Simulation | Endurica LLC

Tolerances in Fatigue Life Prediction with Endurica

November 17, 2023 by William V. Mars, Ph.D., P.E.

I get this question a lot: how well can the Endurica software predict fatigue life? Is it as good as a metal fatigue code, where a factor of 2x is often quoted as a target tolerance?

The answer is yes, fatigue life predictions can reach and beat this level of accuracy. But as always, knowledge and control of the problem at hand is key. We must keep in mind that fatigue behavior varies on a logarithmic scale. It depends on many variables. It depends on how failure is defined in the simulation and in the test. Small variations of an input may lead to large variations of the fatigue life. So, to achieve the best tolerances, careful specification, measurement, and control are required of both simulation and test.

Analysis tolerances depend on whether the analysis workflow is “open loop” or “closed loop”. In an open loop workflow, the analyst is typically in the position of having to accept without question the as-given material properties, geometry, boundary conditions and load history. The analysis is completed and reported. Decisions are made and life goes on. In a closed loop workflow, there are additional steps. These include a careful review of differences between the test and the simulation, as well as identification and correction of any erroneous assumptions (about material properties, geometry, boundary conditions, and load history).

Open loop workflows produce larger tolerances. Every situation is different, but do not expect tolerances tighter than perhaps a factor of 3x-10x in life, when working in open loop mode. There is just too much sensitivity, too many variables, and too little control in this mode. The open loop mode does have a few advantages though. It takes less work, less time, less cost. And it is often useful for ranking alternatives (ie A vs. B comparisons).

For high accuracy, a closed loop workflow is required. It is rarely the case that initial assumptions are sufficiently error-free to support tight tolerances on fatigue life prediction. Therefore, careful measurement and validation of material property inputs, part dimensions, load-deflection behavior, pre-stresses, etc. should be made. Where gaps are found between test and simulation, appropriate amendments to the test and/or to the simulation should be adopted. This approach yields high confidence in the simulation results, and good accuracy in fatigue life predictions. We have seen users hit life predictions to better than a factor of 1.1x with this approach! Although this approach requires more effort, it results in more complete mastery of part design, and it yields a much stronger starting position for subsequent products.

While “right the first time” engineering is possible with either open or closed loop, the closed loop approach benefits from progressive refinement of the analysis inputs and it ultimately gives the highest success rate.

Tire Society Takeaways 2021

June 15, 2022October 15, 2021 by William V. Mars, Ph.D., P.E.

Image of a Tire The Tire Society held its 40^th annual meeting last month with the theme The Virtual Tire. It has always been the place to see up and coming ideas, to see who is pushing into the frontiers of the field, and to renew professional connections across the industry. Endurica was very proud to sponsor this year.

Here’s a brief recap of our favorite talks…

GM’s Mike Anderson, Executive Director of Global Virtual Design, Development and Validation, kicked off the meeting with his keynote lecture, titled “The Move To Virtual”. He spoke of GM’s target to achieve 100% virtual design by 2025. Anderson explained that this doesn’t mean that physical testing will go away, but rather that GM is dead serious about getting to a “right the first time” scenario rather than a “discover and recover” mode. “It’s a measure twice, cut once” culture, he said. He noted that upstart competitors are sprinting ahead in areas like EVs through the use of simulation and that the speed of discovery has increased significantly in the current competitive environment. Simulation drives learning speed, not only because of the opportunity to get engineering answers at the pre-build phase but also because it enables exploration of more of the design space and more of the performance outcomes. He told the conference that “we need to go beyond just replicating physical tests with simulation, we need to leverage the strength of simulation to go beyond test”. In the Q&A, Anderson was asked whether suppliers will also be expected to be virtual. “That’s gonna be tough to play together” for rubber part suppliers that can’t engage via simulation.

There were three talks given by Endurica users at this year’s Tire Society meeting.

Pooya Behroozinia of Maxxis Tires spoke on “Tire Durability Prediction Using Three-Element Layered Mesh for Cord-Rubber Composites”. Behroozinia shared a tire meshing technique for improving representation of interlaminar shearing in their tire model. They used Endurica DT to simulate the damage accruing across all of the 6 steps in a stepped-up load durability test, and they were able to predict correctly the lower sidewall failure mode, the life (45 hours observed, 38 hours predicted), and the crack orientation. They also had a 2^nd validation case in which the loads were increased by 10% in all steps of the test. The simulation again predicted correct failure, and the comparison of experimental life (41000 km) to simulated life (36330 km) was in good agreement.

