MOISTURE EFFECTS ON THE STRAINS DEVELOPED IN GARDEN CHAIR COMPONENTS MADE OF COMPOSITE MATERIALS Camelia CERBU Transilvania University Brașov Rezumat. În acestă lucrare, se analizează efectele umidităţii asupra unor componente fabricate din materiale compozite pe bază de fibre de sticlă şi răşini sintetice din structura unui scaun pentru grădină sau terasă. Unele materiale compozite implicate în studiu, conţin făină de lemn. În acest scop, se vor utiliza datele experimentale obținute prin încercări mecanice, care se referă la efectele umidităţii asupra degradării caracteristicilor mecanice (modulul E, tensiune normală σ e la limita de elasticitate) în cazul a trei materiale compozite: sticlă-e / poliester Heliopol 8431 ATX; sticlă-e / epoxi LY 554; sticlă-e / făină de lemn / epoxi. Se propune un model numeric al structurii de scaun de grădină pentru analiza atât a stăriii de tensiuni cât şi a stării de deformaţie care se dezvoltă în timpul solicitării mecanice. S-a considerat cazul de încărcare în care 75% din greutatea corpului este uniform distribuită pe placa de şezut în timp ce 25% din greutate este distribuită pe spatarul scaunului. Se analizează comparativ rezultatele privind deplasarile, atât în cazul materialelor compozite nedegradate cât şi în cazul materialelor compozite degradate după expunerea de lungă durată în mediu cu umiditate ridicată. Se poate observa că, reducerea modulului de elasticitate E din cauza absorbţiei de umiditate, conduce la creşterea deplasărilor în componentele scaunului fabricate din materiale compozite. În final, articolul recomandă utilizarea materialelor compozite pe bază de răşină poliesterică Heliopol 8431 ATX deoarece caracteristicile mecanice ale unui astfel de material variază puţin după menţinere îndelungată în mediu umed. Cuvinte cheie: material compozit, umiditate, tractiune, incovoiere, simulare. Abstract. In this work, one analysis the effects of the moisture on some components made of composite materials based on glass fibres and synthetic resins, from the structure of a chair for garden or terrace. Some composite materials involved in study contain wood flour. With this purpose in view, it will be used the experimental data previously determinate, that refers to the effects of the moisture on the degradation of the mechanical characteristics (modulus E, normal stress σ e at the elastic limit) in case of three composite materials: E-glass / polyester Heliopol 8431 ATX; E-glass / epoxy LY 554; E-glass / wood flour/ epoxy. One proposes a numerical model of structure of the garden chair for the analysis of the both stress state and strain state that develop during the mechanical loading. It was considered the case of loading for which 75% of the weight of the body is uniformly distributed on the seat plate while 25% of weight is distributed on the seatback of the chair. One comparatively analyses the results concerning the deflections, both in case of undamaged composite materials and damaged composite materials after long-time exposure in environment with high humidity. One may remark that the decreasing of the Young s modulus E due to the moisture absorption, leads to the increasing of the deflections in components of the chair made of composite materials. Finally, the paper recommends the using of the composite materials based on polyester resin Heliopol 8431 ATX because the mechanical characteristics of such as material change a little after long-time soaking in humid environment. Keywords: composite material, humidity, tensile, bending, simulation.
