Behavior characteristics of a dual material wood composite in wet environment

THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 6(79) 2014, VOL 1 31 BEHAVIOR CHARACTERISTICS OF A DUAL MATERIAL WOOD COMPOSITE IN WET ENVIRONMENT ĐẶC TÍNH ỨNG XỬ CỦA VẬT LIỆU KÉP GỖ C[.]

THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 6(79).2014, VOL 31 BEHAVIOR CHARACTERISTICS OF A DUAL MATERIAL WOOD COMPOSITE IN WET ENVIRONMENT ĐẶC TÍNH ỨNG XỬ CỦA VẬT LIỆU KÉP GỖ - COMPOSITE TRONG MÔI TRƯỜNG ẨM ƯỚT Tran Van Luan1, Nguyen Van Dong1, Le Minh Tien2 The University of Danang, University of Science and Technology; Email: tranvluan@gmail.com The University of Danang Abstract - The objective of the aircraft and naval industries is the optimization of structures in terms of lightening and strength Thus, dual material plates, consisting of a light core covered by face sheets, are frequently used instead of solid plates because of their high bending stiffness-to-weight ratio However, the mechanical properties of these materials can be significantly reduced by presence of moisture inside them when they are exposed to a wet environment during long time service In order to analyse the internal stresses induced due to moisture absorption in material, moisture absorption characteristic was investigated for specimens of a dual material wood-composite composed of E-glass/polyester face sheets bonded to a wood balsa These specimens were immersed in distilled water at 40°C, and the weight gain was measured The hygroscopic strains were measured for balsa wood specimens A hygro-mechanical finite element model has been built and is used to simulate the moisture diffusion-induced internal stresses The obtained results allow predict a multi-layer internal appearance stresses to determine the geometrical stability of the structure Tóm tắt - Mục tiêu ngành cơng nghiệp hàng không hàng hải tối ưu sức bền, trọng lượng cấu trúc Do đó, vật liệu ghép cấu tạo từ lõi nhẹ mặt compsite bền, cứng sử dụng để thay thép nặng Tuy nhiên, sức bền vật liệu giảm ảnh hưởng mơi trường ẩm ướt Để phân tích nội ứng suất thấm nước vật liệu, đặc tính thấm nước khảo sát cho mẫu vật liệu kép gỗ balsa – composite cấu tạo từ sợi thủy tinh nhựa polyester Những mẫu nhúng nước tinh khiết 400C, sau tiến hành cân đo để xác định khối lượng Ngoài ra, biến dạng gỗ thấm nước đo đạc cho mẫu gỗ balsa đơn Mơ hình phần tử hữu hạn xây dựng để mô phát triển ứng suất bên vật liệu kép gỗ balsa – composite Từ quy luật phát triển ứng suất, có giải phái cải tiến sử dụng vật liệu hợp lý Key words - moisture diffusion, water content, wood-composite, internal stresses, naval structure Từ khóa - khuếch tán, biến dạng nhiệt, gỗ-composite, nội ứng suất, kết cấu tàu thủy Introduction One of the main objectives of the aircraft and naval industries is the optimization of structures in terms of lightening and strength This may be achieved by substituting metallic materials commonly used with lighter and cheaper materials such as composites In particular, the use of dual material wood-composite with organic matrix provides excellent ratio strength/mass Nowadays, balsa wood are surrounded by composite skins made in polyester resin reinforced with fibres, have a growing expansion The balsa material is a hardwood used mostly in the form of large building plate formed of smaller plates glued together by resin Many studies have already been conducted to investigate and predict the aging of wood-composite subjected to a wet environment, deformed by a mechanical action or thermally degraded Knowledge of variations of the physical, chemical and mechanical properties obtained in these later studies [1, 2, 3, 4, 5] allowed significant optimization of wood manufacturing processes At present, researches are directed towards a better understanding of the moisture diffusion-induced internal stresses of a dual material wood-composite Experimental Section The studied wood-composite material specimens came from commercially plates conventionally used in naval ships for making walls or as reinforced elements Figure shows details of the sandwich glass / polyester / balsa system material The received plates had initially a dimension of about 800.0 x 800.0 x 20.1 mm from which smaller samples