I performed a nick break test on a tee fillet today that had been MIG welded. To my eyes, it looks like there is zero penetration (although it took some whacking to break it following sawing 1-2mm in to the bead).
Here’s the result;
https://photos.app.goo.gl/5DvcNRxit2Fo6cfd9
Here’s the result;
https://photos.app.goo.gl/5DvcNRxit2Fo6cfd9