Fatigue strength of welded components


FKM stands for mathematical proof of strength of machine components. The guideline was based on former TGL standards, the earlier guideline VDI 2226 and other sources and further developed to the new level of knowledge. The guideline is a calculation algorithm consisting of instructions, formulas and tables. Represents a general method for calculating the strength of components in mechanical engineering.

Area of application of the FKM guideline:

  1. In mechanical engineering and related fields.
  2. Mathematical proof for rod-shaped, flat-shaped and volume-shaped components.
  3. Static proof and proof of fatigue.
  4. Verification with nominal stresses or local stresses.
  5. Can be used for components with or without machining or can also be made by welding.

There are 4 calculation tools on our site, that are based on the FKM guideline :

  • Static strength of (un-)welded components
  • Fatigue strength of (un-)welded components
Fatigue strength of welded components

Stress characteristics

Stresses: alternating

Stress amplitudes
σ⊥,a,i [MPa]
σII,a,i [MPa]
τa,i [MPa]
Associated mean stresses
Mean stress perpendicular to the weld seam σ⊥,m,i 0 [MPa]
Mean stress parallel to the weld seam σII,m,i 0 [MPa]
Mean shear stress τm,i 0 [MPa]
Material characteristics
Minimum yield strength Rp - [MPa]
Temperature factor1 KT,D 1 [-]
Construction characteristics
Surface hardening: none
Boundary layer factor KV 1 [-]
Protective layer factor KS 1 [-]
Thickness factor2 ft 1 [-]
Weld seam factor3 αW - [-]
Plastic support number4 npl 1 [-]
Component strength
Infobox: Proof of fatigue strength
Component class5 FAT 90 [-]
Material change resistance σWK,⊥ - [MPa]
Material change resistance σWK,II - [MPa]
Material change resistance τWK - [MPa]
Mean stress factor6 KAK 1 [-]
Fatigue strength factor7 KBK,σ 1 [-]
Amplitude of the component fatigue strength σBK,⊥ - [MPa]
Amplitude of the component fatigue strength σBK,II - [MPa]
Amplitude of the component fatigue strength τBK - [MPa]
Safety factors
Safety factors: applied conservatively
Material safety factor8 jF 1.4 [-]
Load safety factor jS 1 [-]
Overall safety factor jD - [-]
cyclical degree of utilization aBK,⊥ - [-]
cyclical degree of utilization aBK,II - [-]
cyclical degree of utilization aBK,τ - [-]
cyclical comparative degree of utilization aBK,σv - [-]
Notes on the factors
  1. Temperature factor: This takes into account the influence of increased temperatures on the strength of the material.
  2. Thickness factor: This takes into account a reduction in strength due to the thickness of the component
  3. Weld seam factor: takes into account the influence of the load on the weld seam.
  4. Plastic support number: This takes into account plastic bearing reserves, which allow a higher load on the component.
  5. FAT: Categorizes the corresponding weld type, has a direct influence on the permissible stress amplitude
  6. Mean stress factor: This takes into account the influence of residual and medium stresses. With a medium voltage in the train area, the value of 1 can no longer be conservative
  7. Durability factor: takes into account the load on the load spectrum and the required number of cycles. Here fatigue strength and NO fatigue strength proof
  8. Material safety factor: This takes into account the influence of the consequences of damage and the frequency of inspections
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