Elastomers and Composites Vol. 52, No. 3, pp. 167~172 (September 2017) Print ISSN 2092-9676/Online ISSN 2288-7725 DOI: https://doi.org/10.7473/ec.2017.52.3.167 Evaluation on Fatigue Characteristics of Tire Sidewall Rubber according to Aging Temperature Namgyu Jun, Byungwoo Moon, Yongseok Kim, Jae-Mean Koo, Chang-Sung Seok, Ui Seok Hong *, Min Kyeong Oh *, and Seong Rae Kim ** Dept. of Mechanical Engineering, Sungkyunkwan Univ., 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 16419, Rep. of Korea * Polymeric Materials Research Team, Hyundai Motor Group, Hyundaikia-ro, Namyang-eup, Hwaseong-si, Gyeonggi-do, 18280, Rep. of Korea ** R&D Center, Nexen Tire Corporation 226, Bangbae-ro, Seocho-gu, Seoul, 06584, Rep. of Korea (Received March 6, 2017, Revised March 15, 2017, Accepted March 17, 2017) Abstract: Ultra-high performance (UHP) tires, for which demand has recently surged, are subject to severe strain conditions due to the low aspect ratio of their sidewalls. It is important to ensure sidewall material durability, since a sudden tire sidewall breakage during vehicle operation is likely to cause a major accident. In the automotive application of rubber parts, cracking is defined as a failure because when cracks occur, the mechanical properties of rubber change. According to Mars (8), Andre (11) et al., strain and strain energy density (SED) are mainly used as a failure parameters and the SED is generally used as a fatigue damage parameter. In this study, the fatigue life curves of sidewall rubber of tires were determined by using the SED as fatigue damage parameter while the effect of aging on fatigue life was evaluated after obtaining the SED-Nf curves according to aging condition. Keywords: fatigue life, strain energy density, prediction equation, tire sidewall rubber, aging Introduction 최근자동차부품의신뢰성및내구성평가에대한관심이높아지고있으며, 개발단계에서피로내구수명을예측할수있는기술이요구되고있다. 1,2 특히고무는금속과는달리큰변형률에서도탄성을유지하며, 하중과변형률이비선형관계를보이는초탄성 (hyperelastic) 특성과감쇄특성이있기때문에 3 차량진동제어부품에다양하게이용되고있지만고무재료의특성상내구수명예측평가기술이낙후되어신뢰성있는설계및평가가어려운실정이다. 타이어는자동차고무부품구성의대부분을차지하고있으며차체를지지하고진동이나충격을흡수해줄뿐만아니라엔진의동력을지면으로전달하여주행할수있게하는중요한부품이다. 4,5 최근수요가급증한 UHP (Ultra high performance) 타이어는접지면적이넓고횡력에견디는힘이커서선회능력이우수하다는장점이있지만단면높이가낮아서타이어의사이드월부분이가혹한변형률조건에노출되게되는단점이있다. 일반적으로타이어의교체주기, 내구수명을평가할때타이 Corresponding author E-mail: seok@skku.edu 어트레드의마모정도를기준으로평가한다. 6,7 하지만차량운행중급작스런타이어사이드월파손이발생할경우대형사고를유발할가능성이높기때문에타이어사이드월을기준으로한타이어내구수명을평가하는방법의개발이요구되고있다. 