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aluminium laminates after hygrothermal aging
Composites Science and Technology 61 (2001) 1041±1048
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Residual properties of reformed bamboo/aluminium laminates after hygrothermal aging J.Y. Zhang 1, Q.Y. Zeng 2, T.X. Yu, J.K. Kim * Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 11 August 1999; received in revised form 5 October 2000; accepted 9 November 2000
Abstract Following our previous studies on bulk mechanical, interlaminar and static indentation/impact properties in dry conditions, the eects of exposure to various combinations of temperature and humidity on the mechanical response of reformed bamboo/aluminum laminates have been evaluated. The moisture absorption phenomenon of the reformed bamboo was studied in comparison with natural bamboo with and without a protective edge coating. It is shown that protection of the laminate edge with an appropriate polymeric material was eective in reducing the moisture absorption into the bamboo layer and thereby improving the residual mechanical performance of the laminates. The reductions in axial tensile strength and impact strength after prescribed aging treatments were at most about 30% of the original, while that of the aluminium/bamboo interlaminar shear strength was nearly 94% of the original strength. This is explained in terms of the dierence in strength contributions and the preferential concentration of moisture at the laminar interface. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Laminates; Sandwich; Hygrothermal eect; Mechanical properties; Impact behaviour
1. Introduction Bamboo, a natural composite, has attracted signi®cant interest on account of its unique functionally graded microstructure and sound mechanical properties, including high speci®c strength and stiness [1±5]. Apart from traditional applications in its natural form, such as various containers for food or crops, fences, scaolding at construction sites, etc., use of bamboo as a raw material for engineering applications has hitherto been quite limited. This is because of its round shape and degradation of the originally attractive mechanical and structural performance when exposed to the use environment, such as moisture and ultraviolet light. Two approaches have been successfully taken to tackle these problems: one approach is to extract bamboo ®bres from raw bamboo and use them to reinforce
* Corresponding author. 1,2 Visiting scholars from the Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui, and the Institute of Applied Ecology, Shenyang, Liaoning, PR China, respectively, when this work was done.
polymeric matrix materials to form a bamboo-®ber/ polymer-matrix composite [6]. The other is to reform the bamboo, involving a three-step process including softening, compression and ®xation, to produce rectangular bamboo plates that possess a higher density together with a higher strength and elastic modulus than natural bamboo as a result of the increased ®bre volume. To best utilise as load-bearing non-structural and structural engineering components, as well as to protect the reformed bamboo from moisture absorption and rotting when directly exposed to adverse environmental conditions, the reformed bamboo was sandwiched with aluminium face sheets to form reformed bamboo/aluminium sheet laminate [7,8]. The low-cost for materials and production, and the high speci®c strength made the reformed bamboo laminate a potential substitute for conventional materials in many engineering applications, such as cargo containers, mobile homes, ¯oors, and partitioning walls of various purposes. Extensive experimental studies have been carried out to characterize the static mechanical properties in tension, compression, bending and indentation, interlaminar fracture resistance as well as the damage resistance under
0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00232-3
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J.Y. Zhang et al. / Composites Science and Technology 61 (2001) 1041±1048
low-energy impact [9±12]. The above studies con®rmed that the bamboo/aluminium sheet laminates had a speci®c strength six-times greater than monolithic metals, such as aluminium alloys and mild steel. The interface adhesion quality between the reformed bamboo and aluminium sheets was mainly responsible for the initiation and growth of fracture within the laminate, especially in bending and compression. This suggests that a strong bond between these components is the key to sound structural and mechanical performance of the laminate. Amongst the several dierent polymers used to bond the bamboo to the aluminium sheet, a modi®ed high-molecular weight polyethylene was found most suited as the adhesive. The responses of the laminate to static indentation and low-energy impact exhibited was similar in terms of global deformation and the failure process, indicating that these mechanical characteristics are insensitive to loading rate. As part of our ongoing research project on the optimization of the mechanical performance of reformed bamboo/aluminium laminates, the eects of accelerated aging in adverse environmental conditions are speci®cally evaluated in this paper. Following the excursions to three dierent combinations of humidity, elevated and sub-zero temperatures, the reductions in mechanical properties of the laminate, including the interlaminar adhesion strength, tensile strength, ¯exural strength and impact strength, were characterised. Of special interests were the moisture absorption behaviour of the reformed bamboo layer and the laminate, in comparison with that of natural bamboo, and correlation with the residual strengths. 2. Experimental procedures 2.1. Materials and specimens The reformed bamboo/aluminium sheet laminate consisted of two layers of unidirectional or cross-ply bamboo plates (each layer being 5±6 mm in thickness), which were sandwiched between two LY12CZ aluminium sheets (similar to Al-2024T6 alloy sheets) of
thickness 0.3 mm. Schematics of these laminates are shown in Fig. 1. The bamboo layers and aluminium sheets were bonded together with a modi®ed high-density polyethylene (Polybond 3009, supplied by Uniroyal Chemical Company, Inc., USA) using a hot press at a temperature of 160 C and a pressure of 2 MPa for 5±10 min. The detailed fabrication procedures are reported elsewhere [7,8]. Specimens were sawn from the laminates 36 h after the bonding. The details of the specimens' geometry and dimensions for the tests are presented in Fig. 2. 2.2. Moisture absorption Two types of moisture absorption tests were conducted: one under an accelerated aging condition by soaking in water for 9 days; and the other by simply exposing the laminates to an indoor environment for a year. These two dierent conditions represent adverse, outdoor, and long-term, indoor aging conditions, respectively. In the former experiment, the moisture absorption behaviour of the reformed bamboo/aluminium sheet laminates was evaluated using specimens of two dierent sizes: 5050 mm and 100100 mm squares. In the laminates, the aluminium skins protect the core bamboo layer from being exposed directly to the environment, water can only be permeated through the edge surface. In view of this, two dierent surface conditions were prepared: one with a bare, as-cut edge surface without any protection; and the other with an epoxy coating applied to the edge surface as the protective coating. The epoxy coating (Ciba Araldite, Supplied by Ciba Polymers Duxford, UK) was cured at room temperature according to the manufacturer's instruction. During the period of soaking in water at 20 C for 9 days, the weights of laminate specimens were measured every 24 h, and the moisture absorption rate, M, was calculated based on the following equation [13]: M Wr
W0 =W0
1
where Wr and Wo are the weights of specimens after and before soaking in water.
