Design Support

An industrial shower head was developing cracks after several weeks within the field. Stress analyses were performed in order to determine the systems response to loading. Analyses were performed in order to calculate the stress due to assembly, as shown below the assembly stress is significant for long term use. However some of the heads were failing much earlier than predicted using just the assembly stress.

Next the stress caused by thermal expansion of the system was evaluated. The design engineers that developed the product discounted thermal stress as an issue since the plastic material (ABS) has a much greater coefficient of thermal expansion than the steel pipe. 

What the design engineers failed to recognize is that the pipe heats up much faster than the plastic housing, therefore even with the lower coefficient of thermal expansion, the steel expanded increasing the housing assembly stress by at least another 800 psi.

In order to simulate this, MCS performed a transient thermal analysis on the system using the heat input from flowing water at 140 deg F. The results were studied and the temperature profile when the pipe reached 140 deg F was used as input for calculation of the thermal stresses.

Even with the added thermal stresses, the housing should not have failed so soon.

The odor of cleaning solvent was detected around the failed area. Upon visual inspection of the crack surfaces, MCS determined that the mode of failure was environmental stress cracking. The ABS material was reported to have good resistance to the cleaner, however as with most environmental stress cracking, the stressed component degradation was accelerated by the environment. 

The design was modified to become more compliant and bring the stress levels to acceptable values when exposed to the cleanser.

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