The fatigue benefits of neat and interference-fit fasteners (IFF) are well known in the aerospace industry. However, while some filled hole correction factors have been developed and used, the open literature is lacking documented empirical bases to validate analytical approaches the USAF might apply in damage tolerant analyses (DTAs). This presentation discusses investigation into an approach that maps linear and nonlinear 3D finite element stress contour data onto a multipoint crack growth analysis using BAMpF. The method is applied to available IFF fatigue life datasets from a variety of sources including AFFDL-TR-74-47, AFFDL-TR-75-93, recent APES fatigue test data for filled holes, and the soon-to-be tested matrix of specimens in the ERSI IFF round robin effort. Predicted fatigue life enhancement factors (versus open hole life) are compared and correlation quality of the method to available test data is discussed.

The integration of SMART|DT with AFGROW enables more advanced and detailed risk assessments that were previously too computationally expensive to perform. With the addition of an AFGROW interface, SMART|DT can now perform crack growth analyses coordinated with its unique capabilities—such as adaptive importance sampling, generation of crack growth spectra from mixed usage profiles, and inspection schedule optimization. This presentation will focus on how the capabilities of AFGROW and SMART|DT can be used complementarily and explore the possibilities now available that are beyond deterministic crack growth based probabilistic damage tolerance analysis.

This work is the result of a very successful integration of AFGROW with SMART|DT that we are excited to share with the AFGROW user community.

MIL-STD-1530D requires durability analysis for USAF aircraft with some general stipulations as to how they be performed, but leaves many of the details to the discretion of the organizations performing the analyses. APES developed the IDS-MACS (initial discontinuity state – matched analytical crack size) procedure that gives analysts an understanding of the initial flaws that cracks start from and the approximate crack growth rates that are seen from those small flaws. Testing was recently performed for three A-10 materials, and durability analysis was performed with the use of the IDS-MACS data from those tests, with different analytical methods compared against each other. A method is proposed that combines the use of the IDS-MACS results and standard damage tolerance analysis procedures.

The application of residual stress fields in an analysis for a coldworked hole is crucial for extracting the maximum amount of life from a structure. A residual stress field must be correlated to test data prior to its application to a real-life structure. Differing aspects such as geometry, peak stress, material, and spectrum must be considered before applying a validated residual stress field.

This presentation aims to compare three control points on the A-10 containing residual stress fields validated from the same test data. Two control points utilize the same spectra and have similar geometry and material properties and yet produce notably different BAMpF lives. The last control point - while similar in most aspects - contained enough variation to warrant the use and correlation of another RS field. The variation of each control point, application of each RS field, and outcome of each analysis will be presented. Additionally, a comparison of predicted lives between the previous and current revisions of the SA220R0207 report will be made to provide insight on the effect of both analytical approaches.

The project replicates the experimental test and FASTRAN results from the 2020 DTA ASSIST Challenge, (2020 AFROW Conference). Building upon previous iterations of this challenge (2021 AFGROW Workshop Round Robin, 2022 Walker-Newman IRAD, and 2023 Boeing IRAD Testing), this project analyzes the 2022 Walker-Newman IRAD data, focusing on the effectiveness of crack-growth prediction tools for 2024-T3 and 7075-T6 aluminum alloys. The analysis examines the impact of two spike overloads in constant amplitude (CA) loading on a contact tension (C(T)) specimen (R=0.01) within the constraint-loss regime (plasticity-induced crack closure).

Transport aircraft are manufactured using semi-monocoque construction and as a result of weight saving measures it often times employs the full capability of the structure by leveraging its post buckled strength. In the industry, this is primarily associated with static strength however several airframe structures are also permitted to buckle during normal flight. The following presentation provides an illustrative demonstration of one approach for taking into account the change in internal loads to produce fatigue spectra which reflect the buckling effects. Two examples are provided as representative of different airframe structures

Across the global aircraft fleet, many weapon systems operate far beyond their original mission requirements. This is certainly true for the United States Air Force (USAF). Economic pressures, evolving mission demands, and slower replacement rates compared to past decades have driven the USAF’s average fleet age steadily upward. Service life goals for most aircraft now extend well beyond what their designers envisioned, creating significant challenges in both cost management and mission readiness.

The USAF Academy’s Center for Aircraft Structural Life Extension (CAStLE) plays a significant role in contributing to the body of knowledge required to safely operate aircraft to their required service goals. This year CAStLE awarded a new IDIQ contract to support sustainment projects and is available for task order contracts through 2032. Recent efforts under the previous contract include DADTA analysis, material and component testing, repair design and implementation, documentation updates, flight loads data recording, and teardown inspections.

A core part of CAStLE’s mission is integrating operationally relevant research into the cadet curriculum and faculty development, ensuring the next generation of engineers gains hands-on experience with real-world sustainment challenges. This presentation will provide an overview of CAStLE’s capabilities, highlight recent sustainment projects, and explore future opportunities to support the USAF and broader DoD enterprise.

Approved for public release: distribution unlimited. PA number USAFA-DF-2025-838

Accurate values of stress intensity functions K_I (φ) for single and double cracks have been calculated for 20 a/c-, 10 a/t-, 3 b/t-, 8 R/t- and 19 W/R-values for tension, bending and pin loading. The total number of new K-functions are about 60 million.

The ongoing work with developing K-databases for lugs with oblique loading is reviewed. The nonlinear lug problem requires very fast solution procedures in order to derive estimated 7 million accurate K-solutions to available CPU-time.

In the lecture we describe the parameter spaces, the computational procedures (including the nonlinear lug analysis) and the error control schemes used.

A set of 3D FEA fatigue crack growth solutions were performed to demonstrate convergence and validation against marker bands from two experimental datasets. These incremental solutions considered different crack front edge definitions, from a classical 2-DoF (elliptical shape) to a fixed 4-, 5-, 7-, 9-DoF along predefined directions. Solution convergence with respect to an increased DoF crack front increment definition is demonstrated by using an unconstrained multi-DoF crack front representation (120-DoF).

All these solutions are compared to the fractography data to provide a quantitative error assessment as a validation benchmark. For the same accumulated loading cycles, a comparison between the marker bands and all these solutions will be provided to demonstrate that with a larger DoF crack front representation the solution approaches the marker band data.

Accurate determination of residual stress fields is critical for reliable fracture mechanics predictions. This presentation introduces a new, automated method for back-calculating residual stresses from marker band data, using StressCheck to compute stress intensity factors and BAMpF to validate results. By reformulating the inverse problem as a convex optimization, a unique and globally optimal solution can be obtained efficiently. The resulting algorithm achieves sub-second solve times, drastically reducing the analysis time compared to traditional, iterative methods. Results from several datasets demonstrate realistic stress field shape and excellent correlation with experimental crack growth data, validating the accuracy and efficiency of this approach for approximating residual stresses in metals.

Multi-point solutions utilize FEA capability to derive stress intensities for customized scenarios at numerous points along the crack front. The crack growth engine is then fed the stress intensity for each point independently to determine the appropriate amount of crack growth for that point. However, when the stress intensity is passed to the crack growth engine, the crack growth engine is unaware of the geometry constraints around that point and therefore must make assumptions on what the stress state is for that point. A study was completed to evaluate how much error could potentially be introduced due to inaccurate assessments of the stress state. The presentation also highlights that the stress state index used in AFGROW was derived based on toughness data and is assumed to be directly applicable the plastic zone size. A retardation model correlation study was completed to evaluate he sensitivity of retardation model correlation to stress state (and consequently plastic zone size) for a wide array of scenarios.

This presentation will cover the details of the upcoming release, bug reports and all things related to BAMpF!