Fig. 1. Schematic illustrations of bamboo/aluminium laminates.
J.Y. Zhang et al. / Composites Science and Technology 61 (2001) 1041±1048
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Fig. 2. Specimen geometry and dimensions for (a) bamboo/bamboo interlaminar shear test, (b) bamboo/aluminium interlaminar shear test, (c) tensile test, (d) three-point ¯exure test, and (e) un-notched Charpy impact test; h=nominal thickness (10 mm) of the laminate; all units in mm.
In the latter experiment, which was aimed mainly at shedding some insight into the long-term moisture absorption performance of bamboo, bamboo specimens in both reformed and natural forms were stored in an indoor laboratory, continuously monitoring the moisture absorption for an entire year. This experiment was carried out in a laboratory located in Shenyang, the northeastern part of China. The laboratory was not airconditioned, except moderate heating during the winter period from November to March. The moisture absorption rate was calculated using Eq. (1). 2.3. Accelerated aging and mechanical tests In view of the mechanical characteristics and potential applications of reformed bamboo/aluminium sheet
laminates, three dierent types of accelerated aging conditions were selected. These aging conditions have been widely used for accelerated aging of wood-based products as speci®ed by the standards, [14±16], and the details are given in Table 1. For the aging process I, the specimens were treated at 20 C, instead of 12 C as speci®ed in the standard, to simulate an even severer winter temperature. The edges of all laminate specimens prepared for the mechanical tests (Fig. 1) were protected with an epoxy coating before exposure to the aging condition. Having been subjected to each of these accelerated aging environments, the specimens were conditioned at room temperature and a relative humidity of 65% for 48 h before the mechanical tests. The tensile, three-point ¯exural, and lap-joint interlaminar shear tests were conducted using the specimens
Table 1 Accelerated aging conditions Aging process Reference standard
I
II
III
ASTM D1037-96
GB9846.12-88
GB13124-91
1. Boiled in water at 100 C for 4 h 2. Dried in an oven at 632 C for 20 h 3. Boiled in water at 100 C for 4 h 4. Stored at room temperature for 10 min
1. 2. 3. 4.
Aging condition 1. Soaked in water at 49 2 C for 1 h 2. Placed in water steam at 933 C for 3 h 3. Placed in a chamber at 202 C for 20 h 4. Dried in an oven at 992 C for 3 h 5. Placed in water steam at 933 C for 3 h 6. Dried in an oven at 992 C for 8 h 7. Repeat steps 1±6
Soaked in water at 63 2 C for 6 h Placed in a chamber at 20 C for 6 h Dried in an oven at 632 C for 6 h Stored at room temperature for 30 min
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described in Fig. 2 on a MTS universal testing machine at a cross-head speed of 5 mm/min. The details of testing procedures for the tensile and ¯exural tests are given in our previous report [9]. The interlaminar shear strengths were measured for both the bamboo±bamboo and the bamboo±aluminium sheet interfaces based on the lab-shear test according to the speci®cation ASTM D3165 [17]. The details of specimen dimensions shown in Figs. 2(a) and (b) indicate a joint length of 30 mm. Two edges of the specimens were gripped and a tensile force applied until rupture, and the shear strength calculated from the maximum load divided by the joint area. The impact strength was evaluated using un-notched specimens [Fig. 1(e)] on a pendulum impact tester. The energy absorbed divided by the ligament area of the specimen cross-section was regarded as the impact strength. These mechanical properties were also measured without environmental aging. 3. Results and discussion 3.1. Moisture absorption in bamboo and bamboo/ aluminium laminates The principal results from the experiments are presented to establish general trends in moisture absorption and residual strengths with respect to various accelerated aging conditions. The moisture contents for the natural and reformed bamboo measured from the longterm exposure to an indoor environment in Shenyang, China are plotted as a function of time in a year, as shown in Fig. 3. It is clearly seen that moisture was absorbed in humid, hot seasons, i.e. in spring and summer, while it was desorbed in dry, cold seasons, i.e. autumn and winter, in a year. The moisture absorption behaviour exhibited essentially a similar trend for the natural and reformed bamboo. The moisture content in the laminate was functionally proportional to the
Fig. 3. Moisture contents in natural and reformed bamboo as a function of time in a year.
humidity content and temperature of a typical un-airconditioned indoor environment in the place where these experiments were conducted: it reached a peak value in August when the humidity and temperature are generally very high, and it became the lowest in late February when the humidity and temperature are very low. Important to note is that the absolute moisture contents for the reformed bamboo were less than half that of natural bamboo for the whole span of a year, partly proving the bene®cial eect of the reforming process. For example, the maximum absorbed moisture in natural bamboo in summer was 9.1%, while that for the reformed bamboo was just 4.1%. Judging from the large dierence in cellulose ®bre volume fraction between the natural and reformed bamboo (approximately 29 and 44% respectively [7,11], it seems that moisture was preferentially absorbed and stored within the lignin matrix, while the presence of densely packed bamboo ®bres eectively discouraged the moisture absorption. The moisture contents in the bamboo/aluminium laminates measured after soaking in water absorption are plotted as a function of soaking time in Fig. 4. The moisture gain was much larger after a given period of exposure for the specimens with bare edges being directly exposed to water than for those with a protective coating. The moisture content for the latter specimen after 9 days of soaking in water was generally lower than 10%, which is deemed acceptable, while that for the former specimen without edge protective coating was greater than 10%. In particular, it reached as high as 36.4% for the small specimen, because it has a much larger exposed edge area for a given volume than a large specimen. Also associated with the high moisture content, the edges of the former specimen without a protective coating were found to have cracked after soaking for a few days, due presumably to the excessive moisture
Fig. 4. Moisture contents plotted as a function of time after soaking in water for bamboo/aluminium laminates of dierent size: small (5050 mm square) and large (100100 mm square) specimens. w/ and w/o pc, with and without protective coating.
