I really wondered internal stress and I tried to your method which you have tried to see internal stress as well.
-400x100x10mm - Material is "steel" from Simsolid Generic Materials Library.
All connections are "bonded"
-I applied 750 N to each notch
-Here is the distribution of Stress from front view. There is a bit different.
-Here is the internal stress graphic from inside surface
-This result from non-split side. As for me, both results are almost the same.
2) Another work, on the same condions , you can see the distribution of stress . There is a bit difference.