타이어의내구수명을평가하기위해서는일반적으로실제타이어의피로시험및실차주행시험이이용되고있으나, 시간, 경제적제약이크기때문에단기간에타이어내구수명을평가하기어렵다. 또한개발단계에서타이어의내구수명을예측할수있는방법이요구되기때문에고무재료의시편단위에서내구수명을평가하는방법이수행되고있다. 고무재료의내구성능평가방법으로는 KS M6518에서제시하는인장시험, ASTM D4482 에따라수행하는피로시험그리고찢김에너지 (Tearing Energy) 를활용하여균열성장속도를구하는균열진전시험이널리사용되고있으며, 8-10 고무의내구성능을평가하는인자로서변형률또는 SED를활용하고있다. 11,12 타이어는주행시반복적으로가해지는변형에너지이외에도마찰열, 주위환경등으로인한노화도함께가해진다. 또한타이어는장기적으로주위환경에직접노출되기때문에노화에의한영향을반드시고려해야하며그에따른내구특성을평가하여실주행환경에서의타이어의내구성을평가할수
168 Namgyu Jun et al. / Elastomers and Composites Vol. 52, No. 3, pp. 167-172 (September 2017) 있어야한다. Ahagon 등은타이어고무의노화메커니즘을규명하였으며노화온도가 80 o C 이상인경우사슬분리 (Chain Scission) 에의해급격한기계적특성의변화가나타남을밝혔다. 13 본연구에서는차량주행시가해지는변형에너지와노화온도에따른타이어사이드월고무의내구수명특성을평가하기위해다음과같은시험을수행하였다. 타이어사이드월에사용되는상용고무재료에대하여 ASTM 14 에서제시하고있는범위의변형률 (Strain: 0.69, 0.93, 1.16, 1.39) 을선정하고인장시험및피로시험을진행하여 SED에따른인장특성과피로성능을비교분석하였다. 또한, 노화온도와피로수명간의관계를평가하기위하여 25 o C, 70 o C, 90 o C, 100 o C 온도에서 17일씩노화한시편에대해시험을수행하여노화정도에따른피로수명의변화를평가하였으며추가시험을통해검증및보완하였다. Experimental Figure 2. Tensile test machine. 1. 인장실험본연구에서사용된인장시편은타이어사이드월에사용되는상용조성으로만들어진시편이며시편형상은 KS M 6518 15 에준하여아령형 3호로제작하였다. 두께 2 mm인시트를시편모양의절단날로커팅후환경챔버를사용하여노화조건별로노화를실시하였다. 인장시편형상과노화조건별제작된시편정보에대해각각 Figure 1과 Table 1에나타내었다. 인장시험에는 Instron사의 5ton 유압식시험기를사용하였으며, 하중값획득의정밀도를높이기위해 200N 로드셀을설치하여 Figure 2와같이시험장비를구성하였다. 본시험을시작하기전고무분자구조의안정화를위하여시험하고자하는변형률범위까지피로반복을 30회수행하 여 Mullin s effect 16 를제거하였다. 인장시험과정은 ASTM D412-a에준하여수행되었으며, 각조건별 2회씩시험후평균값을구하였다. 2. 피로실험피로시편은인장시편과동일조성으로만들어진시편이며시편형상은 ASTM D4482에준하여제작하였다. 두께 1.4 mm인시트를시편모양의절단날로커팅후전기식퍼니스를사용하여노화온도별로노화를실시하였다. 피로시편형상과노화온도별제작된시편정보에대해각각 Figure 3과 Table 2에나타내었다. 싸인 (Sine) 파형의 1.93 Hz 주기의변위를반복할수있도록크랭크를사용하여전기모터로구동 Figure 1. Tensile test specimen. Table 1. Number of Tensile Specimens. Aging Temperature 23 o C 70 o C 90 o C 100 o C Aging time 17day 0.69 2 2 2 2 0.93 2 2 2 2 1.16 2 2 2 2 1.39 2 2 2 2 Figure 3. Fatigue test specimen. Table 2. Number of Fatigue Specimens. Aging Temperature 23 o C 70 o C 90 o C 100 o C Aging time 17day 40 40 40 40
Evaluation on Fatigue Characteristics of Tire Sidewall Rubber according to Aging Temperature 169 Figure 4. Fatigue test machine. 되는피로시험장치를제작하였으며피로시험을수행하였다. 또한 cycle 수확인을위해근접센서와카운터를설치하였으며, 시편파단현황을녹화하여파단시점의 cycle수를기록, 피로수명을평가하였다. 전체적인시험과정은상온범위에서 ASTM D4482에준하여수행하였으며, 피로시험이진행됨에따라발생하는고무의영구변형을 1,000, 10,000 cycle 그리고매 24시간마다보정하여시험중동일변형률이적용되도록하였다. 전체적인피로시험기의구성은 Figure 4와같다. Results and Discussion 1. 