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!

In this 15-minute pre-recorded webinar, ESRD Chairman Dr. Barna Szabó addresses some of the key issues of simulation governance, including how model development must adhere to the requirements of simulation governance in order to minimize risk and increase reliability.

Following is the abstract of the webinar (via NAFEMS):

Advancements in predictive computational science make it possible to increase reliance of numerical simulation, necessitating fewer physical experiments for substantial savings in time and costs of product development projects. The first and perhaps the most challenging obstacle to full realization of the benefits of predictive computational science is a widespread misunderstanding of what numerical simulation is.

Most managers and many individuals who present themselves as experts in numerical simulation confuse numerical simulation with “finite element modeling” or “numerical modeling“. Those are outdated concepts, responsible for much of the disappointing results that caused widespread loss of confidence in the usefulness and reliability of numerical simulation. Current simulation and data management practices will have to be revised in order to meet the technical requirements of predictive computational science.

The presentation focuses on the central role of simulation governance and management in the coordination of experimental and analytical work necessary for proper use of the tools and techniques of predictive computational science with the objective to maximize the reliability of computed information.

The presentation outlines the methodology of model development in the applied sciences, the essential constituents of which are the formulation, calibration and ranking of mathematical models, data and solution verification, validation and uncertainty quantification. It will be shown that consideration of the size of the domain of calibration is essential. Without such consideration just about any model, even pseudoscientific models, can be calibrated on a sufficiently small domain of calibration.

The presentation also highlights the differences between numerical simulation and finite element modeling. Understanding these concepts and procedures is an indispensable prerequisite to any successful implementation of a Simulation Governance plan. Recognizing that technology changes and the available information increases over time, planning must incorporate data management and systematic updates of simulation practices so as to take advantage of new information and advancements in technology.

Watch the Live Webinar at NAFEMS World Congress (NWC) 2021

The webinar will also be presented live by Dr. Szabó on 10/27/2021 @ 17:30 local Salzburg time (CEST) on-site in Room W, as well as virtually (NWC 2021 is a hybrid event) as part of the Simulation Governance sessions (L6).

To register for NWC 2021 as either an on-site attendee or online attendee, click the below button:

SAINT LOUIS, MISSOURI – December 5, 2017

In ESRD’s November S.A.F.E.R. Simulation post we summarized the business trends in A&D that are driving the need for higher-performing aerostructures that are more efficient, lighter-weight, and more durable and damage-tolerant over longer life spans. This in turn is driving the requirement for higher-fidelity engineering analysis that brings increased accuracy and reliability to the structural engineering function without adding more time and risk to the program schedule.

In this second of our multi-part series on “S.A.F.E.R. Numerical Simulation for Structural Analysis in the Aerospace Industry” we will distill what this means to stress analysis groups and the challenges experienced when using legacy simulation and analysis technologies based on the finite element method (FEM).

Aerospace & Defense budgets are squeezed ever tighter, yet simulation demands and complexities keep increasing…

The Democratization of Simulation

As discussed in our last post on the state of simulation in aerospace, the capabilities of FEA-based software tools have become increasingly more advanced in functionality and richer in features. Not surprising, they have also become more sophisticated to use and difficult to master, even by expert analysts. Training analysts in FEA-based simulation software is a laborious, expensive process, and the results are not always transferable as analysts move to new programs or employers which have their own set of tools, processes, and best practices.

Across the engineering software community there is much discussion about the democratization of simulation; meaning the reliable and routine use of numerical simulation software by non-simulation experts. These non-experts may be mechanical design engineers, occasional users, or new engineering graduates. The hope of democratization is that much of the complexity and risk of FEA-based simulation can be distilled out such that simulation-driven design may be performed with greater confidence by engineers earlier in the design cycle.

