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   Milwaukee Area Lab
N144 W6466 Pioneer Rd
Cedarburg, WI 53012
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Tubing clamp failure
A 304 stainless steel tubing clamp prematurely failed in a residential installation, causing a leak and water damage. Synergy was hired to determine why the clamp failed. The clamp is designed to secure cross-linked polyethylene tubing around a barbed insert fitting to make a pressure tight seal in a potable water system. It’s installed with a special tool that crimps the pinch points of the clamp together. The subject clamp was first examined under the stereo microscope. In addition to corrosion deposits on certain areas of the clamp, extensive secondary cracking was found next to the main fracture, as well as cracking on other areas of the clamp. Click on the photos below to see full size images of these features photographed in the microscope.

The fracture surfaces were then examined under high magnification in the scanning electron microscope (SEM). An image of the fracture surface is shown below. The surface has facets with river-like branched steps or ledges, indicating a cleavage type brittle mode of fracture.
When the clamp was in the SEM, energy dispersive spectroscopy (EDS) was done on the clamp base meatal and corrosion products. One of the elements detected in the corrosion products was chlorine. The failure was determined to be caused by Stress Corrosion Cracking (SCC), which results from the combination of a particular environment, tensile stress in the part, and an alloy that is susceptible to SCC. The clamp was made from stainless steel type 304, which is well documented as being particularly susceptible to SCC in chloride environments, including an evaporated concentrations in the air. In this case, the clamp had been installed inside a wall on piping that fed a bathroom sink. It was later discovered that the residence also contained an indoor swimming pool, which is a competent source of evaporated chlorides in the ambient air.         
 

 


  

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