Welcome and Introduction (Continental Breakfast)

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
• Bearing solution for advanced through crack(s) at a hole

Break

Previous AFGROW presentations discussed the influence of retardation models and the resulting effective stress ratio, highlighting the need for additional crack growth rate data at negative stress ratios. As a result of these investigations, a test program was executed to develop crack growth rate data for common T-38 materials at multiple negative stress ratios. The presentation will review this new data, comparing and contrasting to previous material model extrapolations, as well as revisiting RLO. Concurrently, a structured material fitting process was utilized. Damage Tolerant Analysis comparisons will be presented to review impacts from new crack growth rate data and the material fitting process.

The AFGROW developers and some users have expressed interest in knowing how the Structures Branch (AFLCMC/EZFS) uses AFGROW. This presentation includes an independent analysis performed by EZFS in 2016 using AFGROW to assess potential impacts to a new development program of updated Fatigue Crack Growth Rates (FCGR) properties for Titanium 6AL-4V Beta Annealed (BA) developed by a legacy aircraft.

Lunch Break

In general, fatigue crack growth rate testing is performed using one of a handful of standard specimens, all of which have a through-thickness crack. Testing was performed to develop near-threshold growth rates for 2024-T351 plate and 2024-T3511 extrusion at two different stress ratios (0.1 and -0.3) and two different specimen geometries. The specimens included an edge cracked plate and a centered hole in a plate, both with a corner crack instead of the typical through crack. This presentation will include some background information leading to the motivations for the testing, a description of the testing including the unique requirements necessitated by the two-dimensional nature of the crack, test results, and potential impact on analysis.

Break

Continental Breakfast

This presentation will give an overview of the new updates made to the AFMAT crack growth rate database. Some of the new features added to the database include: expanded search criteria, updates to the crack growth rate plots, and the ability to download tabular look-up material data files for over 50 materials. This presentation will also briefly go over the new test data added to the online database.

With being actively involved within the Afgrow community a number of Gulfstream requests have been implemented in Afgrow. Most of these requests have been related to plug-in development or implementing a secure way to load proprietary material data. With release of Afgrow version 5.3 all current requests have been implemented. At last year’s user workshop, the bending factor for standard solutions was flagged to be too conservative. A brief investigation showed a reasonable correlation using the 2/3-factor for bending solutions for both center and edge crack standard Afgrow solutions. During this investigation, the behavior of cracks close to fillets were also looked at. Further a discussion of a spectrum development problem and concluding with a recent application of Afgrow for an in-service aircraft structure.

Break

Democratization of numerical simulation through the development and dissemination of expert-designed “engineering simulation apps” is rapidly gaining attention for durability and damage tolerance (DaDT) applications. Such apps must satisfy certain technical requirements to ensure their reliable use due to the high accuracy requirements in the computation of fatigue crack propagation life. Proper application of numerical simulation procedures requires expertise in computational engineering that is not widely or readily available. Standardization deployed by means of Smart Engineering Simulation Apps (SESA) can leverage this expertise for recurring analysis tasks, similar to the expertise of specialists in applied mechanics made available through engineering handbooks. Because classical handbooks have limitations in model complexity and scope, SESA developed by expert analysts can deploy verified solutions obtained by numerical means allowing models of much greater complexity for users who do not need to have expertise in numerical simulation technology.

By Smart Engineering Simulation Apps we mean FEA-based software tools developed for standardization and automation of recurring analysis tasks and process workflows for use by non-specialists. Designed to fit into well-defined analysis processes, engineering simulation apps capture institutional knowledge and best practices, can be shared by engineering groups at different geographic locations and produce consistent results by tested and approved analysis procedures. When designed to meet the requirements of Simulation Governance, simulation apps for engineering use are “smart” because their embedded intelligence enables simple, accurate, fast, efficient, and reliable (S.A.F.E.R.) simulations with built-in quality assurance, essential for the non-expert user.

