Fracture toughness
Measures a material's resistance to abrupt or steady crack growth. It indicates how much energy a cracked material can absorb before catastrophic failure.
A crack is like a notch, stress concentrates there. With tough material, the stuff at the tip flows a bit and absorbs energy. Only when stress gets too high does the crack move. With brittle material, that doesn't happen; it breaks without warning. When welding, you make life hard for yourself with residual stresses, notches, and hard zones in the heat-affected zone. There the structure is coarse and not tough. Damp electrodes or grease on the plate introduce hydrogen, which makes it even more brittle. And in cold, it gets really critical, then it breaks without deforming. So use materials suitable for the temperature, and don't make sharp transitions at the weld. Grind smooth the starts and edges. Watch out not to put in too much heat, or the grain gets coarse, but also not too little, or it gets hard. Electrodes and plate must be dry, and for critical parts, post-weld heat treatment helps to relieve the stresses.