Impact of residual stresses on the fatigue strength of welded components

This article sheds light on the often-overlooked phenomenon of residual stresses and their profound impact on the fatigue life of welded components.

Sources of residual stresses

Residual stresses are stresses that remain locked within a material after the original cause of these stresses has been removed. Unlike stresses that arise from external loads or thermal gradients applied to a structure, residual stresses result from manufacturing processes that involve plastic deformations, temperature gradients, or phase transformations. Common sources of residual stresses include machining, casting, forging, and, notably, welding. These stresses can significantly affect a material's mechanical properties, including its strength, ductility, and fatigue life.

How do residual stresses arise in welding?

Welding, a process used to join materials, typically metals or thermoplastics, through coalescence, is a prevalent source of residual stresses. During welding, localized heating and subsequent cooling create temperature gradients within the material, leading to expansion and contraction. This thermal cycle results in non-uniform plastic deformations, as the heated zone expands and the surrounding cool material resists this expansion. Upon cooling, the material contracts, but the non-uniform plastic deformation prevents the material from returning to its pre-welded state, leading to residual stresses (see figure 1).

Cooling down process of a weld resulting in contraction of the weld material and tensile residual stresses
Figure 1. Contraction of the weld material resulting in tensile residual stresses.

Tensile residual stresses are particularly concerning as they can significantly reduce the fatigue life of welded components. Figure 2 shows the residuals stresses, both tensile and compressive, near the weld. Residual tensile stresses in the direction of the weld close to the weld toe can very quickly lead to cracks when the part is subjected to a cyclic loading in the direction fo the weld.

Residual stresses along a weld - the compressive and tensile stresses are shown
Figure 2. Tensile and compressive residual stresses along the length of the weld (green compressive stresses – red tensile stresses).

Residual stresses and their detrimental effects on durability

Residual stresses, especially tensile residual stresses, are detrimental to the durability of welded components for several reasons:

  • Fatigue life reduction: Tensile residual stresses contribute to the initiation and propagation of cracks under cyclic loading conditions, significantly reducing the component's fatigue life.
  • Stress corrosion cracking: In corrosive environments, tensile residual stresses can lead to stress corrosion cracking, where cracks develop and propagate due to the combined effects of tensile stress and corrosion.
  • Distortion: The uneven distribution of residual stresses can lead to distortion or warping of the component, affecting its dimensional stability and fit during assembly.

The influence of residual stresses varies with the application, being either beneficial or detrimental. In some instances, residual stresses are intentionally introduced into designs for their advantageous effects. This is accomplished through methods like shot peening, which applies compressive residual stresses on the surface of an object, thereby enhancing the durability of thin sections or making brittle surfaces more robust. Be aware though that when compressive residual stresses are introduced on one location, due to equilibrium, tensile stresses will arise on other locations.

More often than not, residual stresses yield adverse outcomes. These stresses might go unnoticed by manufacturers unless they cause noticeable distortion of the welded components, yet they can undermine the structural integrity of a part. For instance, structures with thick walls that have not undergone stress-relief treatments are more susceptible to brittle fracture compared to their stress-relieved counterparts.

Additionally, these undesirable stresses impact the fatigue life of materials. In the realm of non-welded materials subjected to fatigue loading, only the tensile parts of the stress cycle are typically responsible for the propagation of fatigue cracks. In contrast, in welded joints, the residual stresses from welding amplify the effects of cyclic stress, meaning the entire cycle, both the tensile and compressive parts, contributes to the incremental fatigue damage (see Figure 3).

Effect of a residual stress on the mean stress of a stress cycle.
Figure 3. Left: Stress cycle with a tensile and compressive part – typically only the tensile part contributes to the propagation of a fatigue crack. Right: A tensile residual stress results in a total stress cycle which has only a tensile part.

How can we estimate residual stresses?

Using Finite Element Analysis (FEA)

Finite Element Analysis (FEA) is a powerful computational tool used to estimate residual stresses in welded components. By simulating the welding process, including the heat input, thermal gradients, and material properties, FEA can predict the development and distribution of residual stresses. This predictive capability allows engineers to modify welding parameters or component design to minimize adverse effects.

Performing measurements

Several experimental techniques are available to measure residual stresses, and can be divided in three categories: destructive, semi-destructive and non-destructive methods.

  • Destructive methods:
    • Contour method: This method provides detailed, two-dimensional maps of residual stresses across a plane of interest with relatively high resolution and accuracy.
      A precise cut through the material of a component is made, letting the material deform slightly due to the relaxation of the residual stresses that were present. The surfaces created by the cut will deform in a manner that reflects the magnitude and distribution of the residual stresses that were present along the cut plane. These surfaces are then carefully measured and the contour data is then analyzed using finite element modeling (FEM) techniques. This analysis reconstructs the residual stress profile that would have caused the observed deformation.
    • Slitting: The slitting method is used to measure through-thickness residual stresses. A narrow cut (or slit) into the material and measuring the resulting deformation around the cut. A precise and narrow cut (or slit) is made in the material, carefully controlling the depth of the slit and gradually increasing it in small increments. The resulting deformation around the cut due to the release of the residual stresses around the slit is measured. The residual stress is then calculated as a function of through thickness position.
  • Semi-destructive methods:
    • Centre hole drilling: This method involves drilling a small hole at the center of the area of interest and measuring the deformations that occur around the hole. These deformations are caused by the release of residual stresses that were present in the material. The method relies on the principle that the material will deform in a predictable manner when the stresses are relieved, allowing for the calculation of the original stresses.
    • Deep hole drilling: This method is used for measuring residual stresses deep within a material. By drilling a small hole in the material and measuring the strain relief around the hole, the original residual stresses can be inferred. This method is particularly valuable for assessing subsurface residual stresses in complex engineering components.
  • Non-destructive methods:
    • Ultrasonic testing: This method utilizes the dependency of sound wave velocity on the stress state of the material to estimate residual stresses.
    • X-ray diffraction: This method is used to measure residual stresses in crystalline materials. It works on the principle that X-rays diffracted by the crystal lattice planes of a material will experience changes in their diffraction angle due to the presence of stresses within the material. By analyzing these changes, the residual stresses can be quantified.
    • Neutron diffraction: This method is a used to measure residual stresses deep within materials, leveraging the unique properties of neutrons to penetrate far deeper than X-rays. This method is based on the diffraction of neutrons as they pass through a material. When neutrons encounter the crystalline structure of a material, they are scattered by the atomic nuclei, creating a diffraction pattern that reflects the arrangement of atoms within the material.

What are possible mitigation strategies?

To improve the durability and fatigue life of welded components, several strategies can be employed to mitigate residual stresses:

  • Pre-welding treatments: Techniques such as preheating can reduce temperature gradients and thus minimize the development of residual stresses.
  • Post-welding treatments: Processes like shot or laser peening, stress relief annealing, or vibratory stress relief can reduce or redistribute residual stresses in the welded components.
  • Optimized welding techniques: Modifying the welding technique, sequence, or parameters can reduce the magnitude of residual stresses. For example, using a lower heat input or employing controlled short welding cycles can be beneficial.

Conclusion

Residual stresses from welding pose a significant challenge to the durability and fatigue life of welded components. Understanding how these stresses arise and their detrimental effects is crucial for engineers and manufacturers. By employing FEA for prediction, experimental methods for measurement, and effective mitigation strategies, the negative impact of residual stresses can be minimized, ensuring the reliability and longevity of welded structures.

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