Crack closure at positive stresses during random load fatigue crack growth

Fatigue crack closure at positive stresses during broad band, random load fatigue crack growth, has been investigated. Tests were performed on centre-notched specimens made from an aluminium alloy (B.S. 2L 71). The closure stress was found to be dependent upon the material thickness, the mode of fracture and the stress ratio Q. Under plane strain fracture mode conditions it was found that the closure stress for 22 mm thick specimens was greater than that found with 6 mm thick specimens. This difference in closure behaviour produced a slower fatigue crack growth rate for the thicker specimens. In addition it was found that for a given r.m.s. stress the fatigue crack closure level only impinged upon the dynamic range at low mean stress levels.

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Characterizing Fatigue Crack Closure by Numerical Analysis

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.33-37.273 ◽

2008 ◽

Vol 33-37 ◽

pp. 273-278 ◽

Cited By ~ 2

Author(s):

Ya Zhi Li ◽

Jing He ◽

Zi Peng Zhang ◽

Liang Wang

Keyword(s):

Finite Element ◽

Fatigue Crack ◽

Numerical Analysis ◽

Fatigue Crack Growth ◽

Crack Length ◽

Crack Growth ◽

Crack Closure ◽

Crack Opening ◽

Crack Tip ◽

Fatigue Crack Closure

The crack closure phenomenon has attracted great attention in the prediction of fatigue crack growth. The finite element analysis of fatigue crack growth has been conducted by many researchers mainly emphasized on the technique implementation of the simulation. In this paper the behavior of plasticity induced fatigue crack closure was analyzed by the elastic-plastic finite element method for middle crack tension (MT) specimen. The material was assumed as linear-kinematic hardening. The crack growth was simulated by releasing the “bonded” node pairs ahead of crack tip in stepwise. The calculations focused on the effects of load cases and crack length on crack opening/closure levels. For constant amplitude cyclic loadings with different load ratios, the crack opening/closure levels increases for a while and then decreases continuously, with the increase of crack length. For the loadings with invariable maximum stress intensity factors (briefly the constant-K loading), however, the crack tip plastic zone sizes at different crack lengths remain unchanged and the crack opening and closing load levels normalized by the maximum load levels keep constants as well. The results indicate that the crack length does not affect the relative opening and closure levels and numerical analysis for the constant-K loading case should play a key role in characterizing the fatigue crack growth behavior.

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Recent Advances in Fatigue Crack Growth

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.510-511.15 ◽

2012 ◽

Vol 510-511 ◽

pp. 15-21 ◽

Cited By ~ 1

Author(s):

A.J. McEvily

Keyword(s):

Stress Intensity Factor ◽

Fatigue Crack ◽

Stress Intensity ◽

Fatigue Crack Growth ◽

Intensity Factor ◽

Crack Growth ◽

Crack Closure ◽

Growth Process ◽

Threshold Level ◽

Fatigue Crack Closure

Many of the recent advances in the understanding of the fatigue crack growth process have resulted from an improved realization of the importance of fatigue crack closure in the crack growth process. Two basic crack closure processes have been identified. One of which is known as plasticity-induced fatigue crack closure (PIFCC), and the other is roughness-induced fatigue crack closure (RIFCC). Both forms occur in all alloys, but PIFCC is a surface-related process which is dominant in aluminum alloys such as 2024-T3, whereas RIFCC is dominant in most steels and titanium alloys. A proposed basic equation governing fatigue crack growth is (1) where where Kmax is the maximum stress intensity factor in a loading cycle and Kop is the stress intensity factor at the crack opening level. is the range of the stress intensity factor at the threshold level which is taken to correspond to a crack growth rate of 10-11 m/cycle. The material constant A has units of (MPa)-2, and therefore Eq. 1 is dimensionally correct. Eq.1 has been successfully used in the analysis of both long and short cracks, but in the latter case modification is needed to account for elastic-plastic behavior, the development of crack closure, and the Kitagawa effect which shows that the fatigue strength rather than the threshold level is the controlling factor determining the rate of fatigue crack growth in the very short fatigue crack growth range. Eq. 1 is used to show that The non-propagating cracks observed by Frost and Dugdale resulted from crack closure. The behavior of cracks as short as 10 microns in length can be predicted. Fatigue notch sensitivity is related to crack closure. Very high cycle fatigue (VHCF) behavior is also associated with fatigue crack closure.

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