To　investigate　the　morphological　changes　and　interaction　mechanism　of　multi-defects　on　coplanar　and　dissimilar　surfaces　in　the　crack　expansion　process　in　the　weld　toe　area　of　steel　bridge　panel　butt　welds,　the　basic　weld　configuration　of　the　simplified　butt　weld　under　the　flange　of　the　U-rib　of　the　steel　bridge　panel　is　used　as　the　research　object　to　explore　the　stress　intensity　factor　KI　distribution　in　the　weld　area　of　the　butt　weld,　and　based　on　verifying　the　reliability　of　ABAQUS　and　FRANC3D　numerical　simulation　methods.　Multi-crack　expansion　analysis　of　the　weld　toe　area　implanted　coplanar　or　heterogeneous　multi-crack　shows　that:　for　the　residual　height　butt　weld　in　the　axial　tensile　load,　there　is　a　stress　intensity　factor　factor　KI　peak　point　near　the　weld　toe　line;　for　adjacent　coplanar　cracks　in　the　process　of　expansion　to　critical　depth,　there　are　three　typical　stages　of　expansion　before　crack　fusion,　during　crack　fusion　and　after　crack　fusion;　different　stages　of　crack　front　morphology　and　expansion　rate　da　/　dN　changes　can　be　reflected　by　the　distribution　of　the　equivalent　force　strength　factor　magnitude　ΔKeff　in　the　crack　front;　before　the　fusion　of　coplanar　cracks,　the　crack　interaction　ratio　factor　of　a　single　crack　near　the　end　point　of　the　long　axis　of　another　crack　is　negatively　correlated　with　the　net　spacing　s1　to　the　crack　depth　a　ratio　in　a　power;　the　crack　at　the　toe　in　the　butt　weld　heterogeneous　crack　has　an　inhibitory　effect　on　the　heat　affected　zone　crack　with　different　crack　size.　With　the　cumulative　increase　of　crack　size　difference,　the　inhibitory　effect　of　crack　at　toe　on　heat-affected　zone　expands　linearly,　and　finally　makes　the　heat-affected　zone　crack　ΔKeff　lower　than　the　threshold　value　ΔKth　and　inactivates　and　stops　expanding;　and　the　above　inhibitory　effect　decreases　with　the　increase　of　net　spacing　s2　on　the　opposite　side.　©　2024　Editorial　Office　of　Railway　Standard　Design.　All　rights　reserved.