Revista ingeniería de construcción
versión On-line ISSN 0718-5073
Rev. ing. constr. v.24 n.2 Santiago ago. 2009
Revista Ingeniería de Construcción Vol. 24 N°2, Agosto de 2009 www.ing.puc.cl/ric PAG. 153- 166
Determination of mechanical properties of fibers of Bambusa Vulgaris to use in bamboo mat board
Lena Mora Rodríguez*¹, Emilio Álvarez Garcia*, Jorge F. Hernández González*
* Universidad Central de Las Villas, Villa Clara, CUBA
At the present time a world tendency exists to the use of the natural fibers as reinforcement material in composite materials. In such a sense presently work you to carry out an analogical and experimental analysis to determine the behaviour of the mechanical properties of the bamboo fibers and in specific the resistance to the rupture, the module of Elasticity and the poisson coefficient. Moreover, it is evaluate the influence of the temperature, the pressure and the time on the percent of humidity lost and about the thickness of the bamboo mat board. Finally one has that for their properties and characteristic the bamboo fiber is a natural fiber that may be an employee as reinforcement in Composite materials.
Keywords: Bamboo mat boards, bamboo fibers, mechanical properties
In the last years, the use of composite materials in the design of machine parts increased considerably. This tendency is attributed to the possibility of designing materials with certain special properties and obtaining, therefore, mechanical qualities that are superior to those of traditional materials. Composite materials present a favorable resistance-to-weight ratio and stiffness-to-weight ratio and, accordingly, they are especially adequate for structures in which weight is an essential variable of the design process Dingle (1974). The structural components that require great stiffness, impact resistance, complex shapes and a large production volume are optimum for manufacturing with composite materials. Therefore, its use in the manufacturing of parts for the aeronautical, aerospatial, naval and automotive industries has greatly increased in the past years.
Composite materials can be made of any combination of two or more materials, either metallic or organic or ceramic. Nowadays, there is a world tendency to use natural fibers as reinforcement of composite materials Van Voorn et al. (2001). Natural fibers are fibrous materials that can be obtained from nature, and they are mainly constituted by cellulose and lignin, and other components in smaller quantities Ishida and Koening (1980) and Alcides (1998). Among those mentioned in the literature concerning these applications, it is possible to find: jute, flax, cotton, hemp, coconut tree, etc. The use of bamboo fiber as reinforcement of these types of materials is not reported, even less in the form of weave, commonly known as bamboo mat boards. Its main applications are found in decorative purposes, in the manufacture of floors and other housing applications.
This paper presents an experimental study for the determination of the mechanical properties of bamboo fibers; samples were taken from the cortex, and the inner fiber, in all cases in sections with knots and without knots, and they were also subjected to a preservation treatment with a solution of tabaquina and to no treatment at all. For the mechanical characterization of the fibers, the following properties were taken into account: rupture strength, Poisson ratio and elastic modulus Janssen (1991) and Valero et al. (2005). In the second stage of research, and starting from an experimental design, the influence of temperature, time, and pressure on the moisture loss of the fiber and on the thickness of the mat board woven with the same fiber is studied. This allows valuing and justifying the proposed weave by evaluating the uniformity and the physical and geometric homogeneity of the interweaving of these fibers in the weave.
2. Characteristic of the trial to determine the mechanical properties
The definition of the mechanical properties of the materials plays an important role in the determination of their ability to resist the external loads to which the materials are subjected to. Their determination is particularly significant for the design of the elements, because the determination of the geometry, the shapes and the dimensions, and the election of the material itself, is closely related to the mechanical properties Nielsen and Landel (1994) and Lee and Joun (1999).
Tensile strength is the resistance that a material offers when beying stretched. This test conforms the ASTM D-638 Standard and the specimens shall be Type II, due to the specimens thickness (between 1.6 and 3.5mm) Figure 1. Before testing starts, these specimens remain 24 hours under the conditions established by the temperature (23±2)°C and moisture (5Q±5%) standards.
The tests were perfirmed in a SZ-500-1 machine (Figure 2) under monoaxial tension in order to measure and register the stress and strain values for the subsequent determination of mechanical properties, such as rupture strength, elastic modulus and Poisson ratio. The loading speed is 2mm/min which is verified with the indicators and through the prevailing means for an increasing strength according to ISO 7500-1: 2004, using a testing equipment calibrated in accordance with ISO 376: 1999.
