The difference between ductile and brittle fracture is that in ductile you get necking (significant reduction of cross section and elongation) of the specimen before breaking in the tensile test. the break is typically cuspate where the specimen is pulled apart by longitudinal shear forces. Breakage is gradual.
In a brittle fracture there is little or no necking, and the fracture is often diagonal across section due to fracture by diagonal shear forces. The fracture surface appears microcrystaline. Breakage is sudden.
Brittle fracture is a cause for concern because you get no warning before it occurs.