Fatigue Modifying Factors
Fatigue Modifying Factors
(OP)
Can I ask a couple of questions about Fatigue Modifying factors when performing a fatigue analysis of an aluminium component:
1. Temperature Factor - I have seen it recommended that a temperature factor of 1 is applied for temperatures up to 450degC. Whereas I have also seen it proposed that the ratio of UTS at operating temperature/UTS at room temperature should be used. Obviously with aluminium there is a huge difference between these factors. What would members on this site recommend?
2. Reliability Factor - Generally a reliability factor of 0.753 is applied to achieve a reliability of 0.999 (3-sigma). However, if the specification requirement has applied a Scatter Factor of 6 to the required number of cycles is it also appropriate to apply a 3-sigma reliability factor, or is this simply applying factors on factors?
Any advise gratefully received, as I seem to end up with some very low material fatigue limits.
Peter
1. Temperature Factor - I have seen it recommended that a temperature factor of 1 is applied for temperatures up to 450degC. Whereas I have also seen it proposed that the ratio of UTS at operating temperature/UTS at room temperature should be used. Obviously with aluminium there is a huge difference between these factors. What would members on this site recommend?
2. Reliability Factor - Generally a reliability factor of 0.753 is applied to achieve a reliability of 0.999 (3-sigma). However, if the specification requirement has applied a Scatter Factor of 6 to the required number of cycles is it also appropriate to apply a 3-sigma reliability factor, or is this simply applying factors on factors?
Any advise gratefully received, as I seem to end up with some very low material fatigue limits.
Peter





RE: Fatigue Modifying Factors
if the customer specifies a safe life factor of 6 (that's high !) then IMHO that should be enough.
Quando Omni Flunkus Moritati
RE: Fatigue Modifying Factors
1) Regarding the temperature factor, use this equation to calculate the temperature factor for ALUMINUM ALLOYS:
For T<=50oC use => CT = 1.0
For T>50oC use => CT = 1- 0.0012.(T-50)2
This specification is the one speficied by FKM-Guideline.
2) I din't undertand your second question.
RE: Fatigue Modifying Factors
Quando Omni Flunkus Moritati
RE: Fatigue Modifying Factors
I've not seen fatigue temperature factors defined in MMPDS, can you provide a reference for the Ct=0.95 that you quoted?
Many thanks,
Peter
RE: Fatigue Modifying Factors
Quando Omni Flunkus Moritati
RE: Fatigue Modifying Factors
Quando Omni Flunkus Moritati
RE: Fatigue Modifying Factors
RE: Fatigue Modifying Factors
at 95C, i might use the ftu (or fty) ratio, looks like a 4% reduction (in the s/n curve). as a stress number, that's pretty small, well within lots of other uncertainities, i'd probably also use an increased safe life factor ... 10? (typically 4 or 5). depends how critical the item is.
Quando Omni Flunkus Moritati
RE: Fatigue Modifying Factors
For T<=50oC use => CT = 1.0
For T>50oC use => CT = 1 - 0.0012.(T-50)