J.Y. Zhang et al. / Composites Science and Technology 61 (2001) 1041±1048
permeation leading to severe expansion of the bamboo layer. All these results prove that the protective coating was eective in retarding the moisture absorption process through the laminate edges. It was also noted that the weight gains due to absorbed moisture were much faster when soaked in water than when just exposed to the laboratory environment (Fig. 3). Although eective in protecting the edges from moisture attack, the coating itself was not sucient once the laminates were exposed to boiling water or water steam as speci®ed in aging processes I and II (Table 1). It should also be mentioned here that the protective coating often fractured during the drying process after soaking in water, due most probably to the high hydrostatic pressure of water steam present within the bamboo layers. The bamboo layer could no longer be properly protected from moisture absorption upon subsequent exposure to water or water steam. This indicates that other better means of protecting the laminate edges should be devised if the intended applications of the bamboo/aluminium laminates are likely to involve environmental conditions similar to these aging processes. The presence of excessive moisture is always detrimental to the mechanical performance and structural integrity of the laminate. Moisture has two major in¯uences on the laminate: (i) water hydrolyses the bamboo core, leading to severe volumetric expansion and generating signi®cant tri-axial internal stresses. This results in mechanical deformation and delamination of the laminate; (ii) water bleaches out the adhesive, deteriorating the interface bond between the bamboo and aluminium layers, eventually causing delaminations. 3.2. Residual tensile and ¯exural strengths after aging The residual tensile and ¯exural strengths of the laminates measured after being subjected to the respective aging processes are presented in Fig. 5. The corresponding strengths and moduli of the laminates with dierent bamboo plate orientations are also given in Table 2. The reductions in tensile strength depended strongly on whether the bamboo layers were unidirectional or cross-plied. For the laminates containing unidirectional bamboo layers, the strength reduction after aging process I was less than 15%, while the residual strengths for those treated with aging processes II and III were almost the same as the original strength. This result is not surprising because the tensile strength of unidirectional bamboo laminate is governed mainly by the strength of the component materials in the direction of loading, and the interface adhesion between these components plays only a minor role. The small reduction also suggests that the bamboo ®bres have survived these severe accelerated aging processes, including soaking in boiling water and drying, without much physical/mechanical degradation.
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Fig. 5. Normalised residual tensile and bending strengths of the laminate after aging.
In sharp contrast, the strength reduction in the laminates with cross-ply bamboo layers was much more signi®cant than those with unidirectional bamboo layers, ranging from 25 to 65%. There are several causes responsible for the strength reductions in cross-ply bamboo laminates due to ageing, namely (i) degradation of bond strength at the adhesive±aluminium and adhesive±bamboo layer; (ii) degradation of Polybond adhesive itself; and (iii) degradation of the bamboo slab. The ®rst two degradation modes aect the interlaminar bond strength and the joint strength between bamboo slabs, while the last degradation mode reduce the transverse strength of bamboo. Degradations of these properties played dierent roles in tension and bending. The details of interlaminar bond strength reductions are presented in the next section. The mechanical behaviour and strengths without prior hygrothermal ageing were characterised in our previous report [10] for the laminates bonded with two dierent adhesives, namely epoxy and modi®ed polyethylene (Polybond), which represent a weak and a strong interface bond, respectively [11]. When the crossply laminate was loaded in tension, splitting of transverse bamboo layer along the bamboo ®bre was a common, major failure mode for both the laminates bonded with epoxy and Polybond adhesives, and there was no signi®cant dierence in tensile strength [10]. This suggests that the adhesion at the aluminium/adhesive and bamboo/adhesive interfaces had a negligible eect on tensile behaviour and strength of cross-ply laminates. On the contrary, when loaded in three-point bending, fracture of the cross-ply laminates bonded with epoxy occurred predominantly by delamination followed by buckling of the top aluminium layer. If Polybond was used as the adhesive, delamination of the aluminum layer was signi®cantly discouraged and failure occurred mainly at the bonded joints of the transverse bamboo
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Table 2 Strengths and moduli measured in tension and bending for the aluminium/bamboo laminatesa Laminates bonded with epoxy
Tensile strength, MPa Tensile modulus, GPa Flexural strength, MPa Flexural modulus, GPa a b c d
Laminates bonded with Polybond
Lb
Tc
Cd
L
T
C
11724 171 1606 221
242 5.90.7 ± ±
879 121 7112 15.40.9
11220 182 26523 271
251 5.60.4 ± ±
7815 11.30.6 1164 14.50.3
Taken from Sui et al. [10]. L=longitudinal direction of the unidirectional laminates. T=transverse direction of the unidirectional laminates. C=cross-ply laminates.
slabs. The ¯exural strength of the former laminate was some 40% lower than that of the latter laminates with strong interface adhesion (Table 2). Thus, the interface adhesion played a signi®cant role in controlling the fracture behaviour of the laminate in ¯exure. In light of the foregoing observations and the degradation in interlaminar shear strength shown in Fig. 6, the following can be summarised, although the property degradations of bamboo plates and adhesive themselves were not speci®cally measured in this study. The substantial reductions of tensile strength after ageing, especially at ageing condition III, appear to be associated, to a certain extent, with the degradation of the bamboo slab's transverse strength. However, the functional similarity between the tensile strength and the bamboo±bamboo interlaminar shear strength with respect to the three ageing conditions (compare Figs. 5 and 6) also suggest that the reduction of joint strength between the bamboo slabs was also partly responsible. The reductions in ¯exural strength for the laminates containing cross-ply bamboo layers were slightly less than those of tensile strength. This may indicate that the loading condition in bending was less sensitive to the transverse strength and the joint strength of bamboo layers than in axial tension, contrary to our previous observations [10] on the laminates without prior ageing. 3.3. Residual interlaminar bond strength and impact strength after aging The residual interlaminar bond strengths of the bamboo/bamboo and bamboo/aluminium interfaces after aging are summarized in Fig. 6. The interlaminar bond strengths, 11.5 and 12.2 MPa, for the bamboo/bamboo and bamboo/aluminium interfaces, respectively, were taken from our previous study [11]. The reductions in interlaminar bond strength were signi®cantly greater than the strengths discussed above, with the residual strengths ranging only about 6±7% of the original for the bamboo/aluminium joint, in particular. Meanwhile, the normalised residual interlaminar shear strengths were much higher for the bamboo/bamboo joint, i.e.
