Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

7.2 Plumbing problem

This case study involving litigation occurred when a small plastic fitting failed suddenly one weekend during November 1988, under a sink in the physics department at Loughborough University (Figure 80). The subsequent flood of water from the cold water main into the computer department immediately below is described briefly in Paper 6. Paper 6 is attached as a pdf which should be printed out (if possible) to gain the maximum benefit from the discussion of this case study (at least it should be kept open on the desktop throughout Section 7).

Click on the 'View document' link below to read Paper 6

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Figure 80: Assembly under the sink where a fitting linking the rising water mains to a tap and a water heater failed. The resulting water leak caused damage on the floor below

The plumbing arrangements were the focus of attention by the loss adjusters and their expert (Figure 1, Paper 6). The acetal fitting was situated below the hot water tap, and was supported in a steel bracket, so that when the hot tap was turned on, cold water flowed through the large water heater to the tap itself.

It was noted that the heavy water heater was attached to the wall by only a single screw, which had come adrift from the wall, and placed the surrounding copper pipes under bending loads. The break in the fitting was just below the lower bend in the steel bracket, where the rising main supplied incoming cold water to the system.

SAQ 23

Draw an FTA diagram to indicate all possible causes of failure that you think could explain the failure of the acetal fitting. The initial theory of the failure suggested the sagging water heater had overloaded the fitting, the load being transferred via the cold water pipe. Using Figure 1, Paper 4, sketch the load path – the path along which load is transmitted – between the water heater and the fractured fitting. From your diagram, comment on the credibility of the theory. Another suggestion could involve downward movement of the rising mains. How likely is this alternative hypothesis?

Answer

Figure 81 suggests some possible failure modes, including overloading by excess pressure from the water heater, overtightening of the joint and loading from the rising water mains pipe.

Figure 82 shows the load path between the water heater and the critical acetal fitting.

With the heater no longer fixed to the wall, the load is supported by the two copper pipes, and is shared roughly equally. The upper horizontal pipes would probably bend.

Figure 81: Fault-tree analysis of the leaking sink
Figure 82: Water heater put bending loads on pipework and joints

The lower pipe will transmit some part of its load to the upper junction of the plastic fitting, and it in turn will load the steel bracket. The angle bracket holding the fitting to the tap junction is made from steel and clearly capable of supporting the part of the total load transmitted to it. If breakage in the plastic fitting were likely, then it would be the upper rather than lower junction that should fail. As it was the lower part that failed, the theory is not credible.

Loading from a downward movement of the water mains pipe is more likely, because that could cause the key junction to be put in tension. However, the load is shared between two pipes, and the smaller pipe extension to the acetal fitting is likely to be less strained than the longer extension to the cold-water tap.

There must have been another reason for the failure. The next suggestion was that the junction had been overtightened by the plumber. The system had been fitted during renovation work some four years earlier. This is an interesting suggestion, because it seems to imply the fitting could be overtightened, and might have been a faulty design in allowing overtightening. It is of course a common way of producing extra stress on a joint, but what did the evidence show?

If screw joints are overtightened, there might be evidence from the rubber washers used in the system. They take any excess stress from tightening, so if the joint had been overtightened, then some damage might be expected. In fact, there was no evidence the washers had broken or distorted excessively at all (Figure 83).

Figure 83: Washers used on the failed part of the acetal fitting showing no evidence of excessive tightening of the joint

But this is where the problem lay when it came to assessing who would pay for the substantial damages to the computers on the floor below. The loss adjuster's expert effectively pointed at overtightening as the cause of failure, possibly exacerbated by the ‘faulty’ screw attachment to the wall. So the loss could be recovered from the plumber or company who installed the system, and possibly the architect for planning the system in the first place.

The architects, or rather their insurers, refused to accept the blame, so proceedings were started by the university against the plumbing company and architects. At this point, the defendants naturally needed their own expert to examine the failure and produce a report – preferably someone with expertise in the failure of plastics materials. The failed acetal fitting was critical, but had not been examined in detail by the first expert. Why not? The reason or reasons are not known. It seems so obvious that the evidence at the heart of a failure, the cracked fitting in this instance, should be subjected to detailed analysis. However, some experts fail to make the leap into detailed inspection, perhaps because they do not have the expertise or laboratory equipment, and so on.

