Stress concentration or the stress will be maximum at the joints such as T joints and L joints.
This results in fracture of materials easily. This is the reason whenever a material is manufactured with complex shapes the manufacturer converts the T and L joints of material to curved shape.
This curved shape in material reduces the chances of fracture.

It is the concentration of stress at a particular point or region such as holes sharp edges corners etc. This is due to the reduced cross section area.
As we know
stress force / area........
The above equation indicates that stress is inversely proportional to area i.e as area decreases stress at that point or region increases and vice versa.

Hence due to smaller cross section area the stress gets accumulated or is generated more hence materials fail or more likely to fail at these points.