Residual stresses arise within a material when it achieves equilibrium after experiencing plastic deformation. This deformation is the result of various forces and changes affecting the material.
Primary Causes of Residual Stress
The following factors contribute to the development of residual stress:
- Applied Mechanical Loads: External forces such as bending, stretching, or impact can cause plastic deformation in a material, which leads to residual stresses when the load is removed.
- Example: Cold rolling of steel sheets imparts residual stress due to the plastic deformation.
- Thermal Loads: Temperature changes can cause materials to expand or contract. If this expansion or contraction is constrained, it results in internal stresses.
- Example: Welding causes localized heating and rapid cooling, creating significant residual stresses.
- Phase Changes: When a material undergoes a phase transformation (e.g., from austenite to martensite in steel), there can be volume changes that induce residual stress.
- Example: Hardening processes like quenching in steel induce phase changes and therefore residual stress.
These are the main reasons a material might be under residual stress, and they all involve some sort of plastic deformation.
How Processes Alter Residual Stress
It's important to understand that these stresses aren't static. Mechanical and thermal processes that a component undergoes throughout its service life can also change its residual stress state. For instance:
- A component with residual stress from manufacturing may experience changes to that stress level due to repeated mechanical loading in service.
- Similarly, elevated temperatures during operation can relax or redistribute residual stress.
Here's a table summarizing the causes of residual stress:
| Cause | Description | Examples |
|---|---|---|
| Mechanical Loads | Application of external forces leading to plastic deformation | Bending, stretching, impacts, rolling |
| Thermal Loads | Changes in temperature causing material expansion/contraction, which is constrained resulting in plastic deformation. | Welding, heat treatment |
| Phase Changes | Volume changes associated with transformations in the material's phase structure. | Quenching, hardening of steel |
Understanding these causes is crucial for managing residual stresses, which can have significant effects on the performance and durability of materials and components.
