Astm stp 694 1979

This paper traces the historical development of tire cords; analyzes forecasts of tire cord consumption; discusses past and present market and societal goals; notes the complex- ities of

TIRE REINFORCEMENT

AND TIRE PERFORMANCE

A symposium sponsored by ASTM Committees D13 on Textiles and F09 on Tires

AIVIERICAN SOCIETY FOR TESTING AND MATERIALS Montrose, Ohio, 23-25 Oct 1978

ASTM SPECIAL TECHNICAL PUBLICATION 694

R A Fleming and

D I, Livingston, Goodyear Tire and Rubber Company, editors

List price $34.50 04-694000-37

#

^AMERICAN SOCIETY FOR TESTING AND MATERIALS

1916 Race Street, Philadelphia, Pa 19103

Copyright © by American Society for Testing and Materials 1979 Library of Congress Catalog Card Number 79-53318

NOTE The Society is not responsible, as a body, for the statements and opinions advanced in this publication

Printed in Baltimore, Md

Dec 1979

Foreword

The Symposium on Tire Reinforcement and Tire Performance was held 23-25 Oct 1978, in Montrose, Ohio ASTM Committees D13 on TextUes and F09 on Tires jointly sponsored the symposium R A Fleming and D I Livingston, Goodyear Tire and Rubber Company, presided as chairmen on the symposium and served as editors of this pubUcation The Program Committee consisted of

C F Brenner, National Highway Traffic Safety Administration, J W Hannell,

E I duPont de Nemours and Co., Inc., and C C McCabe, E I duPont de Nemours and Co., Inc

Related ASTM Publications

Surface Texture Versus Skidding, STP 583 (1975), $12.00,04-583000-37

An Analysis of the Literature on Tire-Road Skid Resistance, STP 541 (1973),

$5.50,04-541000-37

Skid Resistance of Highway Pavements, STP 530 (1973), $12.25,04-530000-37 Annual Book of ASTM Standards, Part 15, Road and Paving Materials; Bitumi-nous Materials for Highway Construction, Waterproofing and Roofing, and Pipe; Skid Resistance (1979), $38.00,01-015079-08

Annual Book of ASTM Standards, Part 38, Rubber Products, ifications and Related Test Methods; Gaskets; Tires (1979), $29.00, 01-038079-20

Industrial—Spec-A Note of Industrial—Spec-Appreciation

to Reviewers

This publication is made possible by the authors and, also, the unheralded efforts of the reviewers This body of technical experts whose dedication, sacri-fice of time and effort, and collective wisdom in reviewing the papers must be acknowledged The quality level of ASTM publications is a direct function of their respected opinions On behalf of ASTM we acknowledge their contribution with appreciation

ASTM Committee on Publications

Editorial Staff

Jane B Wheeler, Managing Editor Helen M Hoersch, Associate Editor Ellen J McGlinchey, Senior Assistant Editor Helen Mahy, Assistant Editor

Contents

Introduction 1 Challenges Facing Tire Cord Engineers Today—c z. DRAVES, JR AND

Steel Cord: Analysis of Used Truck Tires and Simulation of the Found

Phenomena in Laboratory Experiments—c c J DE JONG 69

Adhesion of Steel Tire Cord to Rubber Compounds Mutual Influence of

Belt Test for the Evaluation of the Fretting Fatigue and Adhesion

Laboratory Machine to Evaluate the Resistance of Tire Cords (Textile or

Steel) to Tensile Fatigue or Compressive Fatigue or Both—CESARE

CANEVARI AND A G LALA 1 1 0

Adhesive and Processing Concepts for Tire Reinforcing Materials—

R S BHAKUNI, G W RYE, AND S J DOMCHICK 122

Degradation of Adhesion of Coated Tire Cords to Rubber by Atmospheric

Pollutants—R E HARTZ AND H T ADAMS 1 3 9

Physical Factors in Cord-to-Rubber Adhesion by a New Tire Cord Adhesion

Test—G S FIELDING-RUSSELL, D W NICHOLSON, AND

D L LIVINGSTON 1 5 3

Dynamic Moduli of Continuous Filament Yams Subjected to Low Frequency

Excitation Superimposed on High Initial Longitudinal Strain—

C F ZOROWSKI AND Z P SMITH 1 6 3

Role of Tire Reinforcements on Composite Static and Dynamic

Characteristics—J J voRACHEK 180

Study of Fiber Fracture and Interfacial Chemistry Using the Scanning

Viscoelastic Properties of Tire Cords Under Conditions of Rolling Tires—

Y D KWON, R K SHARMA, AND D C PREVORSEK 2 3 9

Relative Importance of Cords and Rubber in Tire Rolling Resistance—

R K SHARMA, Y D KWON, AND D C PREVORSEK 2 6 3

Design of Radial Passenger Tires with Aramid Cord Belts—D F RYDER 284

Bias Passenger Tires: Effect of Construction on the Cord Deformation

and Temperature Rise During Rolling—D C PREVORSEK,

C W BERINGER, Y D KWON, AND R K SHARMA 2 9 8

Summary 327

Index 329

STP694-EB/Dec 1979

Introduction

This special technical publication comprises the proceedings of the Symposium

on Tire Reinforcement and Tire Performance jointly sponsored by ASTM mittees D13 on Textiles and F09 on Tires The symposium was so titled to stress the important technical relationship between the two aspects of the subject mat-ter and to stimulate contributions in each area, emphasizing the connection between them This theme successfully generated the technological and scientific papers collected here

Com-The goal of the symposium to bring together contributions on the many aspects

of the subject has been well reahzed by these proceedings which have covered the spectrum of tire cord reinforcements and their adhesion to the viscoelastic rubber compounds, two highly dissimilar materials which unite in the composite tire structure to produce its final, unique performance The history of the devel-opment and improvement of tire cords to their present sophistication in materials and configuration, testing their strength and dynamic properties relevant to per-formance in the tire, and a novel method of evaluating their adhesion to the rubber matrix are examples of the content of these proceedings which have served

to further illuminate the subject

The editors acknowledge the assistance of many individuals in ASTM who managed the symposium and ensured the production of this work in its final critically reviewed form We especially thank the Program Conmiittee of the sym-posium who were primarily responsible for its quality The committee members were F C Brenner, National Highway Traffic Safety Administration of the U.S Department of Transportation, C C McCabe, Du Pont Company, and John Hannel, Du Pont Company, who were instrumental in securing many of the papers presented, who ensured a smoothly running symposium, and who partici-

pated prominently in the review process We are grateful to all these people for

their expert help

C Z Draves, Jr} and Leonard Skolnik^

Challenges Facing Tire

Cord Engineers Today

REFERENCE: Draves, C Z., Jr., and Skolnik, Leonard, "Challenges Facing Tire Cord

Engineers Today," Tire Reinforcement and Tire Performance, ASTM STP 694, R A

Fleming and D I Livingston, Eds., American Society for Testing and Materials, 1979,

pp 3-17

ABSTRACT: The basic challenge facing tire cord engineers is to develop a better

under-standing of the relationships between tire cord properties and tire performance This paper traces the historical development of tire cords; analyzes forecasts of tire cord consumption; discusses past and present market and societal goals; notes the complex- ities of relating tire cord properties to tire performance; discusses the optimization

of tire cord tensile strength to burst and bruise a tire; and notes the importance of tire cord dynamic properties to power loss in tires

