Fatigue-crack growth in two aluminum alloys and crack-closure analyses under constant-amplitude and spectrum loading

Simulation of Fatigue Crack Growth in Integrally Stiffened Panels Under the Constant Amplitude and Spectrum LoadinThe paper describes methodology of numerical simulation of fatigue crack growth and its application on integrally stiffened panels made of 2024-T351 aluminium alloy using high speed cutting technique. Presented approach for crack growth simulation starts by the calculation of stress intensity factor function from finite element results obtained using MSC. Patran/Nastran. Subsequent crack growth analysis is done in NASGRO and uses description of crack growth rates either by the Forman-Newman-de Koning relationship or by the table lookup form. Three crack growth models were applied for spectrum loading: non-interaction, Willenborg and Strip Yield model. Relatively large experimental program comprising both the constant amplitude and spectrum tests on integral panels and CCT specimens was undertaken at the Institute of Aerospace Engineering laboratory in order to acquire crack growth rate data and enable verification of simulations. First analyses and verification tests of panels were performed under the constant amplitude loading. For predictions of crack growth using the spectrum loading a load sequence representing service loading of the transport airplane wing was prepared. Applied load spectrum was measured on B737 airplane within the joint FAA/NASA collection program. The load sequence is composed of 10 flight types with different severity analogous to the standardized load sequence TWIST. Before application on the stiffened panels a calculation of crack growth under the spectrum loading was performed for simple CCT specimen geometry. The paper finally presents comparison of simulations of fatigue crack propagation in two-stringer stiffened panel under the spectrum loading with verification test carried out in the IAE lab. The work was performed within the scope of the 6th Framework Programme project DaToN - Innovative Fatigue and Damage Tolerance Methods for the Application of New Structural Concepts.

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Simulation of Fatigue Crack Growth in the High Speed Machined Panel Under the Constant Amplitude and Spectrum Loading

ICAF 2009, Bridging the Gap between Theory and Operational Practice ◽

10.1007/978-90-481-2746-7_55 ◽

2009 ◽

pp. 1005-1018 ◽

Cited By ~ 2

Author(s):

Petr Augustin

Keyword(s):

Fatigue Crack ◽

Fatigue Crack Growth ◽

Crack Growth ◽

High Speed ◽

Constant Amplitude ◽

Spectrum Loading

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Fatigue Crack Growth Behavior in the Threshold Region

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.891-892.327 ◽

2014 ◽

Vol 891-892 ◽

pp. 327-332

Author(s):

Royce G. Forman ◽

Mohammad Zanganeh

Keyword(s):

Growth Rate ◽

Fatigue Crack ◽

Aluminum Alloys ◽

Fatigue Crack Growth ◽

Crack Growth ◽

Crack Closure ◽

Crack Tip ◽

Test Method ◽

Load Shedding ◽

Threshold Region

This paper describes the results of a research study conducted to improve the understanding of fatigue crack growth rate behavior in the threshold growth rate region and to answer a question of the validity of threshold region test data. The validity question relates to the position held by some experimentalists that using the ASTM load shedding test method does not produce valid threshold test results and material properties. The question involves the fanning behavior observed in threshold region of da/dN plots for some materials in which the low R data fans out from the high R data. This fanning behavior or elevation of threshold values in the low R tests is generally assumed to be caused by an increase in crack closure in the low R tests. Also, the increase in crack closure is assumed by some experimentalists to result from using the ASTM load shedding test procedure [1-3]. The belief is that this procedure induces load history effects which cause remote closure from plasticity and/or roughness changes in the surface morphology. However, experimental studies performed by the authors have shown that the increase in crack closure is more a result of extensive crack tip bifurcations that can occur in some materials, particularly in aluminum alloys, when the crack tip cyclic yield zone size becomes less than the grain size of the alloy. This behavior is related to the high stacking fault energy (SFE) property of aluminum alloys which results in easier slip characteristics. Therefore, the fanning behavior which occurs in aluminum alloys is a function of intrinsic dislocation property of the alloy, and therefore, the fanned data does represent the true threshold properties of the material. However, for the corrosion sensitive steel alloys tested in laboratory air, the occurrence of fanning is caused by fretting corrosion at the crack tips, and these results should not be considered to be representative of valid threshold properties because the fanning is eliminated when testing is performed in dry air.

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