Issue 9

Z.H. Qian et alii, Frattura ed Integrità Strutturale, 9 (2009) 105 - 112; DOI: 10.3221/IGF-ESIS.09.11 110 Fig. 8 shows that the cracks developed at the RDDP detail are not typical root crack opening mechanisms. The practice has been to use normal stress, at the bottom of the deck plate, facing the root, assuming a category D (in AASHTO). This detail may, in cases of simple beam, be cycled in compression, the question of the rate of propagation of this detail calls for answers, as does the case for a cantilevered deck. F ATIGUE IMPROVEMENT TECHNIQUES igh tensile residual stress and initial defect exist in as-welded joints in the welded region as a result of the welding process. Advanced technology can reduce defects in weld, while a great benefit can be realized if compressive residual stresses are introduced. In general, fatigue improvement methodologies can be categorized as follows: (1) introduce beneficial compressive stress; (2) reduce stress concentration; (3) remove defects in components; (4) increase the rigidity in the connection. A summary of the various improvement techniques are presented in Fig. 9. Figure 8 : Fatigue cracks at RDDP connection (Camo, 2008). Traditional peening Peening is conventional and economy treatment method to improve the fatigue strength to welded connection. It includes various peening, such as shot peening, hammer peening, needle peening, fluid bed peening, ultrasonic peening, laser peening. Among of these, shot peening is most widely studied and applied. The effectiveness of shot peening is affected by many variables, the control of which are cumbersome and impractical, therefore only two parameters are used to specify the process, Almen intensity and coverage [21]. The major advantage of shot peening is that it covers area at low cost, however, care must be taken to ensure that the shot size is small enough to reach the bottom of all undercuts and weld inter-pass notches. It was reported by Maddox that an increase of 33% in the fatigue strength at two-million cycles of joints with longitudinal attachments and fabricated from 260 and 390 MPa yield strength while the improvement was 70% for higher strength QT steels. Fluid bed peening (FBP) FBP technique is relatively novel treatment to coat metal substrates, change the surface properties, and induce micro- structural changes. Specimens in fluid bed machine are kept in a fixed position on the inner bed, are subjected to strikes from abrasive grains driven by the fluid onto their surfaces. The investigations by Barletta et al. [22, 23] showed a progressive change in both the surface topography of the metal (roundness, roughness and aesthetic aspect) and in superficial properties (surface hardness, residual stress, density of the dislocations) can be induced. Furthermore, progressive smoothing of specimen surface associated with remarkable material removal can be expected, even at low temperature (Fig. 10). After FBP treatment the delay of crack initiation and crack propagation can be expected. The investigations showed as well peening time and alternating stresses were the only two important experimental factors. Therefore, FBP technique is more ease to control because less operational parameters are demanded. H