Residual stresses Flashcards
What is residual stresses
A stress-state found in a material that is not required to maintain its equilibrium with the external environment
We have three different types of residual stress. Which?
Type I: Macroscopic stresses (From tightening/welding/yielding)
Type II: Stresses developed among crystalline grains
Type III: Atomic scale tensions related to effects producing lattice distortion
Type I is the most important when designing. Due to ex. fatigue.
Example on type I - (macroscopic) residual stress?
Beam loaded with a benign moment so high it yields. After unloading the beam is permanently deformed, and has regions where the plastic deformation took place. Since the whole beam is now bent, it is the residual stresses on the inside that keeps it away from going back to its initial equilibrium.
Example on type II - (among crystalline grains) residual stress?
Different grains have different deformability to the same macroscopic loading system, depending on their specific crystal orientation (slip plane).
This creates extra plastic deformation between grains when loading, and residual stressed appear when unloading.
Explain two design changes that can be done to avoid residual stresses causing failure due to shrinkage after casting. (Example with wheel)
If in a wheel, the failure is happening due to shrinkage in the wheel spokes (eiker), the changes could be that the wheel is castes so that the center is offset, seen from the side. The residual stresses will cause tension in the bar, and pull the center closer to the original center position instead of pulling the spoke apart.
If in a wheel, the failure is happening due to shrinkage in the wheel “circle” (outer ring) itself, the changes could be that the wheel is casted so that the spokes has a curvature, seen from the face of the wheel. The residual stresses will hence be able to work in other directions, ex. here rotate the outer ring around the center.
“Residual stresses - Hot Working”
The amount of stress/strain generated from the residual stresses/cooling depends on…
The thermal expansion coeff. of the material -and- the delta T between room temp and the highest temp (often Tm)
“Residual stresses - Hot Working”
The amount of stress/strain generated from the residual stresses/cooling depends on…
The thermal expansion coeff. of the material -and- the delta T between room temp and the highest temp (often Tm)
Which type of treatments of a part can generate residual stresses?
Thermal treatments:
As a cause of phase transformations
-or/and-
As a cause of variations in actual thermal cycles experienced by different regions of the parts, in space and time.
Surface treatments and coating:
Ex. thermal spray coating. Tension occurs in the coating when cooling, and this leads to compression in the material/substrate.
Beneficial use of residual stresses
*Hot pinning -> Improved fatigue life
*Tempered glass -> Subjected to rapid cooling so that it has compressive stress at the surfaces and tensile stress at mid thickness. The compressive stress at the glass surface will contribute to close small cracks -> Improved fatigue life
*Concrete -> Pre tensioning steel bars before the concrete stiffen -> The concrete will get compressed by the stress in the bars, and it will overall have a greater tensile strength.
Is the fatigue life affected by residual stresses?
Yes. It is deeply affected (can be either positive or negative) (Positive -> If we have a tension-cyclic-load will compressive residual stresses be positive, and can keep the component out of the finite-life region.)
Is residual stresses important in ductile materials under static load?
No, they have a relatively low importance on static properties since they are completely erased when the material becomes plastically strained.
How do we measure residual stresses?
We either measure the RS with mechanical methods, X-ray diffraction methods or by indirect methods, and from them calculate the stress of the area approached
How is residual stresses measured with mechanical methods?
Mechanical methods:
1. Removing a small part (ex. a known layer) of simple geometry and analyze it. When the layer/part is removed the part will deflect and we can calculate the stresses
2. Hole drilling strain gauge. Apply a strain rosette (with three strain gauges in different directions), drill a hole in the middle of the rosette, and observe the strain values -> E => Stress.
How is residual stresses measured with indirect methods?
Look at the magnetic or ultrasonic properties of the solid.
Magnetic:
- The ferromagnetic properties are sensitive to the internal stress state.
- Cons: Very rapid, and can also measure biaxial stress
Ultrasonic:
- Rely on the sensitivity of ultrasound waves traveling through a solid.
- Changes in speed are affected by the magnitude and direction of the stress field.
How residual stresses measured with x-ray diffraction methods?
X-ray diffraction methods rely on measurement of lattice strains within a (polycrystalline) material to evaluate internal stresses
- The deformation causes changes in the spacing of the lattice planes to a new value that correlates to the magnitude of the applied stress
- The specimen is irradiated with an high-energy coherent and monochromatic X- ray beam that slightly penetrates the surface. The crystal planes diffract some of these X-rays, according to Bragg’s law
-A detector moves around the specimen to record the intensity of diffracted X- rays. The angular position of the intensity peaks enables stresses to be calculated. The detector moves around the specimen to record different plane-populations of grains due to they’re having different orientations.