Figure 1. Specimens for tensile tests
Figure 2. Tensile test machine
3. Results of the mechanical characterization of the bambusa vulgaris fiber
The objective of the first stage of research is to find out if there are significant differences concerning the behavior of the properties of bamboo as a material, if you take samples of the different parts of the plant. In each case specimens from the cortex and the inner fiber were prepared. In each one, samples were taken from the section with and without knots. Samples with and without treatment were taken from the section of the inner fiber.
The research is planned considering the bamboo fiber with the features described earlier; for a better understanding they can be summed up generically as follows:
BFw/oKw/oCw/oT: Bamboo Fiber without Knot, without Cortex, without Treatment
BFw/oKw/oCwT: Bamboo Fiber without Knot, without Cortex, with Treatment
BFwKw/oCw/oT: Bamboo Fiber with Knot, without Cortex, without Treatment
BFwKw/oCwT: Bamboo Fiber with Knot, without Cortex, with Treatment
BFw/oKwCw/oT: Bamboo Fiber without Knot, with Cortex, without Treatment
BFwKwCw/oT: Bamboo Fiber with Knot, with Cortex, without Treatment
Table 1 summarizes the study of statistical characterization of the response variables for the specimen group.
Table 1. Results of experiments for the studied properties
Figure 3. Behavior of the rupture strength
As it can be observed, the highest values of the rupture strength are reached in the case of the fiber not subjected to treatment and in the section without knots. A similar behavior is observed for the case of the upper cortex of the bamboo cane (outer surface), that is, the section without knot presents a higher strength than the section with knots. The highest value is obtained by the fiber without knot and without treatment. The fiber with knot and with treatment shows the lowest value.
Figure 4. Behavior of the poisson ratio
Here, the variations of the Poisson ratio among the samples being studied are not significant. They range from 0.37-0.46, which coincides with the data reported on the specialized literature that establishes this ratio in different types of woods between 0.41-0.58.
Figure 5. Behavior of the elastic modulus
At present, the Elastic Modulus is the rupture strength of a material and its unitary longitudinal deformations under the effect of tension. Therefore, it has a similar rupture strength behavior in relation to the characteristics of the bamboo fibers concerned. Note that the fiber without knot and without treatment obtained the highest values; the presence of knots in the specimens section and the fiber treatment with tabaquina dissolved in water at boiling temperature, cause a reduction in the properties of the elastic modulus.
4. Study of the Behavior of Moisture Loss and Thickness Reduction of the Bamboo Mat Boards
At this stage, a study is undertaken to evaluate the moisture loss and thickness reduction of the bamboo mat boards, in order to consider the weave compactness, and to assess the influence and its response to technological parameters that determine the manufacturing process of the mat board, such as pressure, temperature and pressing time.
The tested weave coincides with the one used at BMB®, IPIRTI (2009), and by Mora et al. (2008). Unlike articles mentioned earlier, mat boards for this study are woven, pressed and exposed to technological parameters variations without employing adhesives. The idea is to consider only these effects on bamboo, using woven mat boards, with dimensions of 200 x 200 x thickness (mm), put into the hot-plate pressing machine (Figure 6).
Figure 6. Woven mat board and hot-plate press
Considering the previous requirements, we used a Factorial and Response Surface Design type N = 23. The studied variables were: X1-Temperature, X2-Pressure and X3-Time. Table 2 shows the experiment matrix for this case.
Table 2. Experiment matrix for design N = 2K
Processing of the experimental results is done by means of the statistical package STATGRAPHICS PLUS Version 5.1, 2006.
Table 3 shows the results synthetically.
Table 3. Experimental results for moisture loss rate and fiber thickness
Figure 7. Pareto analysis for moisture
The Pareto diagram (Figure 7) shows that time as much as pressure and temperature have an influence upon the moisture loss rate of the sample. Temperature exhibits the greatest influence, showing that while it increases, the moisture loss rate is greater. The influence of time follows, followed by temperature-time interaction. According to the results analysis, the greatest moisture loss is attained by conditions of maximum temperature, maximum pressure and maximum time.
