The O-ring typically fails in its application because of the combined adverse effects of several environmental factors.This article shows and explains them.
Homologous recombination - Wikipedia
Causes for O-Ring Failure
How to Diagnose a Damaged O-Ring and Determine the Root Cause
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How to Recognize and Avoid the Common Causes of O-ring Failure
Troubleshooting O-Ring Failures
A Beginner's Guide to O-ring Troubleshooting
Discover Problems Early On How to Detect Failing O-Rings
A Beginner's Guide to O-ring Troubleshooting