This is particularly important in the area of DaDT where life estimate procedures require accurate computation of stress intensity factors along the crack front and the shape of the propagating crack is solution-dependent. An example of a simulation app for the prediction of fatigue life for cracks emanating from cold-worked holes, developed in support of the AFRL-SBIR Phase III project “Deep Residual Stress Method Demo Program”, and to accelerate the transition of new technology into professional practice, will be presented. The example simulation app has been connected to a residual stress database to facilitate the input of residual stress profiles and communicates with AFGROW via COM to determine the crack growth rate (da/dN) by interpolation, from the available da/dN-ΔK curves of the material, using the values of SIFs for the mechanical load and the residual stresses determined by the app along the crack front.

Engineered residual stress (ERS) is increasingly being applied as a way to extend the durability and damage tolerance performance of primary structure in various industries including aerospace and power generation. To reduce development costs, reduce the development time, and increase the effectiveness of the final design, methods that accurately predict residual stress for surface treatments are being developed. ERStoolbox® is a software suite which predicts full field residual stress for a variety of processes including conventional shot peening (SP), laser shock peening (LSP), and cold hole expansion (CX). An overview of ERS-toolbox® will be presented along with example applications.

The BAMF software, developed by the USAF (A-10/T-38), has evolved over the past seven years to take on unique and challenging fatigue crack growth analysis problems. Recent technological advancements include: complex geometries/crack scenarios, multi-point multi-crack analysis, and inclusion of residual stresses. Hill Engineering plans to facilitate the continued viability of BAMF for internal and external use. Hill Engineering plans to continue to update and maintain the software and support customer needs. This presentation will discuss the current state of BAMF, the near-term transition plan, and long-term thoughts to keep the capability relevant, viable, and growing. The presentation will also provide an opportunity to get input from the user community, similar to the framework established by LexTech with AFGROW.

Lunch Break

Abstract: Research has been underway for the past few years to quantify the effect a fatigue crack has on the residual stress field introduced by the Cold Expansion (Cx) process. In an effort to quantify this impact the Contour Method has been employed to determine the residual stress state in two aluminum alloys (2024-T351 and 7075-T651) after a fatigue crack has been propagated in it. Each coupon had a 0.50inch centered Cold Expanded (Cxed) hole that was designed to hit the “Low” end of FTI’s applied expansion specification. Eight (8) coupons were produced for each alloy and a range of fatigue crack sizes were propagated in them. This presentation will discuss how the residual stresses obtained through the Contour Method have been implemented into BAMF to provide a prediction of a fatigue condition and compare those results to fatigue test data. Two different methods were developed to define the modified residual stress traction stress equations which were utilized in the fatigue life predictions. The effects of the residual stress statistical distribution, as discussed at last year’s AFGROW Workshop, was also be implemented into the life prediction matrix, thus showing the effect of both the modified residual stress traction equation and the statistical distribution of the residual stress state on the final life prediction.

AFGROW is a very powerful crack growth analysis tool which allows the fracture mechanics analyst to efficiently access dozens of standard crack scenarios (with tension, bending, and/or bearing loads) in both through and part-through crack geometries. Stress Intensity Factors (SIFs) for each crack scenario are computed with either curve fits or table lookups—normally derived from finite element solutions or classic crack handbook solutions such as Tada, et al, “The Stress Analysis of Cracks Handbook.” Each of these methods assumes a specific far field load profile which causes a particular uncracked stress state on the intended crack plane—for instance, the tension loading of a plate with an elliptical surface crack assumes a constant tension far field, and causes a constant tension field in the uncracked state at the intended crack plane.

However, the fracture mechanics analyst will often need to have the SIFs for a crack embedded in a non-standard stress field; that is, the uncracked stress state does not match the uncracked stress state of any AFGROW standard solution geometry. Fortunately, AFGROW has an efficient method for adjusting the computed SIFs for the new, nonstandard stress field, either with Normalized Stress correction or Beta Corrections Factors in the Beta Correction Factor interface.

We demonstrate the use of the Normalized Stress correction on an elliptical surface flaw in the fillet of a 3D stepped flat tension bar. First, the uncracked stress at the intended crack plane in the fillet is calculated. Second, some minor calculations and curve fits are made to fit the data to the AFGROW specific format. Lastly, the data is read into the Beta Correction Factor interface and SIFs are computed for a few cracks. For comparison, SIFs are also computed with the p-version finite element software StressCheck.

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