PSI - Issue 19

Wolfgang Schneller et al. / Procedia Structural Integrity 19 (2019) 556–565 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 2. (a) Microstructure of AB condition; (b) Microstructure of HIP condition

3.2. Residual stress measurement The analysis is performed with X-Ray diffraction using a X-RAYBOT from MRX-RAYS. A psi-mounting configuration with Cr-K  radiation is used along with a collimator size of 2 mm in diameter. The evaluation is based on the 2theta - sin 2 (psi) method. The exposure time is set to 30 seconds for each increment, opting for 25 psi increments, with a tilting angle of the X-Ray tube from -40° to +40°. The residual stress analysis is performed on all unprocessed specimens. Measurements are conducted at three positions around the circumference in a distance of 120°. The residual stress condition is evaluated before testing, and specimens that reached the run-out criterion of 1E7 load cycles are measured again. The outcome of this investigation is that there are significant axial tensile residual stresses at the surface of the unprocessed specimens, which stay present after testing at run-out load level. The results are given in Table 3, which represent the mean values of the three measurements positions. Additionally, the residual stresses for the HIP and SA conditions are significantly reduced by the post treatment, what may also positively effect the fatigue behavior.

Table 3. Axial surface residual stress measurement results

Condition

 res,ax,surf (0 LC)

 res,ax,surf (1E7 LC) 0.106±0.023 (Basis)

Difference

UP-AB UP-HIP UP-SA

0.107±0.027 (Basis)

-0.9%

0.049±0.023 (-54.2%) 0.054±0.024 (-49.1%)

+11.0%

0.057±0.026 (-46.7%)

-

-

It is essential to be aware of the residual stresses at the failure origin since this stress condition is responsible for fatigue crack initiation and propagation [24,25]. For each unprocessed specimen, a subsequent fracture surface analysis reveals that the failure originates in every case from the as-built surface. This confirms that the residual stresses are measured at the crack initiation spot, namely at the unprocessed surface. This enables to look at surface residual stresses as a constant mean stress state which alters the stress condition while testing and shifts the intended load stress ratio of R = -1 to an effective stress ratio R eff within the failure initiation area. This reveals an impact of residual stresses on the fatigue behavior of selectively laser melted Al-structures with as-built surface. For this reason, they can not be neglected in order to safely assess fatigue properties.