Fig. 6. Normalised residual interlaminar bond strength of the laminate after aging.
17±33% of its original strength. The above result strongly suggests that the absorbed moisture concentrated preferentially at the bamboo/aluminium interface, and the Polybond adhesive (whose melting temperature is about 125 C) suered signi®cantly the degradation during the temperature excursion between 20 and 100 C and the humidity environment. Previous studies on aramid ®bre reinforced polymer matrix composites showed that moisture is detrimental to the composite interlaminar bond strength [18±20] and the interlaminar fracture toughness [21]. It was suspected that the absorbed moisture tended to concentrate preferentially at the ®bre±matrix interface which is the weakest part of the composite, and that once debonded or delaminated, the interface acted as the pocket or storage for the absorbed moisture, even further deteriorating the bond of the neighbouring interfaces. It was also reported that the amorphous phase within the semicrystalline polymer, such as Polybond employed in this study, tended to swell upon prolonged moisture attack, generating stresses at the amorphous/crystalline interface, eventually leading to the formation of microvoid in the polymer [22]. It is thought that the above two
J.Y. Zhang et al. / Composites Science and Technology 61 (2001) 1041±1048
Fig. 7. Normalised residual impact strength of the laminate after aging.
mechanisms were mainly responsible for the signi®cant degradations of interlaminar bond strengths at the bamboo/aluminium and bamboo/bamboo interfaces. In view of these, aging process III seems to have been most damaging to the aluminium/bamboo interface bond and the strength of Polybond adhesive amongst the aging conditions used. The un-notched impact strength measured on the laminates with unidirectional bamboo layers is shown in Fig. 7. The impact strength reduction was in the range from 12 to 31%, and the residual impact strength varied with aging conditions in a fashion very similar to the tensile strength of unidirectional laminates (Fig. 5). The strength reductions for ageing conditions II and III were much less than that for ageing condition I. This may suggest that the properties of aluminium sheets and bamboo ®bres signi®cantly aect the impact strength of the laminate as for the uniaxial tensile strength. In addition, it is also thought that the reductions in bamboo/aluminium interlaminar bond strength and the strength of Polybond adhesive due to moisture attack also played a certain role. 4. Conclusion The moisture absorption behaviour and the mechanical properties of bamboo/aluminum laminates were evaluated after exposure to several dierent combinations of hot/humid environments and sub-zero temperature. The moisture gain was much faster for the specimens with bare edges being directly exposed to water than for those with a protective edge coating. It was proven that the protection of the laminate edge from direct contact with the environment, using an appropriate polymeric material, was eective in reducing the moisture absorption and thereby improving the residual mechanical performance of the laminates. The
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reduction of aluminium/bamboo interlaminar shear strength due to aging was most signi®cant, often more than 95% of the original strength, which also in¯uenced signi®cantly the tensile strength of the laminate containing cross-ply bamboo layers. It is thought that the absorbed moisture concentrated preferentially at the bamboo/aluminium interface and/or within the adhesive as the weakest part of the laminate, as for the ®brematrix interface in many ®bre-reinforced polymer matrix composites. However, the impaired interlaminar shear strength did not much aect the tensile strength and the impact strength of the unidirectional laminates because these mechanical properties depended mainly on the properties of the aluminium sheet and the bamboo ®bre. The reductions in axial tensile strength and impact fracture toughness were at most about 30% of the original. Acknowledgements The ®nancial support from the Research Infrastructure Grant (RI95/96.EG05) of Hong Kong University of Science and Technology (HKUST) is gratefully acknowledged. Some mechanical tests were conducted with the technical supports of the Advanced Engineering Materials Facility (AEMF) of HKUST.
References [1] Lakkad SC, Patel JM. Mechanical properties of bamboo, a natural composite. Fibre Sci Technol. 1980±1981;14:1319±22. [2] Fengel D, Shao X. A chemical and ultrastructural study of the bamboo species Phyllostachys Makinoi Hay. Wood Sci Technol 1984;18:103±12. [3] Wai NN, Nanko H, Murakami K. A morphological study on the behaviour of bamboo pult ®bres in the beating process. Wood Sci Technol 1985;19:211±22. [4] Nogata F, Takahashi H. Intelligent functionally graded material: bamboo. Compos Eng 1995;5:743±51. [5] Amada S, Munekata T, Nagase Y, Ishikawa Y, Kirigai A, Zhifei Y. The mechanical structures of bamboos in viewpoint of functionally gradient and composite materials. MRS Bulletin 1995;20:35±6. [6] Jain S, Kumar R, Jindal UC. Mechanical behaviour of bamboo and bamboo composite. J Mater Sci 1992;27:4598±604. [7] Li SH, Fu SY, Zhou BL, Zeng QY, Bao XR. Reformed bamboo and reforrmed bamboo/aluminium composite. J Mater Sci 1994;29:5990±6. [8] Li SH, Zhou BL, Tang ZT, Zeng QY. Reformed bamboo and reformed bamboo/aluminium composite, part II Impact properties. J Mater Sci Lett 1996;15:129±31. [9] Sui GX, Yu TX, Kim JK, Zhou BL. Static mechanical behavior of bamboo/aluminium composites for applications in industrial structures. Key Eng Mater 1998;145-149:781±6. [10] Sui GX, Yu TX, Kim JK, Zhou BL. Mechanical behaviour and failure modes of aluminium/bamboo sandwich plates under quasi-static loading. J Mater Sci 2000;35:1445±52. [11] Kim JK, Huang XJ, Yu TX, Chan CM, Bao BH. Interlaminar
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[12] [13] [14] [15] [16] [17] [18]
J.Y. Zhang et al. / Composites Science and Technology 61 (2001) 1041±1048 fracture behaviour of reformed bamboo/aluminium laminate composites. J Adhesion Sci Technol, in press. Zhang JY, Yu TX, Kim JK, Sui GX. Static indentation and impact behaviour of reformed bamboo/aluminium laminate composites. Composite Structures 2000;50:207±16. ASTM D570. Standard test method for water absorption of plastics. ASTM D1037-96, Standard test methods for evaluating properties of wood-base and particle panel materials. GB9846.12-88, Plywood-determination of glue bond strength. GB13124-91, Methods of testing bamboo-mat plywood. ASTM D3165-73, Strength properties of adhesives in shear by tension loading of laminated assemblies. Verpoest I, Springer GS. Eects of moisture on the compressive and interlaminar shear strengths of aramid-epoxy composites. J Reinf Plast Compos 1988;7:23±32.
