AFGROW, Release 5.4 includes several new features and capabilities. The most important new capabilities/features include: a new, advanced solution for combinations of a corner crack and through crack on either side of a hole; new MSD solutions; new weight function solutions for single corner crack and a single corner crack at hole with the stress distribution in C direction and double/single through crack(s) at hole.

Information on the latest research and development efforts and plans beyond AFGROW Release 5.4

A fatigue crack growth measurement conducted on a 2 meter long API X65 piping coupon loaded in a four-point-bend configuration was used to perform a modeling validation study. The test procedure made use of two consecutive loading blocks, R=0.1 for 10000 cycles and, R=0.5 for 5000 cycles to propagate a circumferential surface crack that is initiated from a notch located midplane on the outer surface of the pipe. The alternating loading blocks allowed formation of beach marks that provide crack size and shape instances that can be correlated with the cycle history. The digitized beach marks from fractography were further used as a validation reference for different modeling solutions. The comparison between beach marks and numerical solutions was conducted along three different directions (crack depth, outer surface and at 45 degrees) as well as the entire crack front representation. The numerical solutions made use of the same alternating loading blocks to allow an appropriate comparison with the experimental measurements.

The 3D FE based modeling procedure that resemble the experimental setup employed the fatigue crack growth rate data provided in BS 7910 standard to automatically complete an incremental crack growth solution and achieve validation requirements.

AFGROW’s beta table capability was also used to provide a fatigue crack growth numerical solution. Using the same 3D FEM based procedure (SimModeler Crack), two beta tables were constructed for two sets of elliptical crack sizes and shapes.

In addition to the two types of solutions described above, the 3D FE incremental based procedure was modified to constrain each crack front increment to an elliptical shape to further provide a verification benchmark against AFGROW beta table.

Available finite width correction functions F_w that have been used in aircraft community for decades were developed based on a very limited number of FE-solutions, often of unknown accuracy and by mixing 2D/3D solutions with partly unverified assumptions used for inter-/extrapolation. The accuracy of the F_w-functions obtained in this way was in most cases estimated from comparisons with the FE-solutions that was used to derive the F_w-functions. Hence, the accuracy of the existing F_w-functions in the entire parameter space (R/t, W/R, a/t, a/c) has remained unknown until now.

In the present lecture we derive very accurate finite width correction functions F_w (R/t, W/R, a/t, a/c) for a single quarter-elliptical crack (dimensions (a, c)) at a straight shank hole (radius R) in a plate (thickness t, width 2·W and height 10·W) subject to tension, bending and pin loading.

In 1966, Schijve found that the transition from flat-to-slant (plane-strain to plane-stress) crack-growth behavior occurred at a "constant" crack-growth rate, independent of the stress ratio (R). Others had proposed that ΔK or Kmax controlled the transitional behavior. Newman (1966) had provided additional test data from the NASA Langley Research Center to support Schijve’s conclusion that crack-growth rate was the controlling parameter. In 1968, Elber discovered the crack-closure phenomenon, whereby the crack surfaces are partially-closed under tensile loading. Cracks only grow when the crack tip is fully open. Elber proposed to modify the Paris expression as

dc/dN = C (ΔKeff)ⁿ (1)

Since the crack-closure concept correlated fatigue-crack-growth-rate data on a unique ΔKeff-rate curve, independent of R, the flat-to-slant crack transition is controlled by ΔKeff. In 1992, Newman developed an expression for the flat-to-slant (constraint-loss) regime in terms of ΔKeff, as

(ΔKeff)T = 0.5 σo sqrt(B) (2)

where σo is the flow stress (average between yield stress and ultimate tensile strength) and B is sheet or plate thickness. Currently, the range of the constraint-loss regime (rate at the start of transition and rate at the ending of transition) and the associated constraint factors have to be determined by a trial-and-error method from test data under variable-amplitude loading. The objective of the current study is to “experimentally” determine the constraint-loss regime from constant-amplitude tests by measuring crack-opening stresses under low stress ratio (R = 0 or 0.1) conditions. In addition, it has been determined that the crack-growth delays after a single-spike overload may be caused by constraint-loss behavior. Thus, experiments on both 2024-T3 and 7075-T6 thin-sheet aluminum alloys will be conducted to measure the constraint-loss regime and to conduct repeated single-spike overload tests on M(T) specimens. A method will be developed to experimentally determine the constraint-loss regime either by measurements and/or by single-spike overload tests.