only made with the wood-composite plate were cut Typically, a plate is made of woven E-glass fibers / polyester resin skin bonded, by diffusion process, to a low density balsa wood core The average density of the balsa wood was 126 kg.m-3 with a standard deviation of 30 kg.m3 The balsa core, composed with balsa wood pieces arranged in the form of about 150.0 x 150.0 x 20.1 mm blocks glued to each other, is impregnated with resin The direction of wood fibers is perpendicular to the skins The latter consists of M450/Soric2/Qx868 woven made 3-plies layer surrounding the balsa wood Samples were separate in two distinct sets Set is composed of balsa specimen without any aging process thus the moisture content of these samples was around 5% depending on the laboratory atmospheric conditions Set is constituted of balsa specimen immersed in distilled water maintained at 40°C; continuous monitoring of the moisture uptake was performed over several months until the full water saturation of the specimens To determine the swelling effect of the balsa wood, we used the cubic specimens of the balsa wood to measure the water uptake described above It should be mentioned here the procedure of the swelling measurement The apparatus for measuring swelling is conceptualized, where each cubic specimen of the balsa wood during measuring was placed at the corner of the rigid support of aluminium Aluminium screws of mm in diameter were also used to make point 32 Tran Van Luan, Nguyen Van Dong, Le Minh Tien contacts with the cubic specimen The basic face of the cubic specimen contacted with three points, the face contacted with two points, the face contacted one point We formed these tip point contacts to make sure that the cubic specimen was placed in the same position for each time of measurement Figure The studied glass / polyester / balsa system material used in marine industry The studied materials consist of cm3 cubes of balsa wood infused by resin Dry materials (a) and saturated moisturized materials (b) Theoretical Section 3.1 Fickian Diffusion Authors [7, 8] have reported that the Fickian diffusion model can describe the behavior of moisture absorption in composite sandwich structure, both composite and wood This model represents a simple approach for moisture diffusion It is based on the assumption that there is proportionality between mass diffusion through a surface and its concentration gradient perpendicular to this surface In the case of a thick plate of finite dimensions, the diffusion process through the edges can’t be neglected The 3-D Fick’s law is written as: c  2c  2c  2c = D1 + D 2 + D3 t x y z Where, D1, D2, D3 are respectively the diffusive coefficients governing the speed of moisture absorption through the material in the directions x, y and z, (mm 2/s), c is the moisture concentration (g/mm3), function of the variables of space x, y, z and of the time t Equation (2) is generally solved by supposing that the plate is exposed to a wet environment at six sides In the initial state, the concentration in the plate is constant The mathematical expression obtained for the moisture uptake can be expressed as [3] At the initial time, a stress-free state is assumed The internal stresses induced due to moisture absorption in wood-composite plate composed of E-glass/polyester face sheets bonded to a balsa are computed within the framework of hygro-elasticity We consider an orthotropic hygro-elastic constitutive law The constitutive laws of a hygro-elastic orthotropic material are given as follows: σ = L : ε − β(Δc ) Where, L and β are respectively the tensors of stiffness and hygroscopic expansion coefficients c is the moisture concentration L and β are assumed to be constants A hygro-mechanical finite element model has been built with COMSOL Multiphysics 3.5a ® software to obtain the macroscopic stress states in the wood-composite plate 3.2 Homogeneous Model for Effective Moisture Diffusivity Wood-Composite structures absorb moisture during their life through the composite, and also through the balsa when they are permeable Due to important difference of moisture absorption in the face sheets and in the balsa core, it would be inappropriate to approach the diffusivity in composites as