[19] Haque A, Mooreheed I, Zadoo DP, Jeelani S. Hygrothermal in¯uence on the interlaminar shear strength of Kevlar-graphite/ epoxy hybrid composites. J Mater Sci 1990;25:4639±43. [20] Doxsee LE, Janssens W, Verpoest I, De Meester P. Strength of aramid-epoxy composites during moisture absorption. J Reinf Plast Compos 1991;10:645±55. [21] De Charentenay FX, Harry IM, Prel YJ, Benzeggagh ML. Characterizing the eect of delamination defect by mode I delamination test. In: Eect of defects in composite materials, ASTM STP 836, Philadelphia, (PA): American Society for Testing and Materials 1984. p. 84±103. [22] Bastioli C, Guanella I, Romano G. Eects of water sorption on the physical properties of PET, PBT and their long ®bres composites. Polym Compos 1990;11:1±9.
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Residual properties of reformed bamboo/aluminium laminates after hygrothermal aging J.Y. Zhang 1, Q.Y. Zeng 2, T.X. Yu, J.K. Kim * Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 11 August 1999; received in revised form 5 October 2000; accepted 9 November 2000
Abstract Following our previous studies on bulk mechanical, interlaminar and static indentation/impact properties in dry conditions, the eects of exposure to various combinations of temperature and humidity on the mechanical response of reformed bamboo/aluminum laminates have been evaluated. The moisture absorption phenomenon of the reformed bamboo was studied in comparison with natural bamboo with and without a protective edge coating. It is shown that protection of the laminate edge with an appropriate polymeric material was eective in reducing the moisture absorption into the bamboo layer and thereby improving the residual mechanical performance of the laminates. The reductions in axial tensile strength and impact strength after prescribed aging treatments were at most about 30% of the original, while that of the aluminium/bamboo interlaminar shear strength was nearly 94% of the original strength. This is explained in terms of the dierence in strength contributions and the preferential concentration of moisture at the laminar interface. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Laminates; Sandwich; Hygrothermal eect; Mechanical properties; Impact behaviour
1. Introduction Bamboo, a natural composite, has attracted signi®cant interest on account of its unique functionally graded microstructure and sound mechanical properties, including high speci®c strength and stiness [1±5]. Apart from traditional applications in its natural form, such as various containers for food or crops, fences, scaolding at construction sites, etc., use of bamboo as a raw material for engineering applications has hitherto been quite limited. This is because of its round shape and degradation of the originally attractive mechanical and structural performance when exposed to the use environment, such as moisture and ultraviolet light. Two approaches have been successfully taken to tackle these problems: one approach is to extract bamboo ®bres from raw bamboo and use them to reinforce
* Corresponding author. 1,2 Visiting scholars from the Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui, and the Institute of Applied Ecology, Shenyang, Liaoning, PR China, respectively, when this work was done.
polymeric matrix materials to form a bamboo-®ber/ polymer-matrix composite [6]. The other is to reform the bamboo, involving a three-step process including softening, compression and ®xation, to produce rectangular bamboo plates that possess a higher density together with a higher strength and elastic modulus than natural bamboo as a result of the increased ®bre volume. To best utilise as load-bearing non-structural and structural engineering components, as well as to protect the reformed bamboo from moisture absorption and rotting when directly exposed to adverse environmental conditions, the reformed bamboo was sandwiched with aluminium face sheets to form reformed bamboo/aluminium sheet laminate [7,8]. The low-cost for materials and production, and the high speci®c strength made the reformed bamboo laminate a potential substitute for conventional materials in many engineering applications, such as cargo containers, mobile homes, ¯oors, and partitioning walls of various purposes. Extensive experimental studies have been carried out to characterize the static mechanical properties in tension, compression, bending and indentation, interlaminar fracture resistance as well as the damage resistance under
0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00232-3
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J.Y. Zhang et al. / Composites Science and Technology 61 (2001) 1041±1048
low-energy impact [9±12]. The above studies con®rmed that the bamboo/aluminium sheet laminates had a speci®c strength six-times greater than monolithic metals, such as aluminium alloys and mild steel. The interface adhesion quality between the reformed bamboo and aluminium sheets was mainly responsible for the initiation and growth of fracture within the laminate, especially in bending and compression. This suggests that a strong bond between these components is the key to sound structural and mechanical performance of the laminate. Amongst the several dierent polymers used to bond the bamboo to the aluminium sheet, a modi®ed high-molecular weight polyethylene was found most suited as the adhesive. The responses of the laminate to static indentation and low-energy impact exhibited was similar in terms of global deformation and the failure process, indicating that these mechanical characteristics are insensitive to loading rate. As part of our ongoing research project on the optimization of the mechanical performance of reformed bamboo/aluminium laminates, the eects of accelerated aging in adverse environmental conditions are speci®cally evaluated in this paper. Following the excursions to three dierent combinations of humidity, elevated and sub-zero temperatures, the reductions in mechanical properties of the laminate, including the interlaminar adhesion strength, tensile strength, ¯exural strength and impact strength, were characterised. Of special interests were the moisture absorption behaviour of the reformed bamboo layer and the laminate, in comparison with that of natural bamboo, and correlation with the residual strengths. 2. Experimental procedures 2.1. Materials and specimens The reformed bamboo/aluminium sheet laminate consisted of two layers of unidirectional or cross-ply bamboo plates (each layer being 5±6 mm in thickness), which were sandwiched between two LY12CZ aluminium sheets (similar to Al-2024T6 alloy sheets) of
