- Forensic polymer engineering
The study of failure in
polymeric products is called forensic polymer engineering. The topic includes the fractureof plastic products, or any other reason why such a product fails in service, or fails to meet its specification. The subject focuses on the material evidence from crime or accident scenes, seeking defects in those materials which might explain why an accident occurred, or the source of a specific material to identify a criminal. Many analytical methods used for polymer identification may be used in investigations, the exact set being determined by the nature of the polymer in question, be it thermoset, thermoplastic, elastomericor composite in nature.
One aspect is the analysis of
trace evidencesuch as skid marks on exposed surfaces, where contact between dissimilar materials leaves material traces of one left on the other. Provided the traces can be analyzed successfully, then an accident or crime can often be reconstructed.
Methods of analysis
Thermoplastics can be analysed using infra-red spectroscopy, UV spectroscopy, as NMRand ESEM. Failed samples can either be dissolved in a suitable solvent and examined directly (UV, IR and NMR spectroscopy) or as a thin film cast from solvent or cut using microtomyfrom the solid product. Infra-red spectrosocpy is especially useful for assessing oxidation of polymers, such as the polymer degradationcaused by faulty injection moulding. The spectrum shows the characteristic carbonyl groupproduced by oxidation of polypropylene, which made the product brittle. It was a critical part of a crutch, and when it failed, the user fell and injured herself very seriously. The spectrum was obtained from a thin film cast from a solution of a sample of the plastic taken from the failed forearm crutch. Microtomyis preferable since there are no complications from solvent absorption, and the integrity of the sample is partly preserved. Thermosets, compositesand elastomerscan often only be examined using microtomy owing to the insoluble nature of these materials.
Fractured products can be examined using
fractography, an especially useful method for all broken components using macrophotographyand optical microscopy. Although polymers usually possess quite different properties to metals, ceramics and glasses, they are just as susceptible to failure from mechanical overload, fatigue and stress corrosion crackingif products are poorly designed or manufactured. Scanning electron microscopyor ESEMis especially useful for examining fracture surfaces and can also provide elemental analysis of viewed parts of the sample being investigated. It is effectively a technique of microanalysisand valuable for examination of trace evidence. On the other hand, colour rendition is absent in ESEM, and there is no information provided about the way in which those elements are bonded to one another. Specimens will be exposed to a partial vacuum, so any volatiles may be removed, and surfaces may be contaminated by substances used to attach the sample to the mount.
Many polymers are attacked by specific chemicals in the environment, and serious problems can arise, including
road accidents and personal injury. Polymer degradationleads to sample embrittlement, and fracture under low applied loads.
Polymers for example, can be attacked by aggressive chemicals, and if under load, then cracks will grow by the mechanism of stress corrosion cracking. Perhaps the oldest known example is the ozone crackingof rubbers, where traces of ozone in the atmosphere attack double bondsin the chains of the materials. Elastomerswith double bonds in their chains include natural rubber, nitrile rubberand styrene-butadienerubber. They are all highly susceptible to ozone attack, and can cause problems like vehicle fires (from rubber fuel lines) and tyreblow-outs. Nowadays, anti-ozonants are widely added to these polymers, so the incidence of cracking has dropped. However, not all safety-critical rubber products are protected, and since only ppbof ozone will start attack, failures are still occurring.
Another highly reactive gas is
chlorine, which will attack susceptible polymers such as acetal resinand polybutylenepipework. There have been many examples of such pipes and acetal fittings failing in properties in the USA as a result of chlorine-induced cracking. Essentially the gas attacks sensitive parts of the chain molecules (especially secondary , tertiary or allyliccarbon atoms), oxidising the chains and ultimately causing chain cleavage. The root cause is traces of chlorine in the water supply, added for its anti-bacterial action, attack occurring even at parts per milliontraces of the dissolved gas. The chlorine attacks weak parts of a product, and in the case of an acetal resinjunction in a water supply system, it was the thread roots which were attacked first, causing a brittle crack to grow. The discolouration on the fracture surface was caused by deposition of carbonatesfrom the hard watersupply, so the joint had been in a critical state for many months. When it finally failed, it did so at the worst possible time, at the weekend when no-one was around to sort the problem. The leak flooded computer labs below, and caused substantial damage.
Most step-growth polymers can suffer
hydrolysisin the presence of water, often a reaction catalysed by acidor alkali. Nylonfor example, will degrade and crack rapidly if exposed to strong acids, a phenomenon well known to ladies who accidentally spill acid onto their tights. The broken fuel pipe shown at right caused a serious accident when diesel fuel poured out from a van onto the road. A following car skidded and the driver was seriously injured when she collided with an oncoming lorry. Scanning electron microscopyor SEM showed that the nylonconnector had fractured by stress corrosion crackingdue to a small leak of battery acid. Nylon is susceptible to hydrolysisin contact with sulfuric acid, and only a small leak of acid would have sufficed to start a brittle crack in the injection mouldedconnector by a mechanism known as stress corrosion cracking, or SCC. in lubricity, so skids are common when diesel leaks occur. The insurers of the van driver admitted liability and the injured driver was compensated. Polycarbonateis susceptible to alkali hydrolysis, the reaction simply depolymerising the material. Polyestersare prone to degrade when treated with srong acids, and in all these cases, care must be taken to dry the raw materials for processing at high temperatures to prevent the problem occurring.
Many polymers are also attacked by
UV radiationat vulnerable points in their chain structures. Thus polypropylenesuffers severe cracking in sunlightunless anti-oxidants are added. The point of attack occurs at the tertiary carbon atompresent in every repeat unit, causing oxidation and finally chain breakage. Polyethyleneis also susceptible to UV degradation, especially those variants which are branched polymers such as LDPE. The branch points are tertiary carbonatoms, so polymer degradationstarts there and results in chain cleavage, and embrittlement. In the example shown at left, carbonyl groups were easily detected by IR spectroscopyfrom a cast thin film. The product was a road conewhich had cracked in service, and many similar cones also failed because a anti-UV additive had not been used.
Environmental stress cracking
Forensic electrical engineering
Forensic materials engineering
Stress corrosion cracking
*Peter R Lewis and Sarah Hainsworth, "Fuel Line Failure from stress corrosion cracking", Engineering Failure Analysis,13 (2006) 946-962.
*Lewis, Peter Rhys, Reynolds, K, Gagg, C, "Forensic Materials Engineering: Case studies", CRC Press (2004).
*Wright, D.C., "Environmental Stress Cracking of Plastics" RAPRA (2001).
*Ezrin, Meyer, "Plastics Failure Guide: Cause and Prevention", Hanser-SPE (1996).
* [http://materials.open.ac.uk/mem/index.htm Museum of failed products]
* [http://openlearn.open.ac.uk/file.php/2980/formats/print.htm New Forensic course]
* [http://www.elsevier.com/wps/find/journaldescription.cws_home/30190/description#description The journal Engineering Failure Analysis]
* [http://www.forensic-courses.com/wordpress/?p=42; Forensic science and engineering]
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