directly analogized to thermal/electrical conductivity Thus, the effective diffusivity model relating to both the rate of water diffusion and equilibrium moisture uptake was considered here This model, called homogeneous model is based on the equivalent basic cell presented on the figure Where, Vc is the volume of composite skin, Vb is the volume of balsa, zc, zb are respectively positions of the interfaces composite/balsa and balsa/composite in the thickness direction z, e c, eb are respectively the thicknesses of the composite skin and the balsa wood This modeling considers a volume made up two composite skins surrounding a balsa wood The effective diffusivity is calculated by considering the equations of continuity of the moisture flux and discontinuity of the moisture contents that occur at the boundary between the composite skins and the balsa on the equivalent basic cell Here, applying a continuous moisture flux G on the equivalent basic cell (EBC) enables to obtain the equivalent diffusing behavior ec eb ec Vc Vb Vc zc G zb Figure Equivalent basic cell Deq z = α(1 − v b )Dc + v b Db α(1 − v b ) + v b Where,vc and vb are respectively the volume fractions of composite and balsa Db, Dc, D eq z are respectively the diffusion coefficients of balsa, the composite skin and the homogeneous medium α is a constant standing for the moisture content gap between balsa core and composite skin Results and Discussion 4.1 Moisture Content Figure shows the measured moisture content and the fitted curves as a function of exposure time for the different THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 6(79).2014, VOL specimens Experimental results confirm the Fickian properties of the material beforehand assumed for indentifying The diffusion parameters obtained show in the table 33 indicates the anisotropic hygroscopic character of balsa core Furthermore, swelling characteristic curves for balsa core showed also that the balsa core quickly expanded corresponding to the moisture content in the balsa around 50 % This can be explained that the moisture is first absorbed by diffusion to the boundary of specimen and begins to absorb to the volume of specimen Thus, the moisture absorption period to the boundary induced an important strain This is called free expansion state, (figure 5) To obtain swelling characteristic of balsa core, we used the slope of the linear ranges of the moistureinduced strain curves This linear range corresponds to an actual final state of balsa Figure Moisture diffusion kinetics at 40°C in a resin infused balsa wood cube The measured experimental data (●) and the analytical Fickian 3D solution (▬) are plotted Table Diffusion parameters identified according to the Fick’s model Test specimens Wood-composite plate Balsa wood (single) Polyester resin Moisture content (%) M∞ 80.02 700.03 0.78 a) Diffusion coefficients (mm2/s) D = D2 D3 2.85 x 10-6 7.19 x 10-6 8.98 x 10-6 2.11 x 10-5 2.50 x 10-6 2.50 x 10-6 The table shows the diffusion coefficients and the maximum moisture contents identified on the basis of experimental data The diffusion coefficients and the maximum moisture contents of balsa are much higher than those of the polyester resin or the wood-composite structure The analytical 3D solution of the Fick’s model makes be possible to underline the anisotropic diffusive character for balsa wood or the wood-composite Let us focus on the 3D diffusion parameters of core balsa specimens, such as the balsa cube being analyzed here For the balsa cube specimens, the diffusion coefficient along the direction of the balsa fibers D is three times larger than the perpendicular diffusion coefficients of the D fibers This phenomenon can be explained by the theory of a capillary path of water molecules during diffusion process, where the balsa fibers are assumed to act as the capillary paths to the penetrating water molecules Thus, in the case of balsa cube specimens, the diffusion coefficient along direction of the balsa fibers D is greater than the perpendicular diffusion coefficients to fibers D1 4.2 Moisture Expansion The moisture expansion and the hygro-elastic coefficient of balsa core in the transverse direction are much higher than those in the longitudinal direction This