PSI - Issue 2_B

Vladimir Oborin et al. / Procedia Structural Integrity 2 (2016) 1063–1070 Author name / Structural Integrity Procedia 00 (2016) 000–000

1064

2

1. Introduction The 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 stages of material fracture in the range of gigacycle 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 reliability, in particular, during the exploitation of buildings. An effective method for investigating the role of initial structural heterogeneity, monitoring the accumulation of defects on various scales (dislocation ensembles, micropores, microcracks), and determining 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 invariants was originally proposed by Mandelbrot (1983). This method is based on an analysis of the relief of a fracture surface, which exhibits the property of self-affinity as manifested by the invariant characteristics of the surface relief (roughness) over a broad spectrum of spatial scales. On the other hand, these characteristics reflect a correlated behavior of defects on various scaling levels. 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 by Peters (2000). 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 the understanding of fundamental aspects of multi-scale damage evolution in large range of load intensity for the prediction of the kinetics of nucleation and propagation of cracks in the damaged material with the purpose to optimize the material structure and finding the materials with low sensitivity to random dynamic loads under high- and gigacycle fatigue conditions.

Nomenclature N

number of cycles

l crack length Δ K stress intensity factor A, m experimentally determined constants E Young modulus l sc scale related to the correlated behaviour in the ensemble of defects L pz scale associated with the process zone  universal exponent b Burgers vector K(r) correlation function H Hurst exponent (surface roughness index) z(x) surface relief height r window of size