8.2 Causes of polymer failure
This unit started by looking at simple examples of product failure, and then looked at case studies that are more complex where polymer components or products failed. In the restricted space available in one block of the course, it is impossible to examine all possible failure modes of polymers, which are themselves just one group of materials available to design engineers. Before moving on to learn about metal failures in the next block, it is useful to take a wider view of polymer failures.
Table 6 shows failure modes classified by mechanism, and it indicates the range of ways polymer products can lose their integrity. The most common failure modes are mechanical in origin, for the simple reason most products will bear some load in service – even if it is just its own weight. And many of the other mechanisms will probably lead to mechanical failure in the chain of causation. In this respect, chemical attack will lower strength, and an obvious symptom will be cracking, excessive wear, or some other mechanical effect.
Table 6: Failure modes of polymers by mechanism
|mechanical||fracture (brittle or ductile or mixed mode)|
|creep rupture (static fatigue)|
|fatigue (slow crack growth from cyclic loading)|
|impact (rapid fracture)|
|wear, fretting (abrasion between surfaces)|
|fire, slow combustion|
|chemical||oxidation, ozone attack|
|chlorinolysis (attack by chlorine), fluorinolysis|
|hydrolysis (by water, acid or alkali)|
|stress corrosion cracking (SCC)|
|other chemical interaction (e.g. ZnCl2, ethylene oxide)|
|optical||radiactive attack (e.g. UV attack)|
|ionizing radiation (e.g. gamma radiation attack)|
|electrostatic build-up, sparking|
Table 7 lists causes of failure from raw material processing to consumer use, and it relates the basic mechanisms to the actual ways products can fail. Table 6 would be the starting point of scientific analysis, but Table 7 would be the route to identifying the detailed reasons for a failure. The way an investigator tackles a specific case will also follow the same path, because they will usually be provided with a failed product, which, when analysed, reveals a specific mechanism of failure. When the wider facts of the problem are known, the investigator can relate the circumstances of failure to the specific mechanism already identified.
Table 7: Failure modes of polymers by process
Taking the major cases studied in Sections 4, 5, 6 and 7 of this unit, identify the main mechanisms and failure modes described in Table 6. Then specify the main stage at which failure probably occurred, by reference to Table 7. Which case studies showed one mechanism on initial inspection, but showed a different mechanism on further inspection? What major lessons should be drawn from your conclusions? Indicate the role of forensic methods in each case study.
The major case studies examined in Sections 4, 5, 6 and 7 were:
a ladder accident;
a failed composite car radiator;
a fractured storage tank;
a broken acetal fitting on a rising main;
failed flexible fuel pipes;
a failed catheter.
Apart from the ladder accident, they were all associated with fluid containment in one way or another, so failure was quickly detected by catastrophic consequences. The catheter was somewhat different in that failure of the tip was observed without loss of fluid, although the failure at the proximal end possibly caused loss of the infused drug solution.
The main failure modes are as follows.
The ladder accident was primarily a user failure – failure to heed warnings, failure to follow advice – who leant the ladder at too low an angle against the wall of his house. All new ladders have a warning notice posted on the stile of the ladder at head height. The fracture of the tip was not a cause of the accident, but rather the result of the ladder slipping down the wall and striking a ledge above the patio doors.
The radiator reservoir failure was a manufacturing problem, probably caused by a machine operator wrongly classifying the faulty moulding as a good component. The symptoms of failure included brittle fracture of the composite material, aggravated by frozen-in strains as shown by distortion of the moulding – from the low-temperature conditions at start-up of the machine. The failure was mechanical but stimulated by high operating temperatures, and initiated by weld lines and cold slugs in the wall of the product.
The tank failure was a design problem, where the walls had been built like a barrel rather than like a dam, and didn't resist hydrostatic pressure. The immediate symptom of failure was cracking in the centre of the lower single panel wall, caused by creep rupture at each of the four loading cycles. Initiation occurred from a pinhole defect from welding, probably the final weld made at the manufacturer.
The acetal fitting probably failed by chlorine attack at a pre-existing defect or defects in the inner surface adjacent to the screw threads. The mechanism was probably creep rupture from the normal closure stresses. There were thus manufacturing defects that allowed a stress corrosion cracking mechanism to form the cracks.
The fuel pipe failures were caused by two mechanisms: ozone cracking of the NBR rubber pipes – for which a recall was made – and rapid abrasive wear of the nylon 11 fuel return pipes abrading against hot engine manifolds. They were design faults, the materials were inadequate for their intended function.
The catheter failure was probably caused by a combination of attack by the UV component of sunlight, followed by gamma ray radiation before supply to the hospital. The immediate cause of failure was separation of the catheter near the proximal hole in the tip during the last stages of the epidural injection. It was therefore a manufacturing problem.
Several of the cases considered here initially involved causes that were either not relevant to the reason for failure, or more distantly related than first thought. The broken ladder tip wrongly led the user to suggest the moulding was faulty and therefore caused the accident. The immediate cause of the storage tank failure was crack growth in a weld, but poor welding was not the cause of the incident, rather, it was the basic design of the tank that caused the failure. One investigator suggested the fracture of the acetal fitting was caused by high tightening loads, but deeper analysis showed the failure was actually caused by SCC of a faulty moulding. Initial inspection of the tip of the catheter attributed the cause to the anesthetist withdrawing the catheter wrongly, and so cutting the tip off. In fact, the catheter had been weakened by degradation and showed partly brittle properties, so it could have failed easily during withdrawal.
These examples demonstrate that many failures require deeper thought, and the appropriate analytical method for resolution. Corroboration using independent methods is usually required, but that may not always be possible if the sample is very small, or if it requires independent examination by another expert. The ladder problem yielded to a full reconstruction of events, while the radiator reservoir needed microscopic inspection of the fracture surface to show cold slugs on the weld lines. The tank failure was solved when inspection of the standard showed it had been constructed wrongly. The examination of the fracture surface revealed only that the failure had occurred at the weakest point in the product. The critical method in the acetal fitting failure was finally EDAX analysis of the surface to show the presence of chlorine, corroborating the results of the USA litigation. The fuel pipe failures were solved essentially by simple observation of the failure mode and remedial action in the UK – but not in Ireland. The turning point in the catheter case came from a single, uncorroborated, FTIR microscope spectrum. However, the result confirmed what had already been suggested by the brittle features present in the catheter during the birth, the partly brittle nature of the fracture surface, and the poor mechanical properties of the rest of the catheter when tested later.