell a quarter ton of scrap copper, saying he had more if the price was good enough. The scrap merchant realized that in the load of mixed copper—a few old hot water cylinders, pipes, wire and so on—were pieces of cathode, so he purchased that load and offered to buy all that the driver could deliver. He also notified the police. Within an hour the driver was back with another quarter ton, most of which was cathode. He was arrested, and the police accompanied him back to a small yard where they found the stolen truck containing just over 8 tons of copper cathode.

The man arrested claimed that he was just a small-scale scrap dealer, who had bought all the metal in small lots from local housing estates and small industrial units. It hardly needed a metallurgist to identify it as ex-refinery cathode, which is totally different from swarf or clippings arising from manufacturing processes, or discarded items from plumbing systems and electrical wiring. The thief was obviously unaware that the only uses for cathode copper are in electroplating and for re-melting and alloying with other metals. In no way could the metal in his possession have been collected from housing estates and light industry. It was particularly incriminating that the total amount was 9 tons and was just starting to develop the brown oxide coating. It could only have emanated from the one refinery still operating in the UK, and its color was consistent with it having been produced some three to four weeks before it was recovered. Further inquiries revealed that the man arrested had been living with his family in a caravan not far from the refinery at the time of the theft. The suspect was charged with theft and subsequently served a custodial sentence.

Case Study: Chain from Murder
A woman abruptly left her group of friends in a bar, allegedly because she was being pestered by a man. She walked home and locked her door, securing the chain, but the following day she was found raped and brutally murdered. The chain was hanging broken from the door handle. The man who had been pestering her was known to her friends and lived in the same district. He was questioned by police and claimed that he had walked home soon after the woman left and had watched a late-night movie on television.

The police searched his premises and found a small crowbar under sacking at the side of his house with four links of brass chain twisted tightly round the hooked end, shown close up in Figure 3A. Figure 3B shows part of the broken chain that the woman had used to secure her door alongside the four links twisted round the end of the crowbar.

Forensic examination revealed that the chain was composed of welded links made from brass wire. The diameter of the wire and the pitch of un-deformed links away from the fractures of both exhibits were practically identical. The chemical compositions of one link from the chain on the door and one from the crowbar were not identical but were both within the specified range for 70/30 copper–zinc alloy.

The man was arrested and charged with murder. DNA tests later confirmed that he had raped the woman. At trial, the defense sought to claim that the scientific tests had not established beyond all doubt that the links on the crowbar were indeed from the same length of chain used to secure the door. At this point an independent engineer was called. He found that the links had been electrically welded on the same type of machine, but the damning evidence was that the torsional fracture surfaces where the wires had been twisted to destruction were perfect matches to the crowbar and the chain still hanging from the door.


Figure 3. (A) Links of brass chain wrapped around a crowbar. (B) Twisted links of ornamental brass chain used to secure door.

Case Study: Re-Melting Beer Barrels
A number of public houses in a large city had experienced a spate of thefts of aluminum beer barrels. Whole batches of empty barrels placed in their yards awaiting collection by the brewery had disappeared in the early hours of the morning. In a suburb of the same city, a large warehouse and yard were occupied by small firms in the metal recycling trade, among which were vehicle dismantlers, one of whom had a small metal melting unit used for separating assemblies that contained aluminum alloy parts. As a vehicle was taken apart, components containing alloy were placed onto a sloping hearth under oil burners so that the aluminum alloy melted and ran down into a bath while the iron and steel were left on the hearth to be raked out from time to time. The molten alloy was cast into ingots and sold to a metal refiner.

The police suspected this warehouse yard might have been the destination of the stolen beer barrels, so they kept watch in a number of ways, including helicopter surveillance, but were never able to observe any barrels on vehicles entering the site. (It was later learned that the reason for this was that the barrels were carried in closed vans and the sheet-metal gates of the yard were always kept locked “to prevent entry by nosey parkers and casual passers-by”.)

Although most of the firms on the site were car dismantlers, one of them ran a one-man business stripping old electrical cable to recover lead and copper. He owned the lease of the site and lived a prosperous lifestyle, despite the small throughput of his business. Early one morning, the police raided the site and found the beer barrel melting process in full swing, with a closed van still partly filled with barrels backed up to the door and five men, including the cable stripper, apparently engaged in operating the furnace and casting 50-kg ingots.

All five were arrested, ordered to strip and change their boots, and the burners were shut down by the cable stripper. However, the cable stripper claimed he had nothing to do with the melting process but had merely attended to open the yard and carry on with some cable stripping in a different part of the building while the others were working. He admitted that he did this from time to time but had no idea where the barrels came from and said this job was usually done at night because the furnace was in use by the other firms for car parts during normal daytime hours.

His association with the beer barrel melting became clear after forensic examination of metal particles found on the soles of the boots and on the clothes he was wearing when arrested.

All the suspects’ clothes were brushed, and a number of tiny particles of metal were collected. In addition, there were numerous particles in the heels and soles of their boots, such as illustrated in Figure 4. Examination under the SEM revealed that all were rounded particles characteristic of splashes and droplets such as those emitted by molten metal running into open molds. Every one of the 20 samples taken from the boots and clothing of all five men were found to be aluminum alloy containing low percentages of magnesium and silicon. This composition spread would include the alloy used for beer barrels, but not the ones commonly used for automobile castings, which usually contain greater amounts of silicon. Most significantly, no particle recovered from the cable stripper’s clothing or boots was copper or lead—but, even if there had been any from this source, they would have taken the form of mechanical slivers or clippings, not solidified droplets.


Figure 4. (A) The heel of a suspect’s boots. (B) A closer view showing beads of aluminum alloy embedded in the heel, picked up by walking on a floor close to the metal casting operation.

The clothing exhibited numerous holes formed by hot particles landing on them, particularly below the trouser knees. There were two holes joined together in the side pocket of a nylon jacket worn by the cable stripper. These had been formed by one, or possibly two, splashes of hot metal striking the fabric and melting through the nylon. The lower leg of a pair of corduroy trousers had a series of holes, formed where small, hot particles had landed. Although most of the above observations are essentially scientific, the engineering dimension becomes important in identifying the particles and clothing damage as typical of that expected when workers are standing close to foundry operations. Few scientists have ever observed the splashing that occurs and the distance droplets may carry when a stream of molten metal poured from a furnace first strikes a launder and then runs into an open ingot mold, or the fine spray of droplets that are thrown up as it solidifies when the metal has picked up hydrogen due to being melted under reducing conditions. In this instance, the damage to the clothing worn by the cable stripper clearly established that he must have been standing very close to the stream of molten metal as the ingots were being cast.

This article appeared in the March-April 2020 issue of Evidence Technology Magazine.
You can view that issue here.


About the Author
Colin R. Gagg, M.Sc., holds an honors degree in engineering technology and a master’s degree in management and technology of manufacturing. He is a chartered engineer and professional member of the Institute of Mechanical Engineering. His practical experience includes two years at the Structures Laboratories of Imperial College of Science, Technology and Medicine and four years at the Engineering Department of the University of Toronto. He currently holds the post of research projects officer at The Open University and, for the past 10 years, he has been a member of the Forensic Engineering and Materials Group.

 
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