Using linear elastic fracture mechanics concepts, two steels quenched and tempered to Rockwell C 40-47 were evaluated for application in crawler tractor main frames. These high strength steels were D6-B and an experimental steel whose chemistry was optimized using a digital computer procedure referred to as the CHAT system, an acronym standing for Computer Harmonized-Application Tailored. Fatigue crack growth rate da/dN versus ΔK for constant amplitude loading and plane strain fracture toughness KIc parameters were determined from compact specimens. These parameters together with approximate stress intensity calculations were utilized to estimate component fatigue crack growth life assuming various initial crack sizes. These estimations were compared with accelerated component fatigue life, which included both crack initiation and crack growth.