KEY WORDS: tire cord historical development, tire cord properties and

perform-ance, tire cord market forecasts, tire bruise, tire burst, tire power loss, cotton, rayon, nylon, glass, polyester, steel, aramid, vinal

By many measures the tire industry must be considered mature, yet, technical changes in tires promise to continue at a rapid rate, and at the heart of many of these changes are tire cords

The prime or basic challenge to tire cord engineers is to develop a better standing of tire cord materials and how they function in a tire But before we elaborate on this, and how it relates to the bigger picture, we will first review the historical developments of tire reinforcement and some of the challenges that in-fluenced them Next, we will review some past and present U.S forecasts of future tire cord consumption and comment on the difficulties of making even qualita-tive forecasts beyond three or four years With that background, we wUl then provide some comments about societal challenges as we see them We wiU try to indicate how societal challenges will impact tire and thus tire cord development

under-To illustrate this, we will identify certain tire performance criteria that are lated in some way to tire reinforcement properties Then in a closer look at some

re-' Director, Materials and Compound Development, and senior research and development

associate Tire Group, respectively, The BFGoodrich Company, Akron, Ohio 44318

3

4 TIRE REINFORCEMENT AND TIRE PERFORMANCE

of these criteria, we will illustrate how by improving their understanding tire cord engineers can help resolve today's technical challenges

Historical Development of Tire Cords

Now let us review our history Since that day in 1888, when J B Dunlop

re-inforced his first pneumatic tire [1]^ with Irish flax, there have been many

chal-lenges as to what reinforcements to use in tires, and just as important how to use these reinforcements in tires to optimize tire performance Rubber coated, square woven cotton fabric became the tire fabric of choice in the early tires This was the same type of fabric used to reinforce belts and hose Tire designers soon found that square woven fabric had the disadvantage of abraiding against itself as the tire carcass flexed through the footprint To overcome this challenge,

in 1893 J F Palmer patented [2] the use of all warp (or weftless) cotton fabrics

However, it was not until after World War I, when heavier and more powerful automobiles really became popular, that square woven was completely displaced

in favor of several plies of rubber coated, warp cotton cords

In the 1930s, the first man-made fiber, rayon, a regenerated cellulose, was troduced into tires Rayon in a continuous filament form had greater uniformity and strength as a cord than staple cotton However, it lacked one of the features

in-of cotton—good mechanical adhesion to the rubber compounds During World War 11, the U.S Government demanded the best possible pneumatic tires for its many vehicles and considerable effort was expended in meeting this challenge,

in part by improving the uniformity and strength of rayon cords and their hesion to rubber compounds Industry adopted a water-based, resorcinol-formal-dehyde latex (RFL) adhesive system that had been patented in 1938 by Charch

ad-and Maney at EhjPont [3] By the mid-1950s, rayon had essentially supplanted

cotton as the reinforcing material for tires

However, rayon was found wanting to the challenge of military aircraft Nylon-6,6, a lightweight material that had been developed by DuPont in the 1930s, was in great demand as a replacement for silk in such diverse articles as ladies' stockings and parachutes Its physical properties were such that it was tried in airplane tires where cost was less important than toughness Again ad-hesion was a challenge and another RFL adhesive system, that contained vinyl-

pyridine latex [4], improved cord adhesion to rubber compounds By the end

of World War II, almost all the U.S military airplane tires were made with nylon-6,6 tire cord Nylon rapidly gained acceptance in aU types of tires, espe-cially since its price continued to decrease and both nylon technology and tire technology continued to improve (Nylon still dominates the heavy duty truck, airplane, and off-the-road bias tire markets.) A continuing challenge is to main-tain dimensional stability of nylon tires Much attention was and is still given to processing nylon, for example in heat setting schemes designed to improve its dimensional stability

'The italic numbers in brackets refer to the list of references appended to this paper

Meanwhile, the rayon manufacturers responded to the challenge of nylon by developing rayons that were both more fatigue resistant and over 50 percent stronger than nylon Bias passenger tires, made from nylon-6,6 and the lower melting nylon-6, had one major drawback called flat spotting, which made them unacceptable to automobile manufacturers New cars coming out of showrooms had to have the smoothest possible ride! Thus during the 1950s and most of the 1960s, tires made from improved rayon provided a smooth ride for the auto-mobiles sold by original equipment (OE) manufacturers in the United States However, the search for a new, more marketable, higher strength, more cost ef-fective, but non-flat-spotting textile to replace rayon in OE passenger tires became

a major challenge to tire textile engineers and chemists In 1965, several modified nylons were proposed to improve nylon's flat-spot resistance by raising the

fiber's effective glass transition temperature (To) DuPont tried a block polymeric

blend of 30 percent cycUc aliphatic polyamide (polyhexamethylene amide) and 70 percent nylon-6,6 [5] This fiber, called N-44, was flat-spot resistant when dry, but it failed to Hve up to expectations in the real world The very small amount of moisture always present in tires reduced the fiber's effective glass temperature enough to lose its non-flat-spotting property Allied Chemical Company tried a similar approach with EF-121 by melt spinning a mixture of

isophthal-30 percent polyester (polyethylene terephthalate) and 70 percent nylon 6 [6]

Monsanto proposed N - 88 which was a nylon-6,6, modified by an aromatic polamide based on terephthalic acid [7] Firestone's NF-20 nylon was another

block polymer of polyamide and polyester [8] Finally, Kovac and co-workers

at Goodyear proposed the imaginative approach of using merged fibers of nylon

and polyester [9] A tremendous amount of effort went into the search but

none of these nylon modifications gained acceptance! Why? Only a slight increase

in flat-spot resistance was realized and increased processing problems hardly made this worthwhile

