16th International Conference on New Trends in Fatigue and Fracture (NT2F16)

May 24-27, 2016, Dubrovnik, Croatia

THE EFFECT OF RESIDUAL STRESSES ON FATIGUE CRACK GROWTH

IN WELDED STIFFENED PANELS

A. Firstauthorfamilyname1, Ž. Božić[2,], B.B. Thirdauthorfamilyname3

1University, School, Address, City, Country

2University of Zagreb, Faculty of Mech. Eng. And Nav. Arch., I. Lučića 5, 10000 Zagreb, Croatia

3University, School, Address, City, Country

Abstract:This paper presents models for predicting fatigue crack propagation in welded stiffened panels. The Mode I total stress intensity factor values with respect to crack length were determined based on the superposition rule of linear elastic fracture mechanics by using finite element method. The FEM models included idealized residual stress distribution patterns. By using the Paris law the crack propagation life was simulated for a welded stiffened panel specimen and compared with results of cyclic tension fatigue tests.

Keywords:Modeling, Fatigue, Crack propagation, Residual stress

1. Introduction

The crack growth rate in welded stiffened panels can be significantly affected by the residual stresses which are introduced by the welding process. The high heat input from the welding process causes tensile residual stresses in the vicinity of a stiffener. The tensile stresses are equilibrated by compressive stresses in the region between the stiffeners.

In the present investigation, the fatigue crack propagation in welded stiffened panel specimens was studied taking into account residual stresses. The finite element method was used to determine the Mode I total stress intensity factor Ktot, based on the superposition rule of linear elastic fracture mechanics.The influence of welding residual stress on crack growth rate is accounted for by replacing the nominal stress ratio R in empirical power laws by the effective stress intensity factor ratio Reff[1, 2].

2. Residual stresses modeling

It is well-known that the residual stress in a welded stiffened panel is tensile along a welded stiffener and compressive between the stiffeners. Residual stresses may significantly influence the stress intensity factor (SIF) values and fatigue crack growth rate[3].

The so-called residual stress intensity factor, Kres, is required for the prediction of fatigue crack growth rates in welding residual stress fields. The analysis method is based on the superposition rule of linear elastic fracture mechanics. In the residual stress field, under the cyclic loads, the total SIF range ΔKtot and effective SIF ratio Reff are given as:

(1)

Under cyclic loading, only the Reff changes due to the presence of residual stresses. Welding residual stresses can be idealized by the triangular distribution or the rectangular distribution [4], with peak residual tensile stress equal to the yield strength of the steel. In this study the distribution of welding residual stresses in the stiffened panel specimen is taken into account in a similar manner, as depicted in Figures 1a and 1b.

a) b)

Fig. 1. Welding residual stress distribution: a) rectangular; b) triangular.

Fig. 2. Factor U for the rectangular residual stress distribution.

3. Conclusions

Compressive welding residual stresses decreased the total SIF valuesKtot, and the crack growth rate between the two stiffeners. The Elber and Schijvemodels, which takeinto account the effective stressintensity factor ratio, Reff, were implemented to simulate fatigue crack propagation in the test specimen. The simulated crack growth rate was higher in the region of tensile residual stresses, which is in good agreement with experimental results.

Acknowledgements

This work was supported by the National Science Foundation GrantNo.120-0362321-2198. The support is gratefully acknowledged.

References

[1]Božić Ž, Schmauder S, Mlikota M, Hummel M. Multiscale fatigue crack growth modelling for welded stiffened panels. Fatigue and Fracture of Engineering Materials and Structures 2014;37(9):1043–1054.

[2]Servetti G, Zhang X. Predicting fatigue crack growthrate in a welded butt joint: the role of effective R ratio in accountingfor residual stress effect. Eng. Fract. Mech. 2009; 76: 1589–1602.

[3] Anderson TL. Fracture mechanics, fundamentals and applications. 3rd ed. Norway: CRC Press: Taylor and Francis Group; 2005.

[4] Dexter RJ, Pilarski PJ, Mahmoud HN. Analysis ofcrack propagation in welded stiffened panels. Int. J. Fatigue.2003;25:1169–1174.

1

[2,] Corresponding author

E-mail address: (Ž. Božić)