why is it that holes & notches in engineering componet are more dangerous in brittle than in ductile materials

Answer 1

Fatigue failure is caused by crack propagation.
Cracks starts as microcracks, often not visible to the eye. They grow because of a stress concentration at the tip of the crack.

This growth occurs more readily in brittle than in ductile materials. Also, structures made of ductile materials are better able to distribute loads among many support points by developing tiny zones of plastic deformation. In a brittle material, the tendency is to load each support point to failure, then the next most constraining, etc, so that all of the support points fail sequentially at a fraction of their total load capacity.

Any machining operation that leaves a sharp inside corner, such as a notch, is an excellent initiator for crack growth. However, even a round hole can leave crack initiation sites. Also, in a heavily stressed component, the reduced area that results when a large hole is put in can cause higher stresses in the remainder of the material.

These effects are usually expressed by a "stress riser" or "concentration" factor that expresses the local increase in stress. Although they can be complicated to calculate, roughly, they range from around 2 or 3 for round holes of modest size to 10 and more for large holes or sharp holes. When the nominal stress multiplied by the concentration factor approaches the yield stress of the material, you can expect crack growth and fatigue failure to be a significant failure mechanism.

Answer 2

Because they cause stress points. Stress here loads the material with forces greater than those on the rest of the component. For example, the perforations on toilet paper

Answer 3

Stresses tend to concentrate where the holes and notches are.

Answer 4

becoz as its brittle already holes and notches would weaken it even more