인장실험결과인장시험을수행하여 4가지변형률 (Strain: 0.69, 0.93, 1.16, 1.39) 조건에서노화온도변화에따른응력과변형률 (Stress- Strain) 선도를획득하였다. Figure 5은최대변형률이 1.39인경우에노화온도의변화에따르는응력과변형률선도이다. 모든변형률조건에서노화온도가증가할수록인장응력이증가하는경향이나타났다. 1.1 변형률 -SED 관계식도출초탄성재료인고무는반복하중에의한소성변형이일어나 Figure 6. SED-Strain curve, According to aging temperature. 며이에변형률에너지가축적되어파손이일어난다. 때문에고무재료의경우내구특성을평가하는인자로서 SED를주로사용한다. 17 본연구에서도내구특성평가인자로서 SED 값을사용하기위해인장시험으로부터얻은응력과변형률선도로부터수치해석법적분공식인 Simpson s rule 14 을적용하여 SED값을구하였다. 노화온도별 SED값을 Figure 6에나타내었으며산출한데이터에회귀분석을적용하여노화온도별변형률과 SED 관계식을식 (1)~(4) 과같이나타내었다. 그결과노화온도가증가함에따라서 SED값이증가하는경향을보였다. 이는노화가진행될수록고무가경화되어동일 SED에서변형률이감소하기때문이라고판단된다. SED = 0.802 (Strain 23 oc, 17day) 1.647 (1) SED = 1.088 (Strain 70 oc, 17day) 1.685 (2) SED = 1.530 (Strain 90 oc, 17day) 1.709 (3) SED = 1.639 (Strain 100 oc, 17day) 1.700 (4) 2. 피로실험결과 피로시험은노화온도별 0.69, 0.93, 1.16, 1.39 총 4 구간의변형률구간에서수행하였으며, 각각의변형률마다 10개의시편을사용하여파손시 cycle수를획득하였다. ASTM D4482 의데이터처리방법에따라식 (5) 과같은기하평균 (Geometric mean) 14 을사용하여각변형률구간의피로수명평균값을도출하였으며노화온도별최대변형률과피로수명과의관계를 Figure 7에나타내었다. G = n N 1 N 2 N n (5) Figure 5. Stress-Strain curve, Maximum strain : 1.39. G : geometric mean n : number of specimen N : fatigue life cycle
170 Namgyu Jun et al. / Elastomers and Composites Vol. 52, No. 3, pp. 167-172 (September 2017) Figure 7. Strain-Nf curve according to aging temperature. 2.1 노화온도별피로수명관계식도출앞서획득한노화온도별변형률-SED 관계식에최대변형률을대입, SED와피로수명간의관계를도출하여 Figure 8에나타내었으며회귀분석을통한곡선접합으로 SED와피로수명의관계식 (6)~(9) 을획득하였다. N f = 81318 (SED 23 oc, 17day) 3.298 (6) N f = 239067 (SED 70 oc, 17day) 3.192 (7) N f = 59257 (SED 90 oc, 17day) 3.520 (8) N f = 3469 (SED 100 oc, 17day) 3.482 (9) 2.2 노화온도에따른피로수명예측식 Ahagon 등의연구에따르면타이어고무는노화온도 80 o C 이상에서기계적특성이급격히변화하기때문에피로수명또한급격히감소할것으로판단하였다. 이에피로수명이완만히변화할경우를포함하여두가지모델을이용한노화온도에따른피로수명예측식을구하였다. Figure 9. Nf-Aging temperature curve on the same SED (MJ/m 3 ). 2 T aging N f = 214435 + 16210 T aging 142.23 (10) 피로수명이완만히변화할경우완만한곡선형태인식 (10) 과같은예측식을구하였다. 피로수명이급격히변화할경우변곡점의위치는 80 o C~90 o C 사이로판단되며, 피로수명이선형적으로증감하는형태인식 (11)~(12) 와같은예측식을구하였다. 두모델을이용한예측식의선도를 Figure 9에나타내었다. 그결과노화온도 80 o C 부근에서피로수명의차이가크게나타났으므로 80 o C에서의추가시험을통해두예측식을검증하였다. N f =10 (0.0127 T aging+4.67178) (T aging 83.8 o C) (11) N f =10 ( 0.12325 T aging+15.86556) (T aging 83.8 o C) (12) 3. 추가시험을통한노화온도에따른피로수명예측식검증노화온도에따른피로수명예측식을검증하기위해 80 o C, 17day 노화한시편에대해추가인장및피로시험을실시하 Figure 8. SED-Nf curve according to aging temperature. Figure 10. SED-Nf curve according to aging temperature.
