Publications
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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Authors: WV Mars, JD Suter
Published: 08/10/2019
Journal: Paper C08, Presented at the Fall 196th Technical Meeting of the Rubber Division of the American Chemical Society, Inc.
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Breaking the computational barrier to simulating full road load signals in fatigue
Abstract
In this work, we present the Endurica EIETM nonlinear load mapping procedure, which provides a means by which the strain/stress histories resulting from full road load signals can now be rapidly generated. The procedure utilizes a series of pre-computed finite element solutions to populate a nonlinear map relating global load/displacement inputs to local strains/stresses within each finite element. For each time step of the full road load signal, the nonlinear map is used to obtain stress/strain...
In this work, we present the Endurica EIETM nonlinear load mapping procedure, which provides a means by which the strain/stress histories resulting from full road load signals can now be rapidly generated. The procedure utilizes a series of pre-computed finite element solutions to populate a nonlinear map relating global load/displacement inputs to local strains/stresses within each finite element. For each time step of the full road load signal, the nonlinear map is used to obtain stress/strain results via interpolation. Examples are provided for 1-, 2- and 3-channel signal inputs, and applied to the following automotive components: a sway bar link, a control arm bushing, and a transmission mount. Input signals of several durations were studied as follows: 1000, 10000 and 100000 time steps. The results show that EIE can quickly compute strain histories interpolated from a precomputed set of results with an error that can be controlled to a desired accuracy via map discretization. EIE’s benefit of efficiently interpolating results becomes more pronounced as signal length increases, in this study reaching nearly as high as a 4 orders of magnitude speed-up. However, EIE becomes less efficient as the number of problem dimensions increases (from one dimension to three dimensions). The lower benefit is due to the high cost of producing a higher dimensional map of FEA results for EIE to interpolate from. Even in this case, however, as seen for the 3D transmission mount analysis, the cost of creating the EIE map is still worthwhile when signal length is sufficiently long.
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Authors: William V. Mars, Yintao Wei, Wang Hao, Mark A. Bauman
Published: 01/03/2019
Journal: Tire Science and Technology
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Computing Tire Component Durability via Critical Plane Analysis
Abstract
Tire developers are responsible for designing against the possibility of crack development in each of the various components of a tire. The task requires knowledge of the fatigue behavior of each compound in the tire, as well as adequate accounting for the multiaxial stresses carried by tire materials. The analysis is illustrated here using the Endurica CL fatigue solver for the case of a 1200R20 TBR tire operating at 837 kPa under loads ranging from 66 to 170% of rated load. The fatigue behavio...
Tire developers are responsible for designing against the possibility of crack development in each of the various components of a tire. The task requires knowledge of the fatigue behavior of each compound in the tire, as well as adequate accounting for the multiaxial stresses carried by tire materials. The analysis is illustrated here using the Endurica CL fatigue solver for the case of a 1200R20 TBR tire operating at 837 kPa under loads ranging from 66 to 170% of rated load. The fatigue behavior of the tire's materials is described from a fracture mechanical viewpoint, with care taken to specify each of the several phenomena (crack growth rate, crack precursor size, strain crystallization, fatigue threshold) that govern. The analysis of crack development is made by considering how many cycles are required to grow cracks of various potential orientations at each element of the model. The most critical plane is then identified as the plane with the shortest fatigue life. We consider each component of the tire and show that where cracks develop from precursors intrinsic to the rubber compound (sidewall, tread grooves, innerliner) the critical plane analysis provides a comprehensive view of the failure mechanics. For cases where a crack develops near a stress singularity (i.e., belt-edge separation), the critical plane analysis remains advantageous for design guidance, particularly relative to analysis approaches based upon scalar invariant theories (i.e., strain energy density) that neglect to account for crack closure effects.
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Authors: A. Ramachandran, R. P. Wietharn, S.I. Mathew, W. V. Mars, and M. A. Bauman
Published: 12/10/2018
Journal: Presented at the 2018 Great Lakes Simulia Regional User Meeting
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Experimental Validation of Crystallizing & Non-Crystallizing Models of Rubber Fatigue Behavior
Abstract
The orientation of cracks initiating under cyclic loading in rubber may depend not only on maximizing the energy release rate, but also – in the case of a strain crystallizing rubber – on minimizing the life-lengthening effect of strain-crystallization associated with nonrelaxing loads. Crack orientations in a series of fully relaxing and nonrelaxing fatigue tests were computed with Endurica’s critical plane analysis, and compared with cracks developed in experiments on strain-crystallizin...
