Today's KNOWLEDGE Share
During one of my recent BIMS-1 sessions I was asked to comment about surprising results obtained on notched Charpy tests when using “molded-in notches” vs. post-molding machined notches.
1. First of all molded pre-notched bars should be accurately measured since the effective notch radius will likely be somewhat smoothed out by the material shrinkage and warpage. If effective radius is larger, that will already shift the performance to an apparent better impact
2. Machined notches will typically create some damage on the machined surfaces which might further help initiating the brittle failures that we typically want to see in notched samples. This will likely reduce the apparent performance since crack initiation energy is essentially absent due to the machining pre-damage
3.The single most important reason for the discrepancies however is the fact that machining a molded sample means biting through the frozen skin and the outer layers created by the highest shear stresses:
4. In amorphous materials (where the discrepancy should be a bit less severe) the tip of the notch will be located in deeper layers showing a bit more “physical aging” from the process (lower cooling rate).
5. In semi-crystalline grades the discrepancies can be dramatic since one would typically find the machined notch tip located well below the unique morphology of the frozen skin. As a reminder, the frozen skin typically has a totally different mechanical response due to morphology and orientations
Never forget that notched IZOD or CHARPY are not “material properties” but just the quantitative output of a normalized test intrinsically sensitive to any sample or test variation.
source: Vito leo
