Failure of PVC Pipe
Note: the following analysis and its write-up are property of The Madison Group and cannot be copied and/or distributed in anyway without prior permission from The Madison Group. This analysis in no way suggests that any or all plastic pipe failures occur in the manner described. Each plastic failure is unique and should be treated as such.
Plastic pipe, tubing and other profiles are an extremely popular alternative to copper, steel, aluminum and other materials. In fact, by 2003 it is predicted that 33% of all US pipe production will be made with plastic. Plastic pipes, tubing and profiles are used in a wide variety of industries including, building & construction, automotive, consumer goods, lawn & garden, windows & doors, furniture, plumbing and electrical. One of the most widely used materials for these products is polyvinylchloride or commonly known as PVC. This material is popular in these industries because of the wide range of properties that can be obtain depending on the additives that are mixed with it. PVC can be made to have high strength, rigidity and hardness; good electrical properties; high chemical resistance and be self extinguishing - all this at a relatively inexpensive price. However, depending if a plasticizer additive is used with the PVC, along with what kind and how much, the characteristics of the final part can be dramatically altered to have high impact strength with relatively low hardness and rigidity.
An unplasticized PVC pipe, shown above, is quite rigid with high strength and good chemical resistance. These properties make it attractive for use in above or below ground plumbing applications. However, a very important change in property occurs as the temperature gets colder - the impact strength of PVC drastically changes for the worse. This means that at low temperatures the ability of PVC to dissipate the energy from a sudden blow is limited and may result in part failure. The best way to describe this phenomenon, apart from demonstrating the impact of several PVC pipes at different temperatures, is to graph the impact strength of PVC as a function of temperature, shown in the figure to the right.
The most interesting part of this graph, the area that can explain many plastic failures, is boxed out in gray. Here, you will see a dramatic decrease in impact strength as the temperature gets colder - the part is becoming increasingly more brittle. The impact strength is 4x less at -10oC than at 20oC - a temperature range that is easily experienced in many regions of the US. This phenomenon is one that is not seen with every day metals and is commonly overlooked when designing with plastics.
One can improve this situation by using additives, in this case a plasticizer, that ultimately moves the graph to the left and gives the part a high impact strength at a much lower temperature. However, the gain in one property usually means the loss of other properties, in this case, the loss of stiffness. The figure to the right shows the modulus (stiffness) of PVC as a function of temperature (solid line). The dashed line indicates the temperature at which the modulus will decrease dramatically, approximately 50oC for this PVC. For many uses, 50oC is a temperature that the product would never experience, however, if an additive is used to increase the impact strength (as described above) then this graph will also move to the left lowering the temperature that the stiffness is lost.
Thus, a compromise must be made for how much, if any, additive is to be used for the application and environment in which the product will be used. In the case of PVC pipe, high mechanical strength, rigidity, hardness and high chemical resistance is required at the lowest cost. Plasticizing additives typically add to the cost of a product and are not used in pipe production. Other additives can reduce costs. Unfortunately, these cost reducing additives, such as calcium carbonate, typically make the product even more brittle causing the impact strength graph shown in the first graph to move to the right, making the product more brittle and more susceptible to failure.
An example of a failed PVC pipe is shown in the figure below.
To determine the cause of failure a variety of techniques can be used:
- Visual Inspection of the Failed Part
- Structural Finite Element Analysis
- Dynamic Finite Element Analysis
- Material Evaluation
- Process Evaluation
A visual inspection of the part indicates that this was a brittle failure as opposed to a ductile failure. Many brittle failures occur very quickly, whereas, ductile failures will typically occur over a longer period of time. It was revealed that the pipe was in a cold condition of approximately -5.0oC. The pipe was in an environment where the temperature was low enough that it became very brittle. A force, which could be caused by an external blow or from internal pipe pressure, became too great and the part failed catastrophically. The cause of failure may not be because the engineer specified the wrong pipe for the job or environment, but because the formulation of the material or the processing conditions at the production plant were wrong.
To determine if the pipe had the correct formulation a wide variety of material tests can be performed. One such test is the thermogravimetric analyzer (TGA). This device is often used to identify the components of a plastic part. It works by gradually heating a small sample of the plastic to a very high temperature. At different temperatures the compounds of the plastic will decompose. The TGA accurately records the change of weight with respect to the temperature. The figure to the right shows an example of a TGA test on PVC sample. Here, the decomposition of the different compounds can be seen along with the percent weight loss. Using data from an extensive library, the decomposition peaks are matched with known materials that decompose at the same exact temperature.
To establish the mode and forces of failure, along with providing confidence that a failure took place in the manner that was determined, a finite element analysis (FEA) can be made. This type of analysis allows the engineer to place the part in a realistic environment under normal to extreme conditions and observe what happens to the part - if failure occurs. The animations below show the predicted failure of a pipe caused by an extreme internal pressure using FEA.