Abstracts - AFGROW Workshop 2011, Layton, UT

Workshop Day 1 - September 14, 2011

9:00 – 10:00
Improved Offset Correction for Cracks at Holes Under Bearing Load
James Harter - LexTech, Inc

The offset correction for the axial loading case has been assumed to be applicable for the bearing load case. Recent FEM modeling has shown that a different correction must be used for this case. This presentation will summarize the work done to develop a new offset hole correction for the bearing load case.

10:15 – 11:00
Using the New Multi-Channel Spectrum Format
Alex Litinov, James Harter - LexTech, Inc

To date, AFGROW has used a single channel load spectrum. Because of this limitation, K-solutions for combined loading (axial, bending, and bearing) are limited by the assumption that fractional components of each load case are constant. Several AFGROW Users have expressed a keen interest in the ability to input independent loading spectra for axial, bending, and bearing loads.

This presentation will describe the format and use of the new multichannel AFGROW spectrum. The new format will allow AFGROW to accommodate multiple, independent loading spectra, and will be flexible enough to be expanded in the future as needed. The capabilities of the new Spectrum Design and Editing Tool will also be discussed.

11:00 – 12:00
Managing the Damage Tolerant Reassessment for the A-10 – Impacts from Life Extension, New USAF Requirements, and Emerging Technology
Robert Pilarczyk - USAF AFMC OO-ALC/GHAEJ

The A-10 has experienced a rocky past, with several planned retirements throughout its 36 year service history. The A-10 has proven to be a critical element in providing close air support in recent conflicts. Its performance in these conflicts, as well as the fact that it is the only viable platform for this role in the immediate future, has resulted in the decision to extend the service life through at least 2040. This presentation will discuss the evolution of the damage tolerance analysis for the A-10 over the past 9 years, key efforts to extend its service life in the environment of changing USAF requirements, emerging technologies, and increased analysis capabilities.

1:00 – 1:30
Accounting for residual stress in aircraft damage tolerance assessments
Professor Michael R. Hill - Mechanical and Aerospace Engineering, University of California, Davis
Adrian T. DeWald and John E. VanDalen - Hill Engineering, LLC, Rancho Cordova, CA

Aircraft damage tolerance assessments can be improved by explicitly including the influences of residual stresses induced during manufacture. The presentation summarizes results of building-block studies performed to understand the accuracy of residual stress engineering methods. Key tools for damage tolerance assessments unique to residual stress bearing structure are validated methods for the measurement of residual stress fields, for separation of the effects of applied and residual stresses during fatigue crack growth and residual strength tests, and for combination of applied and residual stress when forecasting fatigue crack growth and residual strength. Coupon-scale experiments in aircraft materials are described that include residual stresses from processes used in life extension of aircraft structure (laser shock peening and hole cold expansion), and focus on measurement of residual stress fields and observation of fatigue crack growth and fracture behavior in a range of coupon conditions. The presentation describes residual stress measurement and fatigue testing methods used in the experiments. Analysis of test data and correlation of observed behavior with that expected for linear material response provide useful validation of the residual stress engineering methods described.

1:30 – 2:30
An Investigation on the Residual Stress Field of a Cold Worked Hole through the Tensile Field

Cold-working has been extensively used in the USAF to extend lives of fastener holes in fatigue critical locations. However, deep compressive stress from the cold-worked hole negate the ability of researchers in running constant amplitude tests representative of a cold-worked hole failure at typical service loads (~10-20 ksi) in the laboratory environment. To change the perspective of the problem, a series of experiments were performed from the far side of the hole to investigate the behavior of a cold-worked hole from the opposite side, a scenario similar to a continuing damage type problem. Overall life trends, two techniques for incorporating residual stress fields into an analysis, and residual stress field solutions will be discussed in detail during the presentation.

3:45 – 4:14
Simulated Notched Hole Betas
Cordell Golson - USAF AFMC OO-ALC/GHAEJ, Zachary Whitman - SWRI

Mechanical damage on the A-10 from routine maintenance and repair is an unavoidable problem. A common problem is longitudinal scoring in the bore of holes due to fastener removal, improper drilling/reaming operation, or fastener installation. To better understand a particular type of damage an investigation was started analyzing the effect of a simulated notch through the bore of a fastener hole. A preliminary investigation was conducted modeling a notch through the bore as a “through the thickness” classical crack model in AFGROW. The findings were extremely conservative and took the calculated risk to unrealistic levels. In order to get a more accurate analytical representation, several notched models were created in StressCheck. The notch models were then used to identify the specific variables that affect the β solution: thickness, diameter, notch depth, etc. These models were used in conjunction with AFGROW to develop an understanding of how a notched hole effects the crack life of a part. This paper will describe the techniques used to develop β solution for notches in AGROW and other conclusions derived from the study.

