Abstracts - AFGROW Workshop 2016, Layton, UT

Workshop Day 1 - September 13, 2016


Welcome and Inroduction (Continental Breakfast)

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.

10:00-10:30 Group Discussion


Stress Ratio Influnce on da/dN with the Generalized Willenbrog Model
Chad King (USFA, T-38 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 point-by-point 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.

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 A-10 ASIP Analysis Group from displacement data provided through A-10 ASIP sustainment engineering funds. Data will be presented for multiple 2024-T351 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 .

11:45-12:00 Group Discussion

Lunch Break

Countersink Bushing Repair-Lower Wing Skin T-38
Luke Bracken (Mechanical Engineering Intern, USAF, T-38 ASIP Analysis Group, Hill AFB, UT)

The lower wing skin on the T-38 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 T-38 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 T-38 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 T-38 group.

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 well-known 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).

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 commercially-available 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.

K-Solutions for Through Cracks at Holes under Bi-Axial Loading
James Harter (LexTech, Inc.)

Many users have expressed interest in the ability to predict crack growth from a fastener hole under bi-axial loading. As a first step, K-solutions (K1 and K2) for a single through crack at a hole have been developed for a few crack orientations and bi-axial 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.

3:00-3:15 Group Discussion


Through-the-Thickness Cracks at an Offset Pin Loaded Hole
Cordell Smith, James Harter (LexTech, Inc.)

Stress intensity factor (SIF) solutions for through-the-thickness cracks at an offset hole subject to bearing loading were determined using a new closed-form solution that will be included in AFGROW, release 5.3. The solution is a modification of the closed-form 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 through-the-thickness crack. One of the modifications is a change to the in-plane bending correction to account for symmetry effects of two, independent through-the-thickness 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.

4:00-5:00 Group Discussion

Workshop Day 2 - September 14, 2016


Continental Breakfast

Benchmarking Problems in Fatigue Crack Growth Analyses
Robert Pilarczyk, (Hill Engineering, LLC, Rancho Cordova, CA)

The evolution of Broad Application for Modeling Failure (BAMF) over the past five years has resulted in a robust capability integrating AFGROW and StressCheck. The ability to handle unique geometries, loading, crack shape evolution, and residual stress is now available. To evaluate and compare the capabilities, multiple benchmark cases were formulated and completed. The results of these cases will be presented.

Recent Development in AFGROW COM and Plug-In Applications
Alex Litvinov (LexTech,Inc.)

This presentation will include practical examples of the Plug-In and COM capabilities of AFGROW, and describe lessons learned during application development. Future changes and additions to COM and Plugin interfaces will be presented and discussed.

9:30-10:00 Group Discussion


Recent Development in Modeling and Simulation of Cold Expanded Holes
Guillaume Renaud, Min Liao (National Reserach Council Canada)

Cold expansion (CX) of fastener holes has been used for decades to enhance the fatigue lives of critical airframe structures. Although the benefits of this technology are undeniable, current requirements for structural life extension and inspection scheduling remain very stringent. This presentation highlights recent development in improving and validating a finite element model used to determine CX residual stresses. This development includes in-progress studies on the mechanics of the CX process, the effects of various analysis parameters on the simulation, residual stress relaxation after overloads and underloads, and results from a recent benchmarking exercise being carried out in partnership with FTI and SwRI. Finally, this presentation discusses future plans to integrate the simulation model with AFGROW to perform integrated crack growth and calculate life improvement factors.

David Wieland (SwRI)

Over the last 10 years or so Southwest Research Institute (SwRI) has been developing and using an Excel COM for analyzing the fatigue critical locations on the T-38.

A brief overview of the development and use of the COM will be provided. SwRI will give a demo of the COM demonstrating its capabilities.

Using the BAMF Framework to Develop a Parametric Lug AFGROW Plug-in
Joshua Hodges (USAF, T-38 ASIP Analysis Group, Hill AFB, UT)

This presentation will outline the process of modifying the MuPMuC (Multi-Point/Multi-Crack) BAMF code to build a parametric model for running within the AFGROW Plug-in framework. The presentation will provide details on the modifications necessary to allow multiple input parameters in the Plug-in. It will also present recent updates in the MuPMuC BAMF code and a few suggestions for Plug-in modifications. The discussion will conclude with a quick demonstration of the Lug Plug-in developed for AFGROW demonstrations.

11:45-12:00 Group Discussion

Lunch Break

Typical flight spedtra from a L/ESS aircraft
Craig Brooks (Analytical Processes / Engineered Solutions)

3 years of data collection on a LESS aircraft from four air stations provide insights into the combinations of major load excursion and significant higher R- ratio excursions.

The importance of understanding the data curves and approximations within AFGROW become apparent. The presentation is to provide an informal discussion and awareness to the AFGROW community of some of the applications from DoD and DHS aircraft usage.

Incorporation of Load Redistribution in Analytical Correlation of Spectrum Crack Growth Test Data
Luciano Smith, Laura Domyancic (SwRI)

Spectrum fatigue crack growth tests were performed to determine the amount of crack growth retardation in fatigue critical locations. These test specimens included multiple fasteners in order to most accurately simulate the aircraft locations and the associated bearing load transfer. Two different geometries were investigated, and finite element analysis was used for both to determine how the loads redistributed through the fasteners as the cracks grew. This presentation covers the steps used for accurate modeling of the test configuration, the implementation of the finite element results into the AFGROW analysis, and the resulting impact on the analytical results.

Current Development Overview
James Harter, Alex Litvinov (LexTech, Inc.)

Information on the latest research and development efforts and plans beyound AFGROW Release 5.3.

2:45-3:00 Break
3:00-5:00 Group Discussion and Adjourn