MATFAT

Bulk Data Entry Defines material properties for fatigue analysis.

Format

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
MATFAT	`MID`	`UNIT`
	`STATIC`	`YS`	`UTS`

Optional continuation lines for SN Fatigue properties:

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
	`SN`	`SRI1`	`B1`	`NC1`	`B2`	`FL`	`SE`
		`FINDLEY`	`TFP`	`MSS1`	`MSS2`	`MSS3`	`MSS4`	`A/R`

Optional continuation lines for SN-based Spot Weld Fatigue properties:

(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
`SPWLD`		`MSS1`	`MSS2`	`MSS3`	`MSS4`	`R`	`A/R`
	`SR1_SP1`	`B1_SP1`	`NC1_SP1`	`B2_SP1`	`FL_SP1`	`SE_SP1`
	`SR1_SP2`	`B1_SP2`	`NC1_SP2`	`B2_SP2`	`FL_SP2`	`SE_SP2`
	`SR1_SP3`	`B1_SP3`	`NC1_SP3`	`B2_SP3`	`FL_SP3`	`SE_SP3`

Optional continuation lines for SN-based Seam Weld Fatigue properties (Volvo Method):

(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
`SMWLD`		`MSS1`	`MSS2`	`MSS3`	`MSS4`		`A/R`
	`SR1_SM1`	`B1_SM1`	`NC1_SM1`	`B2_SM1`	`FL_SM1`	`SE_SM1`
	`SR1_SM2`	`B1_SM2`	`NC1_SM2`	`B2_SM2`	`FL_SM2`	`SE_SM2`

Optional continuation lines for SN-based Seam Weld Fatigue properties (Joint Line Method). The following two blocks identify the SN curves for Seam Weld Joint Line method. The first block is for normal stress SN curve (mandatory) and the second block is for shear stress SN curve (optional):

(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
`SMWLD`	`NORMAL`	`MSSN1`	`MSSN2`	`MSSN3`	`MSSN4`		`A/R`
	`SR1_SMN1`	`B1_SMN1`	`NC1_SMN1`	`B2_SMN1`	`FL_SMN1`	`SE_SMN1`
	`SR1_SMN2`	`B1_SMN2`	`NC1_SMN2`	`B2_SMN2`	`FL_SMN2`	`SE_SMN2`

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
	`SMWLD`	`SHEAR`	`MSSSH1`	`MSSSH2`	`MSSSH3`	`MSSSH4`		`A/R`
		`SR1_SMSH`	`B1_SMSH`	`NC1_SMSH`	`B2_SMSH`	`FL_SMSH`	`SE_SMSH`

Optional continuation lines for SNCM Fatigue properties:

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
	`SNCM`	`A`/`R`/`MAX`	`MCRV_ID`	`SESTS`	`NC1`	`NC`
		`FINDLEY`

Optional continuation lines for EN Fatigue properties:

(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
`EN`	`Sf`	`b`	`c`	`Ef`	`np`	`Kp`	`Nc`
	`SEe`	`SEp`					`A/R`
	`tfp`	`gfp`	`bg`	`cg`	`CoefKp90`	`Coefnp90`	`MXLMSTRN`
	`FSParm`	`BMParm`

Optional continuation lines for Factor of Safety (FOS) analysis:

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
	`FOS`	`Tfl`	`Hss`	`STHETA`	`SSHEAR`

