In this “S.A.F.E.R. Simulation” post we will share the key takeaways for engineers and their managers from a recent ESRD webinar on “Durability and Damage Tolerance Analysis Best Practices in the A&D Industry”. We’ll identify the factors that restrain the wider adoption of computational numerical simulation methodologies, and in particular finite element analysis (FEA) software, when used for detail stress analysis in support of critical engineering tasks such as fatigue life prediction. We hope to lift the fog that exists over the limitations of legacy FEA methods that are encountered by even the most expert simulation analysts. These same challenges make durability & damage tolerance (DaDT) calculations impractical if not risky for the occasional and especially new engineering user to perform.

Why DaDT is becoming ever more important with aging aircraft fleets….

The C130 Hercules transport aircraft depends heavily on reliable DaDT predictions to stay in service

There are numerous fixed wing and rotorcraft platforms that have far exceeded their initial program estimates for years in service. Keeping these aircraft flying safely with ever increasing performance requirements has fueled the need for more reliable and robust computational tools in fatigue life and fracture crack growth calculations to support the DaDT engineering function within repair, maintenance, and sustainment organizations. Aerospace and Defense (A&D) conferences like the Aircraft Airworthiness and Sustainment (AA&S) and Aircraft Structural Integrity Program (ASIP) have become increasingly important in their role to share best practices and new technologies which can improve aircraft life and reduce cost by expanding maintenance intervals. These conferences have revealed the need for new simulation technologies, and software tools based upon them, which improve the fidelity, accuracy, thoroughness, and speed of engineering analysis with improved confidence, reliability, and robustness of results and processes that is independent of the user or model.

To illustrate this growing demand, a strength engineer performing a “typical” stress analysis at the design stage is often delighted with answers within, say, 5% of actual or expected values. But when performing DaDT engineering simulations, being “close” with stress intensity factor (SIF), beta or other fracture mechanics computations is not good enough. For example, as the below figure demonstrates, being off by as little as 5% in SIF predictions can result in a 350% difference in crack growth cycles. The impact of getting this type of prediction wrong can be catastrophic for engineers practicing in the A&D industry (and beyond).

The sensitivity in DaDT life predictions is driven by unknown risks in the input data (e.g. SIF’s)

The application and value of FEA-based tools for numerical simulation is well established in the commercial and military aviation industry. The structural design, loads, strength and stress groups routinely use finite element models to generate internal loads of entire aero structures. FEA tools are then used to perform detail stress analysis to calculate margins of safety on components that are resistant to engineering handbooks, design curves, closed form solutions, and empirical data.

However, in many organizations there is still a preference for quick hand-whipped stress analysis with ample margins of safety when the alternative is constructing complete 3D virtual prototypes with no detail lacking, or to perform more testing on physical prototypes.

Fast high-Fidelity FEA of large aircraft assemblies is still problematic…

Modeling of large multi-scale spanning geometries for capturing SIF’s is unfeasible in most FEA codes

The wider-spread use of FEA tools in fatigue and fracture domains for DaDT calculations is another matter. Over ESRD’s twenty plus years of working within the aerospace community, as well as attending industry conferences like ASIP and AA&S, we have observed a reluctance to turn to the FEA tool kit in the calculation of important DaDT engineering data such as SIF’s. This is surprising as ever-increasing demands on airframe performance and life expectancy are requiring a larger volume of higher-fidelity structural analyses be conducted with improved levels of certainty and confidence. This is occurring at a time where budgetary constraints translates into fewer engineers available to perform analysis work that has become more complex, all with less time for advanced training and fewer resources to rely upon in methods and tools support groups.

In interviewing engineers and their managers who are responsible for DaDT work, we have heard these reservations about the generic use of FEA:

“It takes too long to import and clean up the geometry then build a high-resolution mesh around a high-risk or damaged component.”

“Solving crack propagation problems on my desktop computer takes too long as it is, then I have to go thru many cycles to debug and tune a model to get a result that is believable.”

“The quality of my solutions are a subjective exercise at best, based on my years of experience in handling similar types of analysis problems.”

“My team managers have more faith in historical analysis methods and it’s hard to convince them to let us loose on a digital model.”

