Stress Intensity Factor Compounding
Stress Intensity Factor Compounding
(OP)
A couple of questions have been on my mind for a while. They’re a bit wordy, so apologies in advance!
(1) STRESS INTENSITY FACTOR COMPOUNDING METHODS
When I do SIF compounding, I use the equivalent stress method given in “An Improved Compounding Method for Calculating Stress-Intensity Factors”, Rooke.
I have worked for other companies where they used the equivalent crack length compounding method given in “Approximate Stress Intensity Factors Compounded from Known Solutions”, Cartwright & Rooke. This is also given in ESDU 78036.
Equivalent crack length compounding takes more effort to perform than equivalent stress compounding. Is there a reason why it should be used in preference to equivalent stress compounding? I discussed this with my colleagues at the time, but we never really found a reason why equivalent crack length compounding should be preferred.
(2) CONTRIBUTION OF BOUNDARY INTERACTIONS
Compounding methods usually take the form of a sum of geometry factor terms for the ancillary configurations, plus a “boundary interaction term” that is really just an error correction term and often denoted Ye. Something like...
Yr = Y0 + Y0 • Σ( Yn - 1 ) + Ye
Yr = geometry factor of complex geometry
Y0 = geometry factor of reference geometry
Yn = geometry factors of ancillary geometries
Ye = boundary interaction term
I’ve seen calcs where Ye was quantified, and others where it was neglected. I did some quick work with a specific geometry and I found that Ye was ~7% of the Y0 + Y0 • Σ( Yn - 1 ) term for shorter cracks, and about 3% for larger cracks. In another case, Ye was negligible.
How much effort do others put into assessing the value of Ye? Is it ever just assumed to be a percentage of everything else on the right hand side of the of the equation?
Thanks in advance for any info that you can supply!
FastMouse
(1) STRESS INTENSITY FACTOR COMPOUNDING METHODS
When I do SIF compounding, I use the equivalent stress method given in “An Improved Compounding Method for Calculating Stress-Intensity Factors”, Rooke.
I have worked for other companies where they used the equivalent crack length compounding method given in “Approximate Stress Intensity Factors Compounded from Known Solutions”, Cartwright & Rooke. This is also given in ESDU 78036.
Equivalent crack length compounding takes more effort to perform than equivalent stress compounding. Is there a reason why it should be used in preference to equivalent stress compounding? I discussed this with my colleagues at the time, but we never really found a reason why equivalent crack length compounding should be preferred.
(2) CONTRIBUTION OF BOUNDARY INTERACTIONS
Compounding methods usually take the form of a sum of geometry factor terms for the ancillary configurations, plus a “boundary interaction term” that is really just an error correction term and often denoted Ye. Something like...
Yr = Y0 + Y0 • Σ( Yn - 1 ) + Ye
Yr = geometry factor of complex geometry
Y0 = geometry factor of reference geometry
Yn = geometry factors of ancillary geometries
Ye = boundary interaction term
I’ve seen calcs where Ye was quantified, and others where it was neglected. I did some quick work with a specific geometry and I found that Ye was ~7% of the Y0 + Y0 • Σ( Yn - 1 ) term for shorter cracks, and about 3% for larger cracks. In another case, Ye was negligible.
How much effort do others put into assessing the value of Ye? Is it ever just assumed to be a percentage of everything else on the right hand side of the of the equation?
Thanks in advance for any info that you can supply!
FastMouse





RE: Stress Intensity Factor Compounding
(2) never delt with Ye ... i guess if you know Yr then why bother ? i guess you're making use of test results and trying to extraploate to un-tested configurations.
RE: Stress Intensity Factor Compounding
> (1) not familiar with this method, do you have a reference ?
An Improved Compounding Method for Calculating Stress-Intensity Factors
Rooke, D. P., Royal Aircraft Establishment
Engineering Fracture Mechanics
Volume 23, Issue 5, 1986, Pages 783–792
Available online for a fee:
http://www.sciencedirect.com/science/article/pii/0013794486900901
Rooke wrote a book, and although I have never seen a copy, I *think* that it also documents the method.
Compounding Stress Intensity Factors: Applications to Engineering Structures
(Research Reports in Materials Science, Series 1, Vol 1)
Rooke, D. P.
Parthenon Pub Group, 1986
ISBN-10: 1850701105
ISBN-13: 978-1850701101
With equivalent stress compounding, you don’t need to determine aeq and then get the new geometry. It saves a couple of Excel columns for each ancillary solution. That’s not going to change anyone’s life, but it’s a small plus IMHO.
> (2) never dealt with Ye ... I guess if you know Yr then why bother ?
My problem is that 99% of the time, I do not know Yr, so I need to calculate it. I do the usual compounding calc to find Y0 + Y0 • Σ( Yn - 1 ), and then have to make an informed judgement about the relative importance of Ye. Does Ye equal 0%, 5% or 10% of Y0 + Y0 • Σ( Yn - 1 )?
I recognise that determining Ye is borderline inconsequential. Determining its actual value rather than making a judgement is not going to significantly affect inspection intervals. Rooke’s paper provides guidance concerning how Ye can be calced, but even if the information is available to allow it to be assessed, the time is usually not and a faster, but slightly more conservative judgement must be made.
RE: Stress Intensity Factor Compounding
RE: Stress Intensity Factor Compounding
Your point about quoting an impressive conservatism factor on CPL rather than a less-impressive conservatism factor on ∆K is noted, and you are right, they result in the same thing, but the CPL reduction sounds grander!
RE: Stress Intensity Factor Compounding
I guess it depends on how much risk you are prepared to accept.
With values as high as 7% I think it should be considered so unless you can develop a model that tells you when its value is negligible I would think you are stuck witth doing the calcs.