246 Lucrările celei de-a VIII-a ediţii a Conferinţei anuale a ASTR 1. INTRODUCTION Sometimes, the composite materials are designed to the applications were these are mechanically loaded under the actions of some environmental aggressive factors (humidity, thermal cycles, UV radiations and so forth). For example, such an application could be the furniture for garden or terrace. Usually, these part are made of unreinforced plastics that are not resistant for long time to these environmental conditions. Many works [2,3,5-7] published in the last years, showed that the mechanical characteristics of some polimeric composite materials usually decreased due to the water absorption. However, there are thermoset resins recommended for outdoor applications, which reduces this disadvantage. On the other hand, there is an actual tendancy [1,4] for using of the recycled materials (wood, rubber, textiles) and natural fibres to manufacture new composite materials. The recycled materials may be used either as reinforcement material or as filling material. The work focuses on the analysis of the aspects concerning to the effects of the water absorption on the strains developed in a seat-backrest component of a chair that could be used for garden. In addition to two clasical composite materials, another hybride composite material (E-glass / wood flour/ epoxy) is considered to show a way for using of the wood wastes as a source for filling material for the new class of composites. Moreover the wood component provides a pleasant aspect of the product surface, close to the wood color. 2. WORK METHOD To see the effect of the moisture absorption on the deformation of the seat-chair components mechanically loaded, firstly several composite materials reinforced with glass fibers were mechanically tested before and after long-term immersion in water. 2.1. Materials tested Three composite materials were taken into account in this work: E-glass / polyester Heliopol 8431 ATX; E-glass / epoxy LY 554; E-glass / wood flour / epoxy. To manufacture the last composite material the wood flour was used as filling materials. The wood flour is obtained by milling the wood wastes resulted from wood industry. Moreover, the wood flour added improves the aspect of the products made of such composite materials. The first of all, panels made of composite materials are manufactured by using hand-layup technology. The thickness of the panels was equal to 4 mm. Then, specimens are cut from the panels according to the actual european standards for the following mechanical tests: tensile test [9]; flexural test by using the method of the three points [10]. 2.2 Absorption data In order to quantify the effects of water on the mechanical behaviour of the seat-back component of a chair used outdoor in garden or terrace for example, a part of the specimens were immersed in water. These were periodically weighed to record the absorption curve. The specimens were prepared for immersion according to the European standards [11].
Secţiunea Materiale 247 Two composite materials tested (E-glass / polyester Heliopol 8431 ATX, E-glass / epoxy LY 554) reached the saturation point after approximately 7100 hours of immersion in water while the third composite, E-glass / wood flour/ epoxy, reached that point after 6500 hours. The Table 1 shows only the absorption data recorded at the final time of the immersion because the analysis of the absorption curve is not the main objective of this paper. It may remark that the smallest quantity of the water absorbed is recorded in case of E-glass / polyester Heliopol 8431 ATX. The values of the water absorbed until the saturation were very close in case of the other two composite materials even though the E-glass / wood flour/ epoxy composite contains wood flour that is a hydrophilic material. Absorption data recorded in case of the composite materials involved Table 1 Composite material Immersion time (hours) Water absorption (%) E-glass / polyester Heliopol 8431 ATX 7197 0.568 E-glass / epoxy LY 554 7197 2.147 E-glass / wood flour/ epoxy 6692 1.904 Mechanical property Mechanical properties of the tested materials, measured on the two directions of the glass woven fabric used for reinforcement E-glass / polyester Heliopol 8431 ATX Before After immersion immersion E-glass / epoxy LY 554 Before immersion After immersion Table 2 E-glass / wood flour/ epoxy Before After immersion immersion Young s modulus E in tensile test (MPa) 14073 8449 8827 6178 7793 6605 Tensile stress t (MPa) 118 72 103 62 90 72 Poisson s ratio 0.14-0.16-0.21 - Young s modulus E in flexural test - three points 9385 8913 5825 4543 2901 2629 method (MPa) Flexural stress i (MPa) 154 149 128 89 142 95 3. MECHANICAL