7.2.1 Fracture surface

We have already seen the fracture surface in Figures 23, 24(a) and 24(b), where it was used as an example of how to start assessing the evidence of fracture during macroscopic inspection. The surface showed several old fracture origins, now concealed by debris deposited from the water supply, and implied from flaps left by crack growth (Figures 2 and 3, Paper 6). The detail shown in the latter figure suggests there were at least five different growth regions, with crack growth directions as indicated by the bold arrows. The white areas represent the fresh fracture induced by the loss adjuster's expert when the sample rolled off his desk.

Figure 84: Close-up of the crack tip in the base of the thread showing weld lines ahead of the tip, stained brown by the water supply
Figure 85: Close-up of the crack tip on the inner bore of the pipe, showing crack branching. There were weld lines associated with the crack path

When the areas adjacent to the fracture surface were examined in detail, one significant feature emerged: there were numerous subcritical cracks that appeared to have grown from weld lines in the threads and on the inner surface of the fitting (see Figures 84, 85), (see Figure 3, Paper 6). The weld lines appeared to be associated with flow lines in the outer surface of the moulding, suggesting moulding may have been faulty. Flow and weld lines are indicative of cool tools, so that flow of molten polymer in the mould tool is inhibited. That the fitting had been screwed up to produce a closure stress on the threads was indisputable as Figure 86 shows. So, what effect would such weld lines have when the thread was screwed tight by the plumber?

Figure 86: Macrograph of the brass coupling unit attached to the lower half of the fractured acetal fitting. The section shows the upper surface of the plastic thread touching the brass thread, while the lower surface is free, indicating closure stress on the joint.

SAQ 24

Evaluate the stress concentration at the threads of the fitting using standard diagrams already presented in this unit. The wall is 1.6 mm thick to the base of a thread root, and the root possesses a radius of curvature of 0.2 mm. What is likely to be the effect of having a weld line at a thread root? Assume a weld line is 0.1 mm deep. What light does the analysis shed on the way crack growth occurred? From your analysis, indicate whether cracks grew from the inside out or from the outside inwards.

Answer

The stress concentration at the thread root can be evaluated using Figure 40, Box 14, in a similar way to the calculation of SAQ 10(c). Using the values of wall thickness and radius of curvature of the root, then

r/d = 0.2/1.6 = 0.125

Reading from Figure 40 gives a Kt value of about 2.6, a relatively low value. However, any weld line within the root will magnify the stress further. Suppose, for example, the stress concentration is about 3, the combined effect is to give a stress concentration of 7.8, the product of the two independent stress raisers.

The net effect of the several stress raisers will operate on the final closure stress of the joint made by the plumber, but the evidence from the washers showed there was no excessive force applied. There would have been only a small closure stress on the threads and hence only a relatively small magnified stress at the tips of the weld lines at the thread root. It is interesting to note that subcritical cracks within weld lines were also found on the inside surface of the failed fitting near the fracture. Although the stress concentration will be lower – there is no thread root – it is possible the original cracks grew from inside the fitting as well as from the outside.

However, the overall stresses would have been small and there must be another explanation for the early failure of the joint. Whether or not the cracks grew internally or externally thus remains unknown. The fracture surface itself is of little help.

7.2.2 Water pressure

So tiny weld lines could concentrate the tightening load if any were at the root of a thread. One common observation of failures from screw threads is that failure is often at the last thread but one – a feature confirmed by theoretical analysis of the stress-concentrating effects of thread forms. Thread failures are themselves common because threaded joints and connections are one of the most common ways of joining components.

However, there was no evidence of an excessive closure stress made by the plumber when finally tightening the joint, and the cause of the failure remained unknown at this stage of the investigation. What was clear was that there was no evidence supporting the proposal that the joint had been overtightened – therefore the plumber was not to blame.

Box 20 Environmental stress cracking

ESC is a recurring problem where tough polymers are simultaneously stressed and exposed to organic fluids that can initiate brittle cracks. While many polymers, such as polyethylene and polypropylene, are insoluble in most common organic solvents, some liquids are so aggressive they can be absorbed and swell the surface layers. Swollen polymer is less resistant to crack formation, so that microcracks can develop on exposure. Further exposure can lead to crack growth, which, when it reaches a critical length, leads to catastrophic fracture of the product.

ABS is a tough plastic susceptible to such cracking, often under unusual circumstances. However, there are many organic fluids that can cause ESC in a surface under strain. This is what appears to have happened when an ABS compressed air line suddenly exploded in a factory in 1998. Fortunately, nobody was injured, although there was substantial damage to property. Examination of the large fracture surface showed a characteristic pattern of cracking on the inner wall (Figure 87).

