Issue34

V. Oborin et alii, Frattura ed Integrità Strutturale, 34 (2015) 422-426; DOI: 10.3221/IGF-ESIS.34.47 422 Focussed on Crack Paths Multiscale study of fracture in aluminum-magnesium alloy under fatigue and dynamic loading Vladimir Oborin, Mikhail Bannikov, Oleg Naimark, Mikhail Sokovikov, Dmitry Bilalov Institute of Continuous Media Mechanics of Ural branch of RAS, 614013, Perm, Russia oborin@icmm.ru , mbannikov@icmm.ru, naimark@icmm.ru , sokovikov@icmm.ru, ledon@icmm.ru A BSTRACT . In this paper we investigated the influence of consecutive dynamic and gigacycle fatigue loads on the lifetime of aluminum-magnesium alloy AlMg6. Preloading of samples was achieved during dynamic tensile tests in the split-Hopkinson bar device. Fatigue tests were conducted on Shimadzu USF-2000 ultrasonic fatigue testing machine. This machine provides 10 9 -10 10 loading cycles with the amplitude from 1 to several dozens of microns and frequency of 20 kHz, which reduces dramatically the testing time in the comparison to the classical fatigue testing machines. The New-View 5010 interferometer–profiler of high structural resolution (resolution of 0.1 nm) was used for qualitative fracture surface analysis, which provided the data allowing us to find correlation between mechanical properties and scale-invariant characteristics of damage induced roughness formed under dynamic and gigacycle fatigue loading conditions. Original form of the kinetic equation was proposed, which links the rate of the fatigue crack growth and the stress intensity factor using the scale invariant parameters of fracture surface roughness. The scale invariance characterizes the correlated behavior of multiscale damage provides the link of crack growth kinetics and the power exponent of the modified Paris law. K EYWORDS . Fracture; Gigacycle fatigue; Scaling; Surface morphology; Fractal analysis; Paris law. I NTRODUCTION he assessment of the lifetime of critical engineering structures, in particular those for aircraft engines, poses qualitatively new fundamental problems related to evaluation of the reliability of materials under cyclic loading in excess of 10 9 –10 10 cycles corresponding to the so-called gigacycle fatigue range. This interest is caused by the fact that the fatigue lifetime of critical structures operating under cyclic loading conditions exceeds a gigacycle fatigue range. The gigacycle fatigue range can be characterized by some features, where of special interest is the range pertaining to the number of cycles N≈10 9 . The behavior of materials in this range reveals some qualitative changes in the mechanisms governing both the nucleation of cracks and their propagation. The influence of random statistic and dynamic loads on the lifetime of materials under gigacycle fatigue regime is a subject of much current interest for aircraft motor companies in the context of solving the problem of reliability (longevity) of materials under real operating conditions [1]. For instance, that concerns to the lifetime estimation of gas turbine engine blades during their collision with solid particles usually called foreign object damage. The solution to this problem needs T