Residual stress is the internal stress distribution locked into a material. These stresses are present even after all external loading forces have been removed. They are a result of the material obtaining equilibrium after it has undergone plastic deformation.
Applied stress is generated inside a material due to an external load (often measured with a strain gage). Residual stress is present inside the material regardless of loading. The total stress experienced by the material at a given location within a component is equal to the residual stress plus the applied stress.
TOTAL STRESS = RESIDUAL STRESS + APPLIED STRESS
For example, if a material with a residual stress of -400 MPa is subjected to an applied load of +500 MPa, the total stress experienced by the material is the summation of the two stresses: +100 MPa. Therefore, knowledge of residual stress is important in determining the actual loads experienced by a component.
Compressive (-) residual stress acts by pushing the material together, while tensile (+) residual stress pulls the material apart. In general, compressive residual stress at the surface of a component is beneficial: it tends to increase fatigue strength and fatigue life, slow crack propagation, and increase resistance to environmentally assisted cracking, such as stress corrosion cracking and hydrogen-induced cracking. On the other hand, tensile residual stress at the surface of the component is generally undesirable, as it decreases fatigue strength and fatigue life, increases crack propagation, and lowers resistance to environmentally assisted cracking.
Stresses are characterized as either normal stresses that act perpendicular to the face of a material or shear stresses that act parallel to the face of a material. There are six independent stresses (three normal and three shear) at any point inside a material.
UNITS OF STRESS
• SI unit for stress is the megapascal (MPa)
• US unit for stress is kilo-pounds per square inch (ksi)
6.895 MPa = 1 ksi
Residual stresses are generated, upon equilibrium of material, after plastic deformation that is caused by applied mechanical loads, thermal loads, or phase changes. Mechanical and thermal processes applied to a component during service may also alter its residual stress state.
Examples of processes that can cause residual stress:
MECHANICAL: Plastification of a material during machining.
THERMAL: Difference in solidification of the material (e.g., in a cooling casting).
PHASE CHANGE: Precipitation/phase transformation resulting in a volume change (i.e., austenite to martensite).
The net sum of all residual stresses across any cross-section is always zero. However, across any cross-section of a component, there is typically a residual stress distribution. The residual stress distribution affects performance. It is this distribution that we characterize using XRD.
Residual stress affects the following processes:
Measuring residual stress can provide numerous benefits:
Residual stress can be created during manufacturing by cold-working techniques such as shot peening, laser shock peening, ultrasonic peening, hammering, burnishing, low plasticity burnishing, rolling, coining, and split-sleeve expanding. Residual stress is also created during manufacturing by machining processes such as grinding, milling, and turning, and thermal processes such as welding, casting, forging, and heat treating.
Harmful residual stress can lead to stress corrosion cracking, distortion, fatigue cracking, premature failures in components, and instances of over-design. Techniques such as heat treating, controlled cooling, and localized heating can be applied to help manage potentially harmful residual stresses that were introduced during manufacturing. Other techniques, such as shot peening, are used to introduce beneficial residual stresses into a component to help increase fatigue life. Analyzing the residual stress state of a component will ensure that these processes were successful. Even small changes in residual stress can have a significant effect on the life of a component, which is why residual stress is so important to monitor.