thickness 0.3 mm. Schematics of these laminates are shown in Fig. 1. The bamboo layers and aluminium sheets were bonded together with a modi®ed high-density polyethylene (Polybond 3009, supplied by Uniroyal Chemical Company, Inc., USA) using a hot press at a temperature of 160 C and a pressure of 2 MPa for 5±10 min. The detailed fabrication procedures are reported elsewhere [7,8]. Specimens were sawn from the laminates 36 h after the bonding. The details of the specimens' geometry and dimensions for the tests are presented in Fig. 2. 2.2. Moisture absorption Two types of moisture absorption tests were conducted: one under an accelerated aging condition by soaking in water for 9 days; and the other by simply exposing the laminates to an indoor environment for a year. These two dierent conditions represent adverse, outdoor, and long-term, indoor aging conditions, respectively. In the former experiment, the moisture absorption behaviour of the reformed bamboo/aluminium sheet laminates was evaluated using specimens of two dierent sizes: 5050 mm and 100100 mm squares. In the laminates, the aluminium skins protect the core bamboo layer from being exposed directly to the environment, water can only be permeated through the edge surface. In view of this, two dierent surface conditions were prepared: one with a bare, as-cut edge surface without any protection; and the other with an epoxy coating applied to the edge surface as the protective coating. The epoxy coating (Ciba Araldite, Supplied by Ciba Polymers Duxford, UK) was cured at room temperature according to the manufacturer's instruction. During the period of soaking in water at 20 C for 9 days, the weights of laminate specimens were measured every 24 h, and the moisture absorption rate, M, was calculated based on the following equation [13]: M Wr
W0 =W0
1
where Wr and Wo are the weights of specimens after and before soaking in water.
Fig. 1. Schematic illustrations of bamboo/aluminium laminates.
J.Y. Zhang et al. / Composites Science and Technology 61 (2001) 1041±1048
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Fig. 2. Specimen geometry and dimensions for (a) bamboo/bamboo interlaminar shear test, (b) bamboo/aluminium interlaminar shear test, (c) tensile test, (d) three-point ¯exure test, and (e) un-notched Charpy impact test; h=nominal thickness (10 mm) of the laminate; all units in mm.
In the latter experiment, which was aimed mainly at shedding some insight into the long-term moisture absorption performance of bamboo, bamboo specimens in both reformed and natural forms were stored in an indoor laboratory, continuously monitoring the moisture absorption for an entire year. This experiment was carried out in a laboratory located in Shenyang, the northeastern part of China. The laboratory was not airconditioned, except moderate heating during the winter period from November to March. The moisture absorption rate was calculated using Eq. (1). 2.3. Accelerated aging and mechanical tests In view of the mechanical characteristics and potential applications of reformed bamboo/aluminium sheet
laminates, three dierent types of accelerated aging conditions were selected. These aging conditions have been widely used for accelerated aging of wood-based products as speci®ed by the standards, [14±16], and the details are given in Table 1. For the aging process I, the specimens were treated at 20 C, instead of 12 C as speci®ed in the standard, to simulate an even severer winter temperature. The edges of all laminate specimens prepared for the mechanical tests (Fig. 1) were protected with an epoxy coating before exposure to the aging condition. Having been subjected to each of these accelerated aging environments, the specimens were conditioned at room temperature and a relative humidity of 65% for 48 h before the mechanical tests. The tensile, three-point ¯exural, and lap-joint interlaminar shear tests were conducted using the specimens
Table 1 Accelerated aging conditions Aging process Reference standard
I
II
III
ASTM D1037-96
GB9846.12-88
GB13124-91
1. Boiled in water at 100 C for 4 h 2. Dried in an oven at 632 C for 20 h 3. Boiled in water at 100 C for 4 h 4. Stored at room temperature for 10 min
1. 2. 3. 4.
Aging condition 1. Soaked in water at 49 2 C for 1 h 2. Placed in water steam at 933 C for 3 h 3. Placed in a chamber at 202 C for 20 h 4. Dried in an oven at 992 C for 3 h 5. Placed in water steam at 933 C for 3 h 6. Dried in an oven at 992 C for 8 h 7. Repeat steps 1±6
Soaked in water at 63 2 C for 6 h Placed in a chamber at 20 C for 6 h Dried in an oven at 632 C for 6 h Stored at room temperature for 30 min
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described in Fig. 2 on a MTS universal testing machine at a cross-head speed of 5 mm/min. The details of testing procedures for the tensile and ¯exural tests are given in our previous report [9]. The interlaminar shear strengths were measured for both the bamboo±bamboo and the bamboo±aluminium sheet interfaces based on the lab-shear test according to the speci®cation ASTM D3165 [17]. The details of specimen dimensions shown in Figs. 2(a) and (b) indicate a joint length of 30 mm. Two edges of the specimens were gripped and a tensile force applied until rupture, and the shear strength calculated from the maximum load divided by the joint area. The impact strength was evaluated using un-notched specimens [Fig. 1(e)] on a pendulum impact tester. The energy absorbed divided by the ligament area of the specimen cross-section was regarded as the impact strength. These mechanical properties were also measured without environmental aging. 3. Results and discussion 3.1. Moisture absorption in bamboo and bamboo/ aluminium laminates The principal results from the experiments are presented to establish general trends in moisture absorption and residual strengths with respect to various accelerated aging conditions. The moisture contents for the natural and reformed bamboo measured from the longterm exposure to an indoor environment in Shenyang, China are plotted as a function of time in a year, as shown in Fig. 3. It is clearly seen that moisture was absorbed in humid, hot seasons, i.e. in spring and summer, while it was desorbed in dry, cold seasons, i.e. autumn and winter, in a year. The moisture absorption behaviour exhibited essentially a similar trend for the natural and reformed bamboo. The moisture content in the laminate was functionally proportional to the
Fig. 3. Moisture contents in natural and reformed bamboo as a function of time in a year.
