Abstracts - AFGROW Workshop 2010, Layton, UT

Workshop Day 1 - August 31, 2010

9:00 – 10:00
Lessons Learned While Developing K-Solutions for Pin Loaded Holes
James Harter - LexTech, Inc

The compounding method is commonly used to determine stress intensity solutions for structural geometries for which there are no published alternatives. In the case of a crack growing from an open hole, an offset hole correction has been available for some time. This correction has been applied to cracks at bearing loaded holes, resulting in significant error when compared to detailed FEM solutions. This presentation will summarize the lessons learned during the development of a new offset hole correction for the bearing loaded fastener hole.

10:15 – 11:00
Using the On-Line Material Database
Alex Litvinov, James Harter - LexTech, Inc

Damage tolerance evaluations of aircraft structures are never better than the quality, applicability, and availability of the material properties used for the analyses. The material properties required to perform damage tolerance evaluations of primary aircraft structure include: Plane Strain Fracture Toughness, Plane Stress Fracture Toughness, Fatigue Life (A vs. N), Fatigue Crack, Growth Rate (da/dN vs. delta K), Sustained Load Crack Growth Rate (da/dt vs. Kmax), R-Curve (KR), and K1 (Environmentally Assisted Cracking). The majority of these data have been generated by researchers in the aerospace and other industries; however, there is a no single, searchable database for these fracture mechanics properties.

The AFMAT material database is now accessible for all AFGROW, Version 5 Users. It contains data from 5 different sources: Damage Tolerant Design Handbook (5-Volumes), UDRI, NASA Johnson Space Center in-house data, NASA FLAGRO data, and USAF Aging Aircraft Program data from Purdue University. There are approximately 20,240 different sets of easily searchable and filterable test data available for database users. This presentation will demonstrate how to get the most out of this new resource.

11:00 – 12:00
Investigating Spectrum Crack Growth Retardation Methods and Models for the A-10
Dallen Andrew and Michelle Creps - OO-ALC/GHMEJ, Hill AFB, Utah

AFGROW is the primary tool used by the USAF A-10 ASIP team for crack growth predictions. Two studies were accomplished to investigate the current the A-10 Damage Tolerance Analysis (DTA) parameters. One study looked at modifying material lookup files, while the other focused on comparing different retardation models within AFGROW.

A-10 Generalized Willenborg retardation values (SOLR) have received criticism for being un-conservative. Therefore, material lookup file development was re-evaluated to assess the impact on SOLR and crack growth life. This evaluation required modifying SOLR values to fit test data. Due to the new SOLR values, fatigue critical locations were re-analyzed to determine the effect on life predictions and inspection intervals. Shifts in crack growth curves and the affect on SOLR will be discussed.

Multiple retardation models exist within AFGROW including the Hsu, Wheeler, Fastran, Closure, and Generalized Willenborg model. The Willenborg model is currently being utilized for A-10 DTAs. Little research has been done to investigate the potential use of alternative retardation models. A study was completed to compare some of the models against spectrum test data to understand which retardation model was most representative. For various fatigue critical locations the Hsu, Fastran, Closure, and Generalized Willenborg will be compared.

1:00 – 1:30
Application of AFGROW to Cold-Expanded Holes in Highength Steel
Whitney Ponzoha - Valdez International Corporation (CAStLE)

Cold expansion is a mechanical process that can slow crack propagation in fastener and other thru holes on aircraft structural components. Slower crack growth translates to increased useful life. Cold expansion increases the life of the part thus increasing the time between inspections saving both time and money. It is essential to have the capability to accurately model these critical parts with cold-expanded holes. The Air Force Growth Fracture Mechanics and Fatigue Crack Growth Analysis software tool (AFGROW) was used to obtain this capability. The structural component being considered has several complicating factors (geometry, material, number and orientation of holes, in-plane bending, and residual stresses due to the cold expansion) that require using the advanced features in AFGROW. The purpose of this program was to obtain test data for the part and then determine the necessary parameters for use with the advanced features of AFGROW to model crack growth from a cold-expanded hole (there is not currently a feature within AFGROW explicitly for modeling cold-expansion effects).