Figure 87: ESC on inner wall of ABS pipe

The edge of the pipe showed a crenellated pattern, where numerous subcritical cracks had formed at the stressed surface and grown into one another. The largest crack had grown catastrophically, causing the explosion. But what had initiated the cracks?

When examined closely, the inner surface of the pipe showed faint traces of a fluid contaminant that was always associated with a brittle crack (Figure 88). One possible source of the fluid could have been the oil used in the compressor motor being blown suddenly, for some reason, into the otherwise empty pipe just prior to the explosion.

Figure 88 Traces of fluid causing cracking

Like SCC, the failure mode is not unusual with stressed polymers, perhaps because not enough is known about the phenomenon together with the increasing variety of fluids used in industry. Most manufacturers supply lists of compounds that can cause cracking, but the list is inevitably out of date as potentially harmful reagents are coming into contact with strained surfaces.

The effects are determined by the concentration of fluid in contact with the polymer surface and the degree of strain. There is usually a critical value needed to initiate cracking. A standard test used for testing potentially harmful reagents is the Bell telephone test where bent strips of polymer are exposed to the reagent under standard conditions.

What can be done about the problem? One solution is to coat the strained surface with a resistant polymer, another being to use a grade of higher molecular mass. The latter is always of greater strength, but will increase process costs owing to its greater viscosity.

One additional factor is the internal pressure from the water supply. Although it had not been measured directly, the specification of the fitting allowed for a pressure of up to 25 m head of water. The pressure p would have imposed a hoop stress σH on the wall of the fitting, but what magnitude would it have been? The pressure can be calculated using Equation (9), and the hoop stress with Equation (7), Box 19.

So

The hoop stress, the largest stress acting on the pipe wall from the water pressure, in the system is given by

This stress is small compared with the nominal tensile strength of acetal material, of about 70 MN m−2. Although moulding could reduce the strength somewhat – as seen in the case of the failed radiator – it was difficult to see how the combined effects of internal pressure and screw thread stress raiser could initiate crack growth, and ultimately, catastrophic failure.

7.2.3 Moulding features

What could have created the weld lines in stressed areas of the moulding? A detailed survey was made of new mouldings supplied by the manufacturer. They were smeared with graphite powder to highlight the flow and weld lines. There were considerable variations between the twelve mouldings examined (Figure 89). It was concluded that there was a pattern to the flow lines. The set had probably been made in two batches, each producing a different flow line pattern. Weld lines did exist, but were much less serious than in the failed sample. Moreover, the failed sample showed weld lines where none could be found on any of the new mouldings, especially in the critical threaded areas of the lower inlet pipe. The failed sample had been made in a different batch, and may even have been a maverick sample.

Figure 89: Six of the twelve new fittings compared to show different flow line patterns on the main part of the moulding. The patterns indicate that all six are similar, so the set was probably made in the same batch.

7.2.4 Stress corrosion cracking

One failure mechanism that has been known for many years is known as environmental stress cracking (ESC). Certain fluids can cause brittle cracking of plastic products, even at low imposed stresses. One of the first examples to be discovered occurred in polyethylene when exposed to strong detergent, such as an ionic soap known as Igepal. It was also the probable cause of an explosion in a compressed air line discussed in more detail in Box 20.

Another related failure mechanism is stress corrosion cracking (SCC), where the fluid interacts chemically with the polymer surface it contacts. One example was referred to earlier in Box 4, where radiator seals failed by attack from hot water in the central heating system. There were several subcritical cracks in the failed washers, a characteristic feature of both ESC and SCC failures. For attack to start, something must open the crack. It takes two main forms.

  • Small stresses or strains imposed on the system.

  • Frozen-in strain.

It is also known that the more aggressive the attacking agent, the lower the critical strain needed to grow cracks.

7.2.5 Acetal polymer

In its original form, when polymerized experimentally, polyoxymethylene was not a useful plastic. It easily degraded back to monomer, in this case formaldehyde. This could occur when simply heated, in an injection moulding machine, for instance. Two strategies were adopted to improve the stability of the material to make it suitable for products: copolymerization with another monomer, and end-capping every chain. Both versions of the polymer are available commercially, the failed fitting being made from the latter.

