Most plastic pipe fittings are made of polyvinyl chloride, which is a thermoplastic polymer. This type of pipe fitting is popular in the plumbing industry because it is low cost and is easy to install. The problem with this type of fitting system is that does not bear loads in the same way that metal fittings do. Metal fittings either successfully bear a load or they will fail. Plastic fittings can bear a load at the moment of initial application but, over time, they may fail as cracks initiate and slowly grow. When this happens, the fittings can fail and lead to water leaks. Another problem with plastic fittings is that failure can come from accumulated fatigue damage and this can reduce the burst strength of the fitting. When this type of plastic fitting fails, it can be very costly. If a failure occurs, the entire fitting system will probably need to be replaced and the cost is generally thousands of dollars. In general, failures of pipe fittings usually result from design defects. To prevent this type of failure in pipe fittings, it is usually necessary to use a more robust fitting to reduce or eliminate the stress that causes the fitting to fail. Sometimes fatigue failure of a plastic pipe fitting can show up through a crack in the elbow. When this happens, the pipe will leak. This type of fatigue failure of a plastic pipe fitting is caused when cyclic stresses are applied to the elbow from vibrations or stresses created by the thermal expansion of the attached pipes when hot water flows through them and heats them up. In this case, metal fittings should have utilized to avoid this problem.