Torispherical and semi ellipsoidal heads
The big stresses in dished heads due to internal pressure are always in the knuckle region.  Basically the knuckle section acts like a pivot between the cylindrical shell and the domed section and is therefore subject to bending stress, compressive on the inside and tensile on the outside.  The domed section is considered as a sphere subject to membrane stress only.

Hemispheres
Are subject to pure membrane stress throughout, that’s why they always work out thinner than other head profiles.

External Pressure
If the head is subject to external pressure or vacuum the dome section is likely to collapse.  

Hope this answers your question

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For other than spherical, there is a predominant compressive stress in the knuckle. To visualize this...

Inflate an inflatable toy (a pool doll) and observe what happens in the juntion of a cylinder and a head. (like a leg of the toy) there are wrinkles in the "knucles" that helps you to understand this compressive stress. This is the same that you would see on an actual destructive test.