3.4 Chemical analysis
A common problem investigators face is determining the chemical constitution of the failed sample. A product specification is often absent, so an analysis route must be used. Whether there is a specification or not, the need for analysis is three-fold.
It establishes the nature of the material, and perhaps some of its properties.
It is a check in cases where a specification is available.
If the results show something odd for the material it could indicate material degradation.
Once the material is known, the engineer can form an opinion on whether it was the correct choice for that component. Property tabulations can be consulted for guidance.
To give a simple example, if a component is stressed in service, then it should be tough rather than brittle. Polystyrene is brittle so should not be used for stressed products – such as a crutch. A preferred alternative would be ABS or HIPS plastic, which are much tougher and crack-resistant.
If the material is exposed to a particular environment, then it should be capable of resisting that environment.
A fuel pipe should resist petrol or the fuel being contained, and rubber should resist oil if used for engine seals, water if used for a cooling system.
So what techniques are available? The technique chosen depends first on the nature of the material of construction, whether it be a metal, ceramic glass or polymer.
The first method used for polymeric materials is infrared spectroscopy (FTIR). It is a method that is partially destructive, so it is not used unless permission has first been sought and obtained. However, damage to the failed part can be minimised by selecting an area well away from key features because only milligram quantities are needed. Box 11 describes the use of spectroscopy as a forensic tool.
Box 11 Infrared and ultraviolet spectroscopy
Polymers are characterised by their repeat unit, or units if copolymers. The repeat unit is simply the combination of atoms linked together, which is repeated in a linked linear chain. This repeat unit – a molecular block – has distinct properties that can be detected using spectroscopy, which works on the basis of absorption of electromagnetic radiation: infrared rays for FTIR, and ultraviolet rays for UV spectroscopy.
The sample from the failed crutch produced the spectrum in Figure 30. From that, it was possible to determine the material of construction as polypropylene copolymer. The group that distinguishes the copolymer as polypropylene is labelled on the diagram. The unknown polymer can be identified by comparing its spectrum with a printed compilation of spectra.
The comparison against a reference spectrum can also lead to detection of polymer degradation, as in the spectrum in Figure 31. It shows oxidation of a polyethylene material due to overheating during processing, probably coupled with attack by UV radiation from the sun.
Ultraviolet spectroscopy is another useful method for polymer analysis. The method is especially sensitive to aromatic groups present in a material. These are any structures with benzene rings (C6H6), and are common in UV absorbers and anti-oxidants. A material can be checked for the presence of these protective additives using UV spectroscopy.
Nuclear magnetic resonance (NMR) spectroscopy is another special tool useful in detecting minute amounts of impurities or special additives screened by other compounds.
The method described in Box 11 is not only used for polymers, but also for any organic substance. So it also finds wide use in forensic science for the analysis of drugs and textiles, and any other trace organics at a crime scene.
Spectroscopy is also used on commercial polymers to detect traces of impurities or additives.
3.4.2 Differential scanning calorimetry
Differential scanning calorimetry (DSC) is a tool used to corroborate polymer composition, and it can be used for low-melting-point metals and alloys. It is based on an entirely different principle to spectroscopy. It yields information on the thermal behaviour of materials – specifically the melting point denoted by Tm and the glass transition temperature denoted by Tg. These two pieces of data help characterise the polymer under test. The method is described in Box 12. The comparison of melting temperatures and transition temperatures with the standard values from published data will also show if the values are lower than expected for a high molecular mass material.
Box 12 Differential scanning calorimetry
A method quite independent of spectroscopy is differential scanning calorimetry (DSC), where the absorption or liberation of heat is measured accurately as a few milligrammes are heated up at a regular rate. If the polymer is crystalline, heat will be absorbed as the melting point Tm is approached, with a sharp peak at Tm itself. The glass transition temperature T is observed as a change in slope of the curve. In addition, the degree of crystallisation will be shown by the area under the melting curve. So, if crystallisation was due to exposure of the sample to high temperature, for example, the melting curve will increase in size. This is what was observed with the elastomeric radiator seals in Figure 32.
An increase in the crystallinity of a polymer usually gives a stiffer material, explaining the greater stiffness of the failed seals. No change was detected in the melting point, which might have been expected with any degradation in molecular mass. However, stress corrosion cracking limits degradation to a small amount of polymer at the crack tip, therefore the change would not normally be found when examining bulk material.
3.4.3 Gel permeation chromatography
The last method determines the molecular mass of polymers. One special problem with polymers is the drastic fall in strength with decreasing molecular mass, which often turns tough behaviour into brittle behaviour. If the part is safety-critical, it may not be able to resist normal stresses – let alone abnormal stresses – and will fail.
The method of choice for finding the molecular mass is GPC. It is a method based on absorption and desorption of chain molecules from a special gel. It gives a distribution curve for a typical polymer, from which various molecular masses can be calculated. GPC is used for:
checking molecular mass against the part specification – to establish the grade of polymer;
determining whether degradation has occurred.
Comparison of a new sample with the failed sample will show whether degradation – lowering of molecular masses – has occurred, and it can pinpoint the cause.
Comparison of samples taken from failed battery cases, for example, enabled ultraviolet degradation to be identified as the cause of failure (Box 13).
Box 13 Battery case failures
A common failure mode of polymers exposed to sunlight is degradation from the UV component. It can also happen unexpectedly for other reasons, when the product is normally concealed from sunlight.
Polypropylene (PP) is commonly used in battery cases that are usually hidden from light under the bonnet of a vehicle. Fork-lift trucks used by the Israeli army had lead-acid batteries that leaked after exposure to the sunlight of the Middle East (Figure 33).
