PSI - Issue 59

M. Karuskevich et al. / Procedia Structural Integrity 59 (2024) 642–649 M. Karuskevich et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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(a)

(b)

(c)

Fig. 4. Evolution of surface relief on D16AT clad layer at the multiaxial loading (out-of-phase mode), a = 150 MPa, a = 86.6 MPa: (a) N = 500 cycles; (b) N = 1 000 cycles; (c) N = 5 000 cycles. 3. Surface relief uniaxial fatigue indicators The first surface relief indicator was made of Aluminum single crystal with orientation <221>{110} (Zasimchuk et al., 1992). The 30 mm long fatigue indicator was made of a single-crystal aluminum foil (99.99% Al). The indicator was stuck on a flat specimen 1.2 mm thick. The indicator surface was periodically inspected by means of a microscope with 200x magnification. The indicator sensitivity, i.e., the rate of slip line density increase under cyclic loading, can vary over a wide range by changing the crystallographic orientation of the indicator. In this way, it is possible to select the optimal indicator position for a particular structure, bearing in mind the actual level of stresses and the predicted lifetime of the structure. Experiments also proved that the single-crystal indicator can also be used to evaluate extreme values of singly applied static loads. At a magnification of 200x, primary slip lines were seen at a strain level of 1%. Their inclination angle for a given single-crystal orientation was 82° relative to the tensile axis. If the deformation was increased, the density of the primary slip line became higher. When the strain was equal to .7%, secondary slip lines appeared on the surface. Their direction was 57° relative to the tensile axis. I f the deformation increased to 4.3%, the density for both types of slip lines became higher. Polycrystalline Surface Relief Fatigue Indicator looks like a miniature specimen of aluminum alloy 2024T3 for fatigue test. The alloy composition is presented in Table 1. Table 2 represents the mechanical properties of the alloy presented by Rooy (1990).

Table 1. 2024T3 Chemical composition. Al, wt.

Cu, wt. Mg, wt. Mn, wt. Si, wt.

Fe, wt.

Zn, wt.

Ti, wt.

Other, wt.

[%] 90.7-94.7

3.8-4.9 1.2-1.8

0.3-0.9

Max 0.5 Max 0.5 Max 0.5 Max 0.15 Max 0.15

Table 1. 2024T3 Mechanical composition. Ultimate Tensile Strength (MPa) Yield Stress (MPa) Elongation at Break (%) Modulus of Elasticity (GPa) Shear Strength (MPa) 483 345 18 73.1 283 The intrusion/ extrusion structure is regular only in single-grain boundaries. In some grains, there are no visible regular extrusion/intrusion structures, and some grains have relief very much disturbed by the neighboring grains bodies. Besides the regular slip lines, the black spots are visible, which are areas shadowed by the extrusions jutted above the surface. The current version of the uniaxial fatigue indicator is shown in Fig. 5.