Publications
Authors: Gobi Gobinath, Thomas Ebbott, Shannon Hughes
Published: 23/09/2024
Journal: Tire Science and Technology (2024) 52 (2): 93–122.
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Predicting Residual Casing Life of a Tire following an Impact Event
Abstract
By applying recognized engineering methods, including finite element analysis, the role of impact events on the service life of a tire was studied by varying three factors: speed of impact, treadwear, and angle of impact. The approach combines well-known finite element analysis methods to simulate a tire rolling over an obstacle with the calculation of damage at the tire belt edge imparted by the impact event by using recognized methods of rubber fatigue analysis. An efficient method is develope...
By applying recognized engineering methods, including finite element analysis, the role of impact events on the service life of a tire was studied by varying three factors: speed of impact, treadwear, and angle of impact. The approach combines well-known finite element analysis methods to simulate a tire rolling over an obstacle with the calculation of damage at the tire belt edge imparted by the impact event by using recognized methods of rubber fatigue analysis. An efficient method is developed and used to demonstrate that across a range of impact conditions, some conditions can cause substantial internal damage, whereas other conditions can cause very little damage. The area of investigation is the tire belt edge; thus, although significant internal damage may have occurred, it might not be visually perceptible in the normal operation of a vehicle. In some cases, the nondetectable damage is shown to propagate to a point where the tire loses its structural integrity before reaching its normal operating life defined by treadwear. This study includes the role of mechanical, temperature, and rate effects.
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Authors: William V. Mars, Ph.D., P.E. and Thomas G. Ebbott, Ph.D.
Published: 11/09/2024
Journal: Advances in Understanding Thermal Effects in Rubber
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A Review of Thermal Effects on Elastomer Durability
Abstract
A chapter in the book Advances in Understanding Thermal Effects in Rubber, Pages 251-324
Authors: William Mars, Kevin Barbash, Matthew Wieczorek, Scott Braddock, Joshua Goossens, Ethan Steiner
Published: 09/04/2024
Journal: Society of Automotive Engineers
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Durability of Elastomeric Bushings Computed from Track-Recorded Multi-Channel Road Load Input
Abstract
The qualification requirements of automakers derive from track testing in which road load and moment inputs to a part in x, y and z directions are recorded over a set of driving conditions selected to represent typical operation. Because recorded histories are lengthy, often comprising many millions of time steps, past industry practice has been to specify simplified block cycle schedules for purposes of durability testing or analysis. Simplification, however, depends on imprecise human judgemen...
The qualification requirements of automakers derive from track testing in which road load and moment inputs to a part in x, y and z directions are recorded over a set of driving conditions selected to represent typical operation. Because recorded histories are lengthy, often comprising many millions of time steps, past industry practice has been to specify simplified block cycle schedules for purposes of durability testing or analysis. Simplification, however, depends on imprecise human judgement, and risks fidelity of the inferred life and failure mode relative to actual. Fortunately, virtual methods for fatigue life prediction are available that are capable of processing full, real-time, multiaxial road load histories. Two examples of filled natural rubber ride bushings are considered here to demonstrate. Each bushing is subject to a schedule of 11 distinct recorded track events. Endurica EIETM map building procedures are first used together with a finite element solution to map the 6 channel loading space and to obtain stress/strain solutions at each gridpoint in the load space. EIE is then used with the recorded histories to interpolate from the recorded road load inputs on the bushing to stress-strain history at each element centroid in the finite element model. The interpolated stress-strain histories are then used to compute damage accrual and fatigue life across all 11 events. The computational requirements of this workflow are benchmarked with the outcome that the entire schedule in full detail can be analyzed on a timescale well suited for use at the earliest stages of business development and engineering.
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Authors: Lewis B. Tunnicliffe, Christopher G. Robertson, William V. Mars
Published: 30/12/2023
Journal: Rubber Chemistry and Technology
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A Microscopy Investigation of Rubber Compound Crack Precursors and Tensile Fracture Surfaces
Abstract
Tensile stress–strain testing is used to investigate the fracture behavior of carbon black–reinforced styrene–butadiene rubber, using 50 replicate specimens. Four vulcanized rubber compounds are studied: a CB-filled SBR with standard mixing conditions (control), the same formulation with intentional poor mixing of the CB, and materials identical to the control material but formed by adding minor amounts of 0.5-mm-diameter glass microspheres (beads)—serving as large model defects/inclusio...
