Effect of welding residual stresses on fatigue crack growth thresholds

Abstract Fatigue crack growth thresholds ΔKth were determined for friction stir welded butt joints made from aluminium alloys AA2024 and AA6013. Plotting the thresholds as a function of load ratio R showed distinctly higher amounts for welded joints as compared to those for parent material at small load ratios, but differences became smaller with increasing load ratio, until thresholds became finally identical for the highest R values. Applying Doker’s concept of two controlling parameters, namely ΔK and Kmax [1] , and plotting ΔKth versus Kmax, however, revealed that the effective threshold ΔKth,eff determined at very high R ratios was nearly independent on the alloy and, simultaneously, was identical for parent material and respective welded joints. Thus, differences in threshold behaviour were only caused by the second threshold Kmax,th, which was significantly higher for welded joints as compared to parent material. Differences in Kmax,th coincided with compressive residual stresses determined by cut-compliance measurements in terms of stress intensity factors Krs acting at the crack tip. Based on the analytical approach described by Doker [1] , only one characteristic Krs value was needed to calculate all thresholds of welded joints for 0 ⩽ R ⩽ 1 provided a base material master curve is available.

[1]  G. Biallas,et al.  Monotonic and Cyclic Strength of Friction Stir Welded Aluminium Joints , 2000 .

[2]  H. Döker,et al.  Fatigue Crack Growth Threshold: Implications, Determination and Data Evaluation , 1997 .

[3]  A. K. Vasudevan,et al.  Extension of the Unified Approach to fatigue crack growth to environmental interactions , 2001 .

[4]  M. T. Milan,et al.  Residual Stress Evaluation of AA2024-T3 Friction Stir Welded Joints , 2007, Journal of Materials Engineering and Performance.

[5]  Anthony P. Reynolds,et al.  Residual Stress and Microstructure Effects on Fatigue Crack Growth in AA2050 Friction Stir Welds , 2008 .

[6]  C. Dalle Donne,et al.  Effects of Weld Imperfections and Residual Stresses on the Fatigue Crack Propagation in Friction Stir Welded Joints , 2000 .

[7]  C. Amzallag,et al.  Automation in fatigue and fracture : testing and analysis , 1994 .

[8]  Michael B. Prime,et al.  Measuring residual stress and the resulting stress intensity factor in compact tension specimens , 1999 .

[9]  A. K. Vasudevan,et al.  Analysis of endurance limits under very high cycle fatigue using a unified damage approach , 2007 .

[10]  K. Sadananda,et al.  Residual stress effects on near-threshold fatigue crack growth in friction stir welds in aerospace alloys , 2003 .

[11]  Philip J. Withers,et al.  Dissimilar friction stir welds in AA5083-AA6082: The effect of process parameters on residual stress , 2006 .

[12]  W. Thomas,et al.  Friction Stir Welding – Recent Developments in Tool and Process Technologies , 2003 .

[13]  E. Wolf.  Fatigue crack closure under cyclic tension , 1970 .

[14]  K. Sadananda,et al.  Short crack growth and internal stresses , 1997 .

[15]  I. Finnie,et al.  Experimental determination of stress intensity factors due to residual stresses , 1997 .

[16]  Livan Fratini,et al.  Fatigue Crack Growth in 2024-T351 Friction Stir Welded Joints: Longitudinal Residual Stress and Microstructural Effects , 2009 .

[17]  Rajiv S. Mishra,et al.  Friction Stir Welding and Processing , 2007 .

[18]  K. Sadananda,et al.  A REVIEW OF CRACK CLOSURE, FATIGUE CRACK THRESHOLD AND RELATED PHENOMENA , 1994 .

[19]  C. Dalle Donne,et al.  Investigations on Residual Stresses in Friction Stir Welds , 2001 .

[20]  Volker Bachmann,et al.  Procedure for automated tests of fatigue crack propagation , 1994 .

[21]  Daniel Kujawski,et al.  A two-parameter analysis of S-N fatigue life using Δσ and σmax , 2009 .

[22]  C. Dalle Donne,et al.  Residual stress effects on fatigue crack propagation in friction stir welds , 2001 .

[23]  R. C. McClung,et al.  Advances in fatigue crack closure measurement and analysis: Second volume. ASTM special technical publication 1343 , 1999 .

[24]  A. Ohta,et al.  FATIGUE CRACK PROPAGATION RATES AND THRESHOLD STRESS INTENSITY FACTORS FOR WELDED JOINTS OF HT80 STEEL AT SEVERAL STRESS RATIOS , 1982 .