Definitions

Field	Contents	SI Unit Example
`MID`	Material identification number that matches the identification number on a MAT1 Bulk Data Entry. No default (Integer > 0)
`UNIT`	Defines the units of stress values specified on the `YS`, `UTS`, `SRI1`, `FL`, `Sf`, and `Kp` fields. MPa (Default) PA PSI KSI
`STATIC`	Indicates that static material properties are defined in the following fields.
`YS`	Yield strength. 1 (Real > 0.0, or blank)
`UTS`	Ultimate tensile strength. 1 (Real > 0.0, or blank)
`SN`	Indicates that fatigue material properties for `SN` analysis are following.
`SRI1`	Fatigue strength coefficient. It is the stress range intercept of the `SN` curve at 1 cycle on a log-log scale. No default (Real > 0.0)
`B1`	The first fatigue strength exponent. It can be input in two ways. Real < 0.0 If a negative real number is input, then it is directly used as the slope of the first segment of the `SN` curve in log-log scale. Real > 0.0 If a positive real number is input, then it is internally converted into -1/`B1`. This converted value is used as the slope of the first segment of the `SN` curve in log-log scale. No default (Real ≠ 0.0)
`NC1`	In one-segment SN curve, this is the cycle limit of endurance (see `NC1` in Figure 1). In two-segment `SN` curve, this is the transition point (see `NC1` in Figure 3). No default (Real ≥ 1000.0)
`B2`	The second fatigue strength exponent. It can be input in two ways. Real < 0.0 If a negative real number is input, then it is directly used as the slope of the second segment of the `SN` curve in log-log scale. Real > 0.0 If a positive real number is input, then it is internally converted into -1/`B2`. This converted value is used as the slope of the second segment of the `SN` curve in log-log scale. Default = 0.0 (Real)
`FL`	Fatigue Limit. No damage occurs if the stress range is less than `FL` (see `FL` in Figure 1 and Figure 3). 6 (Real ≥ 0.0, or blank)
`SE`	Standard Error of Log(N). Default = 0.0 (Real ≥ 0.0)
`FINDLEY`	Constant k in the Findley model Default = 0.3 (Real > 0.0)
`TFP`	Shear Fatigue Strength coefficient ( $τ_{f}^{'}$ ) based on range. This value should be twice the value defined for `TFP` on the `EN` continuation line. Default = Blank (Real > 0.0)
`MSSi`	Mean Stress Sensitivity parameters for mean stress correction based on FKM Guidelines. These are used only if the `UCORRECT` field of the `STRESS` continuation line, on FATPARM, is set to FKM/FKM2 or the `MCi` fields of the `MCORRECT` continuation line, on FATPARM, is set to FKM. 11 `MSS2` The `MSS2` default is only applicable when `MSS1`, `MSS2`, `MSS3`, and `MSS4` are all blank. Default = 0.04 (Real > 0.0) `MSS1`, `MSS3`, and `MSS4` Default = blank (Real > 0.0) Note: `MSS1`, `MSS3`, and `MSS4` can be blank only if all of them are blank. If one of them is specified, then all four `MSS1`, `MSS2`, `MSS3`, and `MSS4` should be input.
`A/R`	Defines the interpretation of the defined SN curve. A The SN curve is defined based on Amplitude. R (Default) The SN curve is defined based on Range. Blank
`SPWLD`	Indicates that the fatigue material properties for spot weld fatigue analysis are to follow. The following seam weld properties are only applicable to the Volvo method.
`MSSi`	Mean Stress Sensitivity parameters for mean stress correction based on FKM Guidelines. These are used only if the `UCORRECT` field of the `SPWLD` continuation line on FATPARM is set to FKM or FKM2. 11 `MSS2` The `MSS2` default is only applicable when `MSS1`, `MSS2`, `MSS3`, and `MSS4` are all blank. Default = 0.04 (Real > 0.0) `MSS1`, `MSS3`, and `MSS4` Default = blank (Real > 0.0) Note: `MSS1`, `MSS3`, and `MSS4` can be blank only if all of them are blank. If one of them is specified, then all four of `MSS1`, `MSS2`, `MSS3`, and `MSS4` should be input.
`R`	Indicates the Stress Ratio, R, at which the Spot Weld SN curve is input 2 11 Default = 0.0. or -1.0
`A/R`	Defines the interpretation of the defined SN curve. A The SN curve is defined based on Amplitude. R (Default) The SN curve is defined based on Range. Blank
`SR1_SPi`	Fatigue strength coefficient. It is the stress range intercept of `SN` curve at 1 cycle in log-log scale. Here i=1, 2, 3 represent sheet 1, sheet 2, and nugget, respectively in spot weld fatigue analysis. For default 14 (Real > 0.0)
