Mechanistic dissimilarities between environmentally-influenced fatigue-crack propagation at near-threshold and higher growth rates in lower-strength steels

Characteristics of fatigue crack propagation have been investigated in a low carbon steel and a high tensile strength steel to evaluate the effect of sheet thickness. Crack propagation data are generated over a wide range of growth rates, from 10−8 to 10−3 mm/cycle, for load ratios of 0.05 and 0.70 at room temperature in laboratory air. Particular emphasis is placed on behavior at near-threshold growth rates. Near-threshold fatigue crack propagation behavior is found to show a marked sensitivity to sheet thickness, and near-threshold growth rates decrease and threshold values increase with increasing sheet thickness. Oxide and roughness-induced crack closure models are proposed as a mechanism for the effect of sheet thickness on near-threshold fatigue crack propagation. It is also shown that the requirement for specimen thickness recommended by ASTM, W/20≤B≤W/4, is not always valid for near-threshold fatigue crack propagation.

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Near-Threshold Fatigue Crack Propagation in Ultra-High Strength Steel: Influence of Load Ratio and Cyclic Strength

Journal of Engineering Materials and Technology ◽

10.1115/1.3443519 ◽

1977 ◽

Vol 99 (3) ◽

pp. 195-204 ◽

Cited By ~ 103

Author(s):

R. O. Ritchie

Keyword(s):

Fatigue Crack ◽

Crack Propagation ◽

Fatigue Crack Propagation ◽

Growth Rates ◽

Threshold Stress ◽

Load Ratio ◽

High Strength ◽

Threshold Stress Intensity ◽

Ultra High Strength Steel ◽

Near Threshold

Fatigue crack propagation behavior of an ultra-high strength steel (300-M) has been investigated in humid air over a very wide spectrum of growth rates from 10−8 to 10−1 mm/cycle. Particular emphasis has been devoted to the influence of mean stress (or load ratio R = Kmin/Kmax) and microstructure on fatigue crack growth near the threshold stress intensity for crack propagation, ΔK0. Increasing the load ratio from R = 0.05 to 0.70 was found to lead to increased near-threshold growth rates, and a decrease in the threshold stress intensity. Similarly, increasing material strength, by varying the microstructure through quench and tempering and isothermal transformation, resulted in higher near-threshold growth rates, and a marked reduction of ΔK0. These effects are contrasted with behavior at higher growth rates. The influence of strength on ΔK0 is rationalized in terms of the cyclic hardening or softening response of the material, and hence it is shown that cyclic softening can be beneficial to fatigue crack propagation resistance at very low growth rates. The results are discussed in the light of crack closure and environmental contributions to fatigue crack growth at low stress intensities.

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Effect of Biaxial Static Loads on Fatigue Crack Propagation Behavior under Cyclic Tensile and Torsional Loading

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.321-323.720 ◽

2006 ◽

Vol 321-323 ◽

pp. 720-723

Author(s):

Yong Hak Huh ◽

Philip Park ◽

Dong Jin Kim ◽

Jun Hyub Park

Keyword(s):

Fatigue Crack ◽

Crack Propagation ◽

Fatigue Crack Propagation ◽

Growth Rates ◽

Biaxial Loading ◽

Torsional Loading ◽

Static Loads ◽

Fatigue Crack Propagation Behavior ◽

Propagation Behavior ◽

Crack Propagation Behavior

Fatigue crack propagation behavior under cyclic tensile or torsional loading with biaxial static loads has been investigated. Two different biaxial loading systems, i.e. cyclic tensile loading with static torsional load and cyclic torsional loading with static tensile load, were employed to thin-walled tubular specimens. The crack propagation was measured by two crack gages mounted near the notch and crack opening level was measured by unloading compliance method. The directions of the fatigue crack propagated under respective biaxial loading conditions were examined and the growth rates were evaluated by using several cyclic parameters, including equivalent stress intensity factor range, Keff, crack tip opening displacement range, CTD, minimum strain energy density factor range, Smin. Furthermore, the growth rates were evaluated by effective cyclic parameters considering crack closure. It was found that the biaxial static stress superimposed on the cyclic tensile or torsional loading tests has no influence on the propagation directions of the cracks. Furthermore, it was shown that the fatigue crack growth rates under biaixial faigue loading were well expressed by using the cyclic fatigue parameters, Keq,eff, CTDeff, Smin,eff considering crack closure effect.

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