Tom Ebbott, Ph.D.

Thomas G. Ebbott serves Endurica as Vice President, responsible for go-to-market and growth strategy for the automotive business segment, with a special focus on automotive OEMs. He brings 35 years of leadership and technical experience from his career in the tire industry. Tom received his Ph.D. from the University of Wisconsin where his research focused on viscoelasticity and fracture mechanics in polymers. He entered the tire industry immediately with Goodyear Tire and Rubber company after completing his Ph.D. and has held various technical and leadership positions including a two-year assignment in Europe. Tom has actively contributed to the industry’s technical literature and has presented at various speaking engagements concerning modeling for rubber fatigue, tire durability, rolling resistance, and composite materials. He has also been a major contributor to various technical partnerships and cooperative research with Sandia National Laboratories.

4-in-1

December 30, 2024December 15, 2024 by Tom Ebbott, Ph.D.

Wow – this year has really been one of many firsts for Endurica. We had our first ever Community Conference in April, we started our first sister company – in Europe, and from September 9 – 13, 2024, we presented 4 technical papers – a new Endurica record for one week! The other impressive aspect of this latter feat was that the four presentations were on vastly different topics! I’ll just list the venues and titles and then discuss each one.

International Elastomer Conference 2024, Pittsburgh, PA, USA:

“Heat Build-Up and Thermal Runaway in a Rotating Bending Experiment”

44^th Annual Meeting and Conference of The Tire Society, Akron OH, USA:

“Coupled Multiphysics Strategy to Monitor the Health of Rubbery Structures Using Endurica Tools”
“Critical Plane Analysis of Surface-proximal Fields for the Simulation of Mechanochemical Wear”
“Models, Materials and the Move Towards Virtual Product Development”

Let’s start with the first presentation on heat build-up. Will Mars presented this paper at the IEC in Pittsburgh on Tuesday the 10^th of September. The presentation highlighted a new machine that has been developed by Coesfeld to evaluate the heat build-up behavior of rubber compounds. It uses a hollow rubber tube that is bent to a 60-90 degree arc and then rotated at about 600 rpm to create a tension-compression cycle throughout the tube due to the pre-bending as shown below.

This test offers many advantages over the historical Goodrich Flexometer self-heating test originally developed in 1937. The Heat Build-Up Analyzer is instrumented to measure internal temperature as well as forces and deformations while the test is progressing. The recent advances in the Endurica software and workflows are also equipped to predict the transient behavior in this test. When the rubber reaches a certain high temperature, the rubber starts to break down, often due to the volatilization of low molecular weight additives creating an exothermic reaction, and also due to the reversion of the cross-links. The exothermic reaction and thermal “runaway” condition can also be predicted by Endurica software. The animation below shows the elevated temperatures and the internal pressure rise due to the exothermic reactions. The combination of the HBA test and the Endurica FEA-based analysis will add understanding to the heat-rise behavior of compounds for any company. As with some other Coesfeld machines, Endurica is the sole distributor in the Americas.

The second presentation listed was presented by Mahmoud Assaad, co-authored by others at Endurica and also by Ed Terrill at ARDL. This work aims to provide the combination of a full oxygen diffusion and oxidation reaction simulation and experimental characterization capability. The plot here shows the distribution of reacted oxygen in the crown area of a commercial truck tire. As the oxygen diffuses into the carcass it also reacts with the rubber compounds creating a phenomenon known as Diffusion Limited Oxidation. Mahmoud, Ed Terrill and I worked on rubber oxidation with Sandia National Laboratories when the three of us worked together at Goodyear. Now we have developed a characterization and simulation capability that should be ready for customers to try in 2025!

For the third presentation listed, Will Mars quickly travelled from the IEC in Pittsburgh to the Tire Society in Akron to give a talk on an evolving capability for wear prediction. This work was co-authored by Lewis Tunnicliff and James Kollar at Birla Carbon as well as others from Endurica. For many years, researchers have been trying to link rubber fracture and tearing behavior to surface wear. One of the early works on this topic is shown in the drawing below from Southern and Thomas in 1979.

This work attempted to explain observations from blade abrader experiments. The Endurica/Birla work broadens this concept to different asperity shapes and a cumulative fatigue process that depends on the depth into the surface. Temperature distribution near the surface was also calculated and included in the analysis. Initial results gave similar trends for wear rates as work done by Gent and Pulford in 1983. This new approach also makes it easy to also incorporate any aging effects that may occur on the surface of a rubber product. Development work on this new capability will continue well into 2025. In the meantime, Endurica does have a more basic FEA-based offering for wear prediction that has been used for multiple customers.

