Issue 53

C. Navarro et alii, Frattura ed Integrità Strutturale, 53 (2020) 337-344; DOI: 10.3221/IGF-ESIS.53.26 339 M ETHODS AND RESULTS Methods onsidering that most fatigue failures are initiated at stress concentrations, where there is a stress gradient close to the surface, a series of four point bending tests was designed. 30 prismatic specimens (80x22x10 mm with a radius of 2 mm on corners) were produced by AM with the largest dimension oriented in Z (vertical) direction in the manufacturing chamber. This direction was selected because it is usually the most unfavourable to resist fatigue due to the orientation of the main defects produced during manufacturing [6,13]. The specimens were manufactured in a Renishaw AM250 SLM system with the following parameters: powder size, 15-45 µm; layer thickness, 60 µm; laser power output 200 W; scan rate, 0.86m/s. All specimens were sand blasted and annealed (730ºC-2h; slow cooling in the furnace; high vacuum argon atmosphere) before doing any other treatment or testing directly. The sandblasting treatment was carried out with glass microspheres and the following parameters: direction, 90º to the surface; exposure time 15 s; work pressure, 5 bar; sandblasting distance; 30 mm. In order to analyse the effect of the treatments, the 30 specimens were divided into five groups of six specimens for each group. Each group was subjected to a different treatment before testing: Group 1. Specimens were tested directly after sand blasting and annealing, in order to have a reference. This group will also be called “AS BUILT”. Group 2. All specimens were shot peened with the following parameters: 0.6 mm steel balls; 55-62 HRC balls hardness; intensity 14A and 500% coverage. This group will also be named “SP”. Group 3. In this group, specimens were shot peened before subjected to the treatment called CASE, by Curtis Wright ® . In this treatment, parts are placed in an acid solution with non-abrasive ceramic media and the ceramic media is ‘excited’ using a vibratory bowl. Following, parts are reprocessed in a burnishing solution that restores the chemical stability and polishes the surface. The main objective of this treatment is reducing the roughness produced by shot peening. Actually, the CASE treatment is a kind of what is usually known as REM finishing. This group will also be named “SP+CASE”. Group 4. The specimens were laser shock peened before testing. (wavelength: 1064 nm; pulse length: 10 ns; spot diameter: 2.6 mm; 6 shots at each point; power density 6 GW/cm²). This group will also be named “LP”. Figure 1 shows the residual stresses measured in this group. Group 5. In order to have another reference for fatigue strength to compare the lives obtained with the proposed surface treatments, specimens of this group were treated by HIP (920ºC-2 h; 100 MPa; inert gas atmosphere). This group will also be named “HIP”. Residual stresses produced by the four treatments were measured by the blind hole drilling technique.In all cases, Vishay Micro-Measurement residual stress rosettes type EA-031RE-120 were used .The drilling and measurement process were carried out using a Sint MTS300 equipment, following the Integral Method [21,22]. A residual stress profile was obtained for each specimen. Figure 1 shows the average residual stress profile in axial direction for each group of specimens and the scatter of the 6 measurement at each point represented by  the standard deviation. Figure 1: Residual stress produced by sand blasting, shot and laser shock peening, measured by the blind hole drilling technique. C