 Many distinct factors influence the strength of welds and the material around them , including the welding method , the amount and concentration of energy input , the weldability of the base material , filler material , and flux material , the design of the joint , and the interactions between all these factors . To test the quality of a weld , either destructive or nondestructive testing methods are commonly used to verify that welds are free of defects , have acceptable levels of residual stresses and distortion , and have acceptable heat @-@ affected zone ( HAZ ) properties . Types of welding defects include cracks , distortion , gas inclusions ( porosity ) , non @-@ metallic inclusions , lack of fusion , incomplete penetration , lamellar tearing , and undercutting . 
