폴리프로필렌복합재의동적인장특성평가 김진성 *) 허훈 ) 이강욱 2) 하대율 2) 여태정 2) 박순조 2) 한국과학기술원기계공학과 ) 현대모비스기술연구소 2) Evaluation of Dynamic Tensile Characteristics of Polypropylene Composite Jinsung Kim *) Hoon Huh ) Kangwook Lee 2) Daeyul Ha 2) Taejung Yeo 2) Soonjo Park 2) ) Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Science Town, Daejeon, 5-7, Korea *2) Hyundai MOBIS Technical Research Center, 8-, Mabuk-ri, Guseong-eup, Yongin-si, Gyeonggi-do, 449-92, Korea Abstract : This paper deals with dynamic tensile characteristics for the polypropylene composite used in an IP(Instrument Panel). This polypropylene composite is adopted in the dash board of a car, especially PAB(Passenger Air Bag) module. The dynamic tensile characteristics are important because the PAB module undergoes high speed deformation during the airbag expansion. The operating temperature of a car varies from C to C according to the specification. The dynamic tensile tests are performed at the low temperature( C), room temperature(2 C) and high temperature(85 C). The tensile tests are carried out at strain rates of nine intervals ranged from./sec to 2/sec. The flow stress increases as the strain rate becomes higher. The flow stress decreases at the high temperature while the strain rate sensitivity increases. Finite element analysis of the tensile test with the special specimen was carried out with unmodified stress-strain curve obtained from the ASTM IV specimen directly. The analysis result shows that the load response is almost same with the experimental result but the corresponding strain is rather larger than the experimental result. The stress-strain curves were modified on the basis of the observation of strain distribution along the gage length. The analysis result with the modified stress-strain curves showed good agreement with the experimental result. The material properties obtained from this research will be useful to estimate the airbag expansion accurately in engineering sense. Key words : Dynamic Tensile Test( 동적인장시험 ), Polypropylene( 폴리프로필렌 ), Composite( 복합재 ), True Stress( 진응력 ), True Strain( 진변형률 ), Temperature( 온도 ) 동온도는 - ~ ) 정도이며이온도범위. 서론에서플라스틱의기계적물성은일반적으로온. 도에크게영향을받는다고알려져있다. 변형플라스틱은현대의자동차산업에서그효용이률속도및온도가변형률경화에미치는영향높아지고있다. 특히자동차의내장재는대부분에대해많은연구가진행되어왔다 2)~5). Machida 플라스틱을사용하고있다. 이러한자동차의내장와 Lee 2) 는폴리프로필렌박판의다양한온도에재중에에어백모듈에적용된플라스틱은에어대한딥드로잉에대해연구하였으며 Walley 등 ) 백전개시고속변형하에놓이게된다. 에어백은다양한플라스틱에대한변형패턴을파악하전개시에어백모듈의커버의파단패턴이승객였다. Arruda 등은플라스틱의유동응력은온도상해에영향을미친다. 따라서보다정확한파단가증가함에따라서크게감소하며변형률속도패턴을예측하기위해서는플라스틱의고속변형가증가함에따라증가함을보였다 4). 온도, 변형특성을정확히파악할필요가있다. 자동차의작률속도, 에폭시수지의조성에따른영향에대해연구하였으며 5). 이상의앞선연구결과에서. * soitgoes@kaist.ac.kr 플라스틱이온도와변형률속도에매우민감하
게거동함을알수있다. 플라스틱은일반적인강의변형거동과다르게넥킹이항복시점직후에발생한다. 따라서일반적인방법으로는실험결과로부터진응력-진변형률관계를얻을수없다. 이러한불균일한변형거동을보이는재질의진응력-진변형률관계를얻기위해서는두가지방법이있다. 첫번째는균일한변형거동을보이는시편을설계하는것이고두번째는변형거동의세밀한관찰을통해진응력-진변형률관계를근사하는것이다. Marquez- Lucero 등 6) 은상온에서다양한변형률속도에대해환봉형시편의넥킹전파현상을관찰하였다. 또한 G'Sell 등 7) 은정교한비디오시험법을통해체적변형률을구했다. 본논문에서는온도조건을고려한폴리프로필렌복합재의동적인장특성을평가하고실제공학문제에적용할수있는진응력-진변형률선도를도출하고자한다. 2.2 실험장비및실험조건준정적인장실험은.~./sec의낮은변형률속도로수행되었으며 Instron사의 558모델을이용하였다. 저온및고온의조건을부과하기위해환경시험실 (Environmental Chamber) 을장착하였다. 