The orientation of cracks initiating under cyclic loading in rubber may depend not only on maximizing the energy release rate, but also – in the case of a strain crystallizing rubber – on minimizing the life-lengthening effect of strain-crystallization associated with nonrelaxing loads. Crack orientations in a series of fully relaxing and nonrelaxing fatigue tests were computed with Endurica’s critical plane analysis, and compared with cracks developed in experiments on strain-crystallizing Natural Rubber, and amorphous Styrene Butadiene Rubber. The specimen used for experimentation was a rectangular flat dumbbell prepared according to the ASTM D4482 Standard. Five strain histories were tested, two fully relaxing (0-80% and 0-100%), and three nonrelaxing (20-120%, 50%-150%, and 50%-170%). Critical plane analysis was performed with Endurica CL. As predicted, all fully relaxing tests, for both NR and SBR, developed cracks on the plane perpendicular to the maximum principal stress (90 degrees). Also as predicted, all nonrelaxing tests for the amorphous SBR develop in the same 90 degree plane. Finally, nonrelaxing tests for the crystallizing NR develop on specific planes that were accurately predicted by minimizing the fatigue life with respect to crack orientation.
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Authors: W. V. Mars, C. G. Robertson, R. Stocek, C. Kipscholl
Published: 09/10/2018
Journal: Paper B03, presented at the Fall 194th Technical Meeting of the Rubber Division of the American Chemical Society, Inc., Louisville, Kentucky
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Why Cutting Strength is an Indicator of Fatigue Threshold
Abstract
Crack tip fields during cutting and tensile loading have been computed via finite element analysis, and measured using Digital Image Correlation during experiments executed on the Coesfeld Intrinsic Strength Analyser. The results show that cutting with a sharp blade while the specimen is under a small amount of tension produces a much-reduced dissipative process zone in front of the crack tip, in comparison with the process zone produced by tensile loading alone at nominally similar conditions. ...
Crack tip fields during cutting and tensile loading have been computed via finite element analysis, and measured using Digital Image Correlation during experiments executed on the Coesfeld Intrinsic Strength Analyser. The results show that cutting with a sharp blade while the specimen is under a small amount of tension produces a much-reduced dissipative process zone in front of the crack tip, in comparison with the process zone produced by tensile loading alone at nominally similar conditions. Because the energy released by a growing crack supplies both the process of breaking polymer chains to form crack faces, and the dissipative process at the crack tip, minimizing crack tip dissipation causes the observed remaining energy release rate during a cutting experiment to approach the limit reflecting the breakage of polymer chains. Conveniently, this implies that a relatively brief cutting experiment may be used as an indicator of long-term fatigue behavior.
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Authors: Joshua R. Goossens, William V. Mars
Published: 01/10/2018
Journal: Rubber Chemistry and Technology
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Finitely Scoped, High Reliability Fatigue Crack Growth Measurements
Abstract
Classical procedures for characterizing fatigue crack growth behavior often suffer from uncertainties that make it difficult to plan for productive use of test instrument time, and that can result ultimately in too noisy measurements. An enhanced procedure has been implemented that is based on a fixed test time budget, and that establishes operating conditions that produce crack growth rates down to minimum measurable rates. The procedure features (1) a haversine pulse deformation test cycle fol...
Classical procedures for characterizing fatigue crack growth behavior often suffer from uncertainties that make it difficult to plan for productive use of test instrument time, and that can result ultimately in too noisy measurements. An enhanced procedure has been implemented that is based on a fixed test time budget, and that establishes operating conditions that produce crack growth rates down to minimum measurable rates. The procedure features (1) a haversine pulse deformation test cycle followed by a rest period, (2) a strain peak that ramps linearly over time, (3) minimum and maximum limits on the strain peak chosen to avoid unproductive test time, and (4) a stress–strain probe cycle for purposes of observing strain energy density. A set of replicates of a carbon black filled, natural rubber bushing compound has been characterized via both procedures, and a statistical analysis is made to compare both. The new procedure significantly improves the quality of crack growth rate curve measurements.
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Authors: R. Harbour, A. Fatemi, and W. V. Mars
Published: 01/03/2018
Journal: Journal of Material Science, 43, 1783-1794, 2008
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Fatigue Crack Orientation in NR and SBR Under Variable Amplitude and Multiaxial Loading Conditions
Abstract
The orientations of cracks as they develop in a material indicate the planes that have experienced the maximum damage. For the purpose of fatigue life analysis and prediction, these planes are referred to as the failure or critical planes. In order to study the planes on which cracks develop for different types of loading, the development of cracks was observed during constant and variable amplitude experiments using the multiaxial ring specimen. Two filled rubber materials were compared in this...
The orientations of cracks as they develop in a material indicate the planes that have experienced the maximum damage. For the purpose of fatigue life analysis and prediction, these planes are referred to as the failure or critical planes. In order to study the planes on which cracks develop for different types of loading, the development of cracks was observed during constant and variable amplitude experiments using the multiaxial ring specimen. Two filled rubber materials were compared in this study: NR, which strain crystallizes, and SBR, which does not. Multiaxial test signals composed of alternating blocks of axial and torsion cycles (each of which acts on different critical planes) produced crack orientations that fell between those occurring for signals composed only of axial or of torsion cycles. Plane-specific fatigue damage parameters of cracking energy density and normal strain were evaluated for their ability to predict the experimentally observed planes of crack development.
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