8:00 
Welcome and Inroduction (Continental Breakfast)

8:3010:00 
AFGROW, Release 5.3 Status
James Harter, Alex Litvinov, Cordell Smith (LexTech, Inc.)
AFGROW, Release 5.3 includes several new features and capabilities. The most important new capabilities/features include:
 A new, advanced solution for through crack(s) at holes under bearing loading
 Ability to use different crack growth rate data for each growth direction (a and c)
 Ability to use different crack growth rate data for spectrum defined temperature tags
 Solution for corner cracks at the “knuckle” of a C/S hole
These new capabilities will be discussed in detail, and the timeline for the release will be provided.

10:0010:30 
Group Discussion 
10:3010:45 
Break 
10:4511:15 
Stress Ratio Influnce on da/dN with the Generalized Willenbrog Model
Chad King (USFA, T38 ASIP Analysis Group, Hill AFB, UT)
The Tabular Lookup material model in AFGROW allows the user to input their own crack growth rate (da/dN) curves for one to ten stress ratios. When da/dN is provided for more than one stress ratio, the Tabular Lookup model uses the Walker equation on a pointbypoint basis to interpolate/extrapolate da/dN for any two adjacent stress ratios. The user specifies the da/dN and stress ratio bounds for the extrapolation. The question is presented: What stress ratios would be most beneficial for a da/dN test program?
An example is presented which shows typical stress ratios for a jet trainer usage and how these stress ratios are adjusted when using the Generalized Willenborg model to account for crack retardation. The adjusted stress ratio (Reff or R(final) in AFGROW) is used with ΔK to predict crack growth for each cycle. The need to understand the relationship between the crack growth rate and retardation models is presented. Recommendations are made for crack growth rate test programs.

11:1511:45 
Applacation of Uncertainty Quantification for the Residual Stress Messurement at a Cold Expanded Hole
Scott Carlson (SwRI)
The ability to take analytical advantage of deep residual stress processes in fatigue crack growth analysis
is making great strides forward within the U.S. Air Force. Currently we have analytical tools that can
solve for Stress Intensity Factors (SIF) along the entire crack front, in the presence of an imposed
residual stress field and a far field applied stresses. Through the implementation of these engineered
residual stresses it may be possible to extend both the initial and recurring inspection interval for some
of our most critical structural elements, thus saving program costs while increasing aircraft availability.
One of the questions that rests firming the path of the implementation of these engineered, beneficial
residual stress fields is, what type of confidence bound does one place on the stress/strain field prior to
its implementation into the the crack growth simulation. Due to the dynamic three dimensional
characteristic of most of these residual stress fields, a simple decrees by X% to the stress field may not
be the best way to represent the physics of a “conservative safety factor”. This presentation will
present an overview residual stress measurements developed within the A10 ASIP Analysis Group from
displacement data provided through A10 ASIP sustainment engineering funds. Data will be presented
for multiple 2024T351 coupons with a centered 0.5 inch hole, within a 0.25 inch plate. Each of the
coupons was Cold Expanded (Cx) to the “Low” end of the FTI expansion spec. In addition the
presentation will provide uncertainty results based on a repeatability study of the data, and compare
these results to a single measurement uncertainty methodology. Fatigue crack growth predictions will
be provided demonstrating the effect of shifting the residual stress field based on a statistical
methodology. Questions will be posed regarding other methods that may be used to apply uncertainty
quantification to the application of deep residual stress fields into the damage tolerance paradigm .

11:4512:00 
Group Discussion 
12:001:00 
Lunch Break 
1:001:30 
Countersink Bushing RepairLower Wing Skin T38
Luke Bracken (Mechanical Engineering Intern, USAF, T38 ASIP Analysis Group, Hill AFB, UT)
The lower wing skin on the T38 Talon is attached to the wing structure with countersunk fasteners. Due to high cyclical loads, cracks frequently form at these countersunk fasteners. To repair these cracks the T38 analysis group has been using a countersunk bushing repair. The standard repair on these cracks has been to drill out the countersunk fastener, ream the hole until the crack is removed, and install a countersunk bushing to allow an original size fastener. To determine the betas for these holes with countersunk bushings, the current T38 approach uses a macro written by Southwest Research Institute to determine a beta correction factors and a solution from AFGROW is generated. This presentation will discuss countersunk bushing repairs, effects on crack growth using this countersunk bushing repair, and whether betas generated in this manner are accurate. A Stress check model was also used to determine the betas for cracks growing from this bushing repaired hole. A comparison will be made between these Stress check models and the standard practices employed by the T38 group.