In spite of these setbacks, the search for a new, more marketable, higher strength, more cost effective, yet non-flat-spotting reinforcing textile to replace rayon in OE passenger tires continued Attention turned to polyester as the best candidate to replace both rayon and nylon in passenger tires Because polyester

has a TG much higher than nylon, it is much better for flat spotting Polyester

has been available in the United States since 1953, and most of the tire panies had tried it in experimental tires However, many difficulties were encoun-tered in obtaining good cord-to-rubber adhesion Solvent adhesives based on diisocyanates were practical enough to put polyester in rubber goods such as V-belts, hose, and flat belts But for mass production of tires, water-based ad-hesives were needed The challenge was met by several two-step dip systems and later one-step dip systems, most of which were based on RFL and blocked

6 TIRE REINFORCEMENT AND TIRE PERFORMANCE

tires and some OE automobile manufacturers began to accept tires made from polyester cord By 1970, bias, 2-ply polyester passenger tires were being used almost exclusively by OE in place of rayon tires

A continuing challenge in the 1970s has been to extend the use of polyester into such hotter running bias tires as truck and off-the-road Some headway has been made in light truck tires, but not much in larger tires Apparently no one has yet overcome polyester's inherently poorer dynamic performance at operating

temperatures above those normally found in passenger tires [13] As stated

before, cooler running nylon remains the preferred reinforcement fiber for large bias tires

Meanwhile, Michelin and other European tire manufacturers developed and commercialized radial tire constructions in the late 1940s and early 1950s They had accepted the challenge of inventing a tire construction that would give more stability to the tread and more flexibility to the sidewalls, and permit the effective use of a very high modulus material like steel as a tire cord These con-structions grew in popularity in Europe, so that by the late 1960s radial tires were being made by all the European tire companies (including U.S subsidiaries) The radial construction dominated the entire European market The preferred reinforcing materials for passenger tires were rayon in the carcass and rayon or steel in the belts All steel was preferred in heavy duty tires U.S manufacturers recognized the advantages of the radial constructions, and in 1965, BFGoodrich introduced an all-rayon radial passenger tire to the U.S market But no rapid changeover from bias to radial tires took place The tremendous capital invest-ment required for the changeover by both the tire companies and the automobile manufacturers was just too great Instead, belted-bias tires were developed in the United States since they could be manufactured on existing equipment They served as the major passenger tire in the late 1960s and into the mid-1970s There is no doubt that the belted-bias construction is more efficient than the simpler bias construction It allows the use of high modulus cords such as glass

to reinforce the crown region of the tire This can increase tread life up to 50 percent over conventional bias tires The conversion to the belted-bias construc-tion was facilitated by the development of a new high modulus glass tire cord For many years, ways had been sought to reinforce tires with glass cords, but the nemesis of glass was poor resistance to compression fatigue and interfilament abrasion Owens-Corning successfully met this challenge and introduced glass

as a tire cord by making it into a composite impregnated with 15 to 30 percent

RFL [14\ that coated and protected the individual glass filaments In 1966, the

first commercial glass belted-bias tires were introduced by the Armstrong Tire and Rubber Company Although less popular, other high modulus materials like rayon, vinal, and steel were also used in belts of belted-bias tires Because of polyester's success as a flat-spot resistant carcass material for bias tires, its availability, and its cost effectiveness, it was chosen as the preferred cord for belted-bias carcasses Despite many improvements, glass is not yet suitable for use as a bias carcass cord

By the mid-1970s, U.S industry was ready to produce large numbers of radial passenger tires Urged on by the energy crisis and the demand for these longer wearing and lower power-loss tires, the conversion to radial tires has become a flood Following the European lead, steel is the preferred belt material for most radials But glass, which is lighter and lower in cost, is also being used in the belts of many economy radial tires In high-performance tires, folded rayon belts are being used in increasing amounts Other high modulus fibers, such as aramid and vinal (polyvinyl alcohol) are also competing for use as belt cords when tire designers find a need for the special properties that these high modulus, but Ughtweight, flexible, fibers provide

Rayon is not only being used as a belt textile but extensively for carcass forcement However, as in the belted-bias construction, polyester remains the most used carcass reinforcement in the United States It is interesting to note that nylon, when used as the carcass reinforcement in radial tires, does not cause objectionable flat spotting Nylon manufacturers are presently trying to grow in this market No doubt if radial tires had been introduced sooner in the United States before polyester became generally available, nylon would have captured a much bigger share of the radial passenger tire carcass market Note the impor-tance of "timing." See Fig 1 for today's preferred tire textiles

rein-MAJOR USE OF TEXTILES IN TIRES

CARCASS BELT BIAS

BIAS/BELTED RADIAL

Passenger Heavy Duty Passenger Passenger Heavy Duty

Polyester Nylon Polyester Polyester (Rayon) (All Organlcs)

Glass Steel (Glass and Rayon) Steel

FIG I—Major use of textiles in tires

When DuPont recently commercialized the aramid fiber, Kevlar®' [15], they

achieved a breakthrough in fiber technology and met a challenge put to them by the tire industry in the 1960s Aramid has a tensile strength of 22 grams per denier (GPD), over twice that of nylon, and a very high modulus, approaching that of steel It is interesting to note that aramid is the only organic textile fiber developed especially for the industrial reinforcement market It is being used in the belts of a few premium radial passenger tires and appears to offer advantages

in the carcass of radial heavy duty tires What has limited its acceptance in the tire industry? For one, the industry has not learned how to take fuU advantage

of its high strength Second, it is very expensive (now $5.20/lb) Third, its abiUty is limited Yet the challenge remains to fully utilize its enormous strength, toughness, and modulus As a tire cord it should permit lighter tires, save tire materials, reduce processing energy, and generally improve tire performance

avail-^Registered Trademark of E I DuPont de Nemours and Co., Inc

8 TIRE REINFORCEMENT AND TIRE PERFORMANCE

One wonders if steel would have become so dominant in the United States if aramids had been available five years sooner Note again the importance of "tim-ing!" Because of problems associated with the introduction of aramid into tires, research and development efforts are continuing around the world to develop new fibers in a strength, modulus, and cost range intermediate between aramid and nylon

Meanwhile, new versions of vinal, a textile fiber developed and used in Japan since the 1950s, are already available with tensile strengths and moduli consid-erably higher than nylon, rayon, and polyester In fact, vinal is second only to aramid in strength Vinal has been described as a kind of "super rayon," a cord with the properties the rayon manufacturers would have liked to have developed Vinal tire cord appears to be practical as both radial carcass and radial belt rein-forcement Currently, it is being used successfully in tires in several countries around the world A major penetration of the U.S market will probably have to await a domestic source Is the proper "timing" for vinal now?