변형률속도 ~2/sec에해당하는동적인장실험을위해고속재료시험기 8) 를이용하였으며환경시험실을장착하였다. 온도조건은상온 (2 ), 저온 (- ), 고온 (85 ) 으로부과하였으며각온도에서 ±2 범위내에서온도를일정하게유지했다. 시편의온도가해당온도에도달해야하므로환경시험실내의온도가해당온도에도달한뒤 2분후에인장실험을수행했으며각실험조건당 2~회반복실험을수행했다. 2. 실험준비및결과 2. 시편형상폴리프로필렌복합재의단축인장실험에사용한시편은 Figure 에도시한 ASTM IV 시편이며고속인장실험을위해시편의총길이를 2 mm로늘인 Figure 2의수정 ASTM IV 시편을제작하여실험을수행했다. 시편의게이지길이는 mm, 너비는 6 mm, 두께는 2 mm이다. (a) Figure. ASTM IV standard specimen. Figure 2. Modified ASTM IV specimen. (b) Figure. High Speed Material Testing Machine: (a) before installing environmental chamber; (b) after installing environmental chamber. 2
2.4 실험결과폴리프로필렌복합재의동적인장실험시변형형상을 Figure 4에도시하였다. 변형형상촬영은고속카메라를사용하였으며초당 6프레임의속도로촬영하였다. 변형양상을보면총인장거리가 mm 정도로짧으며게이지부분이균일하게변형함을관찰할수있다. 또한파단면이인장방향과수직임을알수있다. 게이지부분이균일하게변형하므로일반적인진응력-진변형률도출법으로응력-변형률관계를구했다. 실험을통해얻은결과를 piecewise linear 선도를 Figure 5부터 Figure 7에도시하였다. 상온조건인 2 에서의인장실험결과는 Figure 5, 저온조건인 - 의결과는 Figure 6, 고온조건인 85 의결과는 Figure 7에도시했다. 상온과고온에서의응력-변형률선도는변형률속도가증가함에따라응력이점진적으로증가하나저온에서의응력-변형률선도는변형률속도가증가함에따라 유동응력이오르락내리락하여 /sec의유동응력이./sec의유동응력보다낮은결과를보인다. 또한폴리프로필렌복합재는온도가증가함에따라유동응력이급격히감소하는결과를보인다. 각온도조건에서변형률속도./sec의유동응력을비교하면상온에서최대응력이약 64 MPa이며저온에서 6 MPa, 고온에서 24 MPa으로폴리프로필렌복합재가주위온도에따라유동응력의변화가큼을알수있다. 또한각온도조건에서./sec과 2/sec의응력을비교해볼때상온에서는약 % 의증가를보이며저온에서는약 5%, 고온에서는약 7% 의증가를보인다. 따라서변형률속도민감도 (Strain rate sensitivity) 는온도가증가함에따라급격히증가함을알수있다. Engineering stress (MPa) 2 5 5... 2..2.4.6.8..2 Engineering strain Figure 4. Deformed shapes of the ASTM IV specimen at the strain rate of /sec. Figure 6. Engineering stress strain curves at the low temperature(- C) for a polypropylene composite. Engineering stress (MPa) 4 2 8 6 4 2... 2 Engineering stress (MPa) 7 6 5 4 2... 2..2.4.6.8..2 Engineering strain..5..5.2 Engineering strain Figure 5. Engineering stress strain curves at the room temperature(2 C) for a polypropylene composite. Figure 7. Engineering stress strain curves at the high temperature(85 C) for a polypropylene composite.
. 유한요소해석본장에서는인장실험을통해구한폴리프로필렌복합재의기계적물성치를적용한 ASTM IV 시편의인장해석을통해실험을통해구한폴리프로필렌복합재의진응력-진변형률선도가실제공학문제에의적용에적합한가에대해고찰한다. ASTM IV 시편의인장해석을통해실제공학문제에적합하게적용할수있는진응력-진변형률선도를도출한다.. 유한요소해석조건유한요소해석을위해 ASTM IV 시편을 Figure 8 와같이절반을모델링했으며사용된요소는 차원 Brick 요소이며총요소수는 7488개이고두께방향으로 6층을모델링했다. 유한요소해석용프로그램으로 LS-DYNA D ver. 97을사용했다. 해석에적용한물성모델은 piecewise linear plasticity로실험을통해구한진응력-진변형률선도를적용했다. 물성모델에사용한실험결과는앞서실험을통해구한상온에서의폴리프로필렌복합재의진응력-진변형률선도이다. 인장실험조건에부합하도록시편의왼쪽끝을고정하였으며오른쪽끝에서,, 2/sec에해당하는속도로인장해석을수행했다..2 유한요소해석결과유한요소해석결과는 Figure 에도시한하중-변위선도와같다. 해석을통해얻은하중값과실험을통해얻은하중값은같은변위에서차이가큼을알수있다. 이러한현상에대한원인을알아보기위해 Figure 과같이시편의길이방향으로시편의중심의소성변형률을측정했으며측정한결과는 Table 과같다. Figure 과 Figure 2를보면시편의게이지부분의소 성변형률은일정한것으로판단되나소성변형률이시편의길이방향으로넓게분포하고있음을알수있다. 따라서 Table 과같이일반적인진변형률계산법으로구한변형률에비해유한요소해석을통해구한소성변형률이 5~7% 가량낮은결과를보인다. 위결과를통해상대적으 Load (N) 8 6 4 2 8 6 4 2 Experiment /sec /sec 2/sec Finite Element Analysis /sec /sec 2/sec 2 4 5 Displacement(mm) Figure 8. Half model of ASTM IV standard specimen. Figure. Load-displacement curves at different strain rates in the experiment and analysis before modification. Gage region.2 X True stress (MPa) 4 2 8 6 4... 2 Plastic Strain.5..5 Stroke(mm).65. 4.95 6.6 2..2.4.6.8..2 True strain. 2 4 6 8 2 Distance through X-axis(mm) Figure 9. Piecewise linear curves at the room temperature(2 C) for a polypropylene composite. Figure. Distribution of plastic strain along the X- axis of ASTM IV standard specimen. 4