humidity content and temperature of a typical un-airconditioned indoor environment in the place where these experiments were conducted: it reached a peak value in August when the humidity and temperature are generally very high, and it became the lowest in late February when the humidity and temperature are very low. Important to note is that the absolute moisture contents for the reformed bamboo were less than half that of natural bamboo for the whole span of a year, partly proving the bene®cial eect of the reforming process. For example, the maximum absorbed moisture in natural bamboo in summer was 9.1%, while that for the reformed bamboo was just 4.1%. Judging from the large dierence in cellulose ®bre volume fraction between the natural and reformed bamboo (approximately 29 and 44% respectively [7,11], it seems that moisture was preferentially absorbed and stored within the lignin matrix, while the presence of densely packed bamboo ®bres eectively discouraged the moisture absorption. The moisture contents in the bamboo/aluminium laminates measured after soaking in water absorption are plotted as a function of soaking time in Fig. 4. The moisture gain was much larger after a given period of exposure for the specimens with bare edges being directly exposed to water than for those with a protective coating. The moisture content for the latter specimen after 9 days of soaking in water was generally lower than 10%, which is deemed acceptable, while that for the former specimen without edge protective coating was greater than 10%. In particular, it reached as high as 36.4% for the small specimen, because it has a much larger exposed edge area for a given volume than a large specimen. Also associated with the high moisture content, the edges of the former specimen without a protective coating were found to have cracked after soaking for a few days, due presumably to the excessive moisture
Fig. 4. Moisture contents plotted as a function of time after soaking in water for bamboo/aluminium laminates of dierent size: small (5050 mm square) and large (100100 mm square) specimens. w/ and w/o pc, with and without protective coating.
J.Y. Zhang et al. / Composites Science and Technology 61 (2001) 1041±1048
permeation leading to severe expansion of the bamboo layer. All these results prove that the protective coating was eective in retarding the moisture absorption process through the laminate edges. It was also noted that the weight gains due to absorbed moisture were much faster when soaked in water than when just exposed to the laboratory environment (Fig. 3). Although eective in protecting the edges from moisture attack, the coating itself was not sucient once the laminates were exposed to boiling water or water steam as speci®ed in aging processes I and II (Table 1). It should also be mentioned here that the protective coating often fractured during the drying process after soaking in water, due most probably to the high hydrostatic pressure of water steam present within the bamboo layers. The bamboo layer could no longer be properly protected from moisture absorption upon subsequent exposure to water or water steam. This indicates that other better means of protecting the laminate edges should be devised if the intended applications of the bamboo/aluminium laminates are likely to involve environmental conditions similar to these aging processes. The presence of excessive moisture is always detrimental to the mechanical performance and structural integrity of the laminate. Moisture has two major in¯uences on the laminate: (i) water hydrolyses the bamboo core, leading to severe volumetric expansion and generating signi®cant tri-axial internal stresses. This results in mechanical deformation and delamination of the laminate; (ii) water bleaches out the adhesive, deteriorating the interface bond between the bamboo and aluminium layers, eventually causing delaminations. 3.2. Residual tensile and ¯exural strengths after aging The residual tensile and ¯exural strengths of the laminates measured after being subjected to the respective aging processes are presented in Fig. 5. The corresponding strengths and moduli of the laminates with dierent bamboo plate orientations are also given in Table 2. The reductions in tensile strength depended strongly on whether the bamboo layers were unidirectional or cross-plied. For the laminates containing unidirectional bamboo layers, the strength reduction after aging process I was less than 15%, while the residual strengths for those treated with aging processes II and III were almost the same as the original strength. This result is not surprising because the tensile strength of unidirectional bamboo laminate is governed mainly by the strength of the component materials in the direction of loading, and the interface adhesion between these components plays only a minor role. The small reduction also suggests that the bamboo ®bres have survived these severe accelerated aging processes, including soaking in boiling water and drying, without much physical/mechanical degradation.
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Fig. 5. Normalised residual tensile and bending strengths of the laminate after aging.
In sharp contrast, the strength reduction in the laminates with cross-ply bamboo layers was much more signi®cant than those with unidirectional bamboo layers, ranging from 25 to 65%. There are several causes responsible for the strength reductions in cross-ply bamboo laminates due to ageing, namely (i) degradation of bond strength at the adhesive±aluminium and adhesive±bamboo layer; (ii) degradation of Polybond adhesive itself; and (iii) degradation of the bamboo slab. The ®rst two degradation modes aect the interlaminar bond strength and the joint strength between bamboo slabs, while the last degradation mode reduce the transverse strength of bamboo. Degradations of these properties played dierent roles in tension and bending. The details of interlaminar bond strength reductions are presented in the next section. The mechanical behaviour and strengths without prior hygrothermal ageing were characterised in our previous report [10] for the laminates bonded with two dierent adhesives, namely epoxy and modi®ed polyethylene (Polybond), which represent a weak and a strong interface bond, respectively [11]. When the crossply laminate was loaded in tension, splitting of transverse bamboo layer along the bamboo ®bre was a common, major failure mode for both the laminates bonded with epoxy and Polybond adhesives, and there was no signi®cant dierence in tensile strength [10]. This suggests that the adhesion at the aluminium/adhesive and bamboo/adhesive interfaces had a negligible eect on tensile behaviour and strength of cross-ply laminates. On the contrary, when loaded in three-point bending, fracture of the cross-ply laminates bonded with epoxy occurred predominantly by delamination followed by buckling of the top aluminium layer. If Polybond was used as the adhesive, delamination of the aluminum layer was signi®cantly discouraged and failure occurred mainly at the bonded joints of the transverse bamboo
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Table 2 Strengths and moduli measured in tension and bending for the aluminium/bamboo laminatesa Laminates bonded with epoxy
Tensile strength, MPa Tensile modulus, GPa Flexural strength, MPa Flexural modulus, GPa a b c d
Laminates bonded with Polybond
Lb
Tc
Cd
L
T
C
11724 171 1606 221
242 5.90.7 ± ±
879 121 7112 15.40.9
11220 182 26523 271
251 5.60.4 ± ±
7815 11.30.6 1164 14.50.3
Taken from Sui et al. [10]. L=longitudinal direction of the unidirectional laminates. T=transverse direction of the unidirectional laminates. C=cross-ply laminates.