All of the above issues were indeed valid at one time. It is not surprising that an organizational dependency arose on using closed-form solutions and empirically-based handbook tables to predict SIF’s. That was true even for design geometries and load cases that had little resemblance to their textbook surrogates.  Yet, not every analysis can be reduced to well-known cases like a simple plate with a thru-crack. It can be risky to force fit an existing curve or table to meet the needs of an analysis which is clearly well out of its original scope. An example is the use of compounding beta factors to account for variances in geometry and loads which can be precarious to apply and prone to error.

Despite these challenges, many DaDT engineers, rather than changing the legacy processes of their organizations, rely on historical methods no matter however approximate they are.  When these simpler methods failed, they would as a last resort – clearly not the first choice – turn to FEA for modelling complex 3D geometries with a wide variety of loadings, material types, residual stresses, crack shapes, and other complicating features.

Another inconvenient truth…

Despite their longevity in the industry, even legacy FEA methods and software struggle with these more complex classes of problems in DaDT, even when employed by simulation experts.  Obtaining consistently accurate and numerically verifiable solutions with traditional finite element methods has unfortunately added more complexity, time, risk, and cost that was prohibitive for many organizations to endure, especially when they were seeking speed, confidence, and safety.  Only a few highly experienced and well-trained DaDT specialists could perform the work due largely to endless sources of approximations, idealizations, decisions, judgement calls and errors in modelling, analysis, and results interpretation. There was little time available to think about numerical verification, much less understand it was not the same as results validation.

The reason for this state of simulation in DaDT is often obscured by a fog of complexity hiding underneath the hood of legacy FEA codes. The foundational finite element theory, methods, and technology base implemented by nearly all commercial FEA software products has remained largely unchanged over decades. That is not to say there have not been substantial improvements in aspects of FEA such as model creation for faster preprocessing, high performance computing for faster analysis, and improved visualization for post-processing. Yet, these only masked underlying limitations that made FEM an art for the expert masters rather than a reliable numerical computational science for the engineering masses.

These limitations are well known to users of simulation software – as evidenced by the size of the element library – but are less so recognized by their managers who often think this is just the way it has to be. In our next S.A.F.E.R. Simulation post we plan to discuss how these limitations in legacy FEA throttle the wider use and economic value of numerical simulation across the A&D industry. Nowhere is this timelier than in the supply and service chains which have increasing authority for design and analysis, and now new accountability for lifecycle maintenance and program sustainment that requires deeper expertise in DaDT.

Fixing the Holes…

Example 3D crack life calculation using FEA-based methods (ESRD’s Crack Propagation Analysis Tool)

For the last decade ESRD has been at the forefront of advancements in numerical simulation that makes the performance of finite element analysis less a craft of modelling traps, tips, and tricks when practiced by experts, and more S.A.F.E.R. methodology when used by the non-expert. With these advancements it is now possible for DaDT engineers to conduct analyses using more transparent models with greater accuracy, producing faster simulations in more efficient workflow processes which require less re-meshing and debugging, and generating more reliable results from inherently more robust methods independent of the level of expertise of the user or complexity of the engineering problem.

In a future S.A.F.E.R. post on the use of FEA in DaDT we will dive deeper into what makes this now possible in practice. Until then, in the most recent ESRD webinar on DaDT we demonstrated several example fatigue life and crack propagation problems which illustrated that conventional expectations of being “close enough” are no longer “good enough”.  To view this webinar click here. If your corporate firewall prohibits live access please send us an email to webinars@esrd.com and we can provide a link to download.

What do you think…

Related links and conversations…

• ESRD’s DaDT and Fracture Mechanics Application Highlights (via ESRD.com)
  • Learn about the types of DaDT and fracture mechanics applications for which ESRD is best suited, and how A&D engineers solving legacy aircraft problems can benefit from our simulation technology.
• Seven Questions to Ask CAE Vendors of Structural Analysis and Simulation Software (via Engineering.com)
  • Engineering.com Editor Shawn Wasserman highlights what questions are driving simulation technology and development, and what factors should determine the best tool for a simulation (such as DaDT).
• NAFEMS World Congress 2017 Tackles the Big Issues in Simulation and Analysis (via Digital Engineering)
  • DE’s Jamie Gooch describes the events and presentations given at the most recent NAFEMS World Congress, including how ESRD’s Chairman Dr. Barna Szabó warned against democratization of simulation without Simulation Governance.