Lastly, on Friday the 13^th of September, I had the honor of giving the Plenary Lecture for the Tire Society conference. Thanks go to Jim McIntyre and the conference organizers for giving me this unique opportunity to address the society.

In April, we conducted the first ever Endurica Community Conference, and we tied in the Solar Eclipse that passed over Findlay, Ohio on April 8th, to produce a very successful event. I wanted to include the solar eclipse in my Plenary talk and somehow relate it to topics concerning the development of tires. The two concepts I used to make the connection were:

All models are approximations, but some can be very useful, and
Some very good physics theories predict singularities. The singularities reveal our ignorance on the topic and show the area where further work and insights are needed.

The first concept comes from the late George E. P. Box, a statistics professor at the University of Wisconsin. The quote is usually stated as: “All models are wrong, but some are useful”. The second concept makes a tie between fracture mechanics and Einstein’s General Theory of Relativity, which was validated by data taken during a solar eclipse in 1919. Both of these theories predict non-physical singularities but remain extremely useful.

The bulk of my talk was on Virtual Tire Development using tire durability as one of the performances to evaluate without building and testing prototypes. It largely followed my experience and contributions to the topic over the 3+ decades I worked on this at Goodyear with many excellent colleagues and partner organizations like Sandia.

All four of these presentations are available on our website at this location: Fatigue Ninja Frontier – Resources from Endurica’s First Annual Meeting.

Please contact us if you have any questions about these presentations or if you would like to chat with us about anything, including possible work together.

One final note: we are working on a revised website. Our Marketing Director, Pauline Glaza, is heading up a project to develop a new website for us that should make navigating our material and interacting with us much easier. Expect to see our new site in early 2025!

Crack Growth or Continuum Damage?

July 10, 2023 by Tom Ebbott, Ph.D.

The topic of whether to use a crack growth method or a continuum damage method for product fatigue and durability assessment has long been debated. Oftentimes, experts will recommend using a continuum damage approach in the initial phase, when no noticeable cracks are present, and then transition to a crack growth analysis when damage has reached a certain level where cracks are likely to appear. In other applications, most of the product’s life is consumed in the crack or crack growth initiation phase, so a continuum damage method is deemed most appropriate. There are also cases where products are in service with known detectable cracks; in this case fracture mechanics and crack growth analysis is employed to predict how fast the crack will propagate and when it will reach a critical size.

The simplest continuum damage analysis uses Wöhler curves, or S-N diagrams and Palmgren-Miner’s rule. The S-N diagrams are built by running fatigue tests on un-cracked dumbbell specimens at various stress amplitudes, S, and measuring the number of cycles to failure, N_f. Typical S-N diagrams are shown in Figure 1 [1]. The quantity S_f is the Endurance Limit (or Fatigue Limit), below which no failure is predicted to occur.

Figure 1. Typicaly S-N Diagrams [1]

A linear damage rule like the Palmgren-Miner rule states that the amount of damage due to a certain number of cycles, n_i, at a certain stress amplitude, S_i, is a simple linear ratio compared to the number of cycles to cause failure at that stress amplitude, or

(1)

The incremental amount of damage can then be summed over different blocks of cycles at different stress amplitudes to predict failure when

(2)

One of the limitations of this approach is that sequence effects, for example going from a high-to-low stress amplitude vs. going from a low-to-high stress amplitude is not accounted for. Stated another way, the rate of damage accumulation does not depend on the current state of damage. There also tends to be a large amount of scatter in the results. In finite element implementations, the amount of damage is tracked towards failure, and damage can be included as a state variable in the constitutive law to allow the stiffness to evolve as a function of damage.

The Endurica methods of fatigue analysis combine fracture mechanics, crack growth, and continuum damage methods. In most materials, there are crack precursors on the micron, or sub-micron level that serve as crack growth initiators. Filled elastomers are known to have many discontinuities at the micron level due to, for example, voids filled with air, agglomeration of fillers or clumps of additives. These are treated as an initial “pre-cursor” crack with the size c₀ with typical values between 10 and 100 microns. Crack growth analysis is used to predict the number of cycles, or number of repeats of a block of cycles until the crack reaches a length indicative of the end of life of the product or component.

Rather than using stress as the driver for damage as in the S–N diagram, a fracture parameter called Energy Release Rate, or Tearing Energy is used as the driver for crack growth rate. An example plot is shown in Figure 2.

The analogy to the Endurance Limit in the S-N diagram is the Intrinsic Strength, T₀, below which no crack growth is predicted. The power-law portion of the plot with slope “F” can be expressed as

(3)

where r_cis the crack growth rate when T = T_c, the Critical Tearing Energy. In metals, this is termed a Paris Law, in elastomers, it is the Thomas Law [2].

