## 4 Maintenance Schemes  
### Hammer Peeting Strengthening  
Under combined vehicular loading and welding-induced residual stresses, DU welds are subjected to sustained tensile stress states, rendering them particularly susceptible to fatigue crack initiation and propagation [5; 27; 28; 29]. The presence of diaphragm deformation has been demonstrated to exacerbate these tensile stress conditions, thereby significantly elevating the fracture risk at critical weld locations. While conventional remedial measures, including stress-relief hole drilling and notch geometry modifications, have been implemented in operational bridges, these post-cracking interventions are inherently limited by their structural invasiveness and may potentially compromise the original load-bearing integrity. Consequently, the development of non-destructive, rapid implementation preventive strategies has been identified as a critical research priority. To establish a scientifically grounded preventive framework, the principal stress distributions in the DU welds were analyzed, as illustrated in Figure 11. The maximum principal stress was consistently observed to manifest as tensile stress oriented perpendicular to the weld axis, with characteristic angular deviations. Furthermore, as established in Section 3, diaphragm deformation induces measurable stress intensification at DU weld locations. Based on these mechanistic insights, the strategic application of residual compressive stresses through controlled surface treatment techniques was proposed as an effective countermeasure. This approach was designed to neutralize deformation-induced tensile stress components through targeted stress state modification, thereby enhancing fatigue resistance while maintaining structural continuity. Hammer peening represents a non-destructive surface treatment technique that has been demonstrated to effectively delay fatigue crack initiation through two synergistic mechanisms as follows: (1) the enhancement of surface material densification and (2) the introduction of beneficial compressive residual stresses. When applied to DU welds, this technique induces compressive stress fields that modify local stress distributions, thereby extending fatigue life. In the present study, a three-pass hammer peening protocol was implemented at deformation-affected DU weld locations to ensure optimal treatment quality. The treatment parameters were carefully controlled, with a 50 mm peening length along the weld axis, 90 Hz impact frequency, and 5 \(\times\) 5 mm hammer head contact area. The operational stability was maintained by limiting the hammer peening height to below 5 mm during all procedures. The hammer peening process was systematically executed through three distinct stages, as illustrated in Figure 12[11]. The treatment sequence was designed as follows:  
Figure 11: Maximum principal stress distribution of DU weld. (a) vertical impacts were applied to the diaphragm surface, generating localized plastic deformation at the weld-diaphrag intersection zones; (b) subsequent vertical impacts were directed at the weld surface, producing continuous depressed plastic deformation profiles along the weld-diaphrag interface; (c) the final impacts were concentrated at the weld-diaphrag intersection regions to achieve uniform surface deformation profiles and ensure geometric continuity. This sequential approach has been shown to optimize stress redistribution while maintaining structural integrity.