PSI - Issue 39

R. Yarullin et al. / Procedia Structural Integrity 39 (2022) 364–378 Author name / Structural Integrity Procedia 00 (2021) 000–000

366

3

Experimental results were obtained from fatigue tests of thin-walled 7075-T6 and 2024-T3 aluminum tubular specimens with a circular hole by Fatemi et al. (2014). The loading conditions included axial, torsion, combined in phase and out-of-phase axial-torsion, and axial with intermittent torsion cycles. Crack nucleation was observed on planes of maximum shear stress for both notched and smooth (un-notched) specimens. Macroscopic crack growth in notched specimens occurred along planes experiencing the maximum range of nominal principal stress (i.e., mode I crack growth) whereas cracks in smooth specimen grew along maximum shear planes. This was explained in terms of different damage evolution mechanisms in smooth and notched specimens. Aeroengine shafts, wind-turbine shafts, aircraft/helicopter structures have a hollow structure and can be simplified for investigations as a hollow cylinder with a semi-elliptical outer-surface crack. Thickness effects and multiaxial fatigue loading conditions were studied by Citarella et al. 2016 and Lepore at al. 2018. Different commercial software have been adopted for automated 3D remeshing and crack propagation calculation. Crack growth was generally simulated in a stepwise manner, and SIFs were calculated by means of the J-integral technique for each new crack front. However, in the papers mentioned above, the pure torsion loading condition of hollow cylindrical specimens with initial external semi-elliptical surface cracks was not considered because of some difficulties of numerical simulations. One of them is plasticity induced crack closure effect, which becomes significant near free surface of the specimen and delays crack growth rate. Another factor is the effect of K III at the deepest point of surface crack, which results in factory roof on the fracture surface and affects crack growth rate, too (e.g., Kikuchi et al. 2011). Besides, in case of multi-axial loading condition, SIFs proposed by Shahani 2007 and Predan et al. 2013 are not directly applicable to the fatigue crack growth analysis of a thick-walled hollow cylinder because the fatigue crack front continuously changes during a load cycle. The value of maximum SIF is not reached at the same rotation angle for all the points along the crack front profile. Thus, a key point of mixed mode surface crack growth analysis under complex stress state is the knowledge of the SIFs along curvilinear crack front with regard to a three-dimensional geometry. Currently, the procedure of numerical simulation of crack propagation and calculation of SIFs is implemented in many specialized software tools, based on FEM, like FRANC3D, ZENCRACK, ABAQUS, ADAPCRACK3D, or on DBEM (Dual Boundary Element Method) like BEASY (some relevant applications of such approach are shown by Citarella et al. 2013). In any case, the application of such numerical techniques should be based on close interaction with experimental research for the verification of crack growth criterion. 2. Experimental study The test materials are aluminium alloys D16T and B95AT (analogue of 2024 and 7075 aluminium). The first step of the experimental study was the determination of the main mechanical and fracture resistance properties of considered alloys. The tension testing and fatigue crack growth rate tests were performed in accordance with ASTM E8 and E647, respectively. The main mechanical properties and Paris constants are listed in Table 1, where E is the Young’s modulus, σ s is the nominal ultimate tensile strength, σ y is the monotonic tensile yield strength, σ u is the true ultimate tensile strength, δ is the elongation, ψ is the reduction of the cross-section area, n is the strain hardening exponent, α is the strain hardening coefficient, C, m are the Paris constants.

Table 1. Main mechanical properties and Paris constants of aluminum alloys.

σ y , MPa

σ s , MPa

σ u , MPa

δ, %

ψ, %

E, GPa

Material

n

α

C

m

D16T

437

643

665

15

11

80.636

5.86

1.54 2.3651*10 -11 3.325

B95AT 1.46 1.6413*10 -10 2.917 The subject of this study are cylindrical hollow specimens with semi-elliptical surface cracks. The specimen diameter is equal to 28 in the test section and its length is equal to 130 . Using the electro spark method an initial edge notch was cut with initial depth ℎ equal to 3.0 . For this study, specimens with initial aspect ratio equal 518 653 775 14 36 78.596 10.37