= Solution
\b[The characteristic effect is an irradiation-induced upward shift of the <ductile-to-brittle transition> in ferritic <steel>.] A component previously used above its transition can then fail by <cleavage fracture> at the same service <temperature>, with much less warning from plastic deformation.
<Ferrite> has a <body-centered cubic lattice>. At low <temperature>, <dislocation glide> becomes difficult enough that brittle crack propagation can precede substantial plastic relaxation. <Irradiation hardening> raises the <yield strength> by introducing obstacles to <dislocation glide>; the competition with <cleavage fracture> consequently shifts toward brittle failure. <Austenite>, with its <face-centered cubic lattice>, does not normally show this same sharp cleavage transition. This does not make austenitic <steel> immune to loss of <ductility>, <helium embrittlement> or other <radiation damage in materials>.
Examine the fracture surface for <cleavage fracture> facets and river patterns, and compare <Charpy impact test> energy-versus-<temperature> curves from exposed material and an unirradiated reference of the same composition and heat treatment. The reference is important: a low impact energy from one failed specimen cannot establish an irradiation-induced shift. Transition curves can also use lateral expansion or the proportion of ductile fracture appearance.<NIST Software Package for Obtaining Charpy Transition Curves by Enrico Lucon and others>[§2, Background: absorbed energy, lateral expansion, shear fracture appearance and transition curves]{ref}
A direct <fracture toughness> test over the transition region complements the <Charpy impact test>; the two measurements are not interchangeable. Combine these observations with exposure history, local <temperature>, <microstructure> and crack location. Brittle appearance supports the failure mechanism, while a measured transition shift and suitable controls support its attribution to <irradiation>.
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