In this paper, the applicability of the constant life diagram (CLD) methodology for fatigue strengthening of metallic members is described . The CLD methodology is a method that uses the combined effect of alternating stress, mean stress and material properties to predict the material life-time under high-cycle fatigue loading. The method could be used most efficiently to improve resistance to crack initiation in ‘safe-life’ members, which is, in fact, a preventive fatigue strengthening approach. The method can be used to increase the fatigue safety margins of un-cracked metallic members. Furthermore, an innovative pre-stressed un-bonded reinforcement (PUR) system has been developed and patented. The advantages of the developed PUR system are: applicable to unsmooth surfaces (e.g., riveted or corroded), fast on-site installation (i.e., no glue and no surface polymer (CFRP) pre-stress level. The system can be easily disassembled and removed, if necessary, which makes it suitable for historical bridges as a temporary or permanent retrofit technique. Lastly, this paper presents details about fatigues strengthening of riveted girders in a 120-year-old railway (wrought iron) bridge using the PUR system such that the requirements of the CLD approach are fulfilled.