`B1_SPi`	The first fatigue strength exponent. It is the slope of the first segment of `SN` curve in log-log scale. Here i=1, 2, 3 represent sheet 1, sheet 2, and nugget, respectively in spot weld fatigue analysis. For default 14 (Real < 0.0)
`NC1_SPi`	In one-segment `SN` curve, this is the cycle limit of endurance (`NC1` in Figure 1). In two-segment `SN` curve, this is the transition point (`NC1` in Figure 3). Here i=1, 2, 3 represent sheet 1, sheet 2, and nugget, respectively in spot weld fatigue analysis. For default 14 (Real ≥ 1000.0)
`B2_SPi`	The second fatigue strength exponent. It is the slope of the second segment of `SN` curve in log-log scale. Here i=1, 2, 3 represent sheet 1, sheet 2, and nugget, respectively in spot weld fatigue analysis. Default = 0.0 (Real < 0.0 )
`FL_SPi`	Fatigue Limit. No damage occurs if the stress range is less than `FL` (`FL` in Figure 1 and Figure 3). 6 Here i=1, 2, 3 represent sheet 1, sheet 2, and nugget, respectively in spot weld fatigue analysis. (Real ≥ 0.0, or blank)
`SE_SPi`	Standard Error of Log(N). Here i=1, 2, 3 represent sheet 1, sheet 2, and nugget, respectively in spot weld fatigue analysis. Default = 0.0 (Real ≥ 0.0)
`SMWLD`	Indicates that the fatigue material properties for seam weld fatigue analysis are to follow. The following seam weld properties are only applicable to the Volvo method.
`MSSi`	Mean Stress Sensitivity parameters for mean stress correction based on FKM Guidelines. These are used only if the `UCORRECT` field of the `SPWLD` continuation line on FATPARM is set to FKM or FKM2. 11 `MSS2` The `MSS2` default is only applicable when `MSS1`, `MSS2`, `MSS3`, and `MSS4` are all blank. Default = 0.04 (Real > 0.0) `MSS1`, `MSS3`, and `MSS4` Default = blank (Real > 0.0) Note: `MSS1`, `MSS3`, and `MSS4` can be blank only if all of them are blank. If one of them is specified, then all four of `MSS1`, `MSS2`, `MSS3`, and `MSS4` should be input.
`A/R`	Defines the interpretation of the defined SN curve. A The SN curve is defined based on Amplitude. R (Default) The SN curve is defined based on Range. Blank
`SR1_SMi`	Fatigue strength coefficient. It is the stress range intercept of `SN` curve at 1 cycle in log-log scale. Here i=1, 2 represent bending `SN` and membrane SN, respectively in seam weld fatigue analysis. For default 15 (Real > 0.0)
`B1_SMi`	The first fatigue strength exponent. It is the slope of the first segment of `SN` curve in log-log scale. Here i=1, 2 represent bending `SN` and membrane SN, respectively in seam weld fatigue analysis. For default 15 (Real < 0.0)
`NC1_SMi`	In one-segment `SN` curve, this is the cycle limit of endurance (`NC1` in Figure 1). In two-segment `SN` curve, this is the transition point (`NC1` in Figure 3). Here i=1, 2 represent bending `SN` and membrane `SN` respectively in seam weld fatigue analysis. For default 15 (Real ≥ 1000.0)
`B2_SMi`	The second fatigue strength exponent. It is the slope of the second segment of `SN` curve in log-log scale. Here i=1, 2 represent bending `SN` and membrane SN, respectively in seam weld fatigue analysis. Default = 0.0 (Real ≤ 0.0)
`FL_SMi`	Fatigue Limit. No damage occurs if the stress range is less than `FL` (`FL` in Figure 1 and Figure 3). 6 Here i=1, 2 represent bending `SN` and membrane SN respectively in seam weld fatigue analysis. (Real > 0.0, or blank)
`SE_SMi`	Standard Error of Log(N). Here i=1, 2 represent bending `SN` and membrane SN respectively in seam weld fatigue analysis. Default = 0.0 (Real ≥ 0.0)
`SMWLD`	Indicates that the fatigue material properties for seam weld fatigue analysis are to follow. The following seam weld properties are only applicable to the Joint Line method. 17
`NORMAL`	Flag indicates that the SN curve properties in this block are for Normal Stress.
`MSSNi`	Mean Stress Sensitivity parameters for mean stress correction for Normal Stress SN curve based on FKM Guidelines. These are used only if the `UCORRECT` field of the `SMWLD` continuation line on FATPARM is set to FKM or FKM2. 11 `MSS2` The `MSS2` default is only applicable when `MSS1`, `MSS2`, `MSS3`, and `MSS4` are all blank. Default = 0.04 (Real > 0.0) `MSS1`, `MSS3`, and `MSS4` Default = blank (Real > 0.0) Note: `MSS1`, `MSS3`, and `MSS4` can be blank only if all of them are blank. If one of them is specified, then all four of `MSS1`, `MSS2`, `MSS3`, and `MSS4` should be input.
