INVESTIGATION ABOUT DYNAMIC FLEXURAL FRACTURE PROPERTIES OF CARBON - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS INVESTIGATION ABOUT DYNAMIC FLEXURAL FRACTURE PROPERTIES OF CARBON FIBER REINFORCED THERMOPLASTICS T. Matsuo 1 *, J. Takahashi 1 , K. Uzawa 1 , T. Asakawa 1 and K. Kiriyama 2 1 Department of Systems Innovation, School of Engineering, The University of Tokyo, Tokyo, Japan 2 TOYOBO CO., LTD., Research Center, Otsu, Japan * Corresponding author (matsuo@giso.t.u-tokyo.ac.jp) Keywords : carbon fiber reinforced thermoplastics (CFRTP), flexural behavior, strain rate 1 Introduction 2 Observation of Impact flexural behavior in With the development of processing technology of high-speed three-point bending test high performance thermoplastic polymers, light- 2.1 Experimental setup weight structural applications for fiber reinforced thermoplastic composites are expanding rapidly in a In tensile test for carbon fiber reinforced plastics (CFRP), a high-speed photography has great effect number of sectors because thermoplastic composites have potential to achieve high-cycle and low-cost in explanation of the fracture behavior of high- strength CFRP [6]. At first in this study, a high- manufacturing and high recyclability in contrast to speed camera (Hyper Vision HPV-1 by thermosetting composites [1][2]. Above all, polypropylene (PP), by itself or with other polymers, SHIMADZU) observed the dynamic flexural behaviors of the CFRP in high velocity three point has been widely applied to automotive structural bending test by using the impact tower (Dynatup by members such as bumper faces and interior parts because of high toughness and simple fabrication Instron), which has a recording system of load and deflection. A specimen with rectangular cross techniques [3][4]. The wide practical use of such section rests on two supports and is impacted by polymer components has been supported by the advanced vehicle body design with use of various means of dropping a crosshead with a loading nose right above the center of the specimen as shown in computer simulation techniques. Fig.1. Some frames at the instant that the specimens Complying with this trend, Japanese METI-NEDO project has been developing a new type of advanced are fractured by the impact load are took by the high-speed camera. thermoplastic composite materials, carbon fiber reinforced thermoplastic composites (CFRTP), 2.2 Test results - in comparison to carbon fiber which are composed of surface treated carbon fiber reinforced thermosetting composites (CFRTS) (Mitsubishi Rayon) and maleic-acid modified The experiments are conducted for two types of polypropylene (TOYOBO), aiming to apply to the quasi-isotropic laminates. One is constructed from automobile main frame structures for the purpose of the developed CFRTP and the other is from significant reduction of vehicle weight [2][5]. commonly-used CFRTS. From test results, it was In particular, the main frame structural members of found that the flexural fracture behavior of the automobiles are required high energy absorption for CFRTP is clearly different from that of the CFRTS. collision safety. With increasing expectation of wide applications for the developed CFRTP, it is getting more important to predict the impact energy absorption by crash simulation. For this, it is essential to evaluate the mechanical properties and obtain the fracture behavior of the materials applied to the vehicle structure. This study focused on the fracture behavior of the thermoplastic composites, observing the fracture phenomenon and investigating the yield stress dependent on the strain-rate. Fig.1. Observation of impact flexural behavior

high frequent noises by oscillating at impact are Loading nose reduced. 32 m s In addition to an assumption that what causes this 32 m s mechanical behavior is largely dependent on the interfacial adhesive strength between the carbon Specimen Delamination fiber and the matrix resin, it is also necessary to Fig.2. Photographic images at the instant of flexural investigate the mechanism how the interfacial fracture of CFRTS by high-speed camera property affects the dynamic behavior with relation to the strain rate dependency of the composites. Loading nose 3 Relationship between yield stress and strain- 32 m s 32 m s rate of thermoplastic composites Compression Specimen failure 3.1 Effect of strain rate on viscoelastic behavior Fig.3. Photographic images at the instant of flexural of the polymer fracture of CFRTP by high-speed camera In general, the yield stress is assumed by the Eyring theory for the yielding of polymers [4][7], which leads equation (1) and (2). 0.5 Elastic behavior Fracture behavior     0.4 RT      1 * sinh (1)   y  *  V   Load [kN] 0.3 0 developed CFRTP 0.2    H  *        exp (2) 0.1 0 0   RT 0 where,  y is the yield stress, V * is the activation CFRTS -0.1 volume,  is the activation enthalpy,   is the 0 3 6 9 12 15 18   * imposed strain rate at yield, is the constant 0 Deflection [mm] reference strain rate, R is Boltzmann's constant and T Fig.4. Adjusted load-deflection curves is temperature. in impact flexural test For examination about the strain-rate dependency of the materials, the high-velocity three point bending In the case of CFRTS, as illustrated in Fig.2, at just a tests with higher strain rate than 5 s -1 are performed 32 micro second intervals of the impact instant, a by the impact tower mentioned above, and the low- wide range of delamination occurs on the other side velocity three point bending tests with strain rate of the impacted surface. So after the first failure, the ranging from 0.0001 s -1 to 0.1 s -1 are performed by flexural stiffness of the specimen decreases rapidly the universal testing machine (Autograph AGS-X by and the load measured by the load cell attached to SHIMADZU). the loading nose falls down close to zero The load-deflection curves are recorded by the immediately following the maximum in the load- testing machines, and the stress-strain curves are deflection curve, which is represented as a dashed obtained by assuming linear elastic relationship, line in Fig.4. where the stress and the strain are calculated from On the other hand, Fig.3 shows that the CFRTP the load and the deflection taking into account the specimen causes a compression failure of only a few thickness and the width of the specimen and the surface layers at the beginning of the fracture during support span. In a similar way, the strain rate is the same intervals instead of a large delamination. calculated with respect to the crosshead velocity of And, the load expressed as a solid line in Fig.4 is in the testing machine. a relatively gradual decline after the maximum load. 3.2 Verification for the developed PP In other words, the developed CFRTP has its potential to have higher impact absorption than the In the process of developing a new type of CFRTP, CFRTS. Where, both lines are adjusted so that their TOYOBO has developed two types of stiffness and strength are equal to each other and polypropylene whose properties such as modulus,