PFAT

Bulk Data Entry Defines element properties for fatigue analysis.

Format

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
PFAT	`ID`	`Layer`	`Finish`	`Treatment`	`Kf`	`CRTDIST`

Definitions

Field	Contents	SI Unit Example
`ID`	Each PFAT card must have a unique ID. This ID may be referenced from a FATDEF definition. No default (Integer > 0)
`Layer`	Region Layer for shell elements. 1 0 (Default) Worst 1 Top 2 Bottom
`Finish`	Material Surface Finish, a result of manufacturing process. 3 NONE (Default) POLISH GROUND MACHINE HOTROLL FORGE Or any float value between 0.0 and 1.0. When it is a float value, it will be used to modify the fatigue limit by multiplied with the original fatigue limit.
`Treatment`	Material Surface Treatment for Material S-N Curve, a process used to enhance the fatigue life. 3 NONE (Default) NITRIDED SHOTPEEN COLDROLL Or any float value greater than 0.0. When it is a float value, it will be used to modify the fatigue limit by multiplied with the original fatigue limit.
`Kf`	Fatigue strength reduction factor. 2 3 Default = 1.0 (Real ≥ 1.0)
`CRTDIST`	Critical Distance defines the location from the shell surface where the stresses are evaluated. 4 AUTO OptiStruct calculates the critical distance automatically depending on the model. Not supported for Factor of Safety (FOS) run. Real > 0.0 The user-defined critical distance is used. Blank (Default) (Real)

Comments

If shell elements are used, it is necessary to specify the appropriate layer or Surface of results to use Top or Bottom. Worst is the worst result of Top and Bottom (the one with larger damage).
Fatigue strength reduction factor takes into account the effect of notch effects, size effects, and loading type influence.(1)
$K_{f} = C_{n o t c h} C_{s i z e} C_{l o a d i n g}$

Where, $C_{n o t c h}$ , $C_{s i z e}$ , and $C_{l o a d i n g}$ are correction factors for notch effect, size effect, and loading type influence.
Finish, Treatment and $K_{f}$ are ignored in FOS analysis (TYPE=FOS on FATPARM).
The Critical Distance field is only supported for shell elements. Critical distance is not applied to surface stress of solid elements as surface stress is based on pure membrane stress. If this field is blank, there is no critical distance effect. Critical distance is only supported for Static and Transient analysis based Fatigue, it is not currently supported for Random Response Fatigue Analysis.
The Critical Distance field is a way to consider stress gradient effect. The stress evaluation point is at CRTDIST/2 distance below the surface.

For AUTO option, the critical distance is calculated as:(2)
$C R T D I S T = {((7.0 \times 10^{- 4}) (E / (F L)))}^{1.92}$

Where,

$E$

Young’s modulus of the material

$F L$

Fatigue limit defined on the corresponding MATFAT Bulk Data Entry

The units of CRTDIST in Equation 2 is mm. If you define CRTDIST manually, then OptiStruct will directly use it without any unit conversion.

This approach is based on Atzori, Meneghetti and Susmel, "Material fatigue properties for assessing mechanical components weakened by notches and defects" Fatigue and fracture of Engineering Materials and Structures, Vol. 28, 83-97, 2005.
This card is represented as a load collector in HyperMesh.