In case you've forgotten/misplaced your login password and don't like the new one that's been assigned, here are the directions to change your password:

1.      Login with the generated password;

2.      After you login in, navigate to following page. It is not on the menu, so you have to copy and paste the url:

http://www.afgrow.net/profile.aspx

3.      Click edit, go to the right top part of the edit profile page, type your old password, new password twice, and click the “reset password” button.

LexTech & ESRD(StressCheck) shared a exhibitors booth at the AA&S Conference in Baltimore, MD. Since we are close to our next major new version release, we demonstrated several of the new features/capabilities that will be forthcoming. These include: the new multi-channel spectrum tool, advanced model for a corner crack at a countersunk hole, updated solution for the offset correction for bearing load, user notification when K-solution bounds have been exceeded, and several updates to the User Interface (similar to Office 2010).

There was a good amount of traffic and interest in both AFGROW and StressCheck.

ESRD made a significant announcement that it will soon be releasing a capability they call the Handbook Capability which will allow users to run existing StressCheck models at a fraction of the cost of a full StressCheck license. The "fraction" is yet to be made public, but we are told that it will be much more attractive to smaller companies & individuals. This will be very good news to anyone who may want to take advantage of the AFGROW/StressCheck Plug-In capability to run live predictions for cases with an existing StressCheck model. This will open the door for the creation and release of solution libraries for more complex geometries to a much wider user base.

AFGROWCharts2012.pdf (972.55 kb)

A short paper by Mr. James Harter discussing the use of AFGROW to predict the crack growth life of a structure containing internal through cracks in a row of fastener holes is now available on the AFGROW web site. The paper provided details on the use of the Advanced Model Interface and provides a direct comparison with FEM results using StressCheck. To obtain this document (http://www.afgrow.net/userarea/) go to the My AFGROW section.

It goes without saying that load interaction is important if the life prediction is to represent reality and not simply be a go no go measure.  Often times go no go is good enough.  But a lot of the time in the management of aging fleets, a real life prediction can be the difference between retirement or return to service.
 
Retardation can account for 70 percent of the life and that in it self should create wonder!  Well where does this miraculous life enhancement come from?  The neat thing is that nobody really knows.  It is true that few men created (made up) some useful models based upon rational guesses.  These models have been tweaked for years and that’s what makes them so very useful.  They can be tuned to the specific aircraft, usage, and component.  All that’s needed is an attempt at the real spectrum, and the testing equivalent and a test lab.  The number of coupons is left as an exercise.  Of course another avenue is to have service history of cracking and a detail record of how the aircraft was used.  At one time we referred to this as correlating to flying fatigue test articles.  This is because even the OEM has to assume how the customer is going to use the aircraft, and they test to their assumption, reality is sometimes different.
 
So what?  I suppose any one who has even a cursory knowledge of fracture has come across the notion of the fictional “plastic radius.”  It is fictional because the extent of plasticity at a crack tip is definitely not shaped like a radius, so much for the physics of the problem.  A lot of smoke and mirrors and late night sections have been spent on the subject of the plastic radius in all its manifestations, from plane strain plane stress, fracture toughness coupon requirements, to net section yield as a crack propagates. 
 
It is no surprise that load interaction investigators got hooked on the notion of a plastic interaction zone which increases and/or decreases with spectra level and crack length, some later even attempted account for plasticity in the wake of the crack along the flanks.  Simply amazing rational insights but not yet reality.  The point of this is that current state of load interaction models all rely on the notion of plasticity, some “out and out” some “inferred” but all do. 
 
If one digs into the subject even a little he will quickly realize that the benefit arising from plasticity IS dependent upon constraint.  The assumed closure mechanism that all the load interaction models are based on either at the crack tip or along the flanks as the case may be is totally dependent on there being enough elastic material surrounding the crack to provide sufficient strength to squeeze or constrain the plastic deformed region back into its original (undeformed) shape (volume). 
 
The “how much is needed” is why I started this note.  I don’t know the answer but I know its important and possibly the real mechanism of retardation.  But one thing I do know is that when the crack is approaching a free edge, either the part or another hole, there certainly is not sufficient elastic material surrounding it to create the constraint or closure forces.  Without closure forces there is no retardation. 
 