To receive future S.A.F.E.R. Simulation posts…

ESRD is hosting joint webinars on July 17th and July 29th, both at 1:00 pm EST.

Want to learn from the experts in FEA-based Simulation Application (Sim App) development for standardization & automation of complex engineering analysis tasks, such as 3D fatigue crack growth, 3D ply-by-ply laminated composite analysis or other challenging applications you’d like to safely put into the hands of non-experts?

This July, ESRD will be partnering with several industry leaders to provide not one but TWO stimulating webinars on the latest in FEA-based Sim App development.

The Latest Developments in Sim Apps for 3D Crack Growth Simulations

BAMF Example Crack Front Estimate via StressCheck/AFGROW Integration (courtesy Hill Engineering).

First, ESRD is pleased to join Hill Engineering, LLC (developers of BAMF) and LexTech, Inc. (developers of AFGROW) for a joint webinar on Wednesday July 17, 2019 @ 1:00 pm EST. This collaborative webinar will be titled “3D Crack Growth Simulation: Advancements & Applications“, and will detail the latest technological advancements in Sim Apps for accurate simulation of three-dimensional metallic crack growth via coupled finite element analysis (FEA) and fatigue life computations.

During this webinar, you will see the latest in 3D crack growth predictions via Hill Engineering’s Broad Application for Modeling Failure (BAMF) Sim App, which provides a robust integration between StressCheck’s high-fidelity DaDT/fracture solutions and AFGROW’s crack growth life prediction capabilities.

Additionally, ESRD, LexTech & Hill Engineering representatives will explain how each of their respective technologies seamlessly fit together to enable automated, verified & validated (i.e. backed by experimental data) fatigue crack propagation.

The Importance of Simulation Governance in Sim App Development & Deployment

This joint ESRD/Rev-Sim webinar will explore Sim-Gov compliant Sim Apps for democratization of simulation.

Then, on Monday July 29, 2019 @ 1:00 pm EST, ESRD will join the thought leaders at Revolution in Simulation (Rev-Sim) for a joint webinar on why Simulation Governance compliance is essential to the development & deployment of Sim Apps titled “Democratization of Simulation Governance-Compliant Sim Apps“.

In this timely webinar, we will discuss why it is essential that Sim Apps implement Numerical Simulation technologies which enable the practice of Simulation Governance in order for the vision of democratization of simulations to be realized, as well as why it is important for engineering managers to get on the Simulation Governance train sooner rather than later. Strategies will be explored for democratizing engineering simulations via Sim Apps which are: 1) based on the latest Numerical Simulation technologies, 2) available in Commercial Off the Shelf (COTS) form, and most importantly 3) Simulation Governance-compliant.

More ESRD Webinars…

Interested in the complete ESRD webinar listing, including on-demand recordings from past ESRD webinars? Visit our webinars page here:

Demo of BAMpF/StressCheck interation as run through an AFGROW plug-in (courtesy Hill Engineering)

Announcing Hill Engineering and ESRD Agreement to Jointly Market BAMpF & StressCheck Professional

March 12, 2020

Hill Engineering and Engineering Software Research and Development, Inc. (ESRD) have executed a joint marketing agreement to collaboratively promote the combined use of our software tools Broad Application for Multi-Point Fatigue (BAMpF) and StressCheck Professional, respectively, for the engineering applications of fatigue and damage tolerance analysis.

ESRD is pleased to join forces with Hill Engineering and LexTech, Inc. (an ESRD technology partner and developers of AFGROW) to enable state-of-the-art fatigue crack growth capabilities for DaDT engineers.

BAMpF is a software tool developed by Hill Engineering for predicting the growth of fatigue cracks in 3D parts. Starting from an assumed initial flaw, BAMpF automatically combines fracture mechanics solutions from StressCheck with fatigue life calculations from AFGROW to assess fatigue crack growth performance.

Read Hill Engineering’s announcement.