Workshop Day 2 - September 15, 2011

Basics of StressCheck + AFGROW Application Development: An Engineering Perspective
Brent Lancaster, Anil Mehta - ESRD

StressCheck’s flexible COM API and superior fracture mechanics capability make it well suited to be paired with LexTech’s AFGROW for life prediction calculations. Therefore, a partnership has been forged between LexTech, Inc. and ESRD, Inc. to explore AFGROW plug-in tools which use StressCheck FEA as a solver engine. The interoperability between AFGROW and StressCheck allows computation of 2D and 3D elasticity solutions when closed-form or empirical relationships do not exist. This presentation will cover the basic fundamentals of designing and developing a StressCheck plug-in tool for AFGROW, with the target audience being experienced StressCheck users who wish to develop engineering applications driven by AFGROW.

Recent Developments in AFGROW COM and Plug-In Applications
Alex Litvinov, James Harter, Thomas Latta - LexTech, Inc

The COM abilities of AFGROW allow users to automate manual tasks, incorporate AFGROW services into proprietary software, and enable the re-use of code that has been pre-built and tested.

AFGROW Plug-In technology allows the creation of Proprietary, Closed-Form, Tabular / Interpolative / Extrapolative, and External-K (if available) User-Defined custom solutions.

This presentation will include practical examples of the Plug-In and COM capabilities of AFGROW, and describe lessons learn during application development.

Future changes and additions to COM and Plugin interfaces will be presented and discussed.

1:15 – 1:30
Next Generation Crack Growth Predictions - Coupled Finite Element Modeling and Crack Growth Analysis
Joshua Hodges - USAF AFMC OO-ALC/GHMEJ, Kaylon Anderson - USAF AFMC OO-ALC/GHAEJ

Traditionally, stress intensity solution development and crack growth predictions are developed independently. For standard geometries and loading, this typically works quite well. However, for complex geometry and/or loading, varying crack aspect ratios, multiple cracking scenarios, etc., this classic approach doesn’t always fit. Synergies between the factors that affect the overall crack shape and growth are not necessarily captured, and thus can have a significant influence on the crack growth life.

The T-38 and A-10 analysis groups have developed a generic AFGROW plug-in that couples stress intensity development via StressCheck with AFGROW’s crack growth analysis capability. This new capability allows AFGROW to open, update, solve, and extract solutions from parameterized StressCheck models automatically. Solutions are imported into AFGROW, crack growth is calculated, and the new crack geometry is sent back to StressCheck. This process is repeated automatically until a defined failure or stop criteria is reached. This seamless integration allows for more accurate crack growth predictions in complex situations and eliminates many of the assumptions that are required with the traditional approach.

This paper describes the development of the code, keys to building a proper StressCheck model, limitations of the plug-in, as well as future applications and directions of this capability. It also presents comparisons to Classic and Advanced AFGROW solutions. Finally, test data will be presented to help focus the experimental validation process.

1:30 – 2:00
Influence of Secondary Crack Growth in Continuing Damage Cases

Continuing Damage models involve the addition of a secondary, 0.005-inch crack at a hole, directly opposite the primary crack under consideration. These models are currently analyzed by assuming the influence of one crack on the other to be negligible; thus, a 3-phase approach is used with Phase 1 for the primary crack, Phase 2 for the secondary crack, and Phase 3 for the continuing damage crack growth. In order to investigate the effects of multiple flaws on crack growth and how this will impact the current continuing damage approach, several models were developed in StressCheck, including both cracks simultaneously, for two Fatigue Critical Locations (FCLs). The beta factors were calculated and compared to show the effects that are currently neglected in the continuing damage models. Two optional methods for modeling these cases are outlined and performed, and the resulting crack growth curves are compared for the two FCLs under consideration.

2:00 – 2:30
Use of Weight Function K-Solutions to Account for Stress Gradients
Scott Fields - Boeing

A brief overview of weight function solutions will be presented along with a comparison to the use of beta corrections to account for stress gradients. Potential applications using the currently available weight functions in AFGROW will be discussed in addition to possible future development work needed to expand this capability.