`A/R`	Defines the interpretation of the defined Normal Stress-based SN curve. A The Normal Stress-based SN curve is defined based on Amplitude. R (Default) The Normal Stress-based SN curve is defined based on Range. Blank
`SR1_SMNi`	Fatigue strength coefficient for Normal Stress-based SN curve. It is the stress range intercept of SN curve at 1 cycle in log-log scale. Here i=1, 2 represent transverse SN and longitudinal SN, respectively for Normal Stress-based SN curve in seam weld fatigue analysis. For default, 5 (Real > 0.0)
`B1_SMNi`	The first fatigue strength exponent for Normal Stress-based SN curve. It is the slope of the first segment of SN curve in log-log scale. Here i=1, 2 represent transverse SN and longitudinal SN, respectively for Normal Stress-based SN curve in seam weld fatigue analysis. For default, 15 (Real < 0.0)
`NC1_SMNi`	In one-segment SN curve, this is the cycle limit of endurance for Normal Stress-based SN curve (NC1 in Figure 1). In two-segment SN curve, this is the transition point for Normal Stress-based SN curve (NC1 in Figure 3). Here i=1, 2 represent transverse SN and longitudinal SN, respectively for Normal Stress-based SN curve in seam weld fatigue analysis. For default, 15 (Real > 1000.0)
`B2_SMNi`	The second fatigue strength exponent for Normal Stress-based SN curve. It is the slope of the second segment of SN curve in log-log scale. Here i=1, 2 represent transverse SN and longitudinal SN, respectively for Normal Stress-based SN curve in seam weld fatigue analysis. Default = 0.0 (Real < 0.0)
`FL_SMNi`	Fatigue Limit for Normal Stress-based SN curve. No damage occurs if the stress range is less than FL (FL in Figure 1 and Figure 3). 6 Here i=1, 2 represent transverse SN and longitudinal SN, respectively for Normal Stress-based SN curve in seam weld fatigue analysis. (Real > 0.0, or blank)
`SE_SMNi`	Standard Error of Log(N) for Normal Stress-based SN curve. Here i=1, 2 represent transverse SN and longitudinal SN, respectively for Normal Stress-based SN curve in seam weld fatigue analysis. Default = 0.0 (Real > 0.0)
`SMWLD`	Indicates that the fatigue material properties for seam weld fatigue analysis are to follow. The following seam weld properties are only applicable to the Joint Line method. 17
`SHEAR`	Flag indicates that the SN curve properties in this block are for Shear Stress. The Shear Stress block is optional for Joint Line Seam Weld method. 17
`MSSSHi`	Mean Stress Sensitivity parameters for mean stress correction for Shear Stress SN curve based on FKM Guidelines. These are used only if the `UCORRECT` field of the `SMWLD` continuation line on FATPARM is set to FKM or FKM2. 11 `MSS2` The `MSS2` default is only applicable when `MSS1`, `MSS2`, `MSS3`, and `MSS4` are all blank. Default = 0.04 (Real > 0.0) `MSS1`, `MSS3`, and `MSS4` Default = blank (Real > 0.0) Note: `MSS1`, `MSS3`, and `MSS4` can be blank only if all of them are blank. If one of them is specified, then all four of `MSS1`, `MSS2`, `MSS3`, and `MSS4` should be input.
`A/R`	Defines the interpretation of the defined Shear Stress-based SN curve. A The Shear Stress-based SN curve is defined based on Amplitude. R (Default) The Shear Stress-based SN curve is defined based on Range. Blank
`SR1_SMSH`	Fatigue strength coefficient for Shear Stress-based SN curve. It is the stress range intercept of SN curve at 1 cycle in log-log scale. For default, 15 (Real > 0.0)
`B1_SMSH`	The first fatigue strength exponent for Shear Stress-based SN curve. It is the slope of the first segment of SN curve in log-log scale. For default, 15 (Real < 0.0)
`NC1_SMSH`	In one-segment SN curve, this is the cycle limit of endurance for Shear Stress-based SN curve (NC1 in Figure 1). In two-segment SN curve, this is the transition point for Shear Stress-based SN curve (NC1 in Figure 3). For default, 15 (Real > 1000.0)
`B2_SMSH`	The second fatigue strength exponent for Shear Stress-based SN curve. It is the slope of the second segment of SN curve in log-log scale. Default = 0.0 (Real < 0.0)