Table. Plastic strain at a gage region in FE analysis. Stroke(mm) Corresponding Plastic strain True strain in FE analysis.65.488.246..95.525 4.95.4.87 6.6.82.26 True stress (MPa) 4 2 8 6 4 2..2.4.6.8. True strain Modified Figure. Modification of true stress-true strain curves obtained from experiment. 8 6 4 Figure 2. Distribution of plastic strain along the X- axis of ASTM IV standard specimen. 로작은변형률을갖는폴리프로필렌복합재의물성실험시 ASTM IV 시편을사용할경우소성변형률이게이지부분에집중되지않아부정확한응력-변형률선도를얻게될것으로예측할수있다. 하지만이러한문제는변위계를사용하면해결이가능하나고속인장실험과같이변위게장착이힘든실험의경우정확한변형률측정이힘들다. 따라서위와같은유한요소해석결과를바탕으로공학문제에적합하게적용할수있는응력-변형률선도를도출하고자한다.. 진응력-진변형률선도도출앞선유한요소해석결과실험시단순히변위만을측정해얻은변형률은실제시편의게이지부분의변형률과차이가있음을보였다. 따라서실험을통해얻은진응력-진변형률선도에서변형률을기존진응력-진변형률선도의 5% 로가 Load(kN) 2 8 6 4 2 Experiment /sec /sec 2/sec Finite Element Analysis /sec /sec 2/sec 2 4 5 Displacement(mm) Figure 4. Load-displacement curves at different strain rates in the experiment and analysis after modification. 정하고 Figure 와같이새로운진응력-진변형률선도를얻었다. 변형률을조정하여얻은새로운진응력-진변형률선도를유한요소해석에적용한결과는 Figure 4과같다. 변형률속도,, 2/sec에해당하는하중-변위선도에서모두실험결과와부합하는결과를보인것으로판단된다. 본연구결과를통해 ASTM IV 시편으로폴리프로필렌복합재와같은파단연신율이짧은재료의고속인장실험결과는유한요소해석에바로적용하기부적절하며유한요소해석을통해적절한변형률분포를결정하고진응력-진변형률선도를수정하면실험결과와부합하는해석결과를낼수있음을보였다. 5
4. 결론본논문에서는온도조건을고려한폴리프로필렌복합재의고속인장실험을통한동적인장특성을평가하고실제공학문제에적용가능한진응력-진변형률선도를도출했다. 온도조건을부과하여폴리프로필렌복합재의준정적및동적인장시험을수행했다. 폴리프로필렌복합재는온도에따른유동응력의변화가크며온도가증가할수록변형률속도민감도가증가하는결과를보였다. 또한저온에서변형률속도가증가함에따라유동응력이오르내리는결과를보였다. 실험을통해구한동적물성치를유한요소해석에적용한결과실험결과와부합하지않았다. 해석결과에기초하여진응력-진변형률선도를수정하여재해석한결과실험결과와부합하는결과를얻었다. References ) The Korea Society of Automotive Engineers, Technical Handbook of automobile, Seoul, 996. 2) T. Machida and D. Lee, "Deep drawing of polypropylene sheets under differential heating conditions", Polymer Eng. Sci., Vol. 28, pp. 45-42, 988. ) S. M. Wally, J. E. Field, P. H. Pope and N. A. Stafford, "A study of the rapid deformation behavior of a range of polymers", Philos. Trans. Soc. London, Vol. A, No. 28, pp. 78-8, 989. 4) E. M. Arruda, M. C. Boyce and R. Jayachandran, "Effects of strain rate, temperature and thermomechanical coupling on the finite strain deformation of glassy polymers", Mechanics of Materials, Vol. 9, pp. 9-22, 995. 5) W. D. Cook, A. E. Mayr and G. H. Edward, "Yielding behavior in model epoxy thermosets-i. Effect of strain rate and composition", Polymer, Vol. 9, No. 6, pp. 79-724, 998. 6) A. Marquez-Lucero, C. G'Sell and K. W. Neale, "Experimental investigation of neck propagation in polymers", Polymer, Vol., pp. 66-642, 989. 7) C. G'Sell, J. M. Hiver and A. Dahoun, "Experimental characterization of deformation damage in solid polymers under tension, and its interrelation with necking", Int. J. of Solids and Structures, Vol. 9, No. -4, pp. 857-872, 22. 8) J. H. Lim, S. B. Kim, J. S. Kim, H. Huh, J. D. Lim and S. H. Park, "High Speed Tensile Tests of Steel Sheets for an Auto-body at the Intermediate Strain Rate", Trans. of KSAE, Vol., No. 2, pp. 27-4, 25. 6