Excerpt from “The Role of Simulation Governance in the Democratization of Simulation Through Sim Apps in the A&D Industry” presented by ESRD’s CEO Dr. Ricardo Actis at NAFEMS 2017 Aerospace Simulation Engineering: The Big Issues

Indeed, democratization has great potential to compress the product development lifecycle, but is it a realistic objective for the demanding aviation, aerospace, and defense industries? The answer few may want to hear is that it will not be easy to accomplish using legacy FEA-based simulation technologies along with the software tools based upon these technologies.

The results when reviewing the previous attempts to put FEA tools into the hands of the non-expert have not been encouraging.  These schemes included embedding solvers into CAD software to hide complexity, employing scripted templates to insulate users from making errors, and exercising wizards to automate processes. None of these have yet to move most FEA work off of the expert analyst’s desktop and place it into the hands of the design engineer.  Upon closer inspection, most of these approaches failed not because they were bad ideas, but because they were still based on legacy FEA methodologies where creating, debugging, running, and post-processing finite element models was a complex error-prone art form for the expert.

Challenges with Legacy FEA Software

Simulation software providers have continually sought ways to compensate for – and in some cases hide – the inherent complexity of the finite element method (FEM) when applied to analysis problems in computational solid mechanics.  There have certainly been many advancements in the functionality, user interfaces, pre/post processors, high-performance computing, delivery platforms, and licensing options of FEA software over recent years. Yet, none of these individually or collectively removed the intrinsic complexity and challenges of learning and performing FEA by either the expert or novice user.

There are many good reasons why this is so. The underlying theory and methods employed “under the hood” of nearly all FEA software products on the market today are in fact many decades old. As a result, there are near endless sources of assumptions, idealizations, approximations, manipulations and judgement calls, each that add complexity, time, and uncertainty to engineering simulations.

Example of loading fastener holes via Rigid Body Elements (RBE’s). Using RBE’s and other element libraries may be acceptable for expert users, but for non-experts they can be a source of unknown errors.

As an example, the element libraries of most FEA software products contain dozens of variants and odd mutants that must be carefully selected and deployed. When these overly-sensitive elements are used in fragile meshes it is not uncommon for a finite element model to break with even small changes to design geometry or boundary conditions. Rarely can the same elements and meshes be used for different types of physics modelling such as non-linear, contact, heat transfer, dynamics, or fracture analysis.

Often more time is spent in the pre-processing steps of constructing “bad” models, to finally arrive at the “good” ones, than in post-processing the results or optimizing a design. While CAD data is increasingly 3D solid-based, it must often be repaired, defeatured, re-created, or reduced before meshing. It is often unrealistic in legacy FEA simulations to use solid finite element models of large-spanning, multi-scale geometries of built-up components, which are common in aerostructures. Often a series of increasingly granular models must painstakingly be constructed to perform a sequence of multi-fidelity, multi-scale global/local analyses.

Extracting and validating results in traditional FEA is an equally laborious process that is inherently error-ridden. In legacy FEA software mathematical degrees of freedom are nodal based, which means quantities of interest at other locations must be interpolated, extrapolated or massaged in a way that potentially injects additional inaccuracies in engineering data. High-density meshes must be used in areas of stress gradients, which often requires a-priori knowledge of the results and locations of interest, or changing the model once the results are produced and then iterating. It is not uncommon that models are tuned and tweaked such that the computational results align with empirical test data.

Averaged vs. Not Averaged results for Legacy FEA Simulation Technology. Which stress concentration (Kt) is more accurate, if either one? Is this the right mesh density?

All of the above limitations and challenges are so well understood by the expert analyst, who typically has advanced engineering degrees and many years of experience, that they rarely think twice about whether it has to be this complex. They know all the traps, fixes, tricks, and workarounds of finite element modelling. Yet, there is a more fundamental challenge often overlooked; legacy generation finite element methods do not provide a fool-proof measurement of the quality of their solutions. There is no inherent quality assurance, much less explicit support of solution verification. As such, it is up to the individual analyst to assess the applicability, accuracy, and completeness of the computed results. It is no wonder that it always takes an expert in the loop to determine if the results are good and more importantly when they are deficient.