In a 2014 AFGROW User Workshop Mr. Tim Allred identified scenarios for which correlated A-10 SOLR values could lead to unconservative predictions (over predicting more than double the test life in one case). Additionally, the A-10 program recently updated crack growth rate fits. Consequently a re-correlation of SOLR values was necessary. A study was conducted to evaluate multiple approaches for correlating SOLR values based on test results. An overview of that study is presented with key observations and findings as well as an overview of the primary metrics used to define the accuracy of each correlation approach. The presentation also provides insights on relating SOLR values to similar scenarios (different peak stresses).

The A-10 SPO was provided funding to support the B-52 SPO with testing and analysis tasks. A primary requirement for performing some of these tasks was development of a representative spectrum. This presentation discusses how the information available to the SPO and what was provided by the OEM was used to develop a spectrum for several locations on the aircraft. A comparison of the model parameters, differences in crack growth programs, and final crack growth curves are the primary topics covered during this presentation.

The A-10 program was tasked to support the B-52 in further developing the B-52’s organic DTA capabilities and resources. Some of the current B-52 control points are managed with extensive and invasive inspections that drive challenging requirements for fleet management. This presentation explores the various DTA methods, tools, and approaches used to more fully understand one of the B-52’s most critical locations.

An overview of the transition to AFGROW as the main damage tolerant analysis tool within the Gulfstream Fatigue and Damage Tolerance organization is provided in the presentation. AFGROW was introduced to Gulfstream approximately a decade ago, and is now being consistently used to support aircraft certification and maintenance.

Backward compatibility is provided through the plug-in capability of AFGROW. Developmental changes have been implemented over the last decade through continuous product improvement in cooperation with the AFGROW users.

Some of the changes and additions have been implemented by AFGROW upon requests from the Gulfstream F&DT group.

Noteworthy recent Gulfstream developmental focuses have been stiffened panel analyses and load shedding. Stiffened panel analyses based on an existing internal code is currently being upgraded and implemented. In addition to this development, a proof of concept demonstrated that load shedding can be implemented in AFGROW to allow for a cycles vs. SMF correction of any DT analysis. Lastly, manipulation of the XML output file has been standardized, which eliminates the post-processing variability amongst users and lessens the required time needed to report results. All said, AFGROW has become a powerful tool used by Gulfstream to evaluate airframe damage tolerance.

A primary goal of this project was to generate continuing damage test data for comparison to different continuing damage modeling approaches. Two different coupon forms were fatigue tested to specimen failure with crack length measurements taken throughout the testing process. The first sample form was manufactured with an offset hole and had two cracks embedded on each side of the hole. The flaw size closer to the free edge measured 0.050-in., while the crack opposite the short ligament side measured 0.005-in. These coupons more closely replicate the continuing damage model process. The second coupon form had a severed ligament from a previous test project. No artificial flaw was embedded on the side of the hole opposite the short ligament. Several models, including the currently used USAF approach, were compared with the test data.

When a crack at a hole is analyzed with combined axial and bearing loading, AFGROW allows the use of an equivalent width over which the bearing load transfer is effective to help determine the stress intensities. As part of SwRI’s support of USAF damage tolerance analyses, we performed ABAQUS finite element analyses to determine the appropriate equivalent width for various geometric and structural configurations. This presentation gives the background that led to the investigation, the details and results of the analyses performed, and the resulting recommendations.

The currently ongoing NG-11 program seeks to facilitate a more wide-spread adoption of Multi Point Fracture Modeling technologies through a regimented verification and validation effort, consisting of both analytical benchmarks and a building block test program. Additionally, NG-11 seeks to implement commercially available MPFM codes within existing crack growth analysis toolsets, allowing integration of MPFM codes within the framework of existing analysis tool suites and methodologies. This presentation will provide an overview about the different assumptions/FEM codes used by BAMpF vs. FRANC3D, a discussion of their implementation to AFGROW and FASTRAN, a comparison of native BAMpF vs. External BAMpF using AFGROW and FASTRAN, a comparison of FRANC3D’s predictions to those, and then a comparison of FRANC3D implemented with AFGROW and FASTRAN