So you may grab one of the interaction models and do a fantastic job of modeling the cracking scenario and make a perfect prediction but you may be off and in the wrong direction!  Why, all the interaction models assume that there is “enough” elastic material surrounding the crack tip.  There are of course mitigating circumstances such as very little life available due to the steepness of the beta and/or the load level of the spectra.
 
So caution is urged in applying load interaction models carte blanche to the crack growth problems.  Edge ligament cracking is a prefect example of retardation decaying, as are those pesky continuing damage cracks into adjacent holes.  It seems then the only time that the current load interaction models are fully effective is out in the middle of a panel with no discontinuities.
 
And we haven’t discussed relaxation of the plastic zone with time under self weight, perhaps for aircraft that sit on the ramp for weeks between flights. What happens to the load interaction then?
 
I can’t remember anyone really pointing out this subtle nuisance … so I thought I might.

While trying to verify some AFGROW solutions for the Single Corner Crack at Hole in bending, I was comparing the Stress Intensity Factors (SIFs) that I compute in a separate Excel spreadsheeet (VBA). My VBA program computes the SIFs from the original Newman Raju curve fits in Chapter 9 of Computational Methods in the Mechanics of Fracture, 1986. I am using version 4.11.14.0 06/29/2006, which I think is last version before AFGROW got really big (since it didn’t include the Fawaz curve fits for corner crack(s) at a hole)—I am running AFGROW on an old computer which is severely RAM limited compared to 2010 computers.

There are actually 3 references given in the AFGROW help for this crack scenario and the bending load, The first one can be downloaded from the AFGROW site, http://afgrow.net/downloads/ddownload.aspx, the 2^nd can be downloaded from a NASA website, and the third is unavailable.

1) Newman, J.C., and Raju, I.S., "Stress Intensity Factor Equations for Cracks in Three-Dimensional Bodies Subjected to Tension and Bending Loads," Chapter 9, Computational Methods in the Mechanics of Fracture, Elsevier Science Publishers B.V., 1986

2) Zhao, W., J. C. Newman, Jr., M. A. Sutton, X. R. Wu, and K. N. Shivakumar, "Analysis of Corner Cracks at Hole by a 3-D Weight Function Method with Stresses from Finite Element Method," NASA Technical Memorandum 110144, July 1995.

3) Zhao, W. and Newman, Jr., J. C., Electronic Communication, Unpublished NASA Langley Research Center Results, 24 February 1998.

I noticed substantial differences between what I was computing with my VBA (20% or more) and what AFGROW was computing, suggesting that whatever equations were in Ref. 2 and 3 were affecting my results:

My configuration was double (n=2) or single corner crack (n=1), so I show results for both:

                         VBA          VBA                            AFGROW               AFGROW

n                        SIFc           SIFa                                SIFc                         SIFa

1                     3.95091       3.6774                             3.952                        3.678

2                     4.01409       3.73621                           4.112                        3.736

SIFc is the SIF for crack “c” and SIFa applies to crack “a.” VBA are my spreadsheet solutions. Geometry was 2 inches wide, thickness 0.25 inches, hole diameter 0.25 inches, and both crack dimensions are 0.05 inches. Bending stress is 10 ksi, no tension (BSR=1, TSR=0, no bearing). Interesting that all these SIFs are close except for the double corner crack, SIFc.

After double checking all of my equations, I contacted AFGROW support to try to resolve the differences. Turns out there is a 4^th reference not mentioned in Help that summarizes the equations used to modify the bending SIFs from the Ref. 1 above that you can download from AFGROW’s site:

Modification of Stress Intensity Factor Equation for Corner Cracks From a Hole Under Remote Bending by Zhao and Newman.

