Mechanisms and Modeling of Near-Threshold Fatigue Crack Propagation

Abstract

First, this paper proposes a comprehensive framework for the modeling of the intrinsic FCP (i.e. after elimination of any environmental and closure effects). On the basis of numerous experimental data on Al alloys, steels and Ti alloys, three intrinsic crack growth regimes have been identified: i) stage I regime, observed in single crystals or in the early growth phase of short cracks; ii) stage II regime, commonly observed when the crack advance proceeds along a plane normal to the load axis and results from the activation of symmetrical slip systems; iii) crystallographic stage I-like regime which prevails near the threshold. Second, this contribution is dedicated to the description of environmentally assisted propagation and specially focused on the understanding of the role of water vapor and the complex interactions existing between environment and microstructure. The effective FCP behavior is described by superimposing two distinct stage II regimes: i) a propagation assisted by water vapor adsorption which can be operative under very low partial pressure or at very low frequencies; ii) hydrogen-assisted propagation which is operative when some critical conditions are encountered. Constitutive laws are proposed for both intrinsic propagation and water-vapor assisted propagation.

Modelling of fatigue‐corrosion in Ti6246 alloy at 500°c

“…The intrinsic propagation in a high vacuum is in accordance with a propagation law derived by Petit et al 10 from the models initially proposed by Rice 11 and Weertman: 12…”

Effect of actual and accelerated ageing on microstructure evolution and mechanical properties of a 2024-T351 aluminium alloy

“…In any case, a predominance of a stage II, cleavage-like fracture mode, is noticed. However some crystallographic facets, reminiscent of a stage-I like propagation regime [22], were also present on the fracture surfaces in the case of t 0 to t 55 heat-treatments. Typical examples are presented in Figure 12 for a maximum stress of 300 MPa.…”

Influence of surface and internal casting defects on the fatigue behaviour of A357-T6 cast aluminium alloy

“…This oxide layer hinders slip activity at the loading reversal and so an enhanced slip irreversibility would be responsible for the higher fatigue crack growth rates reported in air environment. More recently, Petit et al [21][22][23][24], have identified (based on the experimental results obtained on a wide selection of metallic materials) three main propagation regimes which occur in vacuum environment (during uni-axial fatigue):…”