PSI - Issue 17

C.A.R.P. Baptista et al. / Procedia Structural Integrity 17 (2019) 324–330 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction According to Schnubel et al. (2012), even though the damage tolerant requisites must be met without considering the beneficial effects of crack growth retardation techniques, experience shows that their adoption contributes to reduce the maintenance costs of aircrafts. Among the emerging methods, laser shock processing (LSP) encompasses surface modification techniques aimed at enhancing the resistance to wear, corrosion and fatigue of structural alloys. Two forms of LSP treatments are basically adopted in order to generate compressive residual stresses: i) the conventional process, consisting of high energy (<100J) and long duration (100ns) laser pulses combined with a protective layer over the target material; and ii) lower energy (a few joules or less) pulses with higher overlapping and without ablative layer, usually designated as Laser Shock Peening without Coating (LSPwC). Bergant et al. (2016) put forward that, despite the conventional treatment has proved to be a competitive process, there are still some drawbacks limiting its widespread application and pointing need of developing new laser processing strategies. The basic principles of LSPwC can be explained as follows, see for example, Sano et al. (2006), Mostafa et al. (2017), Ge and Xiang (2016) and Taddia and Troiani (2015): The target material is immersed into a small (about 2 mm) renewable water column (which is transparent to laser). Then a high energy pulsed laser is focused over the surface of the target material by using a lens. When the laser energy exceeds the ablation threshold of the material, the chemical bonds are broken and the material is fractured into energetic fragments, typically a mixture of electrons, atoms, molecules and ions. The target material evaporates through ablative interaction. The water confines the evaporated material, which is immediately ionized forming a plasma. The energy absorption in this plasma generates a high intensity (several GPa) shock wave, plastically deforming the material beneath. Due to the limited laser pulse energy and its short duration, the thermal damage is small or negligible and the shock wave effect prevails. The plastically deformed surface tends to laterally expand, while the subsurface layer restricts this trend, generating as a consequence a residual compressive stress field on the surface. Based on the works by Sano et al. (2006), Mostafa et al. (2017), Trdan et al. (2012), Correa et al. (2015) and various others, the LSPwC treatment usually employs a solid laser of Nd:YAG (  = 1,064 nm), 7-10 ns pulse duration, 0.1 to 3.0 J pulse energy and the generated power density is in the range 2-15 GW/cm 2 . According to Bergant et al. (2016), few research has been devoted to the investigation of the effect of LSPwC on the fatigue crack propagation behavior. In their work, a 2.8 J laser was used to process C(T) specimens of 6082 aluminum alloy. A surface damaged layer of the treated region (10 x 10 mm) was removed before the fatigue tests. Nevertheless, a deleterious effect of the LSPwC treatment was observed. On the other hand, the results of Kashaev et al. (2017) showed a significant retardation on the crack growth rate in 2 mm thick 2024 aluminum alloy treated with a 3 J laser. In this work, LSPwC treatment was performed in both sides of pre-cracked 4.5 mm thick compact tension specimens of aluminum alloy 2024-T3. Like other 2xxx alloys, this material can suffer intergranular corrosion, so a pure aluminum clad is applied in order to impart galvanic protection. The effect of shot peening on the propagation of pre-existing cracks in this alloy was assessed in a previous work by Costa et al. (2014). The objective of the present work was to investigate the effect of the LSPwC treatment and load condition on the crack closure and fatigue crack growth behavior shown by the specimens. 2. Experimental development The AA 2024-T3 alloy with 4.8 mm in thickness was chosen for this work. The microstructure of the material, shown by optical micrograph in Fig. 1 (longitudinal cut), comprises alpha phase (solid solution of copper and other alloying elements in aluminium matrix) elongated (due to cold work) grains, aged to the formation of fine CuAl 2 dispersed precipitates. The material also shows second phase particles (inclusions), mainly of Al-Cu-Fe-Mn and Al Cu-Fe-Si-Mn. Compact tension C(T) test-pieces were electro-discharge machined according to ASTM E647-11 standard having width W = 50 mm and LT crack orientation with notch length a n = 15 mm. The specimens were pre cracked to 1.5 mm crack growth from the notch tip. Three of the pre- cracked specimens were assigned to the “as received” (AR)