Gel permeation chromatography was used to analyse the top and bottom surfaces from the degraded lids, with the results shown in Figure 34. The polymer from the upper surface possessed a much lower molecular mass than that from the lower surface. FTIR spectra showed carbonyl groups in the chains, a normal accompaniment of UV degradation, corroborating the initial assessment of the problem.
It was curious that the most severe degradation was restricted to the welded part of the lids, as can be seen from the chalking of the left-hand lids in Figure 33. The lids are welded to the boxes by heating the contact areas, and then clamping them together. Clearly, some thermal degradation had occurred, making the welded regions more susceptible to UV attack.
The manufacturer accepted responsibility for the problem, replaced the degraded batteries, and modified the material with a UV-absorbent additive to protect it against sunlight. The weld operation was also closely examined and modified to prevent unacceptably high temperatures.
A plastic garden chair made from polypropylene collapsed and injured the user after being outside for several years. The collapse was caused by cracking at an external corner, and there was evidence of extensive superficial cracking.
Construct a fault-tree analysis diagram for the fault. Suggest appropriate analytical methods for determining the most probable cause or causes.
The main possible causes of the cracking are mechanical overload, the presence of a stress raiser, UV degradation, and a moulding defect. Mechanical overload could mean the weight of the user exceeded the specification maximum load, or it could have been caused by impact damage. The stress raiser could be because the design of the corner made it too sharp, or perhaps wear had created a deep scratch. Moulding defects could include voids or contamination near the corner. My diagram is shown in Figure 35.
The polymer is known, so that material determination is not needed. However, the fact it has been exposed outside suggests UV degradation could be the prime cause of the superficial cracking. FTIR should be used first to see if any oxidation has occurred in the outer layers. Comparison with unexposed material – perhaps from the underside of the chair – could indicate the importance of UV degradation, especially if combined with GPC analysis of the two samples.
The critical crack should be examined with an optical microscope to see if there is any connection with the superficial cracks. If a superficial crack was found at the origin on the fracture surface, it is evidence that the UV degradation created the conditions for failure. The FTIR spectrum might indicate whether or not a UV inhibitor had been added to the original polymer. Microscopic examination should also discover any moulding defects such as a void or contamination near the corner, especially if the feature ran into the fracture surface.
An artificial composite leg fractured suddenly when a woman put her weight on it, and she was injured.
The composite was made by injection moulding a thermoplastic polymer – the matrix – reinforced with short glass fibres. The fibres added stiffness to the product and should have be aligned as parallel as possible to the long axis of the prosthesis.
Construct a fault-tree analysis diagram for the fault. Suggest appropriate analytical methods for determining the most probable cause or causes.
There are several possible causes of the failure, including many of those discussed in the previous SAQ, such as mechanical overload, and stress raisers on the surface of the leg. A composite material increases the possibilities because there are two different materials present: the fibre and the resin matrix. In addition, poor fibre distribution during moulding must also be included in the analysis.
My diagram (Figure 36) shows faults for the matrix material only and does not include fibre faults.
It would be appropriate to start by analysing the materials of construction: the polymer matrix and the fibres. FTIR is appropriate for both kinds of material, and a check using standard spectra should reveal the nature of the materials, and whether any degradation is present. GPC may be needed to confirm that the molecular mass of the matrix is appropriate.
The fracture should be examined with an optical microscope to find the nature of the failure, and to determine the origin, or origins. Careful inspection should reveal if the fracture is old and has grown slowly with time. Contamination on the fracture surface is one good indicator of an early initiation event or events. SEM might help if any contamination is found on the fracture surface.
The woman's weight may need to be compared with any specification; if her weight was greater than specification, the failure could simply be mechanical overload. The details of the sequence of events prior to the accident needs to be known to check subcritical damage – for example from an impact – so that all possible causes can be checked in detail. Elimination of the most obvious causes should then lead to the actual cause of the accident.
A battery case leaked acid from a crack in the outer casing, which was injection moulded. A nearby metal pipe leaked brake fluid and led to an accident when the brakes failed suddenly. There was evidence of corrosion on the brake pipe. There were also signs of impact damage near the site of the crack.
Draw a fault-tree analysis diagram to show the connection between the observed failure and the root cause or causes. Suggest the best techniques to pinpoint the problem. Indicate any uncertainties, and what extra knowledge might be needed to find the exact cause or causes.
Figure 37 is one attempt to outline the causal links between the accident and the problems visible in the engine compartment.
It is tempting to jump to the conclusion that the battery leak caused corrosion of the brake pipe. However, it is necessary to establish the chain of causation. For example, is the brake pipe close enough to the battery leak for acid to have made contact with the metal?
Analytical SEM inspection of the corrosion on the brake pipe could resolve the question, because battery acid contains sulphur, which can be detected by ED AX analysis. If it were not found, there is a chance the corrosion was caused by some other agent, such as acid rainwater or salt water.
Another obvious question to ask is whether the corroded pipe was situated vertically below the end of the crack? If so, there seems no problem in showing that acid dripped down onto the pipe. If not, further questions might be posed. Has the pipe been moved since the accident? Can a simple reconstruction show its original position? After all, a pipe in one position for some time will probably leave trace evidence of its position from oil and dirt always present in an engine compartment.
The damage to the battery that caused the crack, and hence the leak, needs careful investigation. Microscopy of the damage might show that the cratering corresponds to a hammer head. If so, the damage was almost certainly deliberate – perhaps to adjust the battery position in its recess.
Why did a brittle crack grow? Was there a stress raiser behind the impact blow? Is the material of sufficient quality to withstand impact damage? Is a hammer blow a normal and expected force that a battery case should be able to withstand? Were there any moulding defects found near or at the crack origin on the fracture surface?