Tensile stress–strain testing is used to investigate the fracture behavior of carbon black–reinforced styrene–butadiene rubber, using 50 replicate specimens. Four vulcanized rubber compounds are studied: a CB-filled SBR with standard mixing conditions (control), the same formulation with intentional poor mixing of the CB, and materials identical to the control material but formed by adding minor amounts of 0.5-mm-diameter glass microspheres (beads)—serving as large model defects/inclusions—using a two-roll mill at two levels, corresponding to average values of 0.78 and 6.24 beads per gauge section region of the tensile test specimen. Microscopy analysis of the resulting fracture surfaces was conducted to complement our recent publication on Weibull failure statistics for distributions of tensile strength and crack precursor size. All 200 fractured specimens from tensile testing at 23°C were imaged with light microscopy and exhibited fracture surfaces characterized by relatively smooth planes perpendicular to the uniaxial loading direction. Most tensile failures originated from the edges of the dumbbell specimens, in line with expectations from fracture mechanics. Light microscopy revealed concentric fracture ring features of high specular reflectance emanating from crack precursors, which are a universal feature of the failure process for these compounds and independent of precursor type, size, or location. Noncontact interferometric microscopy confirmed that the rings resulted from variations in surface micro-roughness, proceeding outward from the precursor as rough–smooth–rough to the edge of the fracture surface. Fracture rings were also observed for tensile tests performed at 80°C. The variation in surface roughness of the fracture surface has parallels to the stick–slip tearing behavior seen for rubbers torn at medium to high rates. To the best of the authors’ knowledge, this is the first time that such striking features have been reported.
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Authors: Mars W.V. (2020)
Published: 03/12/2020
Journal: Heinrich G., Kipscholl R., Stoček R. (eds) Fatigue Crack Growth in Rubber Materials. Advances in Polymer Science, vol 286. Springer, Cham.
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Critical Plane Analysis of Rubber
Abstract
Durability is an essential feature of most elastomer products, directly linked to safety and to perceptions of brand quality. Product designers must therefore consider the impact on product durability of typical and abusive end-user loading scenarios. This can be accomplished using critical plane analysis (CPA). CPA starts by acknowledging that a small crack precursor might exist at any point in a part, and in any orientation, and that the potential development of all crack precursors must be ev...
Durability is an essential feature of most elastomer products, directly linked to safety and to perceptions of brand quality. Product designers must therefore consider the impact on product durability of typical and abusive end-user loading scenarios. This can be accomplished using critical plane analysis (CPA). CPA starts by acknowledging that a small crack precursor might exist at any point in a part, and in any orientation, and that the potential development of all crack precursors must be evaluated. The analysis produces a full accounting of which location and orientation maximizes crack growth (or, equivalently, minimizes fatigue life) at each point, the energy release rate history experienced, and of course the worst-case fatigue life across all possible orientations. This review provides an account of the development of the method over the last two decades and the validation case that has accumulated. This review also suggests directions for further development of the method.
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Authors: Robertson C.G., Stoček R., Mars W.V. (2020)
Published: 01/12/2020
Journal: Advances in Polymer Science
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The Fatigue Threshold of Rubber and Its Characterization Using the Cutting Method
Abstract
Below a limiting value of tearing energy called the intrinsic strength or fatigue threshold (T0), cracks will not grow in rubber due to fatigue; hence, this material characteristic is important to understand from both fundamental and practical perspectives. We summarize key aspects of the fatigue threshold, including the Lake-Thomas molecular interpretation of T0 in terms of fracture of polymer network chains in crosslinked elastomers. The various testing approaches for quantifying T0 are also d...
Below a limiting value of tearing energy called the intrinsic strength or fatigue threshold (T0), cracks will not grow in rubber due to fatigue; hence, this material characteristic is important to understand from both fundamental and practical perspectives. We summarize key aspects of the fatigue threshold, including the Lake-Thomas molecular interpretation of T0 in terms of fracture of polymer network chains in crosslinked elastomers. The various testing approaches for quantifying T0 are also discussed, with a focus on the classic Lake-Yeoh cutting method which was recently revived by the introduction of a commercial testing instrument that applies this procedure, the Intrinsic Strength Analyser (ISA). A validation of the cutting method is also given by demonstrating that a 2-h test on the ISA yields a value of T0 that is essentially identical to the T0 from near-threshold fatigue crack growth (FCG) measurements that require 7.5 months of continuous testing. Compound formulation effects – polymer type, crosslink density, type and amount of reinforcing fillers, and addition of oils/plasticizers – are examined based on the limited published research in this area and our new results. At the end, some insights are offered into using the fatigue threshold to develop highly durable rubber products.
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Authors: William V. Mars, P.E., Ph.D.