BAMpF Resources

The following are helpful resources for learning more about the BAMpF/StressCheck/AFGROW integration for fatigue crack growth:

• BAMpF Home page (via Hill Engineering)
• ESRD/Hill Engineering/LexTech Joint Webinar recording (recorded July 2019)
• AFGROW/BAMpF Training workshop (courtesy LexTech/Hill Engineering/ASIP Conference 2019)
• BAMpF Use Case presentation PDF (courtesy Brian Boeke/AFGROW User’s Conference 2019)

BAMpF Case Study Example

The figure below shows a comparison between the fatigue crack growth predicted near a hole (using BAMpF and StressCheck Professional + AFGROW) and results from a fatigue crack growth test for similar conditions. In this case there is an initial flaw at the edge of the hole and the hole has been cold-expanded to introduce compressive residual stress.

Predicted crack front evolution (blue) compares favorably with the observed experimental result (red):

Want to Learn More? Contact Us:

• Hill Engineering
• ESRD

BAMF running as an AFGROW plug-in (courtesy Mr. Josh Hodges/Hill Engineering, LLC)

On July 17, 2019 a joint webinar on the latest developments in FEA-based 3D crack growth simulation, titled “3D Crack Growth Simulation: Advancements & Applications”, was provided by ESRD’s Brent Lancaster, LexTech’s James Harter and Hill Engineering’s Joshua Hodges.

In this webinar, we detailed the latest technological advancements for accurate simulation of three-dimensional crack growth in metallic structures, with and without residual stresses, via coupled finite element analysis (FEA) and fatigue life computations. Additionally, the webinar expanded further on the DaDT analysis best practices presented in ESRD’s June 2017 webinar, titled “Durability and Damage Tolerance (DaDT) Analysis Best Practices“, and highlighted the importance of mitigating the errors of approximation associated with stress intensity factors (SIF’s) and if applicable, engineered residual stresses (e.g. cold-working).

Webinar attendees from a wide range of industries were treated to a demo of Hill Engineering’s Broad Application for Modeling Failure (BAMF) Sim App, which provides a robust integration between StressCheck’s high-fidelity DaDT/fracture solutions and AFGROW’s crack growth life prediction capabilities. The BAMF demonstration showed how to set up a parametric model for DaDT analysis via StressCheck and then, with limited user intervention, integrate with AFGROW and StressCheck via their respective COM API’s to perform an on-demand 3D crack growth simulation. It was very impressive demo, indeed!

View Webinar Recording

Click the button below to view the 3 part, 65-minute webinar recording (scroll to the bottom of the webinar landing page to find the videos):

View Webinar Slides

Click the button below to view the webinar slides (PowerPoint Show):

Additional Resources

During the webinar, we identified several relevant DaDT and/or crack growth simulation resources that may be of interest:

• “Analytical Modelling of Fatigue Crack Behaviour Under Representative Spectrum Loading in the F/A-18 A/B Y508 Wing Root Shear Tie”, Dr. Kevin Walker (DST Group)
• “Crack Aspect Ratio Investigation, Analytical Methods Subcommittee: Round Robin for Cx Holes”, Mr. Robert Pilarczyk (Hill Engineering, LLC)
• “Benchmarking Problems in Fatigue Crack Growth Analyses”, Mr. Robert Pilarczyk (Hill Engineering, LLC)
• “Numerical Simulation Series: Fracture Mechanics Parameters”, Dr. Ricardo Actis (ESRD)
• “Improving Global-Local Simulation Workflows for High-Fidelity Stress and Damage Tolerance Analysis”, Mr. Eric Buettmann & Mr. Gordon Lehman (ESRD)
• “Computation of SIFs for Cracks in Cold-Worked Holes”, Dr. Ricardo Actis & Mr. Matt Watkins (ESRD), Dr. Scott Prost-Domasky (AP/ES) and Mr. Joshua Hodges (USAF)
• “Crack Propagation Analysis Tool for 3D Crack Simulation: A Simulation App Case Study”, Mr. Matt Watkins (ESRD)
• The Contour Method for Measuring Residual Stresses (Hill Engineering, LLC)

Acknowledgments

As always, many thanks to our attendees for their interest and feedback! And, of course, thanks to LexTech and Hill Engineering for their time and contributions. We hope to collaborate on another webinar in the future!

BAMF Example Crack Front Prediction via StressCheck/AFGROW Integration (courtesy Hill Engineering).