Vidit Bansal of CEAT spoke on “Incremental, Critical Plane Analysis and Experimental Verification for TBR Tyre Bead Endurance Applications”. Similar to the Maxxis paper, CEAT used Endurica DT to simulate a multi-step durability test with loads ranging from 80% to 250%. In this paper, two different truck tire sizes were modeled and tested, a 10.00R20 and an 11.00R20. The analysis correctly predicted the ply turnup as the critical location. The predicted lives of the two tire sizes were predicted at 90-93% of the actual tested life in both cases.

Tom Ebbott and Gobi Gobinath of Goodyear spoke on “A Model for Predicting Residual Casing Life of a Tire Following an Impact Event”. This work demonstrated the consequences on tire damage development of a range of impact event scenarios (3 speeds, 4 impact angles, 3 different wear states) early in the life of the tire. It used Endurica DT to accrue damage from both the impact event (computed with explicit FEA) and subsequent tire runout under steady state rolling conditions (computed with implicit FEA). The crack growth rate curve during the impact was based upon experimental measurements of the critical tearing energy at impact rates. When asked about experimental validation of the simulation results during the Q&A, Ebbott noted that “the modeling work stands on its own – it is based on sound physics”.

We at Endurica were delighted with the significance and innovation on display in all of these talks. We have often been challenged to show validation for tire durability predictions, but such measurements are difficult to obtain without significant tire testing resources. So, the fact that the Maxxis and CEAT papers showed multiple direct comparisons of tire durability tests with simulations, and the fact that excellent predictions of both failure mode and tire life were achieved was a very significant moment for us and for the industry.

The Goodyear paper was significant for a different reason. Their paper showed an application that would have been difficult or impossible to evaluate with physical testing. They showed how getting the right physics into the model builds the trust necessary to leverage simulation to increase the speed and scope of discovery and to go beyond the limits of physical testing. It was the perfect illustration of keynoter Mike Anderson’s point that simulation opens significant opportunities for competitive advantage and ‘right the first time’ engineering.

Click here to download a .pdf summary of this blog post: Endurica Spotlight on The Tire Society 2021 Annual Meeting The Virtual Tire

Virtual vs. Physical in 2021 and Beyond

June 9, 2022August 16, 2021 by William V. Mars, Ph.D., P.E.

An insight to how Endurica stayed connected during Covid-19, by using Microsoft teams to meet virtually. These days everybody’s talking about whether to meet in person or online. There are great tools available for online meetings, and these have helped us navigate Covid-19. Like everyone else, Endurica teammates regularly use online meeting technology. But if there is one thing we learned over the last year, it is that sometimes physical presence really matters. Living and working in isolation is just not healthy in the long run.

The new normal during the pandemic had some benefits that were enabled by the virtual world. Time and energy that used to be consumed by travel were rechanneled into improving our software, testing services, and marketing materials. In our personal lives, we had more opportunities to spend quality time with our immediate families and found more time for fitness activities. We previously talked about our pandemic pivots to bring our training courses online and offer webinars to stay in touch with our existing and potential customers.

But virtual meetings can’t replace the full experience of being together in person. The face-to-face engagement at a trade show, the serendipitous bumping into a client, the spontaneous discussion of ideas with fellow conference-goers with a shared interest, the rapport building that comes from shared experiences. We fundamentally need physical connection. A hug, delivered via Zoom, will never feel the same.

The world of 2021 and beyond is hybrid: part virtual, part in-person. The benefits of the virtual are too great to set aside, and the necessity of the physical is too compelling to neglect. Both are critical to our future, at home and at work.

So, too, with Endurica’s simulation workflows. It was NEVER Simulation OR Build-and-Break. Even the best simulations are not enough to completely skip physical testing. The virtual approach saves significant time and money in product development and design refinement. It allows you to explore a huge space of compound options and of design features before investment in building and testing prototypes. Our simulations enable you to balance difficult trade-offs. Still, before you head into production, you must complete actual physical testing on your rubber part – the physical world is what counts in the end. It is simulation AND build-and-break that are both needed in concert to #GetDurabilityRight.

Just as a Zoom hug will never replace the real thing, software will never replace the role of physical testing. But just as online meetings are creating new opportunities and efficiencies, Endurica’s tools position you for unprecedented success when it’s time to test.

It's a Hybrid World from Here | Endurica's tools position you for unprecedented success when it’s time to test.

Use This One Simple Trick to Ensure Rubber Part Durability

June 9, 2022August 11, 2021 by William V. Mars, Ph.D., P.E.

