Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16; DOI: 10.3221/IGF-ESIS.07.01 8 core: plastic strain amplitudes and cyclic creep were drastically decreased by shot peening. Furthermore, fatigue crack growth rates are markedly reduced by this mechanical surface treatment. Remarkably, as illustrated in Fig. 6, the initial residual stress profile and surface strain hardening were not completely eliminated even by applying high cyclic stress amplitudes [62]. Figure 6 : Residual stress depth profiles of shot peened (Almen intensity 0.175 mmA) AISI 304 before and after cyclic loading with MPa a 320 = σ after N f /2 cycles [62]. Elastic modulus Elastic modulus is another improved characteristic of SP surface crystallized specimens studied by some researchers [44, 88] . In an SMAT experiment conducted on annealed commercially pure titanium, the results demonstrated that maximum value of the elastic modulus at the top surface showed an increase of about 16% in comparison with the untreated sample. The SMAT was performed in air at room temperature with stainless steel balls 3 mm in diameter, for 30 min, at a vibration frequency of 20. The results seem to indicate that the increase of modulus from the bulk to the surface follows the grain refinement and as it can be seen in Fig. 7 the elasticity modulus decreases as a function of distance from the treated surface [88]. Figure 7 : Reduced modulus versus the distance for the sample surface for a load P=10 mN [88]. In another experiment performed on commercial brass, samples were sand blasted under a blasting pressure of 300 kPa for 10 min to produce nanocrystalline surface layers. The sand flow rate was 5 gs -1 . The samples were then annealed at 150, 250, 350, 500 and 600 C, respectively, for 1h and cooled in air to obtain different grain sizes in the sandblasted surface layer. The nanocrystallization resulted in improved elastic behavior. It is said that this happened because the elastic limit or the yield strength was increased when the dislocation motion was retarded by grain boundarie s [44]. -600 -500 -400 -300 -200 -100 0 100 0 50 100 150 200 250 300 350 Depth (micrometer) Residual stress (MPa) Fatigued Initial State 0 20 40 60 80 100 120 140 160 0 100 200 300 400 500 600 700 Distance from the surface (micrometer) Reduced Modulus (GPa)