viernes, 9 de mayo de 2014

Residual stress application for fatigue life improvement

The increase on the fatigue life of components that are subjected to cyclic loads often motivates an over-sizing of their load bearing sections. This design strategy is clearly detrimental for those applications in which the amount of employed material is a critical factor, both if it is for economical (high price alloys) or technical (weight or dimensional restrictions) reasons.

In these situations, alternatives can be found to reduce locally the tension in the weakest areas, avoiding the need of adding material to the part. This can be achieved by means of applying controlled residual stresses that improve load distribution and relieve the critical sections.
A clear example of this technique can be found in high pressure piping and vessels. Stresses are high in the inner surface of these components, while the external area presents way lower values. In order to visualize the phenomenon, consider a pipe of inner radius ri = 6 mm and outer radius re = 10 mm pressurized up to P = 500 bar (Figure 1). In this particular case, the inner surface would be under a stress 1,6 times higher that the external surface. The fatigue resistance of the component is reduced by the uneven load distribution.

Figure 1. Equation for the studied phenomenon and stress-radius curve for the used example. Note how stress decreases when getting closer to the outer diameter.

In an intuitive way, it can be understood as a couple of hikers, one of them carrying 1,6 times more weight than the other. They will stop once the most loaded one gets tired. In case they would want to last longer, they could hire a porter (add material) or distribute better the weight (induce residual stresses).

When considering the pipes, the loads redistribution is performed applying an initial overload pressure. This pressure produces the plasticization of the material in the area close to ri, while the external ring is kept in elastic regime. When the pressure is removed, the outer elastic section shrinks over the plastic zone, inducing the desired residual stresses. Despite the apparent simplicity of overloading the pipe, the control of the effect on the residual stresses and the fatigue life is a technically complex problem.

With the target of offering fatigue life increasing solutions, IK4-AZTERLAN is developing innovative testing techniques for the study the effects that over-pressurizing exerts on thick walled tubes.

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