PSI - Issue 25

R. Baptista / Procedia Structural Integrity 25 (2020) 186–194 Author name / Structural Integrity Procedia 00 (2019) 000–000

192

7

Initial crack propagation direction was also calculated, accordingly to MTS criterion. The FEA obtained results can be compared with the theoretical values for an infinite plate under the same loading. � � �√ � � � � � � (12) �� � �√ �� � �� (13) Table 1 shows the agreement between FEA and theoretical angles, increases with φ . The difference may be explained due to the crack closure effect that happens on the modelled specimen but does not exists on the infinite plate. When φ = 180º, the crack in both models is never closed, therefore the difference is minimal.

Table 1 Normal cruciform specimen fatigue life for different loading conditions.

Initial crack prop. direction IP FEA

Loading Conditions

Fatigue life Loading Conditions

Fatigue life

0º

0º

λ=1, φ=0º, α=0º λ=1, φ=30º, α=0º λ=1, φ=60º, α=0º λ=1, φ=90º, α=0º λ=1, φ=180º, α=0º λ=1.5, φ=0º, α=0º λ=1.5, φ=30º, α=0º λ=1.5, φ=60º, α=0º λ=1.5, φ=90º, α=0º λ=1.5, φ=180º, α=0º

306485 λ=1, φ=0º, α=30º 255548 λ=1, φ=30º, α=30º 181868 λ=1, φ=60º, α=30º 119227 λ=1, φ=90º, α=30º 68480 λ=1, φ=180º, α=30º 482304 λ=1.5, φ=0º, α=30º 380770 λ=1.5, φ=30º, α=30º 197412 λ=1.5, φ=60º, α=30º 109551 λ=1.5, φ=90º, α=30º

303932 290389 187370 115751 111855 106676 197853 102376 60523

-8.2º

-13.1º -46.9º -55.7º -60.0º 31.4º 37.0º -57.3º -61.3º -63.1º

-40.3º -53.2º -60.0º 20.4º 24.7º -55.5º -59.6º -64.1º

51249 λ=1.5, φ=180º, α=30º

44834

3.2. Geometry influence The modified cruciform specimen included the presence of two anti-symmetrical holes. The goal of this specimen was to modify the T-Stress value and to deflect crack propagation. Ayatollahi et al. (2012) also used this approach to modify a specimen introducing negative T-Stress value on a CTS specimen. Fig. 4 the influence of hole location on crack trajectory. If holes are located near the horizontal axis, cracks were attracted, and propagation ended. Fig. 4 a) is an example of such sink-holes, there the corresponding T-Stress values were negative. As holes are placed further away from the horizontal axis, the cracks missed the holes, but deflection still occurred. Fig. 4 b) shows an initial hole attraction, where the corresponding T-Stress values became positive. The crack will eventual miss the hole, and the propagation will return to the horizontal direction, as T-Stress return to negative values.

a)

b)

c)

Fig. 4 Fatigue crack paths on a cruciform specimen with two holes positioned at 7.2 mm from the specimen centre with (a) β=15º, (b) β=30º and (c) β=45º, under λ=1.0, φ=0º, α=0º loading conditions.