In the United States significant conversion of heavy duty and off-the-road tire production from bias to radial construction is just getting underway During the 1980s, as nylon bias tires are replaced by radial constructions for which carcass cord fatigue demands are much lower, all available tire cord materials will

be competing as the carcass cord and all of the high modulus materials will be competing as the belt cord

Forecasting Tire Cord Consumption

In this brief historical review, it is apparent that each textile material has its own life cycle The art of forecasting future tire reinforcement usage depends upon the interpretation of historical trends One usually assumes that a trend will continue until one can foresee any factors which will interrupt the indicated

trends In 1973, Kovacs [16] pubhshed a graph of Ufe cycles for cotton, rayon,

nylon, and polyester (see Fig 2) His curves showed a rather symmetrical sical growth and decay for each tire cord material As we shall see in subsequent graphs, his projections, as indicated by the dashed lines, somewhat overestimated both the rate of growth of polyester and the rate of decay of nylon and rayon

clas-In fact, we must not put too much reliance in any quantitative forecast that goes beyond three or four years To illustrate, we took data from some typical annual industry projections made, starting back in 1971, of expected 1980 con-

sumption of different tire cord materials [17,18] We graphed these data for

nylon, polyester, rayon, glass, steel, and high modulus organics It is apparent when one examines these graphs that forecasting is far from an exact science First look at the graph for nylon (Fig 3) The Xs show the actual yearly con-sumption of nylon tire cord for 1971 through 1977 The circles show the pro-jected consumption for nylon in 1980 made in the indicated year Thus, actual nylon consumption during 1973 was about 300 million pounds and consumption for 1980 was projected at 270 million pounds The square for the year 1980 is

FIG 3-Nylon tire cord, United States

our own latest forecast Notice that we are venturing less than a three-year projection The point that stands out in this graph is that as early as 1971 the forecasters we are reviewing were correctly anticipating the level of nylon con-sumption of the late 1970s as we approach 1980 In other words, the forecasting

of nylon consumption has been pretty good

Now look at the graph for polyester (Fig 4) It is apparent that the forecasts for 1980 made in 1971 and 1972 were too high, but that by 1974 a leveling off

of polyester tire cord consumption, at about 280 miUion pounds per year, had been accurately anticipated

The projection of rayon tire cord consumption for 1980 has not been easy (Fig 5) In 1971, the forecasters were predicting the death of rayon well before

10 TIRE REINFORCEMENT ANDTIRE PERFORMANCE

FIG ^—Polyester tire cord United States

FIG 5—Rayon tire cord, United States

1980 In 1973 and 1974, they changed their minds Then in 1975 and 1976 they again predicted the demise of rayon by 1980 In 1977, they showed some uncer-tainty We do not have time to discuss in detail the reasons for the sharp swing in the rayon forecasts, but certainly there had been a real possibility that rayon tire cord production and use would cease by 1980 in the United States A number of factors had to fall into place to avert that development: an assured supply, demonstrated advantages for certain uses, the entrance of Michelin to U.S production, and the decision of "The Other Guys" to continue to market rayon tires

FIG 1-Steel tire cord United States

The predictions for glass represent another interesting case (Fig 6) In 1971 some forecasters felt glass would be gone by 1980 This was based on an expecta-tion that belted-bias tires made from glass/polyester would disappear However,

by 1973 to 1974 it was apparent that glass-belted radial passenger tires were technically and commercially feasible Forecasts of 1980 glass tire cord con-sumption made in 1973 to 1974 were unduly optimistic, but later forecasts look reasonable

The predictions shown here for 1980 steel tire cord consumption made in the early 1970s were obviously too high by a factor of two (Fig 7) Please remem-

12 TIRE REINFORCEMENT ANDTIRE PERFORMANCE

X X ACTUAL CONSUMPTION

0 — 0 FORECAST for 1980

FIG 8-HMO tire cord United States

ber that a radial passenger belt takes about three times as much steel by weight

as glass or aramid Two factors moderating the growth of steel tire cord sumption have been the continued use of rayon and glass belts in radial passen-ger tires and the slower-than-anticipated conversion from nylon bias to steel cord radial heavy duty tires

con-Forecasting the future consumption of high modulus organic (HMO) tire cord has also been difficult (Fig 8) You can see the extreme swings of optimism and pessimism This is a chicken and egg problem First, an attractive cost-benefit pic-ture has to be demonstrated to both the supplier and the user Second, both have to commit to very heavy technical expenditures Third, consumption can-not go up until supply increases and supply cannot increase until very large investments are made in new production facilities

One generalization that we can make regarding these forecasts is that tire structions and needs are now so diverse that it is unlikely (even impossible) that a single fiber will ever again be in a position to come close to taking over the bulk of the tire cord market (as rayon did when it replaced cotton) In fact the six different materials discussed in these figures show every indication of con-tinued use as tire cords for many years Tire cord engineers are meeting the chal-lenge of finding a preferred use for each What will most likely happen is that

con-they will be joined by new reinforcement materials such as Allied's Metflas [19], high modulus polyesters [20], new HMOs, and the improved vinals [21]

Societal Constraints

The challenges of the past were based on the evolving needs of the marketplace for lower cost and better performing tires Society through the marketplace saw its principal tire performance needs as more tread life, more endurance, good traction, and certain aesthetic considerations such as ride and appearance As we

1 Weftless fabric in place of square woven

2 Continuous filament rayon in place of staple cotton

3 RFL adfiesive on rayon and other cords

4 Processing teclinology of viscoelastic cords

5 Nylon in airplane and fieavy duty tires

6 Post cure inflation of nylon tires

7 Polyester, tfie low cost, higfi strength, non-flatspotting cord

8 Blocked isocyanate adhesives for polyester

9 More efficient, low powerless, radial tire construction

10 Brass plated steel wire cord

11 Belted-bias tires

12 Impregnated glass tire cord

13 Aramid, the lightweight super high strength organic cord

FIG 9—Major commercial developments affecting tire reinforcement

have increased our understanding of how a tire works and the role of tire forcement, improvements have been made in the materials, the processes, and the designs to meet these tire performance needs The marketplace always has controlled the successful commercialization of any changes on a cost-benefit basis Past examples of major changes associated with tire reinforcement and stimulated by major challenges for improved performance and reduced cost (some of which were already mentioned) are shown in Fig 9

rein-Today we find that our concept of needs has enlarged as society has enlarged its priorities We have rearranged and added to the challenges of the marketplace

such societal challenges as (a) energy conservation, (fe) more effective use of

resources, (c) preservation of the ecology, (d) greater safety (both on the job and on the highway), and (e) more reliable information about products

