FIBER LENGTH EFFECT ON TENSILE AND COMPRESSIVE STRENGTH OF SHORT FIBER REINFORCED THERMOPLASTICS

Transcription

1 FIBER LENGTH EFFECT ON TENSILE AND COMPRESSIVE STRENGTH OF SHORT FIBER REINFORCED THERMOPLASTICS Y. Wan a* and J. Takahashi a a Department o Systems Innovation, School o Engineering, the University o Tokyo,7-3-1, Hongo, Bunkyo, Tokyo, Japan * wan-yi@crtp.t.u-tokyo.ac.jp Keywords: CFRTP, Fiber length, Tensile, Compressive. Abstract The mechanism o the iber reinorcement and the mechanical properties o iber reinorced composites are considered important or the manuacture and application. In the present study, randomly oriented short carbon iber reinorced thermoplastics were prepared with compression molding techniques. The tensile properties and compressive properties o this composite were investigated. The iber length shows dierent eect on each mechanical property. The iber eiciency actors o dierent properties were predicted using the modiied rule o mixtures equation and the iber eiciency actor o strength and modulus show dierent sensitivity to the iber length. The ractures o the specimens were analyzed ater the tests, the tensile racture mechanism and the compressive racture modes were discussed. 1. Introduction Carbon iber reinorced thermoplastics (CFRTP) is considered to be promising ultralightweight material or automotive manuacturers not only because it can be applied in the special automobiles to realize extremely driving perormance but also because it can improve the energy eiciency o mass-production vehicles to mitigate the global petroleum consumption and CO 2 emission [1]. To ensure the application o CFRTP on mass production, we need to develop comprehensive knowledge about the mechanical properties o the CFRTP materials, parts and the structures. In the industrial application o CFRTP, the randomly orientated short iber reinorced thermoplastics (RSFRTP) show advantages o good high cycle and complex shaped molding properties compare to the traditional unidirectional laminate and woven materials. However, due to the complex internal structure and the interaction between the ibers and the matrix during the molding process, the mechanism o the iber reinorcement and the mechanical properties o RSFRTP are still under study. The theoretical research o mechanical properties o RSFRTP was begun rom 1990s. Fu et. al. managed to derived the tensile strength and tensile modulus o RSFRTP as unctions o iber length and iber orientation distribution [2, 3]. The compressive strength o laminates and the tensile properties o RSFRTP with dierent ibers were studied then [4, 5]. Since the residual lexure stress is easily generated due to the impregnation process o ibers during the molding 1

2 process o RSFRTP, the mechanical properties o the material is greatly aected by the molding condition[6, 7]. The objective o the present work is to investigate the eect o iber length on the tensile and compressive properties o the RSFRTP. A kind o RSFRTP was manuactured under the considering the reducing o the residual lexural stress. Tensile and compressive tests o RSFRTP with dierent iber length have been conducted. The eect o the iber length to the mechanical and the racture properties have been studied. 2. Materials and methods 2.1 Materials A kind o RSFRTP named as UT-CTT (ultra-thin chopped carbon iber tape reinorced thermoplastics) were used in this study. The UT-CTT is composed with randomly oriented ultra-thin unidirectional prepreg tapes. The tape is manuactured with carbon iber (TR 50S, Mitsubishi Rayon Co., LTD.) and Polycaprolactam (Nylon 6). The tape size is 5 mm in width and about 45 µm in thickness and cut in our dierent tape lengths: 6 mm, 12 mm, 18 mm and 24 mm. The cut tapes were randomly oriented and the UT-CTT plates were manuactured through compression molding. The molding pieces o UT-CTT are shown in Figure 1. And the materials mechanical properties o the components are listed in the Table 1. The iber volume ractions were calculated by ash test ater the molding and the details are listed in the Table 2. The intermediate substrate used in this study were provided by the Industrial Technology Center o Fukui Preecture and AWA PAPER MFG. CO., LTD. Figure 1. The UT-CTT material in the tape length o 6 mm (a), 12 mm (b), 18 mm (c) and 24 mm (d). Materials Carbon iber (TR 50S) Polycaprolactam (Nylon 6) Tensile strength [MPa] Tensile modulus [GPa] Compressive strength [MPa] Compressive modulus [GPa] Table 1. Mechanical properties o materials at ambient temperature. 2

3 Tape length 6 mm 12 mm 18 mm 24 mm 54.4% 56.1% 55.2% 54.5% Void ratio 1.6% 1.7% 1.2% 1.3% Table 2. Fiber volume raction and void ratio o specimens. Tests Thickness [mm] Width [mm] Height [mm] Number o tests Tensile Compressive Table 3. Specimen size or tensile and compressive tests. 2.2 Methods Tensile and compressive tests have been conducted in present study. The specimen sizes were shown in Table 3. Both tensile test and compressive test was conducted with a 10 tons universal testing machine (AUTOGRAPH AG-100kNXplus, Shimadzu CO.). The stroke speed o tensile test was set to 1 mm/min and the stroke speed o compressive test was set to 0.5 mm/min respectively. Ater the tensile and compressive tests, the ractures o the specimens were observed visually under optical microscope and 3D X-ray scanner. 3. Results and Discussions The eect o the tape length on the tensile strength, tensile modulus, compressive strength and compressive modulus were calculated rom the results o the tests separately (Figure 2-5). The eect o the tape length has dierent inluence on the dierent mechanical properties. The tensile strength increase with the increase o the tape length and reached a plateau when the tape length becomes 18 mm (Figure 2), while the tape lengths have no clear eect on the tensile modulus (Figure 3). Both the compressive strength and compressive modulus show increase tendency with the increase o the tape length, but the compressive strength increasing become saturated at the tape length o 12 mm (Figure 4) while the compressive modulus demonstrate a slight decrease when the tape length is longer than 18 mm (Figure 5). Figure 2. Tensile strength o UT-CTT by dierent tape length. 3