`FL_SMSH`	Fatigue Limit for Shear Stress-based SN curve. No damage occurs if the stress range is less than FL (FL in Figure 1 and Figure 3). 6 (Real > 0.0, or blank)
`SE_SMSH`	Standard Error of Log(N) for Shear Stress-based SN curve. Default = 0.0 (Real > 0.0)
`SNCM`	Flag indicating that this MATFAT entry uses multi-mean SN curve for SN fatigue analysis. 12 13
`A/R/MAX`	Stress types: A Amplitude R (Default) Range MAX Maximum stress
`MCRV_ID`	Identification number of FATMCRV or TABLEXN ID FATMCRV ID Multi-mean SN curve wherein each curve on FATMCRV refers to TABLEXN table of x=stress and y=life. TABLEXN ID Indicates that a single SN curve is defined directly via TABLEXN table of x=stress and y=life. No default (Integer > 0)
`SESTS`	Standard Error of Log(Stress) Default = 0.0 (Real > 0.0)
`NC1`	Fatigue transition point. After this point, fatigue strength is offset by the surface correction factor. Before this point, fatigue strength is proportionally reduced. Default = `NC` (Real > 1000.0)
`NC`	Endurance limit. Default = 1.0E+8 (Real > 1000.0)
`FINDLEY`	Constant k in the Findley model. Default = 0.3 (Real > 0.0)
`EN`	Indicates that fatigue material properties for `EN` analysis are following.
`Sf`	Fatigue strength coefficient. No default (Real > 0.0)
`b`	Fatigue strength exponent. No default (Real < 0.0)
`c`	Fatigue ductility exponent. No default (Real < 0.0)
`Ef`	Fatigue ductility coefficient. No default (Real > 0.0)
`np`	Cyclic strain-hardening exponent. No default (Real > 0.0)
`Kp`	Cyclic strength coefficient. No default (Real > 0.0)
`Nc`	Reversal limit of endurance. One cycle contains two reversals. 6 Default = 2.0E8 (Real > 1.0E5)
`SEe`	Standard Error of Log(N) from elastic strain. Default = 0.0 (Real ≥ 0.0)
`SEp`	Standard Error of Log(N) from plastic strain. Default = 0.0 (Real ≥ 0.0)
`A/R`	Defines the interpretation of the defined `EN` curve. A (Default) The `EN` curve is defined based on Amplitude. R The `EN` curve is defined based on Range. Blank
`tfp`	Shear Fatigue Strength coefficient ( $τ_{f}^{'}$ ) based on amplitude. This value should be one half of the value defined for `tfp` on the `SN` continuation line. Default = Blank (Real > 0.0)
`gfp`	Shear Fatigue Ductility coefficient ( $γ_{f}^{'}$ ) Default = Blank (Real > 0.0)
`bg`	Shear Fatigue Strength exponent ( $b_{γ}$ ) Default = $b$ (Real ≤ 0.0)
`cg`	Shear Fatigue Ductility exponent ( $c_{γ}$ ) Default = $c$ (Real ≤ 0.0)
`CoefKp90`	Coefficient value (see Plasticity model for strain-based Fatigue Analysis in the User Guide). Default = 1.2 (Real > 0.0)
`Coefnp90`	Coefficient value (see Plasticity model for strain-based Fatigue Analysis in the User Guide). Default = 1.0 (Real > 0.0)
`MXSTRN`	Maximum Strain value for Strain-Life Approach. The default value is 0.02 (corresponds to 2% strain). 16 In multiaxial fatigue analysis, this value is used as maximum allowable strain in the plasticity model regardless of whether the load is proportional or non-proportional. If accumulated strain is greater than this value, OptiStruct does not calculate actual damage but assigns a larger value of damage (10.0). In uniaxial fatigue, 10% of this value (0.2% by default) is used as maximum possible strain amplitude. If strain amplitude is greater than 10% of this value, a warning message will be issued. Actual damage is still calculated. Default = 0.02 (Real > 0.0)
`FSParm`	Constant k for the Fatemi-Socie model. Default = 0.3 (Real ≥ 0.0)
`BMParm`	Constant S for the Brown-Miller model. Default = 1.0 (Real ≥ 0.0)
`FOS`	Indicates that material properties for factor of safety analysis are defined in the following fields.
`Tfl`	Torsion fatigue limit. A Real or Integer value can be specified. If an integer is input, then it references the ID of a TABLES1 Bulk Data Entry that defines the intersection points. The X-values represent Hydrostatic Pressure, and Y-values represent Shear. 10 No default (Real > 0.0 or Integer)
`Hss`	Hydrostatic stress sensitivity. No default (Real > 0.0)
`STHETA`	Safe zone angle. If the angle of a point in the domain is lower than the Safe zone angle, it is considered safe (`FOS` is 1.0e20). 10 Default = 0.0 (Real ≥ 0.0)
`SSHEAR`	Shear Threshold for the Safe zone. If the microscopic shear stress is lower than this value, it is considered safe (`FOS` is 1.0e20). 10 Default = 0.0 (Real ≥ 0.0)