Many new governmental regulations have been put into effect, and others are even now being considered, that are designed to encourage the tire industry to address itself to these societal challenges Obviously, meeting these new chal-lenges will require improved technology The technical estabUshment can on the one hand help our government make meaningful regulations, and can on the other hand utiUze these regulations as guidelines for future technological pro-gress where both industry and society will benefit

For example, in order for tire technology to contribute efficiently to the more effective use of our energy and resources, if you will, more effective waste reduc-tion, we will need more knowledge about the relationship between tire perfor-mance and tire cord properties We will need to identify sources of waste and set our priorities on our understanding of how to reduce this waste The more knowledge we have, the greater are the chances of success! Do we have enough basic knowledge to meet the new needs? Obviously we have quite a bit, but we are also lacking quite a bit

Tire Performance Related to Tire Cord Properties

In Fig 10, we identified several tire performance criteria, such as burst, bruise, endurance, and power loss, which depend in some way on the tire cord proper-

14 TIRE REINFORCEMENT AND TIRE PERFORMANCE

Tire Size & Shape

Tire Uniformity and

"Fatigue" Resistance, Adhesion, Adhesion Degradation Resistance in Tire Environment, Dynamic Properties (Hysteresis), Uniformity

Modulus, Density Dynamic Properties, Density Dimensional S t a b i l i t y - Modulus, Creep Dimensional Stability ( T Q ) - M o d u l u s , Creep Modulus

Modulus, Density Modulus Modulus, Density Creep

Dimensional Stability (TQ), Hot—Creep and Shrinkage, Moisture Regain, Adhesion, Environmental

Degradation Resistance, Uniformity, Stiffness, Toughness, Moisture Swelling and Shrinkage, Compaction, Tensile

FIG 10—Areas of technical challenges facing tire cord technical people

ties We also estimated the extent of our knowledge about some of these tionships Our estimates are quite conservative The point is not whether you agree on every detail, but rather that there is a lot more to leam

rela-Although, in the short term, real understanding is not always necessary, it is vital to the long term attainment of the most efficient solution of both our pre-sent and future challenges In fact, as we said earher, one can make a very good case that developing "technical understanding" is the one real and all-encom-passing technical challenge

How must we proceed? First, we must reexamine the known facts and evaluate them in hght of today's challenges Thus, we find the gaps with the most poten-tial for exploitation and try to fill them Experience tells us that one way to fill these gaps is to define better the problems and then devise meaningful experi-ments and tests to solve them

Many American Society for Testing and Materials' (ASTM) tests for tire cords are already available Some measure physical properties of tire cords under con-ditions that are easily controlled Often these are not the critical conditions under which tire cords are asked to perform Thus, their usefulness becomes limited Let us look at a few examples

Cord Tensile Strength Related to Tire Burst Strength

Tensile strength is usually tested at 23°C, 55 percent relative humidity, 2 cent per second elongation, as described in ASTM Testing Tire Cords, Tire Cord Fabrics, and Industrial Filament Yarns Made from Man-Made Organic-Base Fibers

per-(D 885) Yet we know that the critical tire conditions where tensile strength is most important are at elevated temperatures [(up to 150°C) and at high speed (over 6000 percent) per second elongation] after cycling many times in an envi-ronment of a rubber compound in the presence of oxygen and moisture under pressure Since we cannot easily simulate all the critical conditions in the labora-tory, we must test the tire cord in the tire and look for some correlation to our laboratory tests

In the case of tire burst strength (the first tire performance criterion listed in Fig 10), what do we know about burst and its relationship to tire cord tensile properties? Quite a bit It turns out that if the cord geometry is know in an in-flated tire, the burst strength of the tire at ambient temperature can be correctly calculated from the cord's tensile strength at the same temperature However, as

we stated before, the critical conditions where tires might burst are actually at elevated temperatures and after many miles of tire operation

What should be the minimum burst strength of a new tire at ambient ature? For bias passenger tires at 20°C we generally find a factor of safety for burst of about 10 However, as previously mentioned, tires do not usually operate with internal temperature at 20°C At 135°C (an extreme short term operating temperature for most tires), the tensile strength for some organic tire cords drops to one-half of the room temperature values If we started with 10, that leaves us with a factor of safety of about 5 Furthermore, when a passenger tire increases in temperature during operation, the inflation pressure also increases say from 165 to 241 kPa (24 to 35 psi) and the effective factor of safety is

temper-reduced further to about Vh Then when the usual 10 to 20 percent degradation

of strength which occurs over a tire's lifetime is also considered, our effective factor of safety has dropped to 3 This still seems rather high As a matter of fact, some bias airplane tires are designed with a minimum initial burst safety factor of about 5 which following the same logic must drop close to 1^ under

extreme operating conditions No doubt starting with 5 and ending up with Wi

would be enough safety factor for bias passenger tires if burst were the only design consideration Thus, if you follow the line of our reasoning, the cord strength in bias passenger tires must reflect some other performance factor need that requires about twice as much cord as is necessary based on considering burst alone

Cord Tensile Strength Related to Tire Bruise Resistance

Is it bruise? Let us look at bruise performance more closely In the field, bruise describes how well a rolling tire can resist impact failure from such obstructions

as rocks or "chuck" holes The Department of Transportation (DOT) laboratory bruise test (49C-FR571-109) is based on a Rubber Manufacturers Association (RMA) procedure developed in 1944 It measures the energy required to break a passenger tire at room temperature when a % in diameter plunger is pushed slowly through the crown of the tire at rated inflation How well can one predict

16 TIRE REINFORCEMENT AND TIRE PERFORMANCE

this energy to break? Not as well as burst The cord-rubber geometry is more complicated It changes as the plunger penetrates the tire However, experience tells us that laboratory bruise can be approximated as a function of the product

of the square of the tensile strength of the cord and some fractional power of cord elongation

