According　to　the　observation　of　fatigue　fractography　for　the　spot-welded　joints,　cracking　behavior　was　modeled　for　specimens　of　different　dimensions　using　3D　FEA　of　a　progressively　growing　crack　that　began　at　the　joining　surface,　progressed　to　the　outside　surface,　and　finally　broke.　The　natural　frequency　ratio　of　experimental　and　modeled　situations　displayed　identical　nonlinear　damage　behavior　for　different　dimensional　specimens.　Thus,　a　new　damage　variable　was　proposed　to　measure　fatigue　damage　using　natural　frequency　nonlinear　behavior,　and　a　nonlinear　fatigue　damage　model　was　developed.　The　results　showed　that　this　model　can　successfully　describe　fatigue　damage　for　spot-welded　joints.　(C)　2008　Elsevier　Ltd.　All　rights　reserved.