Hygrothermal Effect And Adhesive Interface Properties Of Bionic Bamboo Recombining In Original Status

General bamboo recombining technology destroys natural structure advantages of bamboo culm,and it shows low economic and environmental performance for the high pollution,high energy consumption,low utilization rate,and the use of a great deal of adhesive in the process.However,bamboo utilization in original status takes full advantages of its unique structure and mechanical specialty.As one type of multi-sides bamboo recombining in original status?M-BROS?,bionic bamboo recombining in original status is an innovation utilization of bamboo culm.The material inspired by honeycomb is effective for bamboo to be used as large specification material and has vast development prospects.Moso bamboo is the focused object in this research for it is the main species of bamboo industrial and original utilization in China.It is an undeniable fact that environmental temperature and humidity have serious impact on building components and structure materials.Bamboo is sensitive to environmental temperature and humidity,besides that,the adhesive interface constitution of the M-BROS is complicated.While no research on environmental response of the M-BROS is reported so far,the purpose of the study was to explore the environment characteristic based on the service environment of bamboo in original status.The study in this paper was conducted from element to the recombination,the mechanical properties of bionic bamboo element?hexagonal bamboo element?and its adhesive strength under different hygrothermal conditions were researched after the mechanism exploration of bamboo culm annular shrinkage and strain response to environmental temperature and humidity,then the ANSYS modeling was adopted to analyse the stress and strain characteristics of bionic bamboo element under axial and transverse loads,finally the correlated mechanical properties of bionic bamboo elements within the M-BROS were simulated and discussed.All the research work and results are displayed as follows:?1?Shrinkage on cross-section of bamboo culm was annular and centripetal,the relationship between annular shrinkage and moisture content?MC?was cubic estimation?R²>0.83?;the wall of bamboo culm thinned for the fact that the shrinkage degree of bamboo green was larger than that of bamboo yellow.MC of bamboo culm samples declined with time and exhibited ExpAssoc function?R²>0.99?.Higher temperature resulted in larger shrinkage stress,and the most serious crack occurred under temperature 80?for the internal stress release,and wax on the outer epidermis turned melting which was not benifit to moisture evaporation.Node inhibited the crack propagation at some extent.Digital speckle correlation method?DSCM?demonstrated the annular shrinkage by charactering circumferential strain field and radial strain field of bamboo wall and the crack part.Bamboo green was the initial zone of crack induced by shrinkage stress,which differed from the damage crack resulted from outer force that ocurred in the weak part?bamboo yellow?,this provided reversible consideration for bamboo culm anti-splitting.?2?Bamboo culm made immediately response to environmental temperature and humidity changes,the more greater change led to more obvious strain,and the impact of temperature and humidity on bamboo culm mainly embodied by its MC change.The circumferential strain of bamboo culm showed similar change regular with axial strain under the same environment change,while the circumferential strain ranged from-500??to 3000??,the axial strain ranged from-50??to 225??,the difference was nearly an order of magnitude.Bamboo strain response to hygrothermal depended on moisture transfer channel,the circumferential strain of top part was more sensitive than that of middle part where the strain was hysteretic,this phenomenon resulted from stress disequilibrium.Bamboo culm with longer length showed more obvious strain difference between top part and middle part,while the strain between top part and middle part of bamboo culm which was not longer than 300mm showed wane and wax,this was the result of inner stress redistribution.?3?Under different hygrothermal conditions,outer-diameter change ratio of bionic bamboo element was the maximum,followed by inner-diameter change ratio,length change ratio was the minimum.Besides that,bionic bamboo element immersed in water for 30d showed the most obvious dimensional change.Real-time weight of test samples under high hygrothermal?40?90%?environment for 30d displayed linear relation with square root of time.Differing from the contribution of node to control samples,node weakened the axial compress strength of bionic bamboo element after high hygrothermal treatment.Especially,compress strength of samples with node was significantly less?sig.<0.05?than that of internodes samples after both immersed in water for 30d.The compress strength of control group was 50.72MPa,and showed significant difference?sig.<0.05?with that of low hygrothermal?40?20%?treated samples which strength was 79.47MPa.Strength values of samples immersed in water for 30d,treated by high hygrothermal environment for 30d and 15d were all obviously less?sig.<0.05?than that of control samples for main impact of fiber swelling,their strength values were 30.56MPa,28.48MPa and 32.43MPa respectively,while their toughness and degree of crystallinity was slightly increased.Compress strength indicated linearly reduced with hygrothermal time extension?R²=0.6437?.Fiber tear was the main crack morphology and propagated along fiber's direction,while samples treated by low hygrothermal environment showed fiber bridge and fiber rupture with untidy fracture morphology,this consumed the most energy.?4?Shrinkage and swelling asynchronism of bamboo and adhesive was the main reason for hygrothermal stress concentration and interface strength weakening under high-low alternant hygrothermal environment,and the shrinkage and swelling stress on adhesive part margin was the maximum.Shear strength of adhesive interface was influenced by the alternant environment evidently?sig.<0.05?and decreased by 54.36%compared with control samples;while lateral rupture strength of adhesive interface dropped to 15.57 N·mm^-1 from22.19 N·mm^-1 after alternant environment treatment,the difference was significant?sig.<0.05?.The capacity of damage resistance of the bonding samples was downgrade after the alternant treatment,damage occurred at smaller deformation and load,and the fracture energy was decreased by 62.32%.The damage pattern of different test was similar,which meant that initial crack generated in the bonding interface margin and then propagated along the bonding line and suddenly turned to bamboo,and finally destroyed the bionic bamboo element wall in radial.The damage pattern depended on the geometrical shape of bionic bamboo element.While the glued surface of bionic bamboo element was near bamboo green which had many and intensive vascular bundles,this resisted stress deflection at some extent.?5?Bionic bamboo element mechanical simulation with the help of ANSYS soft indicated that,the pressure stress and strain and tensile stress and strain were symmetrical distribution under axial load and transverse load,larger and concentrated Von Mises stress and strain was exhibited on the end parts,pressure part and the thin wall of bionic bamboo element.Seven bionic bamboo elements within the M-BROS sustained different stress when undergoing transverse load,the maximum stress appeared on the upper elements,and the stress distribution on the center one was the most uniformity.Stress of elements on outer were uneven,stress of their glued surfaces was larger than that of their free surfaces.Stress and strain on 12 adhesive parts were different with each other,the maximum stress and strain appeared on the upper adhesive part which was parallelled to load direction,and this certified the necessary and rationality of lateral rupture strength test of bonding interface;Stress and strain of the edge zone of every adhesive part was greater than that of the middle zone.The ANSYS simulation results were consistent with the experiment findings.Finite element method is benefits to analysis failure mechanism of the M-BROS and provide guidance for strength improvement of the M-BROS.