Fatigue remaining life
Fatigue remaining life
(OP)
I want to look for a nondestruction method to evaluate the remaining life of a fatigued structure, because I have to continue to use it after the evaluation test.
Can anyone provide some links or published papers about it?
Thank you in advance.
Can anyone provide some links or published papers about it?
Thank you in advance.
RE: Fatigue remaining life
You can polish the suspect surface and make a plastic replica. Study this in a SEM or a TEM, and you should find some answers.
RE: Fatigue remaining life
How could you determine the number of stress cycles remining till failure by a test of the member? I can't see how that could be possible.
Wouldn't you look in the opposite direction - ie, how many cycles have occurred in the past? Then, if you have a recommended life, you know where you are?
Can you normally see any evidence of fatigue before cracking?
What is SEM and TEM ? What do they tell you from the plastic replica?
Cheers,
John.
RE: Fatigue remaining life
TEM - transmission electron microscope - uses transmitted energy
You cannot "predict" failure point for a single given unit. Failure and fatigue values are based on ensemble statistical behavior.
TTFN
RE: Fatigue remaining life
S1 x Nf1 = K
This is the stress x cycles to failure at that load. The K represents the product of total damage of the material at the test temp. Now:
S1 x N1/Nf1 is the fraction of damage at max load. If N1 is half the cycles to failure, then the material has been damaged 50%. Further:
S1 x N1/Nf1 + S2 x N2/Nf2 +....= K
This is the summation of damages at various loads. The cycles to failure at various loading should be tested.
I am skeptical that micro examination of material will lead to conclusions of life remaining.
RE: Fatigue remaining life
RE: Fatigue remaining life
Fatigue strength doesn't correlate well with YS like it does with UTS.
Tsukuba,
Is the fatigue eval. stressing enough to eventually cause cracking? If you know the stresses and cycles pretty well you might be able to calculate an estimate-assuming the service cond. are also well known.
Otherwise you can use the replica method. It is commonly done on turbine shafts etc. if there is a known area where cracks will start-such as a keyway, etc.
RE: Fatigue remaining life
Metalguy,
I suppose I don't know the previous record of the fatigued structure.
Dose it mean we cannot estimate its remaining fatigue life by a nondestruction method in this case?
RE: Fatigue remaining life
I think the only practical way to use a NDE method is the plastic replica. They can provide amazing detail, and identify very tiny cracks. You'd have to know where the stresses were 9and will be) highest in order to avoid having to replicate the whole thing.
What sort of structure do you have?
RE: Fatigue remaining life
The fatigued structure is stainless steel.
The plastic replica method can provide information about the tiny cracks only on the surface. Even we know these tuny cracks exist, we still cannot say how long the remaining service life of the structure has.
RE: Fatigue remaining life
Periodic NDE, using methods such as UT, eddy current, penetrant testing are what you need to do, unless you are very sure of the future stresses and cycles. Even then it's still a good idea to perform the NDE.
If you don't know much about the stresses except that they do not change (level and cyclic nature), you will be able to predict the usefull life AFTER you build a database via NDE for your particular application.
If you aren't sure the stresses will stay the same during use, you don't have much choice except to perform periodic NDE.
RE: Fatigue remaining life
The plastic replica may be the only NDE method to evaluate the remaining fatigue life.
RE: Fatigue remaining life
A more correct relationship of stress and cycles would be:
S^3 x N = const
If this is evaluated at failure, then
S1^3 x N1 + S2^3 x N2 + ... = const for several stresses and associated cycles.
(My previous post showed S raised to unity power.)