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How to understand the residual stress in a comprehensive way?

Time: 2017-10-31

How to understand the residual stress in a comprehensive way?

Residual stress is defined as “the stress resident inside a component or structure after all applied forces have been removed”.

Compressive residual stress acts by pushing the material together, while tensile residual stress pulls the material apart.

Mathematically, compressive stress is negative and tensile stress is positive. Stresses can also be characterized as normal stresses that act perpendicular to the face of a material and shear stresses that act parallel to the face of a material.

There are a total of 6 independent stresses at any point inside a material represented by it where i is the direction that the stress is acting and j is the face the stress is acting on.

What Causes Residual Stress?

Residual stresses are generated, upon the 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.

What is The Total Stress in a Component?

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 lower resistance to environmentally assisted cracking.

Types of Residual Stress

Residual stresses can be characterized by the scale at which they exist within a material. Stresses that occur over long distances within a material are referred to as macro-stresses. Stresses that exist only locally (either between grains or inside a grain) are called micro-stresses. The total residual stress at a given location inside a material is the sum of all 3 types of stresses.

Type I Stresses: Macro-stresses occurring over distances that involve many grains within a material.

Type II Stresses: Micro-stresses caused by differences in the microstructure of a material and occur over distances comparable to the size of the grain in the material. Can occur in single-phase materials due to the anisotropic behavior of individual grains, or can occur in a multi-phase material due to the presence of different phases.

Type III Stresses: Exist inside a grain as a result of crystal imperfections within the grain

Importance of Residual Stress

Residual stress effects:

• Low cycle and high cycle fatigue performance

• Distortion

• Peen forming (controlled distortion)

• Fretting

• Stress corrosion cracking (SCC) and hydrogen initiated cracking (HIC)

• Crack initiation and propagation. (Damage tolerance)

• Residual Stress distribution is rarely as assumed in FE models and or fracture mechanics; real data is necessary to improve the accuracy and effectiveness of the modeling.

The Benefits of Measuring and Monitoring Residual Stresses

• Optimize process parameters, such as measuring the effectiveness of peening on apart at critical locations.

• 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 present.

• Validate repair area has been “restored” to original specifications.

• More accurate replacement part requirements by tracking residual stress degradation; thus, enabling retirement for a quantitative cause.

• Residual stress information can improve the probability of detection of other nondestructive techniques.

Residual Stress Management

 

There are many ways to introduce and manage residual stresses, among them: cold working techniques, such as shot peening,

laser shock peening, ultrasonic peening, planishing, hammering, burnishing, low plasticity burnishing, rolling, coining and split sleeve expanding, can generate compressive residual stresses. Hot working techniques, such as heat treatment, controlled cooling, and localized heating are often used to minimize or reduce the magnitude of residual stress in components. Coupons or Almen strips are often used to control the process; unfortunately, they do not provide information about the resultant residual stress in the actual component. Peening the Almen strip, for example, gives details about the peening process, but not about resultant residual stress on the part. Additionally, the Almen strip does not account for upstream processing, complex geometry or phase changes.

Surface and Subsurface Residual Stress

1. Fatigue cracks normally initiate at the surface of a component, therefore, surface residual stresses are important in determining the potential for crack initiation and initial propagation. Subsurface crack initiation typically only occurs if a flaw or inclusion is present subsurface.

2. Surface stresses can also strongly influence subsurface crack propagation.

3. Surface and Subsurface stress gradients are required to fully characterize the effects of shot peening, machining or other surface modification processes.

4. In specific cases where machining and other surface modifications are adequately controlled, surface residual stress may be used as an indicator of process quality.

 

 

 

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