An even more important question is, what do we know about the relationship

of laboratory bruise to field performance? Let us consider one case After checking the industry's experience in the field on bruise failure of both bias nylon and rayon passenger tires and after determining the laboratory bruise level

of tires, DOT's minimum requirements were set at 2600 in.-lb for nylon and only 1650 in.'lb for rayon But why the large discrepancy between nylon and rayon minimums? Tire cord engineers were challenged to answer the question They found that bruise failures occur in the rolling tire usually when the tire is relatively "hot." Both load and high speed cause rapid internal heat generation

of tires This tends to weaken nylon but dries and strengthens rayon Rupture energy tests of tire cords at high speeds and elevated temperatures show that rayon becomes equivalent to nylon at about 135°C and 6000 percent per

second elongation [22] This would indicate that rayon tires should compare

more favorably for bruise with nylon tires under hot dynamic conditions Indeed, tests at high speeds on heated tires do confirm that rayon and nylon tires built

to the DOT inch-pound minimums are equivalent for dynamic bruise [23,24]

This situation does represent a perfect case history of the potential danger of setting up design criteria and legal limits without a full understanding of their significance If the DOT limit of 1650 in.-lb for rayon had been in place before the advent of nylon tires and then applied to them, it would have been way too low On the other hand, if rayon had come along after nylon, a 2600 in.-lb limit applied to it would have been ridiculous and if not corrected might have pre-vented the use of a perfectly adequate material It makes one wonder how many illogical design criteria are being used today and are reflected in government standards Here is another area of challenge to tire cord engineers

The above examples show that even apparently simple performance ments such as burst and bruise can be quite complex and present the tire cord engineers with interesting challenges We could discuss many other tire/cord relationships But in finishing, we will limit ourselves to just a few words about power loss—the reduction of which is society's latest challenge to tire engineers Tire cords are the main strength members in tires and as such store most of the potential energy of inflation As the tire is loaded, it deforms and does work

require-to transfer some of this potential energy require-to the axle, etc Some of the work is wasted as heat and contributes to over 90 percent of the power loss of the tire

It is estimated that 20 to 40 percent of a tire's power loss is due to the behavior

of the cords in the tires [25] This in turn depends upon construction of a tire

and the mode of cord deformation in the tire as well as the inherent dynamic properties of the tire cord Since different parts of the tire deform differently

the amount of heat generation and, therefore, power loss also varies around the tire

Can power loss in tires be reduced by improved tire design or by improved dynamic properties of tire materials such as tire cords? How can we do this efficiently? Obviously, it would be nice to know exactly how a tire works, its deformational modes, and the contribution of each portion of a tire to the dyna-mics of its operation Without this information we can only guess and try Yet even without complete knowledge we have made gaint strides Several good examples of changes which have markedly reduced the tire's power loss are: the change from bias to radial construction, the use of higher inflation pressures for tires, and the improvement of rubber compounds, more specifically tread com-

pounds with better dynamic properties [26] The real importance of the

dyna-mic properties of tire cords to the power loss of bias tires was not fully appreciated

by most scientists and engineers in the tire industry until only a few years ago

[25,27] The new challenge is to understand the contribution of cord to tire

power loss, not only for bias tires but for radial tires Therefore, we are pleased

to note that several papers of this symposium are devoted to the dynamic perties of tire cords

pro-In summary, by briefly tracing the history of tire cord development, by pointing out year to year changes in typical tire cord consumption forecasts,

by discussing past and present market and societal goals, and by reviewing the complexities of the relationship between tire cord properties and tire perfor-mance, we have attempted to give tire cord engineers a feeling for the challenges and opportunities that face them today

Dunlap, J B., G B Patent No 10,607,1888; G B Patent No 4,116,1889

Pearson, H., Rubber Tires, India Rubber Publishing Co., N.Y., 1906 (1893 Patents of

J F Palmer)

Charch, W H and Maney, D B., U.S Patent No 2,128,635, 30 Aug 1938

Mighton, C J., U.S Patent No 2,561,215,17 July 1951

Lavern, J A and Zimmerman, J., U.S Patent No 3,195,603, 1964

AUied Chemical Co., Netherland Patent AppUcation No 6,600,608,19 July 1966 Ridgway, J S., U.S Patent No 3,621,002,16 Nov 1971

Robertson, J J., U.S Patent No 3,378,602, 16 April 1968

Kovac, F J., Rye, G W., and Dague, M F., Industrial Engineering and

Chemistry-Product Research and Development, Vol 2, No 279,1963

Thompson, W L and Parke, L W.,Rubber World, Vol 138, 1958, p 588

Aitken, R G., Griffith, R L., Little, J S., and McClellan, J W., Rubber World, Vol

151,No 5,1965,p 58

Timmons,W D., Adhesive Age, Vol 10, No 10, 1967, p 27

Skolnik, L., "Tire Cords," Encyclopedia of Chemical Technology, Vol 20, 2nd Ed.,

Wiley, N.Y., 1969, pp 330, 344

Marzocchi, A., U.S Patent No 3,869,306, 4 Mar 1975

Kwolek, S L., U.S Patent No 3,600,350, 17 Aug 1971; U.S Patent No 3,671,542,

20 June 1972; U.S Patent No 3,819,587, 25 June 1974; U.S Patent No 3,888,965,

10 June 1975

18 TIRE REINFORCEMENT AND TIRE PERFORMANCE

[16] Kovac, F J., Tire Technology, Goodyear Tire and Rubber Co., Akron, Ohio, 4th Ed.,

1973, p 36

[17] Tire Construction and Reinforcement Forecasts, 1971-1977, Monsanto Chemical

Company

[18] World Industrial Yam Market, 1970, 1975,1980, i9S5, E I duPont deNemours and

Co., Inc., Nov 1976

[19] Kavesh, S., U.S Patent No., 3,845,805, 5 Nov 1974

[20] Davis, H L., Jaffe, M L., and Besso, M M., Gev Offen 2,445,528, 27 Mar 1975 [21 ] Kawai, K., "Polyvinyl Alcohol Fibers for Rubber Reinforcements," Technical Sym-

posium, Akron Rubber Group, 1969-70

[22] Lothiop,E.W., Applied Polymers Symposium,No 1, 1965,p 111

[23] Draves, C Z., Jr Kuebler, T P., and Vukan, S F., ASTM Materials Research and Standards, Vol 10, No 6, June 1970, pp 26-29