Published: 28/09/2020
Journal: Rubber Chemistry and Technology
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Rubber Chemistry and Technology , 2010–2020
Abstract
It is with great appreciation that we announce that Dr. Will Mars has stepped aside as Editor-in-Chief. Dr. Mars oversaw the journal for 10 years. He worked with the Editorial Board, the Rubber Division staff, the publisher, and the reviewers and authors who comprise the journal community to grow the scientific quality and influence of the journal. During his tenure, citations to RC&T articles increased enough that the journal's impact factor grew from 0.393 to a high of 1.766, making RC&...
It is with great appreciation that we announce that Dr. Will Mars has stepped aside as Editor-in-Chief. Dr. Mars oversaw the journal for 10 years. He worked with the Editorial Board, the Rubber Division staff, the publisher, and the reviewers and authors who comprise the journal community to grow the scientific quality and influence of the journal. During his tenure, citations to RC&T articles increased enough that the journal's impact factor grew from 0.393 to a high of 1.766, making RC&T the premier journal globally for elastomer-focused research.
Dr. Mars is among the top scientific authorities in the world in the area of elastomer durability and fracture mechanics of rubbery materials. According to Google Scholar, he has published more than 85 papers and holds 3 patents. He has received many awards for his work. He received national recognition in the form of the U.S. SBA's Tibbetts Award for the successful development and commercialization of technology in service to the nation. He received the Rubber Division's Arnold Smith Special Service Award in recognition of service to the Division through the years 2004–2017. During that time, Dr. Mars served on the Science and Technology Awards Committee, the International Rubber Science Hall of Fame committee, the Best Paper Committee, the Education and Publications Committee, and the Program Planning Committee. He was editor of the Tire Society's journal Tire Science and Technology from 2008–2009. He was the 2007 recipient of the Sparks-Thomas Award recognizing outstanding contribution and innovation by younger scientists.
Professionally, Dr. Mars is the founder and President of Endurica LLC, a provider of software and testing solutions for managing durability. Prior to founding Endurica, Dr. Mars held research engineering positions of progressing responsibility in the Research Department at the Cooper Tire and Rubber Company. He began as a co-op student in the Truck Tire Engineering Department in 1992, joined the Research group full time in 1994, and progressed to Advanced Research Engineer before leaving the company in 2011 to lead Endurica.
Dr. Mars earned his M.S. and Ph.D. degrees in Engineering Science from the University of Toledo, and an Honors B.S. Mechanical Engineering degree with Polymer Specialization from the University of Akron.
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Authors: William V. Mars, Govind Paudel, Jesse D. Suter, Christopher G. Robertson
Published: 20/02/2020
Journal: Tire Science and Technology
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Incremental, Critical Plane Analysis of Standing Wave Development, Self-Heating, and Fatigue during Regulatory High-Speed Tire Testing Protocols
Abstract
Tire speed ratings derive from regulatory testing in which tire structural integrity is validated over a series of steps with successively increasing speed. For the FMVSS 139 high-speed standard, there are four half-hour duration speed steps at 80, 140, 150, and 160 kph. Speed ratings from Q through Y may be attained through the UN ECE R30 regulation high-speed testing. For either protocol, a tire must demonstrate the ability to operate without crack development at high speed for a specified per...
Tire speed ratings derive from regulatory testing in which tire structural integrity is validated over a series of steps with successively increasing speed. For the FMVSS 139 high-speed standard, there are four half-hour duration speed steps at 80, 140, 150, and 160 kph. Speed ratings from Q through Y may be attained through the UN ECE R30 regulation high-speed testing. For either protocol, a tire must demonstrate the ability to operate without crack development at high speed for a specified period. After the test, “there shall be no evidence of tread, sidewall, ply, cord, inner liner, belt or bead separation, chunking, broken cords, cracking, or open splices.” A workflow for simulating regulatory high-speed durability performance has been developed based upon (1) recent improvements to the Abaqus steady-state transport formulation that now permit converged solutions to be obtained at high speed (including after the development of standing waves in the tire) and (2) Endurica DT self-heating and incremental fatigue simulations that account for thermal effects and for damage accumulation occurring due to a schedule of load cases. The self-heating calculation features the Kraus model and accurately captures viscoelastic loss modulus dependence on strain amplitude and temperature. For each step of the high-speed procedure, steady-state structural and thermal solutions are first computed. The deformation history in the presence of standing waves is shown to require rainflow counting due to the occurrence of multiple load cycles per tire revolution. Crack growth is finally integrated for each potential critical plane through each step of the test until failure is indicated. Standing waves at high speed induce significant self-heating and damage, rapidly limiting high-speed performance. The temperature dependence of self-heating and strength properties also plays a major role in limiting high-speed durability. The simulations were executed on both a flat surface and on the regulation specified 1.7 m diameter road wheel. As expected, durability testing on the road wheel is more severe, and the beneficial effect of a nylon overwrap is predicted.