There were three main features used in AFGROW for this program: (1) manually inputting calculated residual stresses from FEA; (2) utilizing the Shut-Off Overload Ratio (SOLR) option; and (3) manually inputting β correction factors determined from testing. Inputting the measured values manually for both the residual stresses and β correction factors, required Component Object Model (COM). This interface enhances AFGROW’s capabilities by making it run through information more efficiently. The COM-AFGROW interface also allows for multiple outputs from multiple inputs.

In this program, AFGROW was run with the β correction factors and residual stresses read directly into AFGROW from COM. The resultant crack growth curve and life prediction from AFGROW were then compared to the test data within the AFGROW program. At that point, a SOLR was input utilizing the Willenborg model and the resultant crack growth curve and life predictions were compared to the test data again. This process was repeated until the most accurate crack growth curve and life, in cycles, results were obtained. The results from AFGROW allow those with similar geometry and loading conditions to use the SOLR, β correction factors, and residual stresses to model the impact of cold-expanding a hole.

1:30 – 2:00
EIFS COM programming (raw data, updated guess methods, convergence, tolerance, etc.)
Matthew J. Hammond, P.E. - Valdez International Corporation (CAStLE)

The Durability and Damage Tolerance Analysis (DADTA) approach adopted by the USAF offers engineers applicable tools and guidelines to predict important life management data. One generally accepted practice to determine the applicable life of a structure is to use the Equivalent Initial Flaw Size (EIFS) assumption. An EIFS distribution is a calculated distribution of initial flaw sizes that can be assumed to exist at damage sites at the inception of the structure. These initial flaws will grow to in-service observed crack sizes after some number of flights (load) cycles equivalent to a specified amount of usage. This presentation will outline several of the many factors and considerations that the DADT analyst must consider when using this technique and the benefits of using a commercially available LEFM and Crack Growth program like AFGROW. Discussion about user input data (e.g. final crack size, loading spectrum) and the updated guess and iterative analysis process through the Component Object Module (COM) interface with AFGROW will also be included.

2:00 – 2:30
Investigation of Exceeding Newman-Raju Solutions Thickness to Diameter Bounds: Tensile Loaded Plate with Corner Crack at a Hole
Chad King, Robert Pilarczyk, James Gyllenskog - OO-ALC/GHMEJ, Hill AFB, Utah

AFGROW is currently utilized to perform damage tolerance analysis (DTA) for A-10 and T-38 aircraft in support of fleet management. The most common DTA model used to analyze these aircraft is the classic solution Single Corner Crack at a Hole under tensile loading (75% for A-10, 45% for T-38). This solution is based on the Newman-Raju stress intensity factor equations; equations which were developed from finite element analysis (FEA) solutions for a wide range of parameters. Of particular interest is the ratio of plate thickness to hole diameter (t/D), which ranged from 0.25 to 1.0 in the Newman-Raju FEA.

The A-10 and T-38 have numerous fatigue critical locations where part thickness exceeds hole diameter; therefore, use of the Single Corner Crack at a Hole classic solution at these details is suspect as the Newman-Raju t/D upper bound is exceeded. This calls into question the accuracy of the predicted damage tolerant life.

This study compares the stress intensity factors from AFGROW (solution extrapolated beyond the Newman-Raju bounds) and the FEA software StressCheck for several t/D ratios that exceed the upper limit of 1. Comparison of the damage tolerant life for specific fatigue critical locations will also be presented. Recommendations will be proposed for DTA models where the Newman-Raju bounds are exceeded.

2:45 – 3:45
Converting Experimentally Derived Data into AFGROW Beta Corrections
Scott Carlson - OO-ALC/GHMEJ, Hill AFB, Utah

As the United States Air Force continues to fly their aging aircraft well beyond their originally structural design life it has become necessary for sustainment engineers to sharpen the pencil and utilized all available resources to ensure structural safety while at the same time allow the Warfighter to continue to fly at the required high-paced war-time sortie rate. To accomplish this the A-10 Thunderbolt II’s Aircraft Structural Integrity Program (ASIP) analysis team has been developing experimentally derived Beta (β) corrections to provide more accurate predictions of fatigue cracks propagating from cold expanded holes. These β corrections were then input into AFGROW, one of the USAF’s standard crack growth tools, to develop crack growth curves to model fatigue cracking experienced during testing. The experimentally derived β corrections spoken of in this paper/presentation were developed through lab testing and modeled in StressCheck a Finite Element Modeling (FEA) software and allow for more accurate crack growth predictions of how cracks propagate through the three-dimensional residual stress field produced by cold expansion.