b) Figure Swelling characteristics for balsa core, a transverse hygroscopic expansion, b longitudinal hygroscopic expansion (β1 - transverse moisture coefficient, β3 - longitudinal moisture coefficient) Table Hygroscopic expansion coefficients Transverse moisture coefficient Longitudinal moisture coefficient β1 = 0.002(1) β3 = 0.001(1) Figure Definition of the different hygroscopic strains of balsa, (εi – free expansion, εf – final expansion) 34 Tran Van Luan, Nguyen Van Dong, Le Minh Tien 4.3 Macroscopic Stresses a) b) experimental technique for measuring hygroscopic expansion has been developed and tested for the balsa wood New models are proposed for calculating the effective diffusivities of wood-composite Analytical solutions and numerical solutions based on Fick’s diffusivity process have been studied to identify the diffusion parameters and to simulate the distribution of water content-induced internal stresses As expected, the theory and experiments are well agreed The obtained results allow information that the water diffusion in the balsa wood leads to the diffusivity process in the woodcomposite structure The obtained diffusion parameters of the single balsa are higher than those of the balsa embedded in a wood-composite structure REFERENCES c) d) Figure Variation of hygroscopic stresses through the thickness of the wood-composite plate with (½)eb = 7.95 mm and ec = 2.13 mm Figure shows the evolution of the internal hygroscopic stresses The compressive stresses exist in the balsa core and these can make the composite skins which limit the hygroscopic expansion of the balsa core delaminated The skins are in traction in this case The change of the stress state at interface balsa wood/composite skin shows that the interface is an important sensible area Conclusions Moisture diffusion and hygroscopic aging in the woodcomposite structure and their constituents were investigated in this research The diffusive behavior experimentally observed to identify the parameters controlling the diffusion process such as the diffusion coefficients and the water content at saturation A new [1] Ilcewicz L B, Hoffman D J, Fawcett A J Composite Applications in Commercial Airframe Structures, Comprehensive Composite Materials, Vol 6, pp.87-119, 2000 [2] Apicella A, Migliaresi C, Nicodemo L, Nicolais L, Iaccarino L, Roccotelli S Water sorption and mechanical properties of a glassreinforced polyester resin, Composites Part A: Applied Science and Manufacturing, Vol 13, pp.406-410, 1982 [3] Mouritz A P, Gellert E, Burchill P, Challis K Review of advanced composite structures for naval ships and submarines, Composite Structures, Vol.53, pp.21-41, 2001 [4] Weitsman Y J, Li X, Ionita A Sea water effects on polymeric foams and their sandwich layups, Journal of Sandwich structures and Materials, Vol.7, pp.193-197, 2005 [5] [5] [5] Avilés F, Aguilar M Moisture Absorption in Foam-Cored Composite Sandwich Structures, Polymer Composites, Vol.31, pp.714-722, 2010 [6] Katzman H A, Castaneda R M, Lee H S Moisture diffusion in composite sandwich structures, Composites Part A, Vol.39, pp.887892, 2008 [7] Sadler R L, Sharpe M, Panduranga R, Shivakumar K Water immersion effect on swelling and compression properties of EcoCore, PVC foam and balsa wood, Composite Structures, Vol.90, pp.330-336, 2009 [8] Loh W K, Crocombe A D, Wahab M M A, Ashcroft I A Modelling anomalous moisture uptake, swelling and thermal characteristics of a rubber toughened epoxy adhesive, International Journal of Adhesion and Adhesives, Vol.25, pp.1-12, 2005 (The Board of Editors received the paper on 10/05/2014, its review was completed on 11/06/2014) ... balsa system material used in marine industry The studied materials consist of cm3 cubes of balsa wood infused by resin Dry materials (a) and saturated moisturized materials (b) Theoretical Section... To obtain swelling characteristic of balsa core, we used the slope of the linear ranges of the moistureinduced strain curves This linear range corresponds to an actual final state of balsa Figure... Crocombe A D, Wahab M M A, Ashcroft I A Modelling anomalous moisture uptake, swelling and thermal characteristics of a rubber toughened epoxy adhesive, International Journal of Adhesion and Adhesives,