We’ve just added a new output to the Endurica fatigue solver: Safety Factor. This feature makes it simple to focus your analysis on whether cracks have the minimum energy required to grow. Safety Factor is a quick and inexpensive way to identity potential failure locations. It minimizes the number of assumptions you need to defend, and it is backed by hard science. You don’t need to measure or explain the many influences that together determine how fast cracks grow. You don’t need lengthy materials characterization experiments that take days or weeks. You do need to know your material’s Intrinsic Strength T₀ (ie Fatigue Threshold) and its crack precursor size c₀. The test takes about an hour using the Coesfeld Intrinsic Strength Analyser.

The Safety Factor S is computed as the ratio of T₀ to the driving force T on a potential crack precursor. If the value of the Safety Factor S = T₀/T is greater than 1, it indicates the margin by which crack growth is avoided. If S is less than 1, it indicates that crack growth is inevitable. The calculation of the Safety Factor includes a search for the most critical plane, as we do for our full fatigue life computations.

Although the Safety Factor can’t tell you how long a part will endure, it nevertheless does offer great utility. You can make a contour plot showing the locations in your part where the Safety Factor is the lowest. This is a quick and inexpensive way to identity potential failure locations. You can make statements about the reserve capacity of your design that are easy to communicate and understand with a wide audience.

A vibration isolation grommet operating under small displacement

A vibration isolation grommet operating under large displacement

The images above show a vibration isolation grommet operating under small (Safety Factor 2.6) and large displacements (Safety Factor 0.83). Color contours indicate the Endurica-computed Safety Factor, and use the same scale for both images. Large Safety Factors are shown in blue. Safety Factors approaching 1 are shown in red. Safety Factors smaller than 1 are indicated in black. These results show that the grommet can be expected to operate indefinitely under the small displacements, but that large displacements will produce cracks at some point, in the regions colored black.

Will Mars on the Rubber Industry: A Look Back 10 Years, Where We Are Now, A Look Ahead 10 Years

June 15, 2022November 14, 2018 by William V. Mars, Ph.D., P.E.

Q: With regards to fatigue life prediction methods, where was the rubber industry 10 years ago?

Will: There was plenty of great academic work and good understanding of fundamentals, but the methods were only deployed – if at all – via “homebuilt” solutions that could never support a broad enough audience to really impact daily product design decisions. Simulation methods and experimental methods shared theoretical foundations but they were poorly integrated. They suffered from operational problems, noisy data and open-ended test duration. It was possible to analyze a crack if you could mesh it, but the added bookkeeping and convergence burdens were usually not sustainable in a production engineering context. Mostly, analysts relied on tradition-based crack nucleation approaches that would look at quantities like strain or stress or strain energy density. These were not very accurate and they were limiting in many ways, even though they were widely used. They left companies very dependent on build and break iterations.

Q: Where is the industry today?

Will: The early adopters of our solutions have been off and running now for a number of years. Our critical plane method has gained recognition for its high accuracy when dealing with multiaxial cases, cases involving crack closure, cases involving strain crystallization. Our testing methods have gained recognition for high reliability and throughput. Our users are doing production engineering with our tools. They are consistently winning on durability issues. They are handling durability issues right up front when they bid for new business. They are expanding their in-house labs to increase testing capacity and they are winning innovation awards from OEMs. They are using actual road-load cases from their customers to design light-weight, just-right parts that meet durability requirements. The automotive industry has lead adoption but aerospace, tires, energy, and consumer products are also coming up. We have users across the entire supply chain: raw material suppliers, component producers and OEMs. The huge value that was locked up because durability was previously so difficult to manage is now unlocked in new ways for the first time. This has been the wind in Endurica’s sails for the last 10 years.

Q: Where do you see the industry in 10 years?

Will: In 10 years, OEMs will expect durability from all component producers on day 1, even for radical projects. They will expect designs already optimized for cost and weight. They will push more warrantee responsibility to the supplier. They will monitor durability requirements via shared testing and simulation workflows. Suppliers will pitch solutions using characterization and simulation to show their product working well in your product. The design and selection of rubber compounds to match applications will enter a golden age as real-world customer usage conditions will finally be taken fully into account. Where design and selection was previously limited by the budget for a few build and break iterations, and low visibility of design options, they will soon be informed by an almost unlimited evaluation of all possibilities. Where simulation methods have traditionally had greatest impact on product design functions, we will also start to see rubber part Digital Twins that track damage accumulation and create value in the operational functions of a business. Durability is definitely set to become a strong arena for competition in the next 10 years.