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.

The capabilities available in AFGROW Predict, Preferences menu are often missed by many casual AFGROW users. One very important preference is the growth increment option. The growth increment controls how often AFGROW will recalculate the normalized stress intensity solution (beta value) as a function of the amount of crack growth since the previous calculation. There is a trade-off between the accuracy of the prediction and the total run time. For cases in which crack extension per cycle is very small (say, 1e-6 in.), the change in the beta-value will be minimal (for most cases). For this reason, computer run times can be substantially reduced by taking advantage of the growth increment option.

The default growth increment is set to be 5% of the crack length since the last calculation. This means that the growth increment for each calculation will increase with the crack size. Since most of the calculated life is expended at the shorter crack lengths, this is a reasonable approach. Many users may not realize that the output beta (and associated K) values depend on the growth increment selected. If a user desires the "exact" beta (or K) for each cycle, then it is important to select the appropriate cycle x cycle growth increment. Please refer to the User's Guide or on-line help for more information.

 For example:

Advanced Model (Double Corner Cracked Hole)

Plate Width = 3.25 in., Thickness = 0.25 in., Hole Dia. = 0.15 in., Hole Offset = 1.625 in.

C = 0.0534, A = 0.0655

Constant Amplitude Loading, Smax = 20, R = 0 

The following run times resulted as a function of growth increment (shown for AFGROW Version 4.11.14 and 4.12.15):

Note: The decrease in run times for the new version of AFGROW is attributed to the faster CPU used. Also, the growth increment of 0.0 is the cycle x cycle option. 

As can be seen, there can be a very substantial run time penalty for running a cycle x cycle prediction. However, the increase in prediction accuracy may not warrant the increase in run time. This, of course, can vary from case to case. If a low growth increment is resulting in long run times, it may be helpful to experiment with a larger increment.

The bottom line is that users should be aware of this and use a growth increment that is suited to a given problem.

The following suggestions have been made for ways to improve AFGROW and the AFGROW Web Site:

AFGROW Improvements:

• Add Residual Strength Plotting Capability
• K-Solution for an Offset Loaded Lug
• Stress-Life Crack Initiation Model
• Expand Multi-Crack Capability
• Include the Bending Stress Fraction in the Bearing Stress Fraction Calculator Option
• Ability to Model Cracks in a Bonded Metallic Repair Patch
• Ability to Edit Parameters by Selecting an Item in the Status View
• Include Warning to Save Data Before Closing AFGROW

Web Site Improvements

• Publish AFGROW Tips and Tricks
• Retardation Parameter "Rules of Thumb"
• Add Material Data/Documentation

This will allow users to adjust spectrum tension and compression values independently. The primary purpose of this capability is to allow users to model interference fit pins as a pre-load which may be considered to be a function of crack length. The modifications will be in either equation or tabular form. The general capability will allow for a number of input parameters to be used in order to make the capability as flexible as possible.

Compression Factors for Stress Intensity Solutions

Stress intensity solutions for the crack opening mode are applicable under tensile loading conditions. Examples include a crack at an open hole, and a crack at a hole under bearing load. In the first case, the stress field in the assumed crack plane under remote tension, is quite different in magnitude under remote compression. In the second case, the crack will not see any compression due to the geometry. Although K is not defined under compression loading, the K-values calculated under compression use the same solution as the values calculated under tension loading. These values are then used to determine a crack growth rate for the resulting R-value. The ability to correct this situation may help improve crack growth life predictions for load spectra that includes significant amount of compression loading.

Beta Correction for Advanced Through-the-Thickness Cracks

AFGROW currently allows users to apply beta correction factors to classic models and corner cracked advanced models. The capability will be added to the advanced through crack cases. This will provide additional modeling flexibility for these cases.

K-Solution for an Edge Notch in a Plate

A new advanced solution for corner or through crack at an edge notch in a plate is being developed. This will permit users to continue life predictions for continuing damage cases in which a crack has grown from a hole to the near edge, and another crack was growing simultaneously on the opposite side of the hole. Some have reported using an edge crack to simulate this continuing damage condition, but the result will be extremely conservative since the hole radius will tend to reduce the actual K-value for this geometry. The availability of this new solution will provide better accuracy for these types of problems.

Comments/suggestions are welcome