Why do threaded fittings have lower pressure ratings then socket fittings?

When a pipe or fitting is threaded the wall thickness is decreased. Also, most plastics are notch sensitive, which basically means it will have a greater tendency to break in places that have been notched. When a pipe or fitting is threaded (notched) it will lose a significant percent of its strength.  

See link below for pressure ratings.

 

http://www.usplastic.com/pdfdatafiles/drawings/26300pchart.pdf

Article Title:
Why do threaded fittings have lower pressure ratings then socket fittings?
Article Summary:
Threaded connections have lower pressure ratings than socket connections.
Article Date:
1/29/2008
Article ID:
804
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