Residual stresses

Question 1

What is the definition of residual stresses?

Answer 1

“A stress state found in a material that is not required to keep it in equilibrium with the external environment.”
This means that in a generic body, once the internal forces that are required to counterbalance the external loads are removed, there still exist other internal self-balanced stress systems referred to as residual stresses or internal stresses

Question 2

How are residual stresses generated?

Answer 2

By the differential behavior of portion of the same
body, caused by:
- Heterogeneous thermal field
- Different chemical-physical properties (e.g. between surface and centre)
- Different mechanical properties
- Gradient of plastic deformation experienced

Question 3

What are some examples of residual stresses extending over macroscopic distances?

Answer 3

During:
- shot peening
- bending
-welding
- cold hole expantion

Question 4

What are some examples of residual stresses extending over microscopic distances?

Answer 4

-thermal stresses
-Loading stresses
-transformation stresses
-Intergranual stresses

Question 5

What are the residual stresses devided after?

Answer 5

Depending on the distance over which the stress field extends

Question 6

What are the different groups resiudal stresses are divided into?

Answer 6

• Type I: macroscopic stresses (extended to a
  size of the same order of the size of the part)
• Type II: stresses developed among crystalline grains
• Type III: atomic scale tensions related to
  effects producing lattice distortion

Question 7

When are the different types of residual stresses used? (Type I, Type II and III)

Answer 7

Type I stresses are used in design of structural parts. Type II and III have large effects on material strength and toughness but they are usually considered as intrinsic properties of the materials

Question 8

What is the most likely cause of residual stresses in welds?

Answer 8

Due to contraction of the weld metal from
solidification point down to room temperature

Question 9

What are some of the possible reasons for residual stress field in thermal treatments?

Answer 9

• Phase transformations leading to constituents that have different volume or
  thermal / mechanical properties (type II or III)
• Variations in actual thermal cycles experienced by different regions of the part in space and time (type I)

Question 10

How can surface treatments and coatings induce residual stresses?

Answer 10

they produce a modification of the surface layer of a part that will emphasize differences in properties between surface and core