Through the use of these experimentally derived β AFGROW is able to more accurately predict the physics of fatigue crack growth, as it would be seen in the aircraft structure under the influence of cold expansion’s residual compression and tension fields. Utilizing experimentally derived β provide increased accuracy of the crack growth predictions and can possibly allow the aircraft to stay in the fight for longer periods of time without returning to the depot for maintenance thus saving time and money while ensuring structural integrity.

3:45 – 4:15
Comparison of Advanced Continuing Damage Model & Classic Single Edge Through Crack Model
Tim Allred - OO-ALC/GHMEJ, Hill AFB, Utah

Workshop Day 2 - September 1, 2010

Interoperability between AFGROW & StressCheck
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.

StressCheck has the capability to produce robust stress intensity factor extractions for arbitrarily shaped cracks in 2D and 3D, and pass these values to AFGROW interactively via the Windows COM framework. Additionally, each stress intensity factor can be verified via automatic quality assurance procedures for maximum error control. This is critical, as a moderate error in stress intensity factor can lead to a large error in life prediction.

The concept of using global FEMs to determine component loads and load redistribution, feeding local FEMs with cracks to determine stress intensities and the resulting crack growth life
Robert Pilarczyk, Greg Stowe & Hazen Sedgwick - OO-ALC/GHMEJ, Hill AFB, Utah

Over the past few years the A-10 SPO has encountered cracking in both the wing and fuselage, specifically the lower aft skin of the wing and the upper longeron strap of the fuselage. Throughout the management of this cracking, the A-10 ASIP team has developed a finite element modeling (FEM) and crack growth prediction methodology to better understand the behavior of the structure with various cracking scenarios.

This methodology involves the use of global and local FEMs with and without different crack scenarios, which in turn feed the crack growth predictions. The global FEMs (NX) are used to establish the baseline load distribution in each component as well as the load redistribution as a result of cracks in the structure. The results from the global FEMs provide the inputs for the local single component FEMs (StressCheck) which incorporate cracks. The outputs from the local FEM are stress intensities as a function of crack size, which are used in the crack growth predictions.

The results of this methodology are used to support short and long term repairs, inspection requirements, risk assessments, and depot induction schedules. This presentation will discuss this modeling and crack growth prediction methodology as well as several examples where it was successfully implemented. The presentation will also discuss the path forward to improve the efficiency of this methodology.

10:15 – 12:00
Building AFGROW COM and Plug-In Applications
Alex Litvinov - LexTech, Inc

COM Automation is one of the most popular and frequently used features of AFGROW. Automation is an industry-standard technology that applications use to expose their objects to development tools and macro languages. The COM abilities of AFGROW allow users to save time and money by automating manual tasks, incorporating AFGROW services into proprietary software, and enable the reuse of code that has been pre-built and tested.

The AFGROW Plug-in capability allows Users to Create/Animate their own structural models for use in AFGROW. It provides the capability for users to develop K-solutions that will manage crack size, solution limits and the general configuration of the solution (K-solution parameters, error checking, prediction process, and screen drawing functionality).

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 some examples of how to create Plug-In models.

1:00 – 1:30
Stress Intensity Values for Finite Width, Small Cracks, and Abnormal Aspect Ratios
Matthew J. Hammond, P.E. - Valdez International Corporation (CAStLE)
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It is normal in the engineering research field to push the envelope on the applicability of available data and research techniques. During the course of some research projects at the Center for Aircraft Structural Life Extension at the US Air Force Academy, the need to simulate small crack growth behavior arrived. The crack sizes in question were outside the solution space for the classic Newman-Raju and the database used in the Advanced models developed by Fawaz for an elliptic corner crack at a hole. An automatic FE Mesh generator was used to create models to be used with the Virtual Crack Closure Technique (VCCT) to determine Stress Intensity values along the crack front. With appropriate quality control, analyses indicate that there may be room for improvement in the Stress Intensity values used in AFGROW for large and small aspect ratio cracks, small finite width plates, and very small cracks. Some of these analyses will be presented along with the current AFGROW data.