It All Ends Up On The Engineer’s Desktop…

The confluence of demanding A&D business drivers, higher product performance requirements, and increasing complexity of digital simulation all end up on the structures engineer’s desk. The stress analyst on a modern A&D program ends up owning the burden to produce a larger volume of higher-fidelity analyses, earlier in the NPD cycle, spanning an expanded optimization solution space of structural design variations.

In doing so they are expected to create all-encompassing 3D digital models, with few details left behind to support virtual prototyping and reduce testing, while using more sophisticated tools that take longer to learn and master. And they are expected to perform these analyses in less time with a greater level of confidence in the results and with less tolerance for uncertainty or “fat” factors of safety that were once acceptable in yesteryear’s aerostructure designs.

Compounding the above pressures, today’s analysts may no longer have access to internal support from engineering methods groups which historically provided training, troubleshooted problems, captured institutional knowledge, and shared best practices. It is little surprise that industry associations like NAFEMS report that providing oversight of the simulation function through the practice of Simulation Governance is one of “The Big Issues” for engineering managers who often see their simulation teams struggle to deliver with so many conflicting requirements.

These pressures are not letting up, and current trends do not appear sustainable. The evidence speaks for itself that FEA-based structural analysis often adds so much more time and complexity to engineering processes such that project managers seek to minimize its use when there are other faster methods available. Fortunately, a new generation of simulation software is emerging where that is no longer the case.

Coming Up Next…

In Part 3 of this series we will explain why Numerical Simulation is not the same as Finite Element Modeling and what this means to engineering analysis within the A&D industry. We will describe how the practice of Simulation Governance, enabled by the next generation of software based on numerical simulation, is helping engineering groups respond to an avalanche of complexity in products, processes, and tools.

In our final segment we’ll profile the capabilities of ESRD’s numerical simulation software StressCheck™ and Smart Sim Apps deployed in a Digital CAE Handbook built using StressCheck. Finally, we’ll share use-case examples from A&D that document the benefits to engineers and value to their programs from the use of this newer generation of analysis software that is Simple, Accurate, Fast, Efficient, and Reliable – S.A.F.E.R – for both the expert and non-expert user alike.

Next Week’s Webinar…

On Thursday, December 14th @ 1:00 pm EST an Aerospace & Defense-oriented webinar titled “High-Fidelity Stress Analysis for S.A.F.E.R. Structural Simulation Webinar” will be provided by ESRD’s Brent Lancaster and Gordon Lehman.

Sign Up for Next Week’s High-Fidelity Stress Analysis Webinar

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

ESRD, Inc. will be exhibiting and providing a training course at the AA&S/PS&S Conference 2019 in Washington, D.C. from April 22-26, 2019.  We hope you will drop by our training course and booth to check out the latest!

Training Course

The training course titled “How Do You Verify the Accuracy of Engineering Simulations?” will be held Monday, April 22, 2019 / 3:00 PM – 5:00 PM by Mr. Gordon Lehman, PE. The description is as follows:

Convergence of minimum principal stress in an aircraft keel beam structure (courtesy Digital Engineering)

This Training Class will review strategies for verifying the accuracy of engineering simulation data, including best practices and common pitfalls engineers may encounter when assessing the quality of their engineering simulation results.

We will explore why is the practice of solution verification for FEA results important? And what checks must always be performed before reporting your FEA results as “accurate”?

The class will dive into the four key quality of solution checks that should be examined to confirm solution verification of detailed stress results.

This will be accomplished by examining a variety of publicly-available and industry-applicable case studies, benchmarks and industry examples, to determine the most efficient and reliable methodologies to perform solution verification.

The training course content structure will be based on our January 2019 webinar and recent S.A.F.E.R. Simulation blog article “What Are the Key Quality Checks for FEA Solution Verification?”. You can watch the 38-minute recording and read the article below:

• Webinar Recording: How Do You Verify the Accuracy of Engineering Simulations?
• S.A.F.E.R. Simulation Blog Article: What Are the Key Quality Checks for FEA Solution Verification?

