INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 18, No. 2, pp. 197-202 DOI: 10.1007/s12541-017-0025-4
FEBRUARY 2017 / 197 ISSN 2234-7593 (Print) / ISSN 2005-4602 (Online)
A Comparative Analysis on Fracture Behaviors of 3Point Bending Specimens Made of CFRP and Metal Kyekwang Choi1, Guewan Hwang2, and Jaeung Cho3,# 1 Department of Metal Mold Design Engineering, Kongju National University, 1223-24, Cheonan-daero, Seobuk-gu, Cheonan-si, Chungcheongnam-do, South Korea 2 Department of Mechanical Engineering, Graduate School, Kongju National University, 1223-24, Cheonan-daero, Seobuk-gu, Cheonan-si, Chungcheongnam-do, South Korea 3 Department of Mechanical & Automotive Engineering, Kongju National University, 1223-24, Cheonan-daero, Seobuk-gu, Cheonan-si, Chungcheongnam-do, South Korea # Corresponding Author / E-mail:
[email protected], TEL: +82-41-521-9271, FAX: +82-41-555-9123 KEYWORDS: Carbon fiber reinforced plastic, 3 point bending test, Aluminum, Brass, Structure steel, Fracture behavior
As the composite material of fiber reinforced plastic has the high strength with light weight unlike exiting metal materials, its usage is being gradually increased. In this study, the fracture behaviors are investigated with CFRP and existing metals of aluminum, brass and structural steel by 3-point bending test. As existing metal materials, the maximum reaction forces of 2250 N, 2500 N and 2400 N occurred at aluminum, brass and structural steel respectively. In contrast, the maximum force of about 4000 N was shown at CFRP. This study result can be applied to the durable design of composite structure. Manuscript received: September 19, 2016 / Revised: October 6, 2016 / Accepted: October 11, 2016
1. Introduction In the industry of past, metals were important materials in place of woods. Particularly, the emergence of composite materials is gradually replacing their places as the result of development in material areas. Application areas of composite materials are widely used as a substitute material for metal across the whole area such as leisure, transportation, industry, etc. As the composite materials overcome the limitations of existing materials and have more diversified usage and properties by employing heterogeneous materials together, the uses of composite materials are being the further increased. Among them, the transportation area came to have greater effects on the development of composite materials as strengths on the same level as for the previous use of metal materials were desired as high efficiencies through weight reduction were sought at the energy crisis of past. Accordingly, in recent production of transportation vehicles, diversified attempts are being made with the use of metal and composite material or complete substitution of metal parts, etc. As the most representative example among composite materials, fiber reinforce plastic may be considered. Glass fibers, Kevlar fiber, aramid fibers and carbon fibers are hardened by using resin as the matrix, glass fibers and carbon fibers of these materials have the widest range of application. In fiber reinforced plastics, each fiber hardened in resin is composed of one axis, and numerous fibers become axes to enable the production of damping effects by undergoing the dispersion 1-4
© KSPE and Springer 2017
of loads or forces imposed from the outside. Carbon fibers exhibit the features of such composite materials well, where high strengths and light weights are possessed due to characteristics of the carbon fibers, and the effective damping effects can also be produced in fatigue environments. In this paper, the analytical and experimental results for 3-point bending specimens of fiber reinforced plastic composed of carbon fibers are compared with those for metal materials, and the fracture behavior characteristics of CFRP (carbon fiber reinforced plastic) obtained through this study are identified. For this purpose, 3point bending specimens based on ASTM were produced with CFRP and metal material, and advance analytical results for the specimen configured in 3D model through ANSYS were compared with actual experimental results. Based on the analysis results, the fracture behaviors of CFRP and existing metal materials were identified. Through this study, the basis data on fracture behavior which can occur in bending environment of structures with application of CFRP may be obtained, and it is thought that the contribution can be made to the safe design of structures based on this study data. 5-9
2. Research Method 2.1 Research model In this paper, the specimen was configured on the basis of ASTM
198 / FEBRUARY 2017
INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 18, No. 2
Fig. 1 Configuration of specimen Fig. 2 Experimental setup for static experiment Table 1 Material properties Property (Unidirectional carbon fiber) Density (kg/m ) Young's modulus (MPa) Poisson's ratio Yield strength (MPa) Shear strength (MPa)
Value 1.57 8,980 0.3 1,440 2,580.5
3
Table 2 Chemical components of resin and hardener used at manufacturing CFRP Polyester Resin [LSP - 8020B] 1000 mL
Hardener [Methyl ethyl keton peroxide] 3 mL
as schematically shown in Fig. 1. The specimen had the length of 200 mm, the width of 20 mm and the thickness of 5 mm. Here, the gauge length in the jig was configured to be 120 mm. Table 1 shows material properties of carbon fiber used for experiment and analysis. In manufacturing CFRP, the polyester resin with matrix carbon was hardened with epoxy resin during 10 hours at 75 C through the process of vacuum bag. Table 2 shows the chemical components of resin and the hardener used in order to make the carbon fiber into CFRP.