4 Figure 3. Tensile modulus o UT-CTT by dierent tape length. Figure 4. Compressive strength o UT-CTT by dierent tape length. Figure 5. Compressive modulus o UT-CTT by dierent tape length. 4

5 Figure 6. Fiber eiciency actors or the strength and modulus o UT-CTT by dierent tape length. According to the previous studies, the tensile strength ( ) and the compressive strength ( ) o RSFRTP composites can be predicted using the modiied rule o mixtures equation [2]: t t tv mt (1 V ) c c cv mc (1 V ) (1) where and are the iber eiciency actor or the composite tensile strength and compressive strength taking into account the eects o iber length and orientation. is the iber volume raction. and are the tensile strength o iber and matrix, while and are the compressive strength o iber and matrix, respectively. Similarly, the tensile modulus ( ) and the compressive modulus ( ) o RSFRTP composites can be predicted by the modiied rule o mixtures equation [3]: Et Et E tv E Ec Ec E cv E mt (1 V ) mc (1 V ) (2) where and are the iber eiciency actor or the composite tensile modulus and compressive modulus considering the eects o iber length and orientation. and are the tensile modulus o iber and matrix, while and represented the compressive modulus o iber and matrix, respectively. Figure 7. Cross section o specimen by the tape length o 24 mm. 5

6 For our study, the change in iber orientation can then be neglected since the iber tapes are randomly oriented or all specimens and thus the change in the tape length, otherwise the change in iber length would mainly aect the change o iber eiciency actors. Figure 6 shows the relationship between the tape length and the iber eiciency actor λ o tensile strength, tensile modulus, compressive strength and compressive modulus. It can be seen rom Figure 6 that the λ o tensile and compressive strength/modulus have no signiicant dierence while the iber eiciency actor or the strength is much lower than the iber eiciency actor or the modulus. This can be considered as an evidence o that the modulus o RSFRTP composites is a property o material at low strain and is not very sensitive to the iber matrix interace or iber length while the strength o RSFRTP composites is a property considering the racture and debonding o the interace and more sensitive to the iber length [5]. From the Figure 2-5, it is clear that despite the residual lexure stress o the ibers have been restricted through the molding process, the individual dierence o the specimens is still big in some mechanical properties. This phenomenon is considered lead by the molding quality. For example, the specimens o the tape length 24 mm are generally show poor mechanical properties than the specimens o the tape length 18 mm. Under the observation o the cross section o the 24 mm tape specimens we ound that the tapes were olded thought the thickness direction (Figure 7). This wrinkle o tape is regarded emerged through the molding process and have great inluence to the mechanical properties. Ater the tests, the ractures o the specimens were analyzed. The racture o tensile tests specimens were illustrated in Figure 8. As the tensile strength show increase tendency with the increase o the tape length, the longer tape was considered can prevent the growth o the cracks (Figure 9). In the compressive tests, two main racture modes were observed: the bucking (Figure 10) and the delamination (Figure 11). And in the compressive test, the racture o most specimens included both racture modes (Figure 12). Figure 8. Fracture o UT-CTT in tensile tests. 6

7 Figure 9. Eect o the tape length on the tensile racture. Figure 10. Bucking in the compressive tests. Figure 11. Delamination in the compressive tests. 7

8 Figure 12. General racture o UT-CTT in compressive tests. 4. Conclusion We conducted tensile and compressive tests o RSFRTP in dierent iber length and the iber length show dierent eect on the dierent mechanical properties. The iber eiciency actor o strength is more sensitive to the iber length than the iber eiciency actor o modulus. The iber eiciency actors between tensile properties and compressive properties have no conspicuous dierence. The molding conditions have great eect on the mechanical properties. The tape length shows the property to prevent the growth o the cracks on the tensile strength and two dierent racture modes have been observed in compressive tests. Acknowledgement This study was conducted as a part o Japanese METI project "the Future Pioneering Projects / Innovative Structural Materials Project" since 2013y. Authors would like to express sincerely appreciation to the project members who have provided valuable inormation and useul discussions. Reerences [1] Takahashi J and Ishikawa T, Current Japanese Activity in CFRTP or Automotive Application, in The 13th Euro-Japanese Symposium on Composite Materials [2] Fu S-Y and Lauke B, Eects o iber length and iber orientation distributions on the tensile strength o SFRP. Compos Sci Technol, : p. 12. [3] Fu S Y and Lauke B, The elastic modulus o misaligned short-iber-reinorced polymers. Compos Sci Technol, (3-4): p [4] Adams D F and Welsh J S, The Wyoming combined loading compression (CLC) test method. J Compos Tech Res, (3): p [5] Fu S Y, Lauke B, Mader E, Yue C Y, and Hu X, Tensile properties o short-glass-iberand short-carbon-iber-reinorced polypropylene composites. Compos Part a-appl S, (10): p [6] Ye L, Chen Z R, Lu M, and Hou M, De-consolidation and re-consolidation in CF/PPS thermoplastic matrix composites. Compos Part a-appl S, (7): p [7] Wolrath J, Michaud V, and Manson J A E, Deconsolidation in glass mat thermoplastic composites: Analysis o the mechanisms. Compos Part a-appl S, (12): p