Register

Exhibit Booth

ESRD can be found at Booth 110, conveniently located near the entrance to the exhibit hall.  Feel free to stop by and discuss the training course content, S.A.F.E.R. Simulation, StressCheck Professional, CAE Handbook, StressCheck Tool Box, and What’s New with ESRD!

ESRD’s Andrew Ledbetter talks to an AA&S 2018 attendee.

While you’re at the ESRD booth, be sure to grab one of our giveaways, including this nifty foam flyer:

StressCheck foam plane (it actually flies!)

Contact information for ESRD staff is as follows:

• Mr. Gordon Lehman, PE – gordon.lehman@esrd.com

WATCH THIS WEBINAR

Part 1: Introduction & Implementation Requirements

Part 2: StressCheck & AFGROW Features

Part 3: BAMF Demo & Webinar Summary

Extraction of minimum principal stresses for the aircraft keel section case study. Digital Engineering case study results (bottom right) compared well with StressCheck’s live dynamic extractions (bottom left, images courtesy of Digital Engineering/ESRD).

Introductory Remarks

In recent years, Digital Engineering began publishing articles on practical engineering simulation case studies, walk-thrus and software overviews for a variety of FEA software tools. In each article, FETraining developer and NAFEMS FEA training instructor Tony Abbey selected a commercially-available FEA software tool to analyze a mechanical component under various loads and constraints, and provided commentary, tips, tricks and process workflow of how he arrived at the results. His goal for each article was to focus on general instruction and best practices for FEA rather than in the particular FEA tool used for solving the problem.

With the consent of Mr. Abbey, ESRD selected four (4) case studies from his list of Digital Engineering simulation articles and were provided the geometric description, material properties and boundary conditions to solve these problems in StressCheck Professional.

For comparative and instructional purposes, ESRD then recorded and published video demonstrations of the modeling, solution and live dynamic results extractions of each case study to our Resource Library.

Note: while these were not “blind” case studies (the Digital Engineering case study results were known in advance), the video demonstrations show the unaltered, associated StressCheck process workflow, with solution verification provided for each result to ensure that the approximation error is sufficiently small.

Case Studies in Detailed Stress Analysis

Case Study #1: “Stress in Finite Element Analysis”, Digital Engineering May 2016

Cross brace structure showing constraint and loading set up (image courtesy of Digital Engineering).

In this particular case study, Mr. Abbey’s focus was to examine detailed cross-sectional stresses for a 3D solid cross brace geometry under off-axis loading. This involved defining multiple “zones” for extracting stresses, for a multitude of stress components and local system directions:

Zones of interest A through E and the SX stress distribution (image courtesy of Digital Engineering).

The following video demonstrates how StressCheck was used to reproduce the 3D detailed cross brace analysis and how cross-sectional lines were added after the solution for mesh-independent, curve-based stress gradient extractions. It is shown that StressCheck’s live, dynamic extractions compared favorably with Digital Engineering’s zone-based detailed stress results:

Case Study #2: “Dealing with Stress Concentrations and Singularities”, May 2017

Geometry and mesh of filleted model (image courtesy of Digital Engineering).

For this case study, Mr. Abbey discussed the influence of stress singularities on the FEA solution, and why it is important to model radii and fillets in regions of interest for more accurate stresses if a subsequent fatigue analysis is the goal. He studied the convergence of the stresses in a shoulder fillet geometry by increasing the mesh refinement until the difference in his results was small:

Stress distributions for the filleted model (image courtesy of Digital Engineering).

The following video demonstrates how the 3D shoulder fillet detailed stress analysis was performed in StressCheck, including the steps required to show convergence in the peak stresses. The results were nearly identical with Digital Engineering’s computed peak stress of 65.6 ksi:

Case Study #3: “Siemens FEMAP with NX NASTRAN Overview”, March 2018

Initial imported geometry and cut-out region (image courtesy of Digital Engineering).