[24] GusUtser, R L., "Interaction of a Vehicle Tire with an Obstacle," Trudy

Nauchno-Issledovatel/Skoga Instituta Shinnoi Promyshlennosti, TNSPA, SB 3, pp 154-88, 1957; translated by R J Moseley, Research Association of British Rubber Manu- facturers Translation 817

[25] Collins, J M., Jackson, W L., and Oubridge, P S., "Reference of Elastic and Loss

Moduli of Tyre Components to Tyre Energy Losses," Transactions I.R.U., Vol 40,

No T239, 1964

[26] Walker, R., Rubber and Plastics News, No 8, 6 Mar 1978

[27] Prevorsek, D C, Kwon, Y D., Butler, R H., and Sharma, R K., U.S Patent No

3,893,331,8 July 1975

L Bourgois ,vi

Survey of Mechanical Properties of

Steel Cord and Related Test Methods

REFERENCE: Bourgois, L., "Survey of Mechanical Properties of Steel Cord and

Related Test Methods," Tire Reinforcement and Tire Performance, ASTM STP 694,

R A Fleming and D I Livingston, Eds., American Society for Testing and Materials,

1979, pp 19-46

ABSTRACT: This paper gives a survey of the different testing methods for the

eval-uation of the mechanical properties of steel cord used as tire reinforcement material The paper will deal with the latest findings relative to tension tests, compression tests, rotaflex test, dynamic compression tests, fretting simulation tests, and rubber penetration tests

The steel cord properties measured in those tests will be discussed in relation to tire performance, including steel radial passenger car tires, truck and bus tires, and off-the-road tires The principal steel cord properties will be illustrated

KEY WORDS: reinforcement fibers, steel cord, tests, tension test, compression test,

stiffness test, impact test, rotating beam test, rotaflex test, fretting test, penetration test, rubber, tires, radial

The paper contains a survey of the different testing methods developed at the Bekaert Research Center for the evaluation of the mechanical properties of steel cord used as tire reinforcing material

The paper will deal with static tests (tension test, compression test, stiffness tests, impact test, rubber penetration test) and with dynamic tests (rotating beam fatigue test, rotaflex test, dynamic compression test or "Bartha" test, belt test)

A brief discussion will be given of some differences in the steel cord tions and properties measured in those tests, and, if possible, of their relation to tire performance

construc-Tension Test

The tension test is the most commonly used test method to evaluate ing materials This test permits the determination of the breaking load and the 'Project manager research, N V Bekaert S A., B-8550, Zwevegem, Belgium

reinforc-19

20 TIRE REINFORCEMENT AND TIRE PERFORMANCE

S =!

Rt^geoN

ELONGMIONT/.) aONGAnONIV.)

FIG 2-Tenstle diagram of: 7x3x0.175 regular cord and 3x7x0.175 high elongation cord

elongation at fracture of a cord [ASTM Testing Filaments, Strands, Cords, and

Fabrics Made from Steel (D 2969)] But apart from that, when the tester is

equipped with an electronic extensometer, the yield points at 0.01, 0.05, 0.1,

and 0.2 percent remaining plastic elongation can be derived (Fig 1) Also the

Young's modulus of the cord and the part load elongation at 60 percent of the

breaking load can be calcu'.ated This test also proves that steel cords can be

designed with different moduli as illustrated in Fig 2, which gives the difference

between regular cord and high-elongation cord

Further, it is possible to conduct the test on rubber-embedded samples, for

example rotating beam samples or rotaflex samples Figure 3 gives the same

tensile diagrams of 7X3X0.175 regular cord and 3X7X0.175 high-elongation

cord, but embedded in rubber as rotating beam samples (see further for sample

description) A comparison of Figs 2 and 3 leads to the following

considera-tions: (a) rubber embedment and aging conditions during curing lead to an

in-crease in the breaking strength of the steel cord, and (fc) the penetration of the

rubber into the cord construction, especially high-elongation cord, leads to a

great reduction in the structural elongation of the cord With this test it is also

possible to prove that, in comparison to other tire reinforcement fibers, the

tensUe strength and the modulus of steel cord are the least influenced properties

when the temperature rises up to 200°C

A big problem in tension testing is fractures in the jaws In our research center,

the latest findings are the adaptations of the pneumatic wire tire cord jaws of

Instron A rubber strip is provided between the jaw surface and the cord (Fig 4)

The rubber strip acts as a spring, giving a good distribution of the contact

22 TIRE REINFORCEMENT AND TIRE PERFORMANCE

FIG 3—Tensile diagram of 7X3x0.175 regular cord and 3x7x0.175 high elongation cord,

rubber embedded

forces and avoiding slipping This adaptation leads to a great reduction in failures

in the clamps, particularly for layer constructions such as 3+9+15 A decrease in scatter in the results is also obtained and for some sensitive constructions, an increase in breaking load may be recorded An example: 5 percent higher break-ing load is obtained for the 3+9+15, there is a decrease in scatter of 90 percent, and the failures in the clamps are reduced with 8 percent; this is compared with the results obtained with the same jaws but without rubber strips Another advantage is that, if clamps are provided with rubber strips, the cords with sprial wrap can be tested directly without removing the spiral wrap This means time saving in the laboratory

Compression Test [ i f

Due to their slendemess and high length-to-diameter ratio, normal tire cords (steel cord inclusive) have no resistance to compression, but once embedded in rubber, steel cord can build up considerable compression resistance

There are several factors which influence the compression properties of posite structures As for tires, the design, such as tire shape, tire contour, deflec-tion pattern, rubber (mainly the rubber hardness), amount of steel cord, bias angles, inter-ply distance, etc., play a role Also processing is important, such as type of molds and tire uniformity And, of course, the type of steel cord con-struction plays a role The most important factors are filament size, geometrical

com-^The italic numbers in brackets refer to the list of references appended to this paper

FIG 4—Adapted Instron jaw with rubber strip between cord and jaw surface

configuration, length of lay, and spiral wrap Further, the running conditions such as inflation pressure, loading and riding conditions are important

In our laboratory a cylinder test was developed, which provides fuU tion on the compression properties of rubber-embedded steel cord As a speci-men, a rubber cylinder was selected with a diameter of 30 mm and a height of 48.25 mm (49.50 mm before grinding), reinforced in the center with the test cord (Fig 5) Care is taken that the cord is both straight and exactly in the axis

informa-of the cylinder Therefore, a precision mold is needed in which the cords are slightly tensioned during the curing operation (Fig 6)