Figures

Figure 1. One-segment S-N Curve in Log-log Scale (b2=0). (Nc1 is not Defined or Less Conservative than FL)

Figure 2. One-segment S-N Curve in Log-log Scale (b2=0). (FL is not Defined or Less Conservative than Nc1)

Figure 3. Two-segment S-N Curve in Log-log Scale

Figure 4. EN Curve in Log-log Scale

Comments

UTS or YS is used in mean stress correction (SN) and surface finish correction (SN and EN). If both UTS and YS are defined, UTS will be used. It is not allowed that both UTS and YS are blanks.
SN data defined in the MATFAT card is expected to be obtained from standard experiments that are fully reversed tests on mirror-polished specimens. Fully reversed tests imply that the stress ratio ( $R = S_{\min} / S_{\max}$ ) is equal to -1.0. Therefore, any SN curve input on MATFAT entry should be obtained with a stress ratio (R) equal to -1.0.
Note: Only in the case of Spot Weld SN curve, the R field on the SPWLD continuation line can be used to indicate the stress ratio at which the input SN curve is obtained.
In SN approach, including Spot Weld and Seam Weld, OptiStruct calculates damage based on the Stress Range. If the SN curve is defined based on Stress Amplitude, OptiStruct converts the Amplitude-based SN curve to a Range-based SN curve. ECHO will print the converted SN curves. SN curves are defined in Stress range - Cycle form. Stress range is the algebraic difference between the maximum and minimum stress in a cycle. SN curve is expressed as:(1)
$S_{r} = S R I 1 {(N_{f})}^{b}$
Where,

$S_{r}$

Stress range

$S R 1$

Fatigue strength coefficient

$N_{f}$

Cycle number

$b$

Fatigue strength exponent
Note: For a special case, wherein the following two conditions are satisfied for SN Fatigue (Uniaxial and Multiaxial):
1. SRI1 is greater than 2*UTS, and
2. Stress amplitude after mean stress correction is greater than 90% of UTS.
  Then, the equation used to calculate Fatigue Damage and Life is different from the SN curve mentioned above. Therefore, you may notice a sudden increase in the value of damage for an element when the above two conditions are satisfied for it. This is done to allow for the higher possibility of failure when the corrected mean stress is so close to UTS of the material, and additionally takes into account the situation where the value of SRI1 is extremely high (greater than 2*UTS).
In EN approach, OptiStruct calculates damage based on the Strain Amplitude. If the EN curve is defined based on Strain Amplitude, OptiStruct converts the Range-based EN curve to an Amplitude-based EN curve. ECHO will print the converted EN curves. EN curves are defined in Strain amplitude - Reversal form. Strain amplitude is half of the algebraic difference between the maximum and minimum strain in a cycle, and one strain cycle contains two reversals. EN curve is expressed as: (2)
$ε_{a} = \frac{S_{f}^{'}}{E} {(2 N_{f})}^{b} + ε_{f}^{'} {(2 N_{f})}^{c}$

Where,

$ε_{a}$

Strain amplitude

$S_{f}^{'}$

Fatigue strength coefficient

$E$

Young's modulus

$N_{f}$

Cycle number

$b$

Fatigue strength exponent

$ε_{f}^{'}$

Fatigue ductility coefficient, and c is the fatigue ductility exponent

Empirical formula can be used to estimate SN/EN data from ultimate tensile strength (UTS) and Young's modulus (

E

Table 1. Estimated SN Data from Empirical Formula*. (* Source: Yung-Li Lee, Jwo. Pan, Richard B. Hathaway and Mark E. Barekey. Fatigue testing and analysis: Theory and practice, Elsevier, 2005)
Material	`SRI1`	`b1`	`Nc1`
Steel	4.263*UTS	-0.125	1E6
Aluminum alloys (UTS<336MPa)	2.759*UTS	-0.062	5E8
Aluminum alloys (UTS≥336MPa)	0.131*UTS^1.526	0.379-0.175*log(UTS)	5E8