Through the analysis of the results shown in Figure 8, both temperature and time cause an increase in the moisture loss rate; temperature bears the most significant effect upon it. For any time value, a temperature increase brings along a sudden increase in the moisture loss.
Figure 8. Moisture rate in relation to temperature, pressure, and time
The iso-line graph (Figure 9) shows a nomogram that is highly effective to influence in the best manner, with efficient and/or economical criteria, on the moisture loss process.
Figure 9. Iso-line graph for the moisture rate in relation to temperature and time with constant pressure
This graph allows selecting the different combinations of temperature and time that are necessary to achieve the highest moisture loss rate. In terms of economy, the tendency shall be to decrease as much as possible the time factor, because during the fiber treatment process, the greatest influence on the expenses of energetic carriers will be conditioned by the sample exposure time in the press.
As shown in Figure 10, temperature as much as pressure and time tend to increase the moisture loss rate, being temperature the greatest influence. Note that, moisture loss rate varies from 1.1 to 3.4, from 1.6 to 2.5, and from 1.6 to 2.8, for temperature, pressure, and time, respectively.
Figure 10. Effects of temperature, pressure and time on moisture loss
Another objective of the study was to find out how temperature, pressure and time influence on the thickness variation of the bamboo mat boards.
Time, as much as pressure, influence significantly and independently on thickness (Figure 10). Evidence is shown that what influences most on thickness reduction is the interaction of pressure and temperature. The former coincides with the information available in the literature which sets forth that an increase in temperature brings along a rupture of the chemical structure of bamboo fibers. Temperature by itself does not have a significant influence on the thickness variation.
Figure 11. Pareto analysis for sample thickness
From Figure 11 it can be concluded that with temperature increase there is little variation of the bamboo mat boards thickness, and such tendency determines a slight increase due to expansion. The effect of time is similar to that of temperature, but with a greater intensity; note that the slope is significantly higher. Pressure exhibits a completely contrary effect, because when you increase its value, thickness decreases significantly.
Figure 12. Effects of temperature, pressure, and time on thickness
From a practical point of view, this is very interesting, as it allows to define minimum pressure values to achieve uniformity and homogeneity in the structure of bamboo mat board and, accordingly, of mat boards from different thickness.
Figure 12 ilustrates the effect of pressure and temperature on thickness. It should be kept in mind that, in terms of technology and application of the bamboo fiber, two fundamental aspects are to be considered: thickness reduction and uniformity of the fiber. Note that for the lowest temperature value (80°C), a pressure increase results in a thickness increase. While increasing the temperature, and hence the pressure, the effect changes and a gradual thickness reduction starts to take place. In the same way, you may observe that a persistent thickness reduction is achieved with temperatures greater than 120°C. This behavior seems logical, because at this temperature an almost complete loss of moisture from the fiber, has been obtained.
Figure 13. Effect of pressure and temperature on mat board thickness
Figure 14. Iso-line graph for thickness in relation to temperature and time
The iso-line diagram for thickness (Figure 14) allows selecting the different temperature and pressure combinations in relation to the thickness required for a specific application. For example, if a minimum thickness of 2.3 mm (thick blue line) is needed, this can only be achieved if temperature is varied in an interval of 113 to 120°C and with pressures between 75 and 100 atmospheres.
1. From the experimental determination of the behavior of the mechanical properties of Bambusa Vulgaris fibers, according to the findings of this study, it is concluded that rupture strength varies in the interval of 80.6 to 221.3 MPa. The elastic modulus varies from 13 to 29 GPa, and Poisson ratio from 0.37 to 0.46. These values agree with those reported in the specialized bibliography and in works done by other authors for other types of bamboo.
2. Temperature, pressure, and time have a pronounced effect on the moisture loss rate, being temperature the one that shows the greatest positive influence on the moisture loss rate.
3. Pressure and temperature have a direct influence on the thickness of the bamboo mat board. In relation to this parameter, the greatest influence is shown by the pressure-temperature interaction.
4. It is demonstrated that it is possible to achieve uniformity and homogeneity in the thickness of the mat board woven with fibers, if you start by evaluating the influence of technological parameters such as pressure and temperature.
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