Because of the high stress levels experienced by the spokes, surface flaws which act as stress 'concentrators' can decrease the average stress that a spoke can stand before fracture. For an elliptical crack s_max = 2 s(c / r )^0.5 where 2c is the major axis length and r the crack tip radius of curvature. For a 'typical' crack, s_max is about three times the average stress

If the material is brittle a crack will propagate rapidly when the fracture stress is reached. Most metals have some ductility and there is a region of plastic deformation in front of the crack which makes the crack tip less sharp and increases the work required to form the crack. This process is illustrated in the lower parts of the diagram. The joining of voids formed in front of the crack in the ductile case is known as 'ductile tearing.'

From: Ashby & Jones, "Engineering Materials 1,"
Pergamon (1986)

Ductile tearing consumes a lot of energy making the material "tougher." Toughness is measured by the work done to cause ductile fracture.