Crack Propagation in welded joints in low alloy carbon steel under cyclic loading.

Crack Propagation in welded joints in low alloy carbon steel under cyclic loading.

Sergio Antonio Muñoz Pinzón

The quality of welding is a repeated issue and one of the key parameters in assuring the integrity of a structure. Welds not only join two parts of a component together, they also provide a source of defects, and they provide a continuous path for the propagation of a crack throughout a component. Weld quality has the potential to be extremely variable, as good control is only obtained for factory welds and inevitably many welds have to be performed in site conditions.In pipelines, long sections of pipe are welded together on site, often in hostile conditions, making them more difficult to control. In long pipelines, there are many km of weld, and defect-free ones cannot be guaranteed.[1].

Most of welded structures are subjected to an environment that may affect their integrity, some structures may even be exposed to corrosive environments such as seawater, acids and chemicals from spills or process fluids [2]. In addition they are also subjected to loads of variable type whose origin may be vibrations, waves, winds, pressure changes or temperature, starts and stops of equipment that can cause a phenomenon known as fatigue [3]. Fatigue is a common phenomenon linked to cyclic loading, which occurs in service operation of structural components and can result in premature failure of the material compared to quasi-static conditions. [4] .

The understanding of the growth law of a fatigue crack is essential to predict the progress of the fatigue cracking process in the damage-tolerant design. The rate of the fatigue crack propagation is governed by a loading parameter which is primarily a function of applied stress, σ, and the crack length, 𝑎  as  [5]

da/dN= f(σ,a)

The fatigue life of structural materials comprises two periods, the crack initiation and crack growth until failure. The first growth of microcracks is a surface phenomenon, at a later stage  microcrack penetrates around width on the surface and at depth away from the surface, then the crack growth stops depending on the surface condition to be a function of the resistance to crack growth as a bulk material property. Several models have been developed to explain and predict the growth of fatigue cracks, mechanistic models based on fracture mechanics [6], dislocations models that answers the question of how and where dislocations are generated explaining the plasticity, this method is applicable to describe the behavior of changes in the crack size scale of the crystal lattice parameters [7]. Finally, numerical models, mainly the finite element method with which we have simulated crack growth in two and three dimensions, this method has served as the basis for designs in the aerospace industry [8].

The fatigue of welded joints is generally considered to be a specific problem, it is different from general structural fatigue problems because of the various welding techniques and the large variety of applications [9]. To assess fatigue in welded joints global approaches have been developed that are based on nominal stress with S-N curves and local approaches that are based on local stress, local approaches are necessary when there are differences in the parameters used in the global approach, which happens in most cases or when there are no data for use in the comprehensive approach [10]. IIW has recently published recommendations for evaluating fatigue in welded joints used the approach notch stress, it considers the local stress in the notch formed by the foot or root of the weld based on the theory of elasticity, usually this stress is calculated by the finite element method, the results are correlated with the fictitious notch radius used in the model; notwithstanding that this method is useful for evaluating the severity of the stress level in the various welded joints in the design stage and involves the use of finite element method as a design tool [11].

Welding presents phenomena affecting the structural integrity such as stress risers and misalignments, defects in the weld joint, non-uniformity in the mechanical properties in the weld metal and the heat affected zone compared to the base metal and residual stresses; the welded joint presents particular complexities then reflected in the dispersion of the results of tests to determine the influence of these factors on the structural integrity thereof so that research is still far from being complete [12]. All these problems are accentuated in the presence of a corrosive environment, the crack growth rate is quite high and eventual failure of the structure is inevitable; fracture mechanics is useful to develop test methods, but in the presence of an aggressive environment the crack growth rate vs. the change of the stress intensity factor (da/dN vs ΔK) can vary depending on parameters such as chemical composition and microstructure, frequency and waveform of the load, electrochemical potential, crack geometry, welded joint geometry [3].  Until now research in this field has focused on butt welded joints, then there is much research to do in other geometries of welded joints widely used as overlaps, branches and fillet welds; regarding to fatigue caused by cyclic impact loads or with different frequency components, or varying amplitudes not enough studies have been made to allow the understanding of crack propagation phenomena, so there are research opportunities in this field.

REFERENCES

[1] Milne, I.,  Karihaloo, B., Ritchie, R.O. Structural Integrity Assurance. Comprehensive structural integrity, cyclic loading and fracture, vol. 1. Elsevier; 2003. p. 1–25.

[2] Hobbacher, A. (2008). Recommendations for fatigue design of welded joints and components. IIW Document XIII-1823-07. International Institute of Welding.

[3] M.A. Wahab, M. Sakano (2001), Experimental study of corrosion fatigue behaviour of welded steel structures, Journal of Materials Processing Technology.

[4] Casas-Rodriguez, J. P. (2008). Damage in adhesively bonded joints: sinusoidal and impact fatigue (Doctoral dissertation, © JP Casas-Rodriguez)

[5] Tanaka, K. (2003). Fatigue Crack Propagation. Comprehensive structural integrity, cyclic loading and fracture, vol. 4. Elsevier;. p. 95–127

[6] Tanaka, K., Akiniwa Y. (2003). Modelling Fatigue Crack Growth: Mechanistic Models. Comprehensive structural integrity, cyclic loading and fracture, vol. 4. Elsevier;  p. 165–190.

[7] Pippan, R. (2003). Modelling Fatigue Crack Growth: Dislocation Models. Comprehensive structural integrity, cyclic loading and fracture, vol. 4. Elsevier; p. 191–208.

[8] Newmann J.C. (2003). Modelling Fatigue Crack Growth: Mechanistic Models. Comprehensive structural integrity, cyclic loading and fracture, vol. 4. Elsevier;  p. 165–190.

[9] Schijve, J. (2014). The significance of fatigue crack initiation for predictions of the fatigue limit of specimens and structures. International Journal of Fatigue, 61, 39-45.

[10] Radaj, D., Sonsino, C. M., & Fricke, W. (2006). Fatigue assessment of welded joints by local approaches. Woodhead publishing.

[11] Fricke, W. (2012). IIW recommendations for the fatigue assessment of welded structures by notch stress analysis: IIW-2006-09. Woodhead Publishing.

[12] Zerbst, U., Ainsworth, R. A., Beier, H. T., Pisarski, H., Zhang, Z. L., Nikbin, K., ... & Klingbeil, D. (2014). Review on fracture and crack propagation in weldments–A fracture mechanics perspective. Engineering Fracture Mechanics, 132, 200-276.