PROPERTIES OF THE MATERIALS Then, the wet specimens are subjected to the both tensile test and the flexural tests to determine their mechanical properties: tensile stress at maximum force; Young s modulus in tensile test; flexural stress at maximum load; Young s modulus in bending. The results were compared with the ones obtained in case of the dried specimens like it is shown in the Table 2. Table 2 also shows the values of the Poisson s ratios that are required in the second part of this work regarding to the simulation and analysis of the mechanical behaviour of a seat-back component made of such composite materials. It is mentioned that to determine the Poisson s ratios the tensile test was combined with the method of the digital image correlation (so-called DIC method). Digital image correlation method provides an optical solution for measuring of the deformations. To simultaneously record the specific
248 Lucrările celei de-a VIII-a ediţii a Conferinţei anuale a ASTR deformations on the both longitudinal and transverse directions of the tensile specimen, the images of the specimen were acquired by using two cameras controlled by the Aramis system located within the Department of Strength of Materials at the University Polytechnic of Bucharest. Aramis is an optical system produced by GOM (Gesellschaft für Messtechnik Optische). 4. SIMULATION OF THE MECHANICAL BEHAVIOUR OF THE SEAT-BACK COMPONENT Herein, the mechanical behaviour of the seat-backrest component made of the composite materials tested, is analysed. The seat-backrest component is a part of a chair assumed to be used outdoor (garden or terrace) were the effects of the water is inevitable. It may be mentioned that the data shown in the Table 2 are consider of course, the results obtained after accelerated absorption tested (total immersion for long time). The effects of the moisture absorbed during long-time immersion could be equivalent with some years of outdoor using of the product made of such composite materials. Taking into account that the thickness of the laminated composite material of the seat-backrest component is small compared to the other dimensions, this component belongs to the category of the laminated thin shells, it means that component can be modeled as laminated material or as an orthotropic material having small thickness. In that case, it is necessary to specify the mechanical characteristics: the elastic moduli E 1 and E 2 on the directions 1 and 2, respectively; Poisson's ratio in the plane of lamina; transversal modulus G 12 in the lamina plane; interlaminar transversal elastic moduli G 13, G23. The chair is mainly composed of a composite component that is fixed on a steel support by using bolts. a) b) Fig. 1. Numerical model and scheme of loading. The numerical model analysed is shown in the Figure 1,a. A dummy model was used for simulation of the mechanical loading (Fig. 1,b). It was assumed that 75% of the gravity force corresponding to a body having 100 kg, acts on the seat while 25% is applied on the backrest of the composite component. To model the seat-backrest component shell elements having four nodes were used. The pre-loading forces that acts to the level of the bolts, were considered as bold load type.
Secţiunea Materiale 249 Mechanical characteristics shown in Table 2 were used to define on the elastic domain, the orthotropic materials that are equivalent with the structure of the composite materials tested. In order to model the material behaviour on the plastic domain, the coordinates of the points located on curve in plastic, were used in FEA (finite element analysis). 5. RESULTS Herein, the detailed results for numerical modeling will be shown only for the case of the chair whose seat-backrest component is made of glass / wood flour / epoxy composite material. For this case, the figure 2 shows the distribution of the normal stress 1 on the direction of Ox axis. In the following, it is shown only the results of the finite element analysis for the seat-bakrest component made of the hybride composite material (glass / wood flour / epoxy): distribution of the normal stresses 1, 2 on Ox and Oy directions, respectively (Fig. 3); distribution of the normal strains 1, 2, on Ox and Oy directions, respectively (Fig. 4); displacements u3 on direction of Oz axis that is perpendicular on the seat-chair component (Fig. 5,a); displacements u1 on direction of Ox axis. For the seat part the directions Ox, Oy coincide with the reinforcement directions of the composite material. Fig. 2. Distribution of the normal stresses 1. a) b) Fig. 3. Distribution of the normal stresses in component made of glass/wood flour/epoxy composite material: a normal stresses 1 ; b normal stresses 2.