1:302:00 
Weight Functions for a finite width plate with a radial crack at a circular hole
Jihw Kim, Michael R. Hill
(Department of Mechanical and Aerospace Engineering, University of Califorina)
This work develops accurate weight functions for a single crack at a hole in a finite width plate for various hole sizes. In order to develop an accurate weight function, we first obtain accurate stress intensity factors, using the finite element method (FEM), for a reference load case of uniform stress on the crack line. Following the earlier approach for developing a weight function suggested by Wu and Carlsson, we fit the reference stress intensity factor data from FEM to a smooth analytic function; however, for the open hole it is necessary to adopt a piecewise polynomial to fit the stress intensity factor data, in place of the single polynomial suggested by Wu and Carlsson. We validate the new weight function for the case of remote uniform applied stress, which induces a stress field on the crack line exhibiting the wellknown stress concentration at the hole, and for which we have accepted stress intensity factor solutions. The new weight functions provide stress intensity factors that agree very well with results from two commercial fracture mechanics software packages (AFGROW and NASGRO).
Comparing results from the new and earlier weight functions shows good agreement some crack line stress fields, but errors of a few percent for other stress fields, with the new weight function providing more reasonable results. The improved quality of the new weight functions is due both to the new reference solution for uniform crack line stress and to the piecewise fit to the reference stress intensity data. Trivial changes to the FEM model allow us to provide additional weight functions for the cases of symmetric double cracks at a hole (by adding a symmetry plane to the FEM mesh) and a single crack at a hole in a square plate (by reducing the length of the FEM mesh).

2:002:30 
Development of a Compendium of Mechanical Properties of USAF ASIP Materials
Dallen Andrew, Carl Popelar, Ph.D.(SwRI)
Southwest Research Institute (SwRI) has performed basic mechanical characterization of aerospace materials in support of damage tolerance analyses for several USAF aircraft. To make this data available and usable to others, summary reports were compiled that contain the tensile strength, fracture toughness, and fatigue crack growth data for many commerciallyavailable materials that have been characterized by SwRI over the past decade. This presentation gives a brief overview of the steps that were taken to make this valuable treasure trove of data available to others.

2:303:00 
KSolutions for Through Cracks at Holes under BiAxial Loading
James Harter (LexTech, Inc.)
Many users have expressed interest in the ability to predict crack growth from a fastener hole under biaxial loading. As a first step, Ksolutions (K1 and K2) for a single through crack at a hole have been developed for a few crack orientations and biaxial load ratio cases. These solutions will be presented along with discussion on how this type of solution could be implemented in a future AFGROW release.

3:003:15 
Group Discussion 
3:153:30 
Break 
3:304:00 
ThroughtheThickness Cracks at an Offset Pin Loaded Hole
Cordell Smith, James Harter (LexTech, Inc.)
Stress intensity factor (SIF) solutions for throughthethickness cracks at an offset hole subject to bearing loading were determined using a new closedform solution that will be included in AFGROW, release 5.3. The solution is a modification of the closedform solution developed by Bombardier and Liao at NRC. The Bombardier and Liao solution is based on compounding and superposition of known solutions for a crack at an open hole and a wedge loaded throughthethickness crack. One of the modifications is a change to the inplane bending correction to account for symmetry effects of two, independent throughthethickness cracks at a hole located at various positions along the plate width. There were also corrections made to the infinite plate wedge solution for relatively small cracks (C/R < 1) and a minor correction for larger cracks based on available FEM solutions. This presentation will briefly discuss the modifications made to the solution and show comparisons of the modified AFGROW solution and available finite element model (FEM) results.

4:005:00 
Group Discussion 