Fatigue Crack Growth in an Al-Mg-Si Alloy under Negative Load Ratio

Abstract:

The usual approaches for Fatigue Crack Growth (FCG) assessment are based on semi-empirical models that consider the stress intensity factor range of fracture mechanics, ΔK, as the governing driving force for crack propagation. However, FCG data incorporating compressive loads have brought discussions and controversies. The compressive loads are not taken into account in the usual life prediction codes and the negative portion of the loading cycle is neglected. In the present work, constant amplitude fatigue crack growth tests with load ratios-1 and 0 are performed, for two diferent stess levels, in AA 6005 alloy centre-notched middle tension M(T) specimens and the obtained results are discussed. It is shown that the crack closure concept, that usually work well for positive ratios, is not enough to satisfactorily explain the negative load ratio effects. The results also showed how the negative part of the load affected the propagation rate of the crack, mainly for larger cracks and smaller loads. Finally, it is shown that a previously proposed expression for the stress distribution ahead of the crack can shed some light on this question.

• Periodical: Key Engineering Materials (Volume 754)
• Pages: 43-46
• DOI: https://doi.org/10.4028/www.scientific.net/KEM.754.43
• Online since: September 2017
• Authors: Marcelo A.S. Torres*, B.R.L. Silva, D.H.S. Costa, C.A.R.P. Baptista, V. Pastoukhov
• Keywords: AA 6005 Alloy, Fatigue Crack Propagation, Negative Stress Ratio

References

[1] D. Kujawski A fatigue crack driving force parameter with load ratio effects. International Journal of Fatigue Vol. 23 (2001), pp.239-246.

DOI: 10.1016/s0142-1123(01)00158-x

[2] S. Dinda, D. Kujawski. Correlation and prediction of fatigue crack growth for different R-ratios using Kmax and ΔK+ parameters. Engineering Fracture Mechanics Vol. 71 (2004), n. 12, pp.1779-1790.

DOI: 10.1016/j.engfracmech.2003.06.001

[3] C.A.R.P. Baptista et al. Describing fatigue crack growth and load ratio effects in Al 2524 alloy with enhanced exponential model. Mechanics of Materials. Vol. 51(2012), pp.66-73.

DOI: 10.1016/j.mechmat.2012.04.003

[4] M. Mehrzadi, F. Taheri. The influence of negative and positive stress ratios on crack growth rate in AM60B magnesium alloy. Materials Science and Engineering A, vol. 545(2012), pp.68-77.

DOI: 10.1016/j.msea.2012.02.091

[5] S. Ishihara et al. The effect of Load Ratio on Fatigue Life and Crack Propagation Behavior of an Extruded Magnesium Alloy. International Journal of Fatigue Vol. 31 (2009), pp.1788-1794.

DOI: 10.1016/j.ijfatigue.2009.02.034

[6] F.S. Silva. Crack Closure inadequacy at Negative Stress Ratio. International Journal of Fatigue Vol. 26 (2004), pp.241-252.

DOI: 10.1016/s0142-1123(03)00162-2

[7] D. Nascimento. Estudo do comportamento em fadiga de baixo ciclo e fadiga sob cargas multiaxiais das ligas de alumínio AA605, AA6063 e AA6351. Lorena: (2015).

DOI: 10.11606/t.97.2015.tde-21052015-153422

[8] F. S. Silva. The importance of compressive stresses on fatigue crack propagation rate. International Journal of Fatigue Vol. 27 (2005), pp.1441-1452.

DOI: 10.1016/j.ijfatigue.2005.07.003

[9] C.A.R.P. Baptista, V.A. Pastoukhov. A damage accumulation model of fatigue crack growth in titanium. Journal of the Mech. Behavior of Materials Vol. 14 (2003), pp.73-85.

DOI: 10.1515/jmbm.2003.14.2-3.73

[10] S.V.P. Espezua, C.A.R.P. Baptista, A.M.B. da Silva, V. Pastoukhov, M.A.S. Torres. Study of Fatigue Crack Growth in Al-Mg-Si Alloys Using a Predictive Model under Positive and Negative Load Ratios. Advanced Materials Research (Online), Vol. 891-892 (2014).

DOI: 10.4028/www.scientific.net/amr.891-892.1785