Mechanical behaviors of molded pulp material

Transcription

1 Mechanical behaviors of molded pulp material Hongwei Ji *a, Huaiwen Wang a, Jinlong Chen b a School of Mechanical Engineering, Tianjin University of Commerce, Tianjin 0014, PR China b Department of Mechanics, Tianjin University, Tianjin 00072, PR China ABSTRACT Many mechanical phenomena of interest for web-like materials, such as molded pulp, take place at the micro-scale. A SEM (scanning electron microscope) with SHIMADZU electrohydraulic servo experimental system was employed to study the micro-scale mechanical behavior of molded pulp materials. Uniaxial tension tests of molded pulp specimens were carried out, resulting in the stress-strain curves. Experimental results indicated that the material is not only elasticplastic, but also emplastic. The surface morphology evolution of the tensile specimen was visually monitored during the process of loading, and some SEM micrographs were captured under different load levels. Full-field deformations over an area of μm 2 were obtained using the digital image correlation method. The higher strains occurred at the fibre fines zone or around voids whereas the lower strains were obviously found at long fibres, demonstrating that the strain distribution is obviously uneven. The reason may be due to the random orientation and the fraction of the fibres, and the presence of impurities and voids as well. Keywords: molded pulp, tension experiment, micro-scale mechanical behavior 1. INTRODUCTION The greater increase of the demand for environmental friendly materials in packaging industry leads to the larger interest in the reusable and recyclable materials. Molded pulp, which is made of waste newspaper or magazines, is recognized as a kind of recycled, environmentally friendly green packaging materials for its potential to contribute to the conservation of natural resources. It is regarded as a substitute for polymeric packaging materials such as EPS (Expandable Polystyrene) and has been widely used to give-in transit protection to electronic equipments, farm products and household electrical appliances, etc 1-5. In response to the rapid development of molded pulp packaging products, research on mechanical behaviors, shock absorbing properties and cushioning mechanism of the molded pulp has drew attention in the field of mechanics and materials, which has led to publications with new information on molded pulp. For example, Noguchi et al developed a shredded pulp-plastic microsphere composite with modified starch binder. The mechanical properties of the material, including compressive stress-strain properties and shock absorbing properties, were investigated. Hoffmann experimentally investigated the relationship between the geometry of a molded pulp packaging and the resulting static and dynamic strength. The effect of the shape of the molded pulp sample on mechanical characteristics was also examined 7. Ma et al proposed a structural factor approach which allows a modular design method to be used for molded pulp packaging design 8. Eagleton et al studied the cushioning properties of molded pulp material. They constructed the cushioning curves for a molded pulp and compared them with those of EPS 9. Ji et al investigated the mechanical behaviors of molded pulp material by use of a photomechanical method 10. Gurav SP et al investigated the mechanical properties of paper-pulp packaging. The tensile and compressive strength of paper-pulp packaging were tested, and FEM was used to analyze the effect of change in geometry of rib on its mechanical behavior 11. Sorensen G and Hoffmann J investigated the stacking strength of moulded-fibre trays. Both compressional creep and static compression strength were determined at constant and varying humidity conditions 12. However, as to micro-scale mechanical behavior of molded pulp material, little work has been reported in the literature. In this study, a uniaxial tension experiment and SEM observation of molded pulp material were carried out first. In order to investigate the micro-scale mechanical behavior of molded pulp, the digital image correlation method (DICM) is employed to analyze the deformation in micro-zone of the specimen. * jhwei@tjcu.edu.cn; phone ; fax ICEM 2008: International Conference on Experimental Mechanics 2008, edited by Xiaoyuan He, Huimin Xie, Yilan Kang, Proc. of SPIE Vol. 775, 775D 2009 SPIE CCC code: X/09/$18 doi: / Proc. of SPIE Vol D-1

2 2. EXPERIMENTAL DEVICE AND SPECIMEN Uniaxial tension experiment was performed with molded pulp material. The purpose of tensile test was to obtain the stress strain curves and mechanical properties for the material. JSM-5410LV scanning electron microscope with SHIMADZU electrohydraulic servo experimental system is used for the testing purpose as shown in Figure 1. (a) SEM (b) Loading instrument for SEM Fig. 1. JSM-5410LV SEM and loading instrument The specimen was cut out of a particular molded pulp product. The specimen dimensions and the clamps are shown in Figure 2. The density and thickness of the specimen were 55.2kg/m and 1.mm, respectively. The atmospheric conditions for all tests were, temperature: 22±1 oc, relative humidity: 80±%. R y x 5 Molded pulp 7. The clamps 55 Fig. 2. Specimen and clamps for tensile test. EXPERIMENT AND RESULTS JSM-5410LV scanning electron microscope and the digital image correlation method (DICM) were used to measure strain distributions on the surface of molded pulp specimen. A SHIMADZU electrohydraulic servo experimental system was used to load. The tensile test was set as the elongation rate of 0.mm/min. The stress-strain curve is given in Figure. The surface morphology evolution of the tensile specimen was visually monitored during the process of loading. In order to capture the SEM images of the specimen under different load levels, the stroke was stopped when capturing images. We indicated i (i=1,,7) in Figure where some SEM images were captured. The first four of these SEM images are given in Figure 4. Proc. of SPIE Vol D-2