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Authors: Christopher G. Robertson, Jesse D. Suter, Mark A. Bauman, Radek Stoček, William V. Mars
Published: 07/02/2020
Journal: Tire Science and Technology
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Finite Element Modeling and Critical Plane Analysis of a Cut-and-Chip Experiment for Rubber
Abstract
Rubber surfaces exposed to concentrated, sliding impacts carry large normal and shearing stresses that can cause damage and the eventual removal of material from the surface. Understanding this cut-and-chip (CC) effect in rubber is key to developing improved tread compounds for tires used in off-road or poor road conditions. To better understand the mechanics involved in the CC process, an analysis was performed of an experiment conducted on a recently introduced device, the Instrumented Chip an...
Rubber surfaces exposed to concentrated, sliding impacts carry large normal and shearing stresses that can cause damage and the eventual removal of material from the surface. Understanding this cut-and-chip (CC) effect in rubber is key to developing improved tread compounds for tires used in off-road or poor road conditions. To better understand the mechanics involved in the CC process, an analysis was performed of an experiment conducted on a recently introduced device, the Instrumented Chip and Cut Analyzer (ICCA), which repetitively impacts a rigid indenter against a rotating solid rubber wheel. The impact process is carefully controlled and measured on this lab instrument, so that the contact time, normal force, and shear force are all known. The numerical evaluation includes Abaqus finite element analysis (FEA) to determine the stress and strain fields during impact. The FEA results are combined with rubber fracture mechanics characteristics of the material as inputs to the Endurica CL elastomer fatigue solver, which employs critical plane analysis to determine the fatigue response of the specimen surface. The modeling inputs are experimentally determined hyperelastic stress-strain parameters, crack growth rate laws, and crack precursor sizes for carbon black–filled compounds wherein the type of elastomer is varied in order to compare natural rubber (NR), butadiene rubber (BR), and styrene-butadiene rubber (SBR). At the lower impact force, the simulation results were consistent with the relative CC resistances of NR, BR, and SBR measured using the ICCA, which followed the order BR > NR > SBR. Impact-induced temperature increases need to be considered in the fatigue analysis of the higher impact force to provide lifetime predictions that match the experimental CC resistance ranking of NR > SBR > BR.
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Authors: C.G. Robertson, L.B. Tunnicliffe, L. Maciag, M.A. Bauman, K. Miller, C.R. Herd, W.V. Mars
Published: 13/01/2020
Journal: Polymers
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Characterizing Distributions of Tensile Strength and Crack Precursor Size to Evaluate Filler Dispersion Effects and Reliability of Rubber
Abstract
Undispersed filler agglomerates or other substantial inclusions/contaminants in rubber can act as large crack precursors that reduce the strength and fatigue lifetime of the material. To demonstrate this, we use tensile strength (stress at break, σb) data from 50 specimens to characterize the failure distribution behavior of carbon black (CB) reinforced styrene-butadiene rubber (SBR) compounds. Poor mixing was simulated by adding a portion of the CB late in the mixing process, and glass beads (...
Undispersed filler agglomerates or other substantial inclusions/contaminants in rubber can act as large crack precursors that reduce the strength and fatigue lifetime of the material. To demonstrate this, we use tensile strength (stress at break, σb) data from 50 specimens to characterize the failure distribution behavior of carbon black (CB) reinforced styrene-butadiene rubber (SBR) compounds. Poor mixing was simulated by adding a portion of the CB late in the mixing process, and glass beads (microspheres) with 517 µm average diameter were introduced during milling to reproduce the effects of large inclusions. The σb distribution was well described with a simple unimodal Weibull distribution for the control compound, but the tensile strengths of the poor CB dispersion material and the compounds with the glass beads required bimodal Weibull distributions. For the material with the lowest level of glass beads—corresponding to less than one microsphere per test specimen—the bimodal failure distribution spanned a very large range of σb from 13.7 to
22.7 MPa in contrast to the relatively narrow σb distribution for the control from 18.4 to 23.8 MPa. Crack precursor size (c0) distributions were also inferred from the data, and the glass beads introduced c0 values in the 400 µm range compared to about 180 µm for the control. In contrast to σb, critical tearing energy (tear strength) was unaffected by the presence of the CB agglomerates and glass beads,because the strain energy focuses on the pre-cut macroscopic crack in the sample during tear testing.
rather than on the microscopic crack precursors within the rubber. The glass beads were not detected
by conventional filler dispersion measurements using interferometric microscopy, indicating that
tensile strength distribution characterization is an important complementary approach for identifying
the presence of minor amounts of large inclusions in rubber.
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