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 gauge). 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
If a material with a residual stress of a -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, or +100 MPa. Therefore, knowledge of the residual stress state is important to determine the actual loads experienced by a component. In general, compressive residual stress in 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. Tensile residual stress in 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.
Compressive (-) residual stress acts by pushing the material together, while tensile (+) residual stress pulls the material apart. 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 a total of 6 independent stresses (3 normal and 3 shear) at any point inside a material.
UNITS OF STRESS
• SI unit for stress is the Mega Pascal (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.
MECHANICAL: Plastification of a material during machining.
THERMAL: Difference in solidification of the material. (i.e. in a cooling casting)
PHASE CHANGE: Precipitation / Phase transformation resulting in a volume change (i.e. Austenite to Martensite)
Net sum of all residual stresses across any cross section is always zero. Across any cross section of a component there is typically a residual stress distribution. Residual stress distribution affects performance. It is this distribution that we characterize using XRD.
Residual stress affects:
• Low cycle and high cycle fatigue performance
• Peen forming (controlled distortion)
• Stress corrosion cracking (SCC) and hydrogen initiated cracking (HIC)
• Crack initiation and propagation. (Damage tolerance)
Optimize process parameters, such as measuring the effectiveness of peening on a part at
• Provide a quantitative metric to enable specifications and Go/No-Go decisions.
• Improve product quality, substantiate supplier quality, engineering source approval (ESA)
• Improve safety and reduce catastrophic failures.
• Extend component or structure life by ensuring sufficient compressive residual stress is
• Validate repair area has been “restored” to original specifications.
• More accurate replacement part requirements by tracking residual stress degradation; thus, enabling retirement for
• Residual stress information can improve the probability of detection of other nondestructive techniques.
• Validate residual stress distribution from FE models and or fracture mechanics
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 treatment.
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 are applied to help manage potentially harmful residual stresses created during manufacturing. Other techniques, such as shot peening, are used to introduce beneficial residual stresses into a component to help increase fatigue life. Knowledge of the residual stress state is required to ensure that these processes have been correctly applied. Small changes in the residual stress state can often have a significant effect on the life of a component.