slabs. The ¯exural strength of the former laminate was some 40% lower than that of the latter laminates with strong interface adhesion (Table 2). Thus, the interface adhesion played a signi®cant role in controlling the fracture behaviour of the laminate in ¯exure. In light of the foregoing observations and the degradation in interlaminar shear strength shown in Fig. 6, the following can be summarised, although the property degradations of bamboo plates and adhesive themselves were not speci®cally measured in this study. The substantial reductions of tensile strength after ageing, especially at ageing condition III, appear to be associated, to a certain extent, with the degradation of the bamboo slab's transverse strength. However, the functional similarity between the tensile strength and the bamboo±bamboo interlaminar shear strength with respect to the three ageing conditions (compare Figs. 5 and 6) also suggest that the reduction of joint strength between the bamboo slabs was also partly responsible. The reductions in ¯exural strength for the laminates containing cross-ply bamboo layers were slightly less than those of tensile strength. This may indicate that the loading condition in bending was less sensitive to the transverse strength and the joint strength of bamboo layers than in axial tension, contrary to our previous observations [10] on the laminates without prior ageing. 3.3. Residual interlaminar bond strength and impact strength after aging The residual interlaminar bond strengths of the bamboo/bamboo and bamboo/aluminium interfaces after aging are summarized in Fig. 6. The interlaminar bond strengths, 11.5 and 12.2 MPa, for the bamboo/bamboo and bamboo/aluminium interfaces, respectively, were taken from our previous study [11]. The reductions in interlaminar bond strength were signi®cantly greater than the strengths discussed above, with the residual strengths ranging only about 6±7% of the original for the bamboo/aluminium joint, in particular. Meanwhile, the normalised residual interlaminar shear strengths were much higher for the bamboo/bamboo joint, i.e.
Fig. 6. Normalised residual interlaminar bond strength of the laminate after aging.
17±33% of its original strength. The above result strongly suggests that the absorbed moisture concentrated preferentially at the bamboo/aluminium interface, and the Polybond adhesive (whose melting temperature is about 125 C) suered signi®cantly the degradation during the temperature excursion between 20 and 100 C and the humidity environment. Previous studies on aramid ®bre reinforced polymer matrix composites showed that moisture is detrimental to the composite interlaminar bond strength [18±20] and the interlaminar fracture toughness [21]. It was suspected that the absorbed moisture tended to concentrate preferentially at the ®bre±matrix interface which is the weakest part of the composite, and that once debonded or delaminated, the interface acted as the pocket or storage for the absorbed moisture, even further deteriorating the bond of the neighbouring interfaces. It was also reported that the amorphous phase within the semicrystalline polymer, such as Polybond employed in this study, tended to swell upon prolonged moisture attack, generating stresses at the amorphous/crystalline interface, eventually leading to the formation of microvoid in the polymer [22]. It is thought that the above two
J.Y. Zhang et al. / Composites Science and Technology 61 (2001) 1041±1048
Fig. 7. Normalised residual impact strength of the laminate after aging.
mechanisms were mainly responsible for the signi®cant degradations of interlaminar bond strengths at the bamboo/aluminium and bamboo/bamboo interfaces. In view of these, aging process III seems to have been most damaging to the aluminium/bamboo interface bond and the strength of Polybond adhesive amongst the aging conditions used. The un-notched impact strength measured on the laminates with unidirectional bamboo layers is shown in Fig. 7. The impact strength reduction was in the range from 12 to 31%, and the residual impact strength varied with aging conditions in a fashion very similar to the tensile strength of unidirectional laminates (Fig. 5). The strength reductions for ageing conditions II and III were much less than that for ageing condition I. This may suggest that the properties of aluminium sheets and bamboo ®bres signi®cantly aect the impact strength of the laminate as for the uniaxial tensile strength. In addition, it is also thought that the reductions in bamboo/aluminium interlaminar bond strength and the strength of Polybond adhesive due to moisture attack also played a certain role. 4. Conclusion The moisture absorption behaviour and the mechanical properties of bamboo/aluminum laminates were evaluated after exposure to several dierent combinations of hot/humid environments and sub-zero temperature. The moisture gain was much faster for the specimens with bare edges being directly exposed to water than for those with a protective edge coating. It was proven that the protection of the laminate edge from direct contact with the environment, using an appropriate polymeric material, was eective in reducing the moisture absorption and thereby improving the residual mechanical performance of the laminates. The
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reduction of aluminium/bamboo interlaminar shear strength due to aging was most signi®cant, often more than 95% of the original strength, which also in¯uenced signi®cantly the tensile strength of the laminate containing cross-ply bamboo layers. It is thought that the absorbed moisture concentrated preferentially at the bamboo/aluminium interface and/or within the adhesive as the weakest part of the laminate, as for the ®brematrix interface in many ®bre-reinforced polymer matrix composites. However, the impaired interlaminar shear strength did not much aect the tensile strength and the impact strength of the unidirectional laminates because these mechanical properties depended mainly on the properties of the aluminium sheet and the bamboo ®bre. The reductions in axial tensile strength and impact fracture toughness were at most about 30% of the original. Acknowledgements The ®nancial support from the Research Infrastructure Grant (RI95/96.EG05) of Hong Kong University of Science and Technology (HKUST) is gratefully acknowledged. Some mechanical tests were conducted with the technical supports of the Advanced Engineering Materials Facility (AEMF) of HKUST.
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