The damage rate in this case is the crack growth rate, dc/dN. Also, the “damage” is tracked as the predicted length of a growing crack. The summation of the damage over a given set of cycles can be written as

(4)

The Tearing Energy in a single edge cracked tension specimen is given by

(5)

where W is the strain energy density far from the crack and k is a constant depending on strain level. In a general three-dimensional state of deformation, Endurica uses the Cracking Energy Density, W_c such that,

(6)

In each of these cases, the Tearing Energy, and thus the crack growth rate is predicted to depend on the crack length, c.

Combining equations 6 and 3, we see that the damage rate, dc/dN, in this analysis, will depend on the current state of damage, c, and thus be able to represent sequence effects as part of the analysis.

In the finite element implementation with the Endurica software, there is typically no explicit crack in the FEA model. Thus the calculation of damage in the form of a growing crack is like a continuum damage approach on the macro-scale. A co-simulation workflow is also available where the stiffness of each element in the FEA model evolves with the calculation of crack length in each element.

The Endurica analysis methods can be viewed as a continuum damage method on the macro-scale, while using fracture mechanics and crack-growth analysis on the micro-scale. The use of fracture mechanics provides many advantages including a well-developed and validated theory for elastomers, less scatter in fatigue experiments, nonlinear damage evolution and sequence effects, and the easy ability to include many other aspects such as temperature, aging, and strain crystallization.

References

[1] Stephens, R. I., Fatemi, A., Stephens, R.R., and Fuchs, H.O., Metal Fatigue in Engineering, 2^nd edition, John Wiley & Sons, 2001.

[2] Thomas, A.G., “Rupture of Rubber IV. Cut Growth in Natural Rubber Vulcanizates,” Journal of Polymer Science, Vol 31, pp 467-480, 1958.

Introducing Tom Ebbott, the new Vice President at Endurica!

December 14, 2022December 14, 2022 by Tom Ebbott, Ph.D.

Thomas G. Ebbott, Ph.D. Vice President Endurica LLC

Hello everyone. I am really excited to be writing this as the newest member to Team Endurica! I am really enjoying my on-boarding with Will and the team thus far. I continue to learn all the capabilities that the Endurica software has to offer, along with all the services that Endurica the company offers. I’m looking forward to using my knowledge and experience with modeling and simulation combined with expertise in fatigue and fracture in polymers to bring value to Endurica’s customers.

Endurica’s software and services enable customers to monitor, predict and improve the endurance of products. This has a positive impact on many of today’s contemporary questions. For example, for sustainability, customers need to evaluate the durability impact of using a material with a more sustainable source, or one with better recyclability or re-useability in place of an existing material. Even re-designing a component to use less material, or to last longer is more sustainable. For electric vehicles, many of the elastomeric components are called on to carry higher loads and higher torques in the case of tires. And, for fleet operations, Endurica can be used to monitor the health of elastomeric systems and predict when maintenance will be needed.

I feel I have a good background to help both Will and the team at Endurica as well as Endurca’s many and wide-ranging customers. As many of you know, I recently retired from Goodyear after nearly 36 years with that great company. While I was at Goodyear, I worked with many wonderful and capable people, and I was fortunate to have many fulfilling experiences and roles. Some include developing fundamental technology, developing products–specifically Aviation Tires and Retreads, various people leadership roles, and finally a high-level technical leader role responsible for technical strategy. While at Goodyear, I was able to publish several papers on topics such as fracture mechanics of rubber in tires, temperature distribution and rolling resistance prediction for tires, crack growth in twisted rubber disks, and continuum damage analysis of cord-rubber structures. I served on The Tire Society Executive Committee for 8 years as Treasurer. One of my long-term contributions at Goodyear was to the 30-year partnership with Sandia National Laboratories.

For my formal training, I spent 10 years at the University of Wisconsin-Madison that culminated in a PhD in Engineering Mechanics. My masters work focused on structural dynamics while my PhD research was on crack growth in polyethylene. The application of my PhD work was for the durability evaluation of natural gas distribution pipelines. The crack growth evaluation in (high density, high molecular weight) polyethylene required development of viscoelastic material laws and characterization as well as crack growth measurement systems, means to measure strain distributions, and use of viscoelastic crack growth theories.

On a personal note, my wife Sheri and I have two adult children. Amanda is teaching 2nd grade at a school near Columbus, OH, and Zachary is a junior pursuing a Finance degree at Regis University in Denver. One of my passions is flying, and I’ve had my private pilot’s license for many years. One of my most memorable flying trips was to New Mexico and Colorado. The photo shows my plane with the sun rising over the Sandia mountains in Albuquerque, NM.

I’m looking forward meeting and talking with Endurica’s customers in the coming months and learning about their needs and challenges concerning the use of elastomers and polymers for component design.