Breaking down new features of upcoming AFGROW release 5.2

Fatigue cracks in welded and notched machine components and structures are subjected to complex stress fields induced by the external load and residual stresses resulting from the manufacturing processes. The stress fields are often non-uniform and available handbook stress intensity factor solutions for such configurations are usually unavailable, especially in the case of surface breaking cracks at notches. Cracks in patch (welded and adhesive) repaired components are subjected to similarly complex stress fields. The proposed method makes it possible to calculate stress intensity factors for cracks subjected to arbitrary stress fields by using the generalized weight function technique. Therefore the mathematical and physical nature of available weight functions is to be discussed, including the principles of the weight function technique used for the determination of stress intensity factors. The general weight function and calculated stress intensity factors will be validated against various numerical and analytical data. The numerical procedure for calculating stress intensity factors for cracks subjected to non-linear stress distributions such as those near notches and welds will be briefly discussed as well. It will be also shown that the proposed weight functions make it possible to calculate the Crack Opening Displacement field used in the evaluation of the critical load or the critical crack size. The method makes it also possible to calculate stress intensity factors for irregular non-elliptical cracks along the entire crack contour. The method is particularly suitable for modeling fatigue crack growth of arbitrary two-dimensional surface and embedded cracks and the fatigue growth of cracks in welded structures.

Finally examples of the weight function application for the determination of the critical crack size and the simulation of fatigue crack growth will be demonstrated using in-house computer software.

Accurate quantification of crack tip stress intensity values is paramount in the analysis of damage tolerant structures. The present analytical investigation seeks to determine the stress intensity solutions for crack geometries outside the existing valid solution space and expand the analyst’s ability to capture representative crack growth behavior. The primary focus of this investigation is to better quantify the crack growth behavior of single quarter-elliptical corner cracks from centrally located holes in finite width plates under various loading conditions (remote tension, bending, and pin loading). Some current finite width corrections, such as the Newman-Raju corrections, have been calculated from straight through cracks and universally applied to all locations along the quarter-elliptical crack front. Early investigations into the validity of this application seem to indicate that this correction procedure produces stress intensity values +/- 30% from reality for relatively narrow plates (width/depth<6). Furthermore, the crack depth to length ratio and depth to thickness ratio can significantly influence the applicability of the current finite width corrections. The analytical investigation utilizes the three dimensional virtual crack closure technique (3D VCCT) and a well-structured completely hexahedral element mesh. Stress intensity values are generated for a range of crack depth to crack length ratios (a/c = 0.01 to 10), crack depth to sheet thickness ratios (a/t = 0.01 to 0.99), hole radius to sheet thickness ratios (r/t = 0.1 to 10), and sheet width to hole diameter ratios (W/D = 1.1 to 20). This effort is being executed under a DoD Technical Corrosion Collaboration program.

A series of tests were performed to empirically determine a shut-off overload ratio (SOLR) for varying thicknesses of 7075-T7351 aluminum alloy using the Generalized Willenborg Retardation Model to provide data for aircraft wing crack growth analysis for a trainer aircraft. The test matrix consisted of three different plate thicknesses: 0.125 in. (3.18 mm), 0.375 in. (9.53 mm), and 0.625 in. (15.88 mm). Each 4 in. X 20 in. (102 mm X 508 mm) plate had a 0.25 in. (6.35 mm) center hole with a corner crack (EDM notch). All specimens were machined from the mid-thickness of the same 0.75in thick plate of material. A positive correlation was found between plate thickness and SOLR. The AFGROW software tool was used to model the tests which, through an iterative process, were matched to model results by changing the SOLR, thus generating an empirically determined SOLR value for each thickness. The SOLR for each respective thickness was found to be about 1.93 for the 0.125in thickness, 1.99 for the 0.375in thickness, and 2.09 for the 0.625in thickness for the c-length cracks. The a-length cracks were not found to be well correlated with the simulations, growing faster in the tests than in the AFGROW simulations.

The current Advanced Solution in AFGROW for multiple thru-cracks is a collection of curve fit solutions to FEM results for a large matrix of geometric variables. The curve fits were based on a selection of parameters that appeared to have the most influence on the solution. The curve fit solution was reasonable for most cases, but was off by as much as 10% for some extreme cases. Recent User inquiries have prompted a review of the solution for the case of 2 through cracks in a plate, and significant improvements have been made. This presentation presents the current state of the solution, the new proposed solution, and the resulting improvement in accuracy.