ESRD is pleased to join Hill Engineering, LLC (developers of BAMF) and LexTech, Inc. (developers of AFGROW) for a joint webinar on July 17, 2019 @ 1:00 pm EST. This collaborative webinar will be titled “3D Crack Growth Simulation: Advancements & Applications“, and will detail the latest technological advancements for accurate simulation of three-dimensional metallic crack growth via coupled finite element analysis (FEA) and fatigue life computations.

During this webinar, you will see the latest in 3D crack growth predictions via Hill Engineering’s Broad Application for Modeling Failure (BAMF) software tool, which provides a robust integration between StressCheck’s high-fidelity DaDT/fracture solutions and AFGROW’s crack growth life prediction capabilities.

Additionally, ESRD, LexTech & Hill Engineering representatives will explain how each of their respective technologies seamlessly fit together to enable automated, verified & validated (i.e. backed by experimental data) fatigue crack propagation. You don’t want to miss it!

WATCH THIS WEBINAR

Part 1: Introduction & Implementation Requirements

Part 2: StressCheck & AFGROW Features

Part 3: BAMF Demo & Webinar Summary

ESRD Co-Presents at ERSI Workshop 2019

Interactive K-solver crack propagation (CPAT) in StressCheck used for the prediction of fatigue growth rates.

ESRD’s Dr. Ricardo Actis attended and co-presented with technology partner Analytical Processes/Engineered Solutions (AP/ES) at the Engineered Residual Stress Implementation (ERSI) Workshop 2019 in Clearfield, UT.  This annual workshop is focused on the development of a damage tolerance methodologies for fatigue crack growth through deep Engineered Residual Stress (ERS) fields.

Sponsored by the US Air Force’s A-10 Airframe Structural Integrity Program (ASIP), the goal of this workshop is to develop a roadmap for the implementation of the beneficial effect of engineered residual stresses within the United States Air Force (USAF) Damage Tolerance Analysis (DTA) fatigue crack growth and lifing programs.

At ERSI Workshop 2019, Dr. Thomas Mills (AP/ES) and Dr. Ricardo Actis (ESRD) co-presented “Fatigue Life Modeling at CX and Interference Holes”.  The presentation summary is as follows:

• Crack closure experimental observations and modeling.
  • Continuation of previous year’s work.
• Residual stress redistribution at interference filled holes.
  • Task evolved from USAF goals dictated at ERSI.
  • Goal is to better understand interference fit fasteners and their interaction with residual stresses from cold-working (CX).

A PDF of the presentation may be downloaded at the following link:

ESRD Sponsors the WashU Racing Team

Congratulations to the Washington University in St. Louis Racing Team on a successful season! It was a pleasure to sponsor the team as they improved their global standings in multiple events, and increased their team to over 50 members. Below is a signed image of the racing team members, as well as a letter of appreciation sent to the racing team sponsors:

The 2019 Washington University Racing Team.

Washington University Racing Team Letter to Sponsors.

We are pleased to have a positive impact on our local engineering community, and look forward to our continued sponsorship of the team!

Corner crack at the edge of the hole inserted after cold-working. Contour show the progression of crack opening as the external load is increased from 0 to 27 ksi. First noticeable crack opening appears around 19 ksi.

ESRD’s Dr. Ricardo Actis attended and presented with technology partner Analytical Processes/Engineered Solutions (AP/ES) at the Engineered Residual Stress Implementation (ERSI) Workshop 2018 in Clearfield, UT.  This annual workshop is focused on the development of a damage tolerance methodologies for fatigue crack growth through deep Engineered Residual Stress (ERS) fields.

Sponsored by the US Air Force’s A-10 Airframe Structural Integrity Program (ASIP), the goal of this workshop is to develop a roadmap for the implementation of the beneficial effect of engineered residual stresses within the United States Air Force (USAF) Damage Tolerance Analysis (DTA) fatigue crack growth and lifing programs.

At ERSI Workshop 2018, Dr. Ricardo Actis (ESRD) and Dr. Thomas Mills, Dr. Scott Prost-Domasky, and Mr. Craig Brooks (AP/ES) presented “RS Crack Closure: Experimental Observations and Modeling”, the PDF of which can be downloaded below:

The ERSI Workshop 2018 attendees are shown below:

Attendees to the Third ERSI Workshop.

ESRD, Inc. continues to work closely with AP/ES and the USAF to develop and implement accurate solutions for challenging Durability and Damage Tolerant applications.