However, both the commercial polymers are susceptible to attack, especially by oxidizing agents. It was known that acetal could not be used in swimming pool plumbing, where chlorine levels can be high. Chlorine is a powerful oxidizing agent, which is why it is used for water purification – it attacks bacteria. Could the much lower levels present in potable water have attacked the material?

7.2.6 Problems in the USA

While the various expert reports were being digested by the several parties now in the action – university, plumbers, architects – one of the investigators happened to come across a report in a technical newspaper that suggested there could indeed be a problem with plastic plumbing materials. The article is shown in Figure 90, and is dated 18 March 1991. It reports the settlement before trial of a case involving domestic hot water pipes, where polybutylene pipes had been connected with acetal copolymer fittings. They had failed and caused flooding. The newspaper reported a class action, an action taken by the affected families against the manufacturers and suppliers of the materials. Both the pipe and fittings had suffered extensive cracking on their inner surfaces, a problem expert opinion considered to be caused by chlorine and dissolved oxygen in the water.

A further class action in Texas came to trial in 1992, and resulted in a large settlement for the numerous plaintiffs. This case (Babb v. US Brass, Shell, Hoechst-Celanese, DuPont) also attracted the attention of the expert acting on behalf of the plumbers in the UK case. Contact by the instructing solicitor with the American attorneys produced not only copies of the expert reports in the case, but also the transcripts of the trial – where the expert evidence had been tested in public. It was clear from those expert reports that even low levels of chlorine down to 0.3 parts per million in the water could cause serious stress corrosion cracking of acetal.

Figure 90: Article about a settlement in the USA by home owners against manufacturers and material suppliers (Chemical Reporter, 18 March 1991)

Such chlorine levels can occur in drinking water, so it seemed as though the problem of the cracked acetal fitting may have been caused directly by chlorine. A check with the local water company produced extensive analyses, both from 1993, and from when the fitting had failed in 1988. The records showed free chlorine was variable at different sampling points, but could rise as high as 0.9 parts per million. One reason for high levels was given as being caused by slugs of chlorine being added by the water company when there were accidental breaks in the pipes – a precautionary measure to prevent contamination.

7.2.7 Direct confirmation of SCC

A direct experiment was then carried out on the failed fitting using the EDAX facility of the scanning microscope. Analysis of the surface showed the presence of chlorine in significant quantities. Presentation of the new evidence to the other parties in the dispute produced a rapid settlement of the action, with the parties walking away from the action, and bearing their own costs.

The USA actions were appealed by the defendants, but were lost in the Texas Court of Appeal, and the many plaintiffs received full compensation. This was incidentally one of the largest class actions – outside automobile actions – in the US and ultimately cost the companies millions of dollars in damages awarded plus litigation costs.

7.2.8 Afterword

What did the failure at Loughborough University show? There appeared to be no widespread failures of similar fittings in the UK, so it would be reasonable to conclude that the particular fitting was indeed a maverick. Perhaps it was supplied by mistake, for the same reason that the faulty radiator reservoir was fitted to a new car in the previous case study.

The failure could be attributed to low levels of chlorine in the water supply, which led to rapid creep rupture at weld lines on the inner bore adjacent to the threads. The several cracks thus initiated eventually merged as they grew slowly. Crack growth probably slowed down as the closure stress was relieved by their formation. During this period, water leakage was slow and allowed deposits to grow on the fracture surfaces, helping to stem the leak. This helps to explain why the leak was not noticed, or if noticed, dismissed as insubstantial.

Some trigger over the weekend of 6–7 November 1988 allowed the crack to split open and the water damage resulted. Movement of the mains inlet pipe, or perhaps water hammer – a sudden surge in water pressure often caused by valve a closing or opening – could have provided the trigger (Figure 5, Paper 6).

The case shows how important it is to be aware of problems that may have occurred in other countries using the same material in a similar way. The USA case also revealed, in the expert reports, how the manufacturing companies had actually tested the materials with low levels of chlorine long before they were promoted as fit for water plumbing fittings. The laboratories in those companies had shown that levels as low as 0.3 ppm of chlorine could cause cracking and hence destroy the integrity of the materials. Why that information was not acted upon before they launched the plumbing fittings and pipe remains unknown.

Polybutene has been withdrawn from the USA market by the makers as a direct result of the US litigation, although acetal remains, and is widely used for fittings both in the USA and in the UK. Now, though, manufacturers should be fully aware of the possible problems that can ensue if low quality mouldings are supplied to users.

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