Evaluation on Fatigue Characteristics of Tire Sidewall Rubber according to Aging Temperature 171 도가증가할수록동일변형률에서 SED값이증가하였다. 이는노화가진행될수록고무가경화되기때문이라고판단된다. 3. 피로시험을수행하여노화온도별 SED-피로수명관계식을도출하였다. 그결과노화온도가증가할수록피로수명이선형적으로증가하다 80 o C 이후부터급격히선형적으로감소하는경향을확인하였으며동일 SED(MJ/m 3 ) 에서노화온도에따른피로수명예측식을제시하였다. 4. 추후연구를통해앞서제시한노화온도에따른피로수명예측식을보완하고아레니우스식을통해피로수명이감소하기시작하는상온에서의노화일수를도출한다면타이어사이드월내구성평가에기준자료로사용할수있을것으로판단된다. Figure 11. Nf-Aging temperature curve on the same SED (MJ/ m 3 ). Acknowledgements 였다. 노화온도별 SED-피로수명선도인 Figure 8에 80 o C, 17day 노화한시편의시험결과를추가하여 Figure 10과같이나타내었으며, 노화온도에따른피로수명변화선도인 Figure 9에추가하여 Figure 11로나타내었다. 그결과노화온도에따라피로수명이증가하다감소하기시작하는변곡점은 80 o C~ 90 o C 사이에서존재함을확인하였다. 또한노화온도에따라피로수명이완만한곡선을그리며변하는것이아니라선형적으로증가및감소함을확인하였다. 따라서식 (11)~(12) 와같이선형적으로피팅하는것이타당하다. 노화온도 80 o C 부근까지는가교결합 (Crosslinking) 에의해수명이증가하였지만, 그이상의온도에서는사슬분리 (Chain Scission) 에의한급격한기계적특성의변화로인해피로수명이급격히감소하는것으로판단된다. 13 따라서두요인에의한수명증감효과를정량화한다면더욱정확한피로수명예측이가능할것으로판단된다. 추후연구를통해데이터를추가확보하여예측식의정확도를높이고노화온도와시간과의관계를아레니우스식을통해규명한다면피로수명이감소하기시작하는변곡점의노화정도가상온에서는며칠에해당하는노화정도인지예측가능하기때문에타이어내구성평가에기준자료로사용할수있을것으로판단된다. Conclusion 본연구에서는타이어사이드월에사용되는사용고무재료에대해서노화온도별인장시험및피로시험을수행하고노화온도에따른내구특성을평가하였다. 1. 인장시험결과모든변형률조건에서노화온도가증가할수록인장응력이증가하는경향을나타냈다. 2. 인장시험결과에 Simpson s rule 을적용하여 SED값을구하였으며, 변형률-SED관계식을도출하였다. 그결과노화온 본논문은현대자동차의지원을받아수행된연구임. References 1. G. Y. Park, J. W. Choi, K. C. Lee, and Y. K. Jin, Certification Technological Trends in Aerospace tire, Korea Aerospace Research Institute, Current Industrial and Technological Trends in Aerospace, 4, 68 (2006). 2. S. J. Hong and H. G. Lee, Investigation of Market Status of Imported Tires and Quality 1169 Evaluation, Transactions of KSAE, 13, 1 (2005). 3. W. D. Kim, C. S. Woo, K. S. Kim, and J. D. Kwon, An Experimental Study on the Dynamic Characteristics of Rubber Isolator, Elastomer, 37, 183 (2002). 4. Institute of Machinery and Materials, 2004, Development of Integrated Design System for Mechanical Rubber Components, M1-9911-00-0014. 5. T. K. Lee and B. S. Kim, Vibration Analysis of Automobile Tire Due to Road Impact, The Journal of the Acoustical Society of Korea, 22, 505 (2003). 6. C. Wright, G. L. Pritchett, and R. J. Kuster, Laboratory Tire Wear Simulation Derived from Computer Modeling of Suspension Dynamaics, Tire Science and Technology, TSTCA, 19, 136 (1990). 7. S. R. Chung, B. H. Choi, and K. J. Cho, Tire Wear Influence on the Vehicle Dynamics, 한국자동차공학회춘추계학술대회논문집, 408 (1995). 8. W. V. Mars and A. Fatemi, A Literature Survey on Fatigue Analysis Approaches for Rubber, International Journal of Fatigue, 24, 949 (2002). 9. R. J. Harbour, A. Fatemi, and W. V. Mars, Fatigue crack growth of filled rubber under constant and variable amplitude loading conditions, Fatigue and Fracture of Eng. Materials
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