250 Lucrările celei de-a VIII-a ediţii a Conferinţei anuale a ASTR a) b) Fig. 4. Distribution of the normal strains in component made of glass/wood flour/epoxy composite material: a normal strains 1 ; b normal strains 2. a) Fig. 5. Displacements recorded in component made of glass/wood flour/epoxy composite material: a displacements u3 on direction of Oz axis that is perpendicular on the seat-chair component; b displacements u1 on direction of Ox axis. It is well-known that the rigidity of a part is affected by the value of the modulus of elasticity E (Young s modulus) of the material. It follows that the decreasing of the Young s modulus E leads to the increasing of the values of the deflections (displacements). The decreasing of the Young s modulus E is caused by the effects of the moisture absorbed during the immersion in water in case of the composite materials tested (Table 2). Figure 6 shows the maximum value of the deflection u 3 recorded in each case analysed when the seat-backrest component is made of one of the composite materials tested before immersion (undamaged composite material). Moreover, Figure 6 shows the same quantity recorded in case of the damaged composite materials. b)
Secţiunea Materiale 251 Fig. 6. Maximum value of the deflection u3 in case of both undamaged and damaged composite materials. The composite materials involved in this study may be used for other outdoor applications when the values of the displacement are much more important. 6. CONCLUSIONS In case of the undamaged composite materials, it may be remarked that the greatest value of the displacement u 3 was obtained in case of E-glass / wood flour / epoxy composite while the lowest value was recorded in case of E-glass / polyester Heliopol 8431 ATX. On the other hand, the greatest value of u 3 was 0.3256 obtained in case of E-glass / epoxy LY 554 composite material after 7197 hours of immersion in water. This can be attributed to the different types of epoxy resins used in case of the two composite based on epoxy resin. In fact, the epoxy resin used in case of the hybride composite (E-glass / wood flour / epoxy) is recommended for applications in environments with high humidity. The values of the normal stresses (Fig. 3) do not exceed the values recorded at rupture in case of the specimens made of composite materials before or after immersion in water. Acknowledgement The author wishes to thank to the colleagues from the Department of Strength of Materials from the University Polytechnic of Bucharest, especially to prof. Dan Mihai Constantinescu, for support in measuring of the strains by using Aramis 2M measurements with the DIC method. References [1] AdhikarY, K. B., Pang, S., Staiger, M. P. (2007). Long-term moisture absorption and thickness swelling behaviour of recycled thermoplastics reinforced with Pinus radiata sawdust. In: Chemical Engineering Journal, doi: 10.1016 /j.cej.2007.11.024. [2] Cerbu, C. (2006). Materialele compozite şi mediul agresiv. Aplicaţii speciale. Editura Universității Transilvania, Brașov.
252 Lucrările celei de-a VIII-a ediţii a Conferinţei anuale a ASTR [3] Cerbu, C. (2007). Aspects concerning the degradation of the elastic and mechanical characteristics of glass / polymer composite materials due to the humidity absorption, In: Revista Materiale Plastice, 44 (2): 97-102. [4] Cerbu, C., Ciofoaia, V., Curtu, I., Vișan, A. (2009). The effects of the immersion time on the mechanical behaviour in case of the composite materials reinforced with E-glass woven fabrics. In: Revista Materiale Plastice 46 (2): 201-205. [5] Corum, J.M., Battiste, R.L., Ruggles, M.B., Ren, W. (2001). Durability bassed design criteria for a chopped-glass-fiber automotive structural composite, In: Composite Science and Technology, 61: 1083. [6] Pomies, F., Carlson, L.A., Gillespie, J.W. (1995). Marine environmental effects on polymer matrix composites, In: Composite Materials - Fatique and Fracture, 5: 283. [7] Springer G. S. (1984). Environmental Effects on Composite Materials, Vol. 2, Technomic Publishing Inc., Lancaster, PA. [8] *** Tutoriale soft Abaqus. [9] *** EN ISO 527-2: 2000, Determination of the tensile properties of the plastics - Part 2: Test conditions for moulding and extrusion plastics, European Committee for Standardization, Bruxelles, 2000; [10] *** EN ISO 178 (2001). Plastics. Determination of flexural properties, European Committee for Standardization, Brussels. [11] *** EN ISO 62 (2008). Plastics. Determination of water absorption, European Committee for Standardization, Brussels.