Fig. 3 Boundary conditions for the simulation analysis
o
2.2 Experiment condition Fig. 2 shows experimental conditions for specimens set up for experiments, where the fixed points at the gauge length of 40 mm are supported on both sides with the load cell in the upper part descending at a rate of 1 mm/min. Experiments of a test specimen are conducted for 3 times for each material. Since fracture of CFRP at each ply cannot be observed when the proceeding speed of the load cell is too high, the speed was designated based on the tensile test results from the previous experiment. 2.3 Boundary condition for the simulation analysis Fig. 3 is shown with 3D model configured for analysis in the finite element program of ANSYS, where the load cell in the upper part is descended at the speed of 1 mm/min which is the same as the condition in actual experiment. Also, for the calculation of result values in test specimens through an accurate equation, the shape of finite elements The numbers of elements and was generated as full-quad mesh. nodes on metal specimens are 28,300 and 127,715. Also, the numbers of elements and nodes on CFRP specimens are 36, 074 and 198,574 As the sheet of carbon fiber has the very thin thickness, Z axis is not be considered. The analysis can be carried out on two dimensional plane 10-13
Fig. 4 Formation of axes through each fiber of CFRP
by using the shell element. So, the fracture behavior can be investigated through this analysis.
2.4 Material characteristics of CFRP In the previous characteristics of CFRP, each fiber withstands loads and forces applied from outside through the role of axis, which is schematized in Fig. 4. For CFRP specimens used in this paper, the unidirectional carbon fiber allowing the determination of lamination angles was employed, where the lamination was made at the lamination angle of 60º having the highest result value in the applied lamination
INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 18, No. 2
FEBRUARY 2017 / 199
angle based on previously conducted tensile tests. Through the application of such lamination angle, the fracture behavior characteristics for metal materials with isotropy and CFRP with directionality can be comparatively analyzed. 14-16
3. Study Results 3.1 Experimental and analytical result in aluminum Fig. 5 is shown with the experimental result for reaction force due to displacement in aluminum specimen, where the reaction force having begun at an early displacement showed the maximum value of 2480 N at the displacement of 20 mm. Subsequently, the specimen fractured showing the maximum bending at the displacement of 25 mm, which may be compared with the analytical result in Fig. 6. In the result of Fig. 6, the maximum reaction force of 2500 N occurred at 10mm, showing the result where the reaction force was maintained to 2300 N at the displacement of 25 mm. Also, at this time, the equivalent stress was found to be 31 MPa at an early displacement of 7mm and became 126 MPa at 10 mm, followed by occurrence of the maximum stress of 620 MPa at the displacement of 25 mm. Here, as shown in Fig. 7, the maximum stress can be seen to occur at the part where the load cell of upper part comes into direct contact. At the analysis result the maximum stress does not happen at the part right on the middle of specimen. The maximum stress is shown to happen at left and right of specimen because V bending occurs at the middle during the process of 3 point bending. This phenomenon is shown with the similar as Figs. 10, 13 and 16. In the comparison between experimental result and analytical result, while the test specimen used for the experiment shows the result where reaction force is maintained to a certain extent in the actual bending accompanied by a slight increase, the analytical result shows a slight decrease due to the excluded result value as the point exceeding the maximum stress was processed as a failed part because of the exclusion from the result output. 3.2 Experimental and analytical results in brass Fig. 8 shows the experimental result for reaction force due to displacement in brass specimen, where 380 N occurred at 2 mm, about 2400 N was observed at 10 mm, followed by the maximum reaction force of 2500 N at about 27 mm for a fracture while showing the maximum bending of the specimen as the result of continued displacement. Fig. 9 is shown with the analytical result for this specimen, where 310 N was shown at an early displacement of 5.4mm and then 2480 N at 12 mm as the result of progression of displacement, followed by the linear decrease to 2340 N at 25 mm. Also, in terms of equivalent stress value at each displacement, 63 MPa occurred at an early displacement of 5.4 mm, and 797 MPa at 12 mm as a result of continued displacement, with the maximum stress of 820 MP shown to occur at the displacement of 25 mm. Here, as shown in Fig. 10, the maximum stress can be seen to occur at the part where the load cell of the upper part comes into direct contact. 3.3 Experimental and analytical results in structural steel Fig. 11 shows the experimental result for reaction force due to displacement in structural steel specimen, where 280 N occurred at an
Fig. 5 Experimental result for reaction force due to displacement in aluminum specimen
Fig. 6 Analytical results for reaction force due to displacement in an aluminum specimen
Fig. 7 Contour of equivalent stress at the maximum facture at Fig. 6
early displacement of 2 mm, and then about 1900 N at 4 mm as the displacement was gradually increased, followed by the fracture accompanied by the maximum bending of the specimen while showing the maximum reaction force of 2000 N at about 18 mm. Fig. 12 is shown with the analytical result, where 400 N is shown at an early displacement of 3.2 mm, and 2030 N at 7 mm with the progression of displacement, followed by the liner decrease to 2000 N at 25 mm. Also, for equivalent stress value at each displacement at this time, 28 MPa
200 / FEBRUARY 2017
INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 18, No. 2
Fig. 8 Experimental result for reaction force due to displacement in brass specimen
Fig. 9 Analytical result for reaction force due to displacement in brass specimen
Fig. 11 Experimental results for reaction force due to displacement in structural steel specimen
Fig. 12 Analytical result for reaction force due to displacement in structural steel specimen
Fig. 13 Contour of equivalent stress at the maximum facture at Fig. 12 Fig. 10 Contour of equivalent stress at the maximum facture at Fig. 9
occurred at an early displacement 3.2 mm, and then 126 MPa at 7 mm with the progression of displacement, followed by the maximum equivalent stress of 653 MP observed at the displacement of 25 mm.As shown in Fig. 13, the maximum stress can be seen to occur at the part where the load cell of the upper part comes into direct contact.