Table 2. Estimated EN Data from UTS and E**. (** Source: Anton Baumel and T. Seeger, Materials Data for Cyclic Loading, Elsevier, 1990)
	Unalloyed and Low-Alloy Steels	Aluminum and Titanium Alloys
$σ_{f}^{'}$	1.5*UTS	1.67*UTS
$b$	-0.087	-0.095
$ε_{f}^{'}$	0.59 $Ψ$	0.35
c	-0.58	-0.69
K'	1.65*UTS	1.61*UTS
n'	0.15	0.11

Ψ = {\begin{cases} 1.0 U T S / E \leq 3 \times 10^{- 3} \\ 1.357 - 125 * U T S / E U T S / E > 3 \times 10^{- 3} \end{cases}}

For one-segment SN curve (b2=0.0), if FL is blank, the fatigue limit is the stress range at Nc1. If both Nc1 and FL are defined, the more conservative value (larger damage) will be used (Figure 1).
For two-segment SN curve, if FL is blank, the fatigue limit is 0.0.

When fatigue optimization is performed, fatigue limit FL of SN data and reversal limit Nc of EN data will be ignored in order to get continuous changes in fatigue results when stress/strain changes.
If tfp or gfp are not available, OptiStruct calculates this automatically. See Fatemi-Socie Model in the User Guide.
Although tfp is defined in EN, it can be used both in EN (FS model) and SN (Findley). tfp should be defined based on the amplitude.
If tfp is not defined for SN, OptiStruct calculates this automatically. See Findley Model in the User Guide.
The Tfl field can be used to define either a value (constant slope) or a table (multiple slopes) to specify the Failure zone. Additionally STHETA and SSHEAR fields can be used to determine safe-zones for FOS calculation.

Figure 5.
See Mean Stress Correction in the Fatigue section of the User Guide for more information.
SN and SNCM continuation lines cannot be specified together.
For SNCM fatigue, both uniaxial SN and multiaxial SN fatigue analyses are supported. Static, Transient, and Random Response Analysis are only supported. Seam and Spot Weld Fatigue are not supported with SNCM fatigue.
If SRI_SPi, B1_SPi, and NC1_SPi are not defined for Spot Weld Fatigue, then the following values are used as the default for the SN curve.
- Sheet 1: SRI_SP1=28218.0 MPa, B1_SP1=-0.34, NC1_SP1=2000000.0
- Sheet 2: SRI_SP2=28218.0 MPa, B1_SP2=-0.34, NC1_SP2=2000000.0
  These default SN curves are based on a stress ratio (R) equal to 0.0
  
  Only sheet damage (sheet 1 and sheet 2) at spot weld locations will be analyzed regardless of the value of SPTFAIL field on PFATSPW entry.
  
  Mean stress correction for R=0.0 will be carried out using FKM guidelines regardless of the value of CORRECT field on SPWLD continuation line on FATPARM entry.
If SRI_SWi, B1_SWi, and NC1_SWi are not defined for Seam Weld Fatigue, then the following values are used as the default for the SN curve.
- Bending SN curve: SRI_SW1=3254.0 MPa, B1_SW1=-0.1429, NC1_SW1=2000000.0
- Membrane SN curve: SRI_SW2=6094.0 MPa, B1_SW2=-0.2270, NC1_SW2=2000000.0
  These default SN curves are based on a stress ratio (R) equal to -1.0
In uniaxial fatigue, the calculated damage needs further checking, because the excessive strain implies that original analysis (static or transient) result could be beyond linear range.
For the Joint Line Seam Weld method, two SN curve blocks are available for input.
The first block (starting from field NORMAL on the SMWLD continuation line), defines the SN curves based on Normal Stress. There are two SN curve lines for this, the first is for Transverse Stress SN curve, and the second is for Longitudinal Stress SN curve. The Transverse Stress SN curve is mandatory, while the Longitudinal Stress SN curve is optional (if not input, then Fatigue Damage/Life is not calculated for Longitudinal Stress).

The second block (starting from field SHEAR on the SMWLD continuation line), defines the SN curve based on Shear Stress. This second block is optional (if not input, then Fatigue Damage/Life is not calculated for Shear Stress).
This card is represented as a material in HyperMesh.