Jim Harter gave me this document, which contains modifications to 3 sub-equations in the Newman Raju curve fits, coded those into my VBA; still had a difference. I went to another computer that had a more recent version of AFGROW, 5.1.5.16 07/26/2010, which gave me the following comparison:

                                                                                AFGROW               AFGROW

                         VBA          VBA                              5.1.5.16                    5.1.5.16

n                        SIFc           SIFa                                SIFc                         SIFa

1                     3.95091       3.6774                             3.9509                      3.6774

2                     4.01409       3.73621                           4.0141                      3.7362

Turns out there is a small undocumented bug in this old version of AFGROW; mystery solved.

Several Users have asked me to post some helpful "rules of thumb" that may be useful when performing life prediction tasks. While I am happy to do it, I must first state that while I have found this information to be useful, each user must take full responsibility for their own life predictions. Neither I nor LexTech take any responsibility for how this information is used.

Typical Retardation Parameters

Willenborg SOLR

Aluminum (2.5 to 3.0)

Titanium Alloys (2.2 to 2.7)

Steel (1.8 to 2.2)

Closure Model

Aluminum (3.8 to 4.2)

Crack Growth Rate Data

Aluminum Alloys

Threshold @R=0 (1 to 2) Note, while this is the typical range, I have seen reputable data as high as 3 in a few cases. Always look at data for a similar alloy if there are no reliable data for the alloy of interest.

A typical crossing point for most aluminum alloys is approximately 1.0 E-5 in/cycle at Delta K = 10 Ksi-sqrt(in) for R=0

Also, note that a "double-knee" is typical for aluminum alloys tested in air. The first "knee" generally ocurrs at approx. 4.0E-07, and the second around 2.0E-8 in/cycle.

As we approach the first commercial release of AFGROW, it makes sense to do a little "code cleaning." Based on user input (or lack thereof), it appears that the bonded repair analysis capability is not being used. Therefore, we are planning to remove the capability from the new release.

We are asking for input from anyone who would be adversly affected by this decision.

The current AFGROW release (V4.12.15) was built as a 32 bit application for VISTA and XP. Under 64 bit Windows 7, it will try to install itself in the Program Files(x86) directory. However,only the AFGROW and NASGRO database directories will be copied to this location. The work-around is to copy all of the files in the AFGROW directory from a VISTA or XP machine to the AFGROW directory on the Windows 7 box. In order to save user-created input files, it is necessary to install MSXML 4.0 (see the blog entry, also under tips and tricks). To use AFGROW COM capabilities, the afgrow.tlb file must be registered manually.

AFGROW won’t show up under your list of programs to run, but you can copy a shortcut to your desktop. This should not be a problem with the upcoming AFGROW release (V5.01.16)

Spectrum Filters

There are situations in which it may desireable to modify the stress intensity solution for a given geometry depending on whether the applied stress is tensile or compressive. The new sprectrum filters will allow a user to modify the tension and compression values in a given stress (or load) spectrum. This will, in turn, modify the resulting stress intensity solution.

Users may select the spectrum filter option as shown below:

 JavaScript engine

While AFGROW includes an option to enter modification factors in the usual tabular form, it is now possible to use JAVA scripting language within AFGROW to calculate the filter values. This will allow for more complex fuctions to be used to define the filters. JAVA scripting tutorials are available on the internet, but a simple example follows:

Example Life Prediction With and Without the Tension Filter

Countersunk K-Solutions

The next AFGROW release will include countersunk stress intensity solutions for a single or double, symmetric corner crack under tension loading.

This is the first installment for the countersunk geometry and has been provided by Dr. Scott Fawaz (USAFA). We understand that many users have been waiting for this solution, and will also be anxious to have the bending and bearing solutions. We will include additional solutions as soon as possible.

Continuing Damage Solution

A new, closed-form solution for corner and thru cracks at a u-shaped notch under tension loading. The solution was developed for edge distances from 0 to 49.5. Bending and bearing solutions are not yet available, but the tension solution should provide a significant improvement in life prediction capability for continuing damage problems.

We added an Installation Notes document on our Documentation and Supporting Information Page.

If you are experiencing problems saving or reading an AFGROW input file, you need to download and install MSXML 4.0 Service Pack 3 (Microsoft XML Core Services), as described in the AFGROW Installation Notes document.