3 Stress σx / MPa Experimental Results Strain εx / % Fig.. Stress-strain curve of the material under static tension It can be found from Figure that: 1) the material is an elastic-plastic material. At the beginning of the curve, the stressstrain relation is linear. With the increase of stress, plasticity appears; 2) the stress decreases when the stroke was stopped to capture the SEM image. This is so called stress relaxation behavior, which indicates that the material is an emplastic material as well. Image 1 Image2 Image Image 4 Fig. 4. The SEM images of the material surface under static tension In order to investigate the mechanical behavior of molded pulp, DICM is employed to analyze the deformation in microzone of the specimen. The method involves recording, digitizing and processing a pair of images of an object under different deformation states to determine in-plane displacement and in-plane strain. As shown in Figure 4, first, the first image (image 1) is selected as the sample image and other images respectively as the target image. Then, the deformation components are calculated between the target image and the sample image by DICM. In this test, the strain components in micro-zone of the specimen (Figure 5a) were analyzed corresponding to the deformation between the image 4 and the image 1, and the change of load is 8N. Figure shows the distributions of the normal strain components εx at line 1, line 2 and line (Line 1, line 2 and line shown in Figure 5b). Line 1 Line 2 Strain analysis micro-zone 100µ a The location of strain analysis zone Line y x µm b Zoom picture of analysis zone Fig. 5. The location of strain analysis zone in the SEM image Proc. of SPIE Vol D-

4 Strain ε xx 0.0 Strain ε x 0.0 Strain ε x x / Pixels x / Pixels x / Pixels (a) The strain components on line 1 (b) The strain components on line 2 (c) The strain components on line Fig.. The distributions of the normal strain components in micro-zone It can be found from Figure that the strain distributions at the test lines ( line 1, line 2 and line shown in Figure 5 ) show obviously uneven characteristic. The higher strains occurred at the fibre fines zone or around voids whereas the lower strains were obviously found at long fibres. The mean value of the strain components ε x for the test line 1 is 8480, and for the test line 2, and for the test line. The reason may be due to the random orientation and the fraction of the fibres, and the presence of impurities and voids as well (shown in Figure 7). Fig. 7. The impurities and voids of material 4. CONCLUSIONS The uniaxial tension experiments of molded pulp materials were carried out. The experiment results indicate that the material is not only an elastic-plastic, but also emplastic. Many mechanical phenomena of interest for web-like materials, such as molded pulp, take place at the micro-scale. A SEM (scanning electron microscope) is therefore essential. A SEM provides a convenient way to arrange both the imaging and illumination of the small field-of-view. A SEM with SHIMADZU electrohydraulic servo experimental system can be employed to study the micro-scale mechanical behaviors of molded pulp material. The surface morphology evolution of the tensile specimen can be visually monitored during the process of loading, and some SEM micrographs were captured under different load levels. Full-field deformations over an area of μm 2 are obtained using the digital image correlation method. The strain distributions in micro-zone show obviously uneven characteristic due to the random orientation and the fraction of the fibres, and the presence of impurities and voids as well. Proc. of SPIE Vol D-4

5 ACKNOWLEDGEMENTS This work was supported by Tianjin Science and Technology Development Project (Grant No.: 0YFGPSH04100) and Tianjin Natural Science Foundation (Grant No.: 08JCZDJC1100). The support is gratefully acknowledged. Special thanks should go to Beijing Key Laboratory of Fracture and Damage Mechanics of Rocks and Concrete at China University of Mining and Technology for the experiments. REFERENCES [1] [2] [] [4] [5] [] [7] [8] [9] [10] [11] [12] Cao, G. and Zhang, Y., "Progress in the development of moulded pulp products," Packaging Engineering 27 (1), 21-2 (200). Gong, G.F., "Application in transport package of molded pulp products," China Packaging Industry (), 52-5 (2005). Zhang, X.C., Liang, J., Zhou, F.G. and Sun, B.Q., "The present situation and development on the pulp modeled for industrial packaging in China," Packaging Engineering 24 (1), 4-8 (200). Pan, M.J., Chen, Y.M., Chen, G.L. and Zhang, X.C., "Research and experimentation on the molded pulp package of HP Pavilion PC mainframe," Packaging Engineering 27 (4), 8-40 (200). Yang, B., Han, J. and Zhang, X., "Structure design and cushioning performance of molded pulp cushion," Packaging Engineering 29 (2), (2008). Noguchi, T., Miyashita, M., Seto, J., Tan, M. and Kawano, M., "Development of molded pulp materials for the packaging of electronic equipment," Packaging Technology and Science 10(), (1997). Hoffmann, J., "Compression and cushioning characteristics of molded pulp packaging," Packaging Technology and Science 1(5), (2000). Ma, X., Soh, A.K. and Wang, B., "A Design database for molded pulp packaging structure," Packaging Technology and Science 17(4), (2004). Eagleton, D.G. and Marcondes, J., "Cushioning properties of molded pulp," Packaging Technology and Science 7(2), 5-72 (1994). Ji, H.W., Yu, B.N., Ping, Y.M., Shao, W.Q. and Teng, L.J., "Investigation of the mechanical properties of molded pulp with DICM," Chinese Packaging Engineering 25(4), (2004). Gurav, S.P., Bereznitski, A., Heidweiller, A. and Kandachar, P.V., "Mechanical properties of paper-pulp packaging," Composites Science and technology, (200). Sorensen, G. and Hoffmann, J., "Moisture-induced effects on stacking strength of moulded-fire packaging in varying environmental conditions," Packaging Technology and Science 1, (2004). Proc. of SPIE Vol D-5