In　Cu/Cr　multilayers　with　modulation　period　(gamma)　ranging　from　10　to　250　nm,　maxima　are　observed　for　both　tensile　ductility　and　fracture　at　a　critical　lambda　similar　to　50　nm,　different　from　the　monotonic　lambda　dependence　known　for　monolithic　films.　This　unusual　behavior　is　explained,　via　quantitative　assessments,　based　on　micromechanical　model,　by　considering　the　competing　thickness　effects　on　the　size　of　the　microcracks　Initiated　ill　the　Cr　layers　and　oil　the　role　of　the　ductile　Cu　layer　in　blocking　crack　propagation　(C)　2009　Acta　Materialia　Inc　Published　by　Elsevier　Ltd　All　rights　reserved