Fundamentals and measurement of residual stress and techniques for improving fatigue strength of equipment parts

Residual stress is a common phenomenon occurring in various materials and equipment parts during manufacturing processes such as welding, casting, and machining.
Understanding and managing residual stress is crucial because it can significantly impact the performance, durability, and fatigue strength of components.
This article delves into the fundamentals of residual stress, how it is measured, and the techniques available to improve the fatigue strength of equipment parts.

Understanding Residual Stress

Residual stress refers to the stresses that remain in a material or component after the original cause of the stresses has been removed.
These stresses can exist without any external load being applied and are usually self-equilibrating.
Residual stresses can be either beneficial or detrimental depending on their nature, magnitude, and distribution within the material.

Types of Residual Stress

Residual stresses are generally classified into three types:

1. **Macro Residual Stress**: These stress types vary over a large distance and affect the material’s overall structure.

2. **Micro Residual Stress**: Found at the grain level, micro residual stresses do not affect the entire structure but can influence individual grains.

3. **Sub-Micro Residual Stress**: These stresses exist at the atomic level, usually as a result of imperfections in the crystal lattice structure.

All three types can impact the material’s properties and its behavior under mechanical loads.

Measurement of Residual Stress

Measuring residual stress is vital for assessing the structural integrity and performance of components.
Various techniques can be employed, each with its advantages and limitations.

Non-destructive Techniques

1. **X-ray Diffraction (XRD)**: XRD is a popular non-destructive technique used to measure residual stress at the surface level of a material.
It utilizes X-rays to analyze the crystalline structure of the material and identify stress-induced changes.

2. **Neutron Diffraction**: Unlike XRD, neutron diffraction can measure residual stress deeper into the material.
This method is ideal for large components where surface measurements alone are insufficient.

3. **Ultrasonic Testing**: Ultrasonic waves are employed to determine residual stresses by analyzing wave velocity changes in the material.
It is a suitable method for bulk measurements.

Destructive Techniques

1. **Hole-Drilling Method**: This widely used method involves drilling a small hole in the material and measuring the strain relief around the hole to calculate residual stress.

2. **Sectioning and Contour Method**: In this method, the component is cut into sections, and the deformation caused by stress relief is measured to evaluate the initial residual stress.

3. **Layer Removal Technique**: Layers of the material are gradually removed, and the resulting deformation is measured to deduce the residual stress.

Each measurement technique has specific applications depending on the type of material, the size of the component, and the depth of stress measurement required.

Impact of Residual Stress on Fatigue Strength

Residual stress can significantly affect the fatigue strength of equipment parts.
While residual compressive stress can enhance fatigue strength by impeding crack initiation and growth, tensile residual stress can have the opposite effect, promoting fatigue failure.
Thus, control and manipulation of residual stresses play a critical role in enhancing fatigue strength.

Techniques for Improving Fatigue Strength

To improve the fatigue strength of components, it is essential to alleviate or redistribute detrimental residual stresses.
Several techniques are used in industry to achieve this goal.

Shot Peening

Shot peening involves bombarding the component with small spherical media (shots).
This process induces compressive residual stresses on the surface, which can significantly enhance fatigue strength.

Surface Hardening

Techniques such as carburizing, nitriding, and induction hardening are used to increase the hardness of the component’s surface layer while also introducing beneficial compressive residual stresses.
These techniques enhance resistance to surface wear and fatigue.

Heat Treatment

Heat treatment processes such as annealing, stress-relieving, and quenching are employed to modify the residual stress distribution within the material.
These processes can alleviate undesirable tensile stresses and replace them with compressive stresses.

Laser Shock Peening

Similar to shot peening, laser shock peening uses laser-induced plasma shock waves to introduce compressive stresses on the surface of the material, enhancing its fatigue strength without causing significant deformation.

Conclusion

Residual stress plays a fundamental role in determining the durability and performance of equipment parts.
Understanding its measurement and management is crucial for industries focused on reliability and longevity of components.
By utilizing techniques like shot peening, surface hardening, and heat treatment, manufacturers can effectively improve the fatigue strength of their components.
This ensures not only their longevity but also their safe and efficient performance in various applications.