The test consists of recording a force versus deformation diagram The load of the reinforced rubber cylinder is taken up by both the rubber and the steel cord The force taken up by the rubber is derived from an identical, compressed, and unreinforced rubber cylinder (Fig 7), from which the elastic spring constant

F, is deduced

24 TIRE REINFORCEMENT AND TIRE PERFORMANCE

STEELCORD

AFTER GRINOrNG ( 8 25 mm

FIG 5—Compression sample

FIG 6—Mold for compression samples

The modulus in compression E^ is derived from the diagram (Per definition,

£|oo is the secant modulus at an axial compressive stress of 200N/mm^.) more, the cord kinking stress Ok and the deformation at instability Wk are deter-mined Two different types of compression curves are obtained

Further-First Type

Curves with a sharp buckling point are represented by the example given in Fig 8 The interpretation of curves with pronounced points of instability are easy, because they show a peak corresponding to the maximum compression force taken up by the steel cord The corresponding deformation w^ can also be accurately measured The cord buckling stress is obtained by the equation

Ck =

^tot - P^ Rubber

(1)

t

/ /

GRUBBER

DEFORMATION (v.) r,; 1 _J

FIG %—Compression diagram of 2+7x0.22+1

where

^Rubber = 0.4825 WkF„N,

Ok = compression stress, N/mm^,

Aot ~ total compressive load of the sample at deformation 1%, N,

Vf = rubber spring constant, N/mm,

S'eff = cross-sectional area of steel cord without spiral, mm^, and

Wk = deformation at instability, %

Second Type

Curves with a smooth buckling, as shown by the example given in Fig 9, need

a special geometrical construction for the determination of the kinking point The point of maximum load sustained by the cord is graphically determined by

drawing a parallel (a) (see Fig 9) to the rubber spring constant F, through the

FORCE ( N )

W k x l 5 2 •/

I , DEFOfMfTlON C / f l ^

FIG 9—Compression diagram of 5X0.25

point of the diagram where the tangent to the diagram coincides with this

paral-lel In this point, o^ and w^ are computed by means of Eq 1 The fact that the

point corresponds with the geometrical construction may be checked by X-ray

Figure 10(a), (b), and (c) give the sequences for the 2+7X0.22+1 construction; Fig 11(a), (b), (c), and (d) those for the 5X0.25 construction; and Fig 12 for

high-elongation cord

2+7x0,22+1

bianco

2+7x0.22+1 0,5 %

FIG 10—X-ray pictures of cylinder sample during compression, reinforced with cord

2+7x0.22+1

The cord 2+7X0.22+1 shows a sharp buckling point in the compression gram, which is lower than 0.5 percent; the cords 5X0.25 and high-elongation cords (Fig 13) show a smooth buckling In the last case, the cords act as springs

dia-This is clearly visible on Fig 11(d)

In some cases, mainly for high-elongation cords, the buckling stress is much lower than 200 N/mm^, and then the definition of the secant modulus is changed

In those cases, the secant modulus is derived at 100 N/mm^ or at 50 N/mm^ (Fig 13)

The most important factors to increase the Young's modulus in compression are longer lay lengths and spiral wrap The factors which are important to permit

a big compression deformation without leading to kinking are constructions without spiral wrap and core strand, and with short lay lengths, such as high-elongation cords So again this test proves that widely different compression properties may be reached with steel cord due to the varied geometrical cord construction

The compression properties of steel cord are of great importance in belt design Cords with a high compression modulus lead to lower tread wear as compared to low compression modulus cords Also the lateral stiffness of the belt may be influenced by differences in cord tension and compression modulus

Stiffness Tests

In order to describe the behavior of steel cord in bending, it is necessary to

determine two properties: (a) the amount of force necessary to give the cord a

given bending deformation, called the degree of stiffness, and (b) the degree to

28 TIRE REINFORCEMENT AND TIRE PERFORMANCE

3x1x0,15 3.1 M.f SS

30 TIRE REINFORCEMENT AND TIRE PERFORMANCE

i ( & f e ^

' i ^

FIG 14—Taber stiffness tester

which Steel cord springs back after having been subjected to bending or the degree

of elasticity

Two tests have been considered in our laboratory, namely, the well-known taber stiffness test (Fig 14) and the three-point-bending test (Fig 15) The results prove that there is a good correlation between both tests, but for research purposes the three-point-bending test is more suitable because of its accurateness and full information diagram The disadvantage of the three-point-bending test is that it takes more time

The diagram of a three-point-bending test (Fig 16) gives the following different states in bending measured on the second loading cycle

1 A completely anchored state caused by the frictional forces between the wires, resulting in great stiffness, represented by ev, which is the product of the bending moment and the curvature radius in that state

Sl^^^

¥IG 15—Three-point bending tester

2 A transition area, represented by the position of the knee in the curve, and

called knee value M^

3 An area with a low slope, which occurs after a very slight bending moment

In this state frictional forces are insufficient to resist the shear stresses produced

in the cord The wires start shifting and suddenly the cord becomes much more flexible The shifting between the filaments leads to abrasion (fretting) as dis-cussed further in the paper; the state is characterized by ek, which is again the product of the bending moment and the curvature radius

4 The loss in energy at 0.5 percent deflection is taken as a measure for the elasticity of the cord

When the cords are embedded in rubber, shifting will be partially hindered However, because the rubber shear modulus is low, hindrance will be small Big movement between the wires caused by severe bending may generate failures in the rubber leading to local separations

32 TIRE REINFORCEMENT AND TIRE PERFORMANCE

/£y^77N.MM^

DEFORMATION

AREA AHCA , „ ,.,,

* ' AREAABflA '''°° " ' ' 0.8-

FIG 16-nree-point bending diagram of 7x4x0.175+1

Again as the diagram proves (Fig 16), the spiral wrap is very important to increase the nonelastic behavior of steel cord The elastic part of the stiffness of

a cord is proportional to the fourth power of the filament diameter of the wire and the total number of wires

Impact Test

A transverse impact test method for steel cord was developed to determine the resistance to cutting with a view to their use as tire belt reinforcement (puncture resistance) By doing so, the proper steel cord constructions for tire belt protec-tion can be selected

The test consists in a modified "Charpy test" adapted with a sharp knife (Fig 17) The sample is a rubber cylinder with a 3 mm diameter reinforced in the center with a steel cord (Fig 18) (the same sample is used for the rotating beam fatigue tester) The total amount of absorbed energy is measured The impact force is recorded with a piezoelectric transducer on a digital scope