Objective　Ultra–high–performance　concrete　(UHPC)　is　widely　used　in　long-span　bridges,　airport　pavements,　and　explosion-resistant　structures,　which　are　often　subjected　to　high　strain　rate　impact　loads　during　service.　Given　that　the　tensile　properties　of　concrete　are　generally　weaker　than　its　compression　properties,　and　tensile　failure　is　a　main　damage　mechanism　in　concrete,　it　is　crucial　to　investigate　the　tensile　properties　of　UHPC　under　impact　splitting　tensile　load.　Currently,　research　on　UHPC　primarily　focuses　on　materials,　mixing　ratios,　and　mechanical　properties　under　static　load,　with　less　focus　on　dynamic　impact　and　microscopic　damage　mechanisms.　Methods　Digital　image　correlation　(DIC)　is　an　innovative　optical　detection　technology　that　monitors　the　deformation　and　displacement　of　a　test　object＇s　surface　by　collecting　grayscale　digital　images　and　analyzing　the　changes　over　time.　A　split　Hopkinson　pressure　bar　is　utilized　to　conduct　dynamic　tensile　tests　at　different　strain　rates.　The　damage　and　crack　development　of　the　specimen　were　analyzed,　and　the　crack　growth　rate　is　measured　and　calculated　using　DIC　technology.　The　UHPC　material　used　in　the　experiment　is　steel　fiber　ultra–high–performance　concrete.　The　experiment　utilized　a　100　mm　large　diameter　split　Hopkinson　compression　rod　as　a　dynamic　loading　device,　Different　strain　rates　were　applied　by　adjusting　the　impact　air　pressure,　with　three　repeated　specimens　are　selected　under　different　air　pressures　to　minimize　errors.　Based　on　one-dimensional　stress　wave　theory,　the　stress,　strain,　and　strain　rate　of　the　specimen　under　dynamic　impact　splitting　tensile　load　were　calculated　from　the　stress　wave　data　collected　by　strain　gauges　on　the　incident　and　transmission　rods.　This　research　explores　the　changes　in　failure　mode,　dynamic　compressive　strength,　dynamic　elastic　modulus,　dynamic　dissipated　energy,　and　damage　mechanism　of　UHPC　under　different　strain　rates,　providing　theoretical　support　for　the　service　performance　and　damage　assessment　of　UHPC.　Results　and　Discussions　The　failure　modes　of　UHPC　specimens　under　different　impact　strengths　show　that:　as　the　strain　rate　increases,　the　degree　of　fracture　in　the　specimens　becomes　complete;　the　addition　of　steel　fibers　results　in　a　significant　difference　in　the　impact　fracture　morphology　between　UHPC　and　ordinary　concrete.　From　the　fracture　surface,　it　can　be　observed　that　there　are　numerous　concrete　fragments　bridged　by　steel　fibers　and　pulled-out　steel　fibers　on　the　fracture　surface,　indicating　that　the　sources　of　UHPC　impact　damage　include　the　failure　of　the　UHPC　concrete　matrix　and　the　pulling　out　or　breaking　of　the　steel　fibers.　The　splitting　and　tensile　failure　process　of　UHPC　under　different　impact　strengths　was　captured　by　a　high-speed　camera.　Under　impact　pressure,　the　splitting　crack　first　develops　at　the　center　of　the　specimen　and　gradually　extends　toward　both　ends　of　the　loading　end.　The　degree　of　expansion　of　the　main　crack　increases　with　the　continuous　increase　in　impact　strength.　Simultaneously,　some　secondary　cracks　develop　near　and　parallel　to　the　main　crack　as　the　main　crack　expands,　demonstrating　the　bridging　effect　of　steel　fibers　in　the　damage　process　of　UHPC　under　impact　load.　The　strain　cloud　maps　of　specimens　under　different　impact　strengths　and　the　curves　of　the　main　crack　width　of　specimens　under　different　impact　strengths　as　they　expand　over　time　were　analyzed　based　on　DIC　technology.　The　above　DIC　analysis　explains　that　the　higher　the　impact　strength,　the　faster　the　crack　development　speed,　and　there　are　distinct　differences　in　the　damage　characteristics　of　specimens　under　different　impact　strengths.　The　stress-strain　curves　of　UHPC　under　different　impact　loads　show　that:　the　peak　strain　rate　during　the　impact　process　of　the　specimen　increases　with　the　increase　in　impact　load.　As　the　peak　strain　rate　increases,　the　dynamic　tensile　strength　(i.e.,　stress　peak)　of　UHPC　also　increases.　At　high　strain　rates,　the　dynamic　tensile　strength　of　the　concrete　specimens　is　significantly　higher　than　the　quasi-static　tensile　strength.　Unlike　ordinary　concrete,　which　exhibits　splitting　failure　after　the　stress　reaches　its　peak,　the　rate　of　stress　reduction　in　UHPC　gradually　slows　down,　and　a　platform　segment　appears.　This　phenomenon　demonstrates　the　bridging　effect　of　steel　fibers　in　the　damage　process　of　UHPC　under　impact　loads.　By　calculating　the　dynamic　improvement　factor　(Dft)　under　different　strain　rates　and　comparing　it　with　the　concrete　Dft　curve　provided　by　existing　research,　it　is　evident　that　the　strain　rate　sensitivity　of　UHPC’s　ultimate　tensile　strength　is　higher　than　that　of　ordinary　concrete.　Based　on　the　law　of　conservation　of　energy,　the　dissipation　energy　and　energy　dissipation　ratio　of　UHPC　under　different　strain　rates　are　calculated.　The　results　showed　that　as　the　strain　rate　increased,　the　dynamic　dissipation　energy　of　UHPC　also　increased;　the　energy　dissipation　ratio　shows　a　trend　of　first　increasing　and　then　decreasing　with　the　increase　in　strain　rate.　Conclusions　The　research　results　indicate　that:　1)The　impact　resistance　of　UHPC　is　significantly　enhanced　compared　to　ordinary　concrete.　UHPC　exhibits　high　strength　and　strong　deformation　capability　under　impact　loads,　and　its　performance　in　dissipating　impact　energy　is　superior.　There　are　two　ways　for　UHPC　to　dissipate　impact　energy:　one　is　based　on　the　failure　of　the　concrete　matrix,　and　the　other　is　the　pulling　out　or　breaking　of　the　steel　fibers.　2)As　the　impact　strength　increases,　the　degree　of　failure　of　UHPC　gradually　increases,　and　the　extension　of　the　main　crack　along　the　center　of　the　specimen　continues　to　increase　until　the　specimen　completely　fractures.　Except　for　the　main　crack,　the　development　of　secondary　cracks　is　significant　during　the　failure　of　UHPC　with　strain　rates　in　the　range　of　60　to　80　s−1;　when　the　strain　rate　is　higher　than　100　s−1,　the　secondary　crack　development　of　UHPC　is　weaker.　3)The　dynamic　ultimate　tensile　strength　of　UHPC　increases　with　the　increase　in　strain　rate.　The　strain　rate　sensitivity　of　UHPC’s　ultimate　tensile　strength　is　higher　than　that　of　ordinary　concrete.　As　the　strain　rate　increases,　the　dynamic　dissipation　energy　of　UHPC　also　increases,　but　the　energy　dissipation　ratio　experiences　a　trend　of　first　increasing　and　then　decreasing.　4)These　analysis　results　fully　demonstrate　that　UHPC　is　still　a　high-performance　concrete　material　suitable　for　resisting　impact　loads　and　dissipating　impact　energy　under　dynamic　splitting　and　tensile　loads.　©　2024　Sichuan　University.　All　rights　reserved.