3.4 Experimental and analytical results in CFRP Fig. 14 shows the experimental result for reaction force due to
displacement in CFRP specimen, where 1500 N occurred at an early displacement of 4 mm and the maximum reaction force of about 4000 N at 7 mm as the result of a gradual increase in displacement. With continued displacement, the shape of a great up and down change is observed in the reaction force graph as the delamination of fiber layers constituting CFRP occurs. Subsequently, the fracture of specimen occurred with occurrence of 2000 N at 13 mm. CFRP is laminated in order and withstood as one layer supports another layer. So, the higher reaction force occurs with the greater deformation. At Figs. 14 and 15,
INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 18, No. 2
FEBRUARY 2017 / 201
Fig. 14 Experimental result for reaction force due to displacement in CFRP specimen
Fig. 16 Contour of equivalent stress at the maximum facture at Fig. 15
by fracture can be seen to occur simultaneously at several places due to delamination of each fiber layer at the bottom unlike the previous results with metal materials. As the result of this study applies to the real structure, the safe design and the evaluation can be possible through the fracture shape.
4. Conclusions
Fig. 15 Analytical result for reaction force due to displacement in CFRP specimen
the fluctuation of rise and fall is shown as this phenomenon. As CFRP has the brittle fracture property, the fluctuation of rise and fall in this figure is shown as the fracture at each laminated layer. Fig. 15 is shown with the analytical result for this specimen, where 3600 N was observed at an early displacement of 3.5 mm, and then 4200 N at 5 mm with continued displacement, showing a reaction force graph of the same shape as that of the experiment where 2000 N was observed at 8 mm, and 1000 N at 10 mm. Analytical result for CFRP represents the result value observed by the reflection of analysis result for each shell element as one model is composed of a few tens of elements through pretreatment process of laminating several sheets by using the thin finite element model which simulated carbon fibers as shell elements among the existing finite element models. Here, for the equivalent stress value at each displacement, 1397 MPa occurred at an early displacement of 3.5 mm, and then 2300 MPa at 5 mm with continued displacement, followed by 736 MPa at 8 mm and 15 MPa at 10 mm. Thus, unlike the case of metals where the shape was maintained due to ductility, the stress values in the brittle material of CFRP can be seen to be reduced due to fracture. Fig. 16 is shown with the stress distribution shape at the point of maximum reaction force in Fig. 15, and the stress values caused
This paper is investigated with the experiment and analysis through 3-point bending tests for 3 types of metals including aluminum, brass and structural steel as CFRP composite material, and the following conclusions could be drawn. 1. According to the results of the experiment and analysis with 3point bending specimens, while the maximum reaction force values of 2480 N for aluminum, 2500 N for brass and 1900 N for structural steel occurred, the high reaction force value of 4000 N could be seen to occur for CFRP. A higher reaction force was observed in the fracture behavior of CFRP than with existing metals through the load distribution of each fiber layer, indicating the higher stability for external load. 2. According to the results of the experiment and analysis for 3point bending specimens, while stress values occurred at the fracture points of specimens were 620 MPa for aluminum, 820 MPa for brass and 653 MPa for structural steel, the corresponding value for CFRP specimen could be seen to be 2300MPa. Since the stress to be reached before the occurrence of a fracture in CFRP was high, the structural stability of the latter could be seen to be excellent in comparison with metals. 3. Based on the result of this study related to the experiment and analysis for 3-point bending specimens, the basis data for fracture behavior due to load applied to CFRP structure could be secured, through which the contribution to safety design is considered possible.
