Considering Fatigue in the Safety Factor of an Analysis
Considering Fatigue in the Safety Factor of an Analysis
(OP)
I am trying to determine how the industry handles fatigue when designing to a pre-determined safety factor. If I want to design to a SF=3, it would seem purtinent to ensure this safety factor exists over the full life of the system. In order to determine this, you would need to know that the failure stress of the material after being loaded by a lower stress (<1/3 UTS), 1E6 times. For example you would end up knowing the following: After being loaded to 1E6 to 10ksi, the failure stress drops from 36ksi (static UTS) to 30ksi thus resulting in a SF=3 (30ksi/10ksi) over the life of the system. I've derived a way of obtaining conservative approximation of this from S-N curves, but I'm quite sure this isn't generally how it is typically handled in the industry. Does anyone have a basis for how fatigue is handled in the industry and possibly a resource to back it up? Some examples I've seen treat fatigue independently of the system safety factor - as long as the system doesn't fail in fatigue over the useful life of the system, the designer is happy. It seems that this is overlooking the safety factor of the system near the end of the useful life of the system. On the other hand the information is not readily avaible to obtain the numbers that I mentioned above.





RE: Considering Fatigue in the Safety Factor of an Analysis
In ASME section VIII div 2 the safety factor is 2 on stress or 20 on cycles , plus the base values are based on 2 standard deviations worse than average properties, but based on unnotched specimens, parent material. While this might seem like a high safety factor, in real life the actual failure likely occurs at the heat affected zones of welds whcih typicaly have pre-existing microcracks , so it is not that conservative.
In the European Union PED, they recognize the likely location of failure is the weld HAZ and account for this in a manner similar to the BS. IN those codes which assume failure in the weld at a preexisting crack, then the failure mode is actually crack growth and not fatigue per se. Yu can freely choose the safety factor in those design codes.
RE: Considering Fatigue in the Safety Factor of an Analysis
RE: Considering Fatigue in the Safety Factor of an Analysis
RE: Considering Fatigue in the Safety Factor of an Analysis
I will review the ASTM code Monday when I am back in the office.
Cheers
RE: Considering Fatigue in the Safety Factor of an Analysis
"The load-bearing structure components of a lifter shall be designed to withstand the stresses imposed by its rated load plus the weight of the lifter, with a minimum design factor of three, based on yield strength of the material, and the stress ranges that do not exceed the values given in ANSI/AWS D14.1 for the applicacable condition. ...."
This paragraph determines the maximum allowable stress for materials used as load-bearing structural components. Analysis or actual max working stresses must be determined based upon working conditions including impact or other factors pertinent to the application. Actual max working stress must be equal to or less than the allowable stress.
The intent of Section 20-1.2.2 is that the load suspension parts of a lifter shall be designed so that the static stress calculation for the rated load shall not exceed 33% of the yield strength.
RE: Considering Fatigue in the Safety Factor of an Analysis
RE: Considering Fatigue in the Safety Factor of an Analysis
Aluminiums orinarly run from about 30 to 40 % of tensile strength.
I have beed involved with several failure investigations where the designers used the static strength/sf in fatigue applications. The prudent designer uses the design strength at the estimated cycles, then applies the appropiate SF.