For this case study, Mr. Abbey utilized Siemens FEMAP with NX NASTRAN to model and solve a 3D solid tie rod under axial loading. He also recorded videos for how to perform the pre-processing, solution and post-processing and deployed them on his website, FETraining.net.

Below are the von Mises stress contours from the NX NASTRAN results, with an unaveraged peak von Mises stress of ~43.6 ksi:

von Mises stresses (image courtesy of Digital Engineering).

The following video demonstrates how the full 3D tie rod geometry is imported into StressCheck, modified to remove all solid material outside of the cut-out region, automeshed with curved tetra elements, assigned the appropriate boundary conditions and solved with a p-extension process to increase degrees of freedom (DOF) on the fixed mesh.

Then, the converged peak von Mises stress was computed from the hierarchic sequence of linear solutions via a live dynamic extraction request and was observed to compare closely to Digital Engineering’s refined mesh NX NASTRAN result:

Case Study #4: “SOLIDWORKS Simulation Overview”, July 2018

Applied loads and boundary conditions (image courtesy of Tony Abbey/Digital Engineering).

For this case study, Mr. Abbey utilized SOLIDWORKS from Dassault Systèmes to analyze a 3D solid aircraft keel section, in which several load cases (vertical and lateral loads) were applied to the structure’s attachment points via sinusoidal bearing distributions. From Mr. Abbey about the origin of this aircraft keel section structure:

The keel section is on the lower centerline of a combat aircraft fuselage. It transmits undercarriage loads into the fuselage. It also provides a load path through the lower fuselage section in overall bending and torsion loading due to maneuvers. The geometry has been created in SOLIDWORKS. Many of the smaller fillet radii have been defeatured in preparation.

Of interest were the peak von Mises stresses and minimum principal stresses in the left-hand end upper fillet radii (38.46 ksi and ~-37.5 ksi, respectively):

Von Mises stress distribution in the left-hand end upper fillet regions (image courtesy of Digital Engineering).

Minimum principal stress, P3, plotted around the left-hand end upper fillet (image courtesy of Digital Engineering).

The following video demonstrates how the 3D solid aircraft keel section was imported in StressCheck as a Parasolid and analyzed for detailed stresses in the left-hand end upper fillet regions for the vertical/lateral load cases and end abuttment/bolt radial constraints. The end abuttments were represented by symmetry constraints, and the bolt radial by stiff normal springs. The vertical and lateral load cases would be represented by sinusoidal bearing load distributions applied to the lug holes.

Again, the solution was obtained by increasing the polynomial order of the approximation functions on the fixed mesh, and the solutions for each load case were available for live dynamic processing.

It can be observed that the peak von Mises stress in the left-hand end upper fillet radii converges tightly to a value similar to that produced in Digital Engineering’s results, and the live dynamic principal stress gradient extractions were also quite similar in nature to those produced by Mr. Abbey in SOLIDWORKS probes:

Case Study Summary and a Note on Benchmarking

Many thanks to Mr. Abbey, FETraining.net and Digital Engineering for providing the model files to ESRD. The outcome of these case studies reinforce that only with careful engineering analysis methodologies, appropriate modeling assumptions, and tight control of approximation errors is meaningful to perform comparisons of stress results produced by different analysts using different FEA software tools.

Also, as we highlighted in the above case studies, the practice of solution verification is especially important for any benchmarking, round-robin or other comparisons involving the computation of the results of interest by numerical means for a well-defined mathematical problem. Reporting results without objective measures of the size of the approximation errors does not meet the technical requirements of Simulation Governance.

Finally, if you are interested in results of published benchmark problems solved with StressCheck, check out our solutions to the Standard NAFEMS Benchmarks:

The Standard NAFEMS Benchmarks: Linear Elastic Tests