202 / FEBRUARY 2017
INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 18, No. 2
ACKNOWLEDGEMENT This work was supported by the research grant of the Kongju National University in 2016.
REFERENCES 1. Kim, S.-S., Han, M.-S., Cho, J.-U., and Cho, C.-D., “Study on the Fatigue Experiment of TDCB Aluminum Foam Specimen Bonded with Adhesive,” Int. J. Precis. Eng. Manuf., Vol. 14, No. 10, pp. 1791-1795, 2013. 2. Harris, J. A. and Adams, R. A., “Strength Prediction of bonded Single Lap Joints by Non-Linear Finite Element Methods,” International Journal of Adhesion and Adhesives, Vol. 4, No. 2, pp. 65-78, 1984. 3. Michailidis, N., Stergioudi, F., Omar, H., and Tsipas, D. N., “An Image-based Reconstruction of the 3D Geometry of an Al Open-Cell Foam and FEM Modeling of the Material Response,” Mechanics of Materials, Vol. 42, No. 2, pp. 142-147, 2010. 4. Pradeep, K. R., Rao, B. N., Srinivasan, S. M., and Balasubramaniam, K., “Interface Fracture Assessment on Honeycomb Sandwich Composite DCB Specimens,” Engineering Fracture Mechanics, Vol. 93, No., pp. 108-118, 2012. 5. Aymerich, F., Onnis, R., and Priolo, P., “Analysis of the Effect of Stitching on the Fatigue Strength of Single-Lap Composite Joints,” Composites Science and Technology, Vol. 66, No. 2, pp. 166-175, 2006. 6. Ohno, N., Okumura, D., and Niikawa, T., “Long-Wave Buckling of Elastic Square Honeycombs Subject to In-Plane Biaxial Compression,” International Journal of Mechanical Sciences, Vol. 46, No. 11, pp. 1697-1713, 2004. 7. Nguyen, D.-T., Kim, Y.-S., and Jung, D.-W., “Finite Element Method Study to predict Spring-Back in Roll-Bending of Pre-Coated Material and Select Bending Parameters,” Int. J. Precis. Eng. Manuf., Vol. 13, No. 8, pp. 1425-1432, 2012. 8. Lee, J.-M., Lee, K.-H., Kim, B.-M., and Ko, D.-C., “Design of Roof Panel with Required Bending Stiffness Using CFRP Laminates,” Int. J. Precis. Eng. Manuf., Vol. 17, No. 4, pp. 479-485, 2016. 9. Stauder, B. J., Kerber, H., and Schumacher, P., “Foundry Sand Core Property Assessment by 3-Point Bending Test Evaluation,” Journal of Materials Processing Technology, Vol. 237, pp. 126-138, 2016. 10. Goncalves, J. P. M., De Moura, M. F. S. F., and De Castro, P. M. S. T., “A Three-Dimensional Finite Element Model for Stress Analysis of Adhesive Joints,” International Journal of Adhesion and Adhesives, Vol. 22, No. 5, pp. 357-365, 2002. 11. Lee, H. K., Pyo, S. H., and Kim, B. R., “On Joint Strengths, Peel Stresses and Failure Modes in Adhesively Bonded Double-Strap and Supported Single-Lap GFRP Joints,” Composite Structures, Vol. 87, No. 1, pp. 44-54, 2009.
12. Batra, R., and Peng, Z., “Development of Shear Bands in Dynamic Plane Strain Compression of Depleted Uranium and Tungsten Blocks,” International Journal of Impact Engineering, Vol. 16, No. 3, pp. 375-395, 1995. 13. Thipprakmas, S. and Boochakul, U., “Comparison of Spring-Back Characteristics in Symmetrical and Asymmetrical U-Bending Processes,” Int. J. Precis. Eng. Manuf., Vol. 16, No. 7, pp. 14411446, 2015. 14. Cho, J. U., Hong, S. J., Lee, S. K., and Cho, C., “Impact Fracture Behavior at the Material of Aluminum Foam,” Materials Science and Engineering: A, Vol. 539, pp. 250-258, 2012. 15. Zhao, O., Gardner, L., and Young, B., “Buckling of Ferritic Stainless Steel Members Under Combined Axial Compression and Bending,” Journal of Constructional Steel Research, Vol. 117, pp. 35-48, 2016. 16. Kim, S.-S., Han, M.-S., Cho, J.-U., and Cho, C.-D., “Study on the Fatigue Experiment of TDCB Aluminum Foam Specimen Bonded with Adhesive,” Int. J. Precis. Eng. Manuf., Vol. 14, No. 10, pp. 1791-1795, 2013.