is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible.This is an author-deposited version published in: http://sam.ensam.eu Handle ID: .http://hdl.handle.net/10985/10516To cite this version :V. OBORIN, M. BANNIKOV, O. NAIMARK, Thierry PALIN-LUC -Scaling invariance of fatigue crack growth in gigacycle loading regime -Tech. Phys. Lett. -Vol. 36, n°11, p.1061-1063 -2010 Any correspondence concerning this service should be sent to the repository Administrator : archiveouverte@ensam.euThe task of assessing the working resource of important structures, in particular, those for aircraft engines, poses qualitatively new basic problem related to evaluation of the reliability of materials under con ditions of cyclic loading in excess of 10 6 -10 10 cycles, which refer to the field of so called gigacycle fatigue. This interest is related to the fact that the resource of loading for many important parts operating under conditions of cyclic loading exceeds the so called multicycle range. the behavior of materials in the range of gigacycle fatigue reveals some qualitative changes in the laws governing both the nucleation of cracks (in the bulk of a sample) and their propagation, which are related to changes in the mechanisms of fatigue crack nucleation and propagation. In the range of gigacycle loading, the fatigue curve exhibits discon tinuities and the behavior shows evidence of a signifi cant increase in the role of environment, so that the problem acquires an interdisciplinary character.The stages of material fracture in the range of giga cycle loading are classified based on the structural signs of damage related to a broad spectrum of spatial scales, including persistent slip bands (PSBs), fatigue striations, microcracks (formed as a result of PSB crossing), and grain boundary defects. The main damage refers to the defect scales within 0.1 μm-1 mm, which are significantly smaller than those detected by the standard methods of nondestructive testing used for the conventional monitoring of reli ability, in particular, during the exploitation of build ings.An effective method for investigating the role of initial structural heterogeneity, monitoring the accu mulation of defects on various scales (dislocation ensembles, micropores, microcracks), and determin ing critical conditions for the transition from dispersed to macroscopic fracture is offered by the quantitative fractography. This technique reveals the characteristic stages of fracture (crack nucleation and propagation), thus providing a base for evaluating the temporal resource of materials and structures under conditions of gigacycle loading.The approach to characterization of the fracture surface morphology in terms of spatiotemporal invari ants was originally proposed by Mandelbrot [1]. This method is based on an analysis of the relief of a frac ture surface, which exhibits the property of self affin ity as manifested by the invariant characteristics of the surrace re...
The objective of this work is to provide the link between the fatigue behaviour of pre-strained aluminium alloys and the scaling properties of damage induced on the fracture surface. Fatigue tests performed on pre-strained aluminium alloys revealed a large difference in their residual fatigue resistance linked to the material: the Al-Cu alloy demonstrated a sharp decrease of HCF life-time due to the pre-straining whereas the insensitivity of the Al-Mg alloy was clear. For the Al-Cu alloy, the investigations made at a 'mechanical' scale allow us to associate the strain energy absorbed during the prior loading with the aspect of the surface and the residual HCF life-time. The statistical characterisation of the fatigue damaged zone was done from the measurement of the surface roughness. Scaling properties were established that allowed the conclusion of the universality of HCF damage kinetics as the mechanism controlling the sensitivity of Al-Cu alloy whatever the pre-straining history.
Fatigue (high-and gigacycle) crack initiation and its propagation in titanium alloys with coarse and fine grain structure are studied by fractography analysis of fracture surface. Fractured specimens were analyzed by interferometer microscope and electronic microscope to improve methods of monitoring of damage accumulation during fatigue test and verify the models for fatigue crack kinetics. Fatigue strength was estimated for high cycle fatigue (HCF) regime using the Luong method [1] by "in-situ" infrared scanning of the sample surface for the step-wise loading history for different grain size metals. Fine grain alloys demonstrated higher fatigue resistance for both HCF and gigacycle fatigue regimes. Fracture surface analysis for cylindrical samples was carried out using optical and electronic microscopy method. High resolution profilometry (interferometerprofiler New View 5010) data of fracture surface roughness allowed us to estimate scale invariance (the Hurst exponent) and to establish the existence of two characteristic areas of damage localization (different values of the Hurst exponent). Area 1 with diameter ~300 µm has the pronounced roughness and is associated with damage localization hotspot. Area 2 shows less amplitude roughness, occupies the rest fracture surface and considered as the trace of the fatigue crack path corresponding to the Paris kinetics.
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