Practical structural parts often have three-dimensional cross sections, but are modeled as flat plates in AFGROW because the available fracture mechanics models do not include the true geometries. In order to determine the appropriate fracture model width to use when the true geometry is not flat, a study was performed comparing the stress intensities for flat plates of varying widths against those for T, L, Z, and J sections of various dimensions. From these results, guidance is given for determination of an appropriate flat plate (model 1030/2020) width to use in AFGROW depending on the actual structural configuration.

Gulfstream's current damage tolerance analysis tools are no longer supported. A replacement tool is required that will increase efficiency, accuracy, and reliability of analyses as well as generate report-ready results. Also, the new tool should be able to work with Gulfstream's currently established methods. Afgrow allows for user defined solutions to be incorporated as plug-in models. All existing in-house solutions, as well as material databases, were converted for use with Afgrow and analysis results using Afgrow compare favorably with Gulfstream's existing methods. Afgrow XML output files are used to generate report-ready analysis results, including residual strength plots, crack growth plots, analysis summaries and tabular outputs of the crack growth data. Afgrow has proven to be an effective solution for modernizing Gulfstream's crack growth analysis methods.

Since the first K-solution for bearing loading was included in AFGROW, continued improvements have been made to address a number of issues that were discovered as a result of comparisons to more detailed FEMs. It was initially assumed that the bearing solution for a wide plate could be compounded with the finite width correction for an open hole. This was very quickly shown to be incorrect based on a comparison to the open hole solution for a narrow plate with an equivalent remote gross section stress. Later, the offset correction for the bearing load case was also shown to be significantly different from the open hole solution. The latest issue that has been discovered for the bearing load case is the effect of an in-plane bending constraint on the solution for a single thru-crack at a hole in the center of a plate. This presentation will discuss the influence of the in-plane bending constraint on the K-solution for a cracked hole under bearing loading and the implications for the current AFGROW bearing solutions.

The desire for breakout, or global to local, modeling is becoming more common for finite element analysts who wish to use global PATRAN (or similar) nodal output as boundary conditions for application to detail, high-resolution local models. This is especially true for investigations of cracked components, as the problem becomes multi-scale and high resolution in the crack region is necessary for accurate representation of SIF’s. This presentation covers unique methodology for using the StressCheck TLAP (Total Load at a Point) methods to apply PATRAN nodal output data to a local, organically-cracked (e.g. non-elliptic) StressCheck model for calculation of accurate SIF's/Betas.

The T-38 and A-10 analysis groups have been developing an AFGROW plug-in that allows the user to ascertain a more physics based life prediction by coupling Finite Element Modeling (FEM) and AFGROW's crack growth analysis capability. Within StressCheck this new tool meshes, solves and extracts Stress Intensity Factors (SIF) along the crack face at specific points defined by the user. The resultant SIFs are imported into AFGROW and the crack is allowed to evolve at the rate defined by the SIF extracted at each point along the crack front. This seamless integration allows for a more accurate prediction of the crack shape and rate at which the crack propagates within a body.

This presentation will be a follow-on discussion from the 2012 AFGROW Workshop presentation and will describe the recent developments within the COM, increased stability of the code and features for analyzing the results. Discussion will also include plans for the future development of the COM along with AFGROW and StressCheck. The presentation will conclude with a demonstration on the process of building, meshing solving, extracting SIFs and the crack growth life prediction of a complex geometry.

The solution for a single primary crack with a series of secondary cracks in a row of holes in an infinite plate is one of the most requested solutions we get at our training classes. Discussions with A-10 Engineers have indicated that this solution may also be useful for the Ogden ALC as well. Development work is on-going at AFRL, and the solution will be made available as soon as it is released to the public. The purpose of this presentation is to provide more information about the details and status of the solution(s), and to discuss some interesting effects that have been discovered.

The presentation focuses on correlation of 3D fatigue crack growth in residual stress bearing aircraft structural details, comparing test data against predictions based on linear elastic fracture mechanics. The main focus of the presentation is a review of recent work regarding laser shock peened titanium wing attach lugs. Test information includes data on residual stress present in test articles in addition to more typical information (test article geometry, load spectrum, etc.). Test data are reported as fatigue crack growth as a function of applied cycles. An initial calculation of full field residual stress in test articles due to laser shock peening was developed using finite element analysis (ABAQUS® and ERSToolbox®). Crack growth from an initial flaw, due to applied cyclic and residual stresses, was predicted using boundary element analysis (BEASY®), by repeatedly calculating stress intensity factors and incrementally extending the crack. The methods allow for arbitrary crack front evolution in 3D. Comparisons of observed and predicted crack growth behavior provide useful insight. The final element of the presentation describes current work to extend the modeling capability to high-strength aluminum lower wing areas, with fatigue crack growth from cold expanded holes.

Information on the latest research and development efforts and plans beyound AFGROW Release 5.2.