Stress-strain Relationship And Strength Criterion Of Parallel Strand Bamboo

Parallel strand bamboo(PSB) is a potential building material which integrating social, economic and ecological benefits. It is well known as bio-composites which was made from original bamboos and shaped through truncating, softing, defibering into fiber bundles, drying, gluing, assembling and hot-compacting procsses. Due to the lack of the references about PSB material performance, a large amount of targeted experiments were proceed to solve the problems such as materials strength, bearing capacity and stiffness of components which designers concerned before applied into engineering construction. Production cycle and cost of the construction projects were increased by those experiments, which impeding the large-scale engineering promotion for PSB. In this paper, in order to service PSB structure engineering design, with that starting point, the basic mechanics performance of PSB was studied by conducting tensile, compression and shearing mechanics experiments, failure modes, mechanical parameters and stress-strain curves of PSB under different stress state were obtained, correlation theory including mechanics of composite materials, elastic-plastic mechanics and statistics were utilized to analyzed those experimental data and experimental phenomena. The main conclusions as following:(1)Vast majority of PSB destructions were caused by the failure of interface bonding between bamboo fiber bundles and matrixs, in order to improving the mechanical property of PSB, improving the interface bonding strength was the key to success.(2)The tensile and compression mechanical property of PSB in parallel-to-grain direction was superior to those in perpendicular-to-grain direction, on the contrary, due to the high tensile performance of bamboo fiber bundles, the shear mechanical property in perpendicular-to-grain was superior to parallel-to-grain and transverse-to-grain-plane direction; furthermore, there were wide differences in tensile, compression or shear performance in the same martial direction. those discovery certified that PSB was a oriented fiber reinforced composite by bamboo fiber bundles, as long as under the stress state where making use of the high mechanical property of bamboo fiber bundles, the mechanical property of PSB would improved.(3)The growing trend of PSB stress-strain curves were influenced by the developments of the cracks, low speed and short path crack development would lead to brittle failure for material without obvious signs and the relationship between stress and strain was elastic; high speed and long path crack development would lead to plastic failure for material with ductility and the relationship between stress and strain was elastic-plastic. The stress-strain curves obtained by experiments in different stress states could be simplified into three models, curves which were calculateed by analytical expressions of three models fitted those by experiments well.(4)Using experimental mechanical parameters above, the yield and failure envelope curves in 1-2 PSB material plane were figured according to Tsai-Hill、Norris and Hoffman strength theories respectively, Tsai-Hill envelope curves were the most smooth between Norris and Hoffman criterion under any stress state, when taking the conservative into considersion, Tsai-Hill was recommended in the absence of a large number of multiaxial experimental studies at present, The collusion that PSB material strength would improved under biaxial tension or biaxial compression situation obtained by Norris criterion and Hoffman criterion must be verified by further tests.(5)Samples of PSB experimental strength data submitted to gaussian distribution, in the case of small samples, the standard values of material properties calculated by unilateral confidence lower limit coefficient method with 90% assurance rate was the same as calculated by reliability code method with 95% assurance rate, so unilateral confidence lower limit coefficient method was recommended for small samples condition.(6)The allowable design stress of PSB material was calculated on the basis of experimental data and the adjusted allowable stress reduction factor, referring to limit state design method for wood, the standard value of strength of PSB small clear material specimen was reduced to be the reference value for PSB component design.