One aspect is the analysis of trace evidence such 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.
Thermoplastics can be analysed using infra-red spectroscopy, UV spectroscopy, as NMR and 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 microtomy from the solid product. Infra-red spectrosocpy is especially useful for assessing oxidation of polymers, such as the polymer degradation caused by faulty injection moulding. The spectrum shows the characteristic carbonyl group produced 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.
Microtomy is preferable since there are no complications from solvent absorption, and the integrity of the sample is partly preserved. Thermosets, composites and elastomers can often only be examined using microtomy owing to the insoluble nature of these materials.
Scanning electron microscopy or ESEM is 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 microanalysis and 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.
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 cracking of rubbers, where traces of ozone in the atmosphere attack double bonds in the chains of the materials. Elastomers with double bonds in their chains include natural rubber, nitrile rubber and styrene-butadiene rubber. They are all highly susceptible to ozone attack, and can cause problems like vehicle fires (from rubber fuel lines) and tyre blow-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 ppb of ozone will start attack, failures are still occurring.
Another highly reactive gas is chlorine, which will attack susceptible polymers such as acetal resin and polybutylene pipework. 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 allylic carbon 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 million traces of the dissolved gas. The chlorine attacks weak parts of a product, and in the case of an acetal resin junction 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 carbonates from the hard water supply, 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.
Polycarbonate is susceptible to alkali hydrolysis, the reaction simply depolymerising the material. Polyesters are 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 radiation at vulnerable points in their chain structures. Thus polypropylene suffers severe cracking in sunlight unless anti-oxidants are added. The point of attack occurs at the tertiary carbon atom present in every repeat unit, causing oxidation and finally chain breakage. Polyethylene is also susceptible to UV degradation, especially those variants which are branched polymers such as LDPE. The branch points are tertiary carbon atoms, so polymer degradation starts there and results in chain cleavage, and embrittlement. In the example shown at left, carbonyl groups were easily detected by IR spectroscopy from a cast thin film. The product was a road cone which had cracked in service, and many similar cones also failed because a anti-UV additive had not been used.