Experimental And Analytical Study On Intralaminar/Interlaminar Fracture Of Laminated Veneer Bamboo Under Mode ? Loading

Laminated veneer bamboo(LVB)is a kind of fibrous composites,which is made by gluing rectangular bamboo strips together along the grain direction.LVB has a high strength-to-weight ratio and is suitable for structural members such as beams and columns.There are some initial defects in the LVB material,which come from the raw material or formed during the manufacturing process.During the service time,the presence and growth of the defects leads to the degradation of stiffness and the decrease of the strength,which may result in unexpected fracture failure of LVB structural members.Therefore,the knowledge of the fracture mechanism for LVB and the establishment of fracture failure-based criteria are key issues in the design of LVB structures.The defects,including micro cracks,pores,etc are randomly distributed in the matrix layers and the adhesive layers.Under opening(mode-? fracture)loadings,there are two different modes in LVB materials: intralaminar fracture and interlaminar fracture.Mode-I interlaminar fracture of LVB material is characterized by linear fracture behaviour,and its failure mechanism is relatively simple.In contrast,the nonlinear behaviour of mode-? intralaminar fracture is obvious.In mode-? intralaminar fracture,there are some fiber bridging at the crack tip and the failure mechanism is extremely complicated.Therefore,this study focused on the mode-? intralaminar and interlaminar fracture problems of LVB materials.The displacement filed at the crack tip and the fracture energy was solved using the double cantilever beam(DCB)test and theoretical analysis.The origination and main achievements of this study are as follows:(1)The mode-? fracture test(DCB test)was carried out using the digital image correlation(DIC)and high-speed observation technology.The failure behaviour of mode-? intralaminar and interlaminar fracture was studied.The load-displacement curve of intralaminar fracture composed of two parts: the linear rising part and the descending one which was corresponding to crack propagation.The load-displacement curve of interlaminar fracture consisted of three parts:the linear rising part,the nonlinear rising part and the descending part.In the nonlinear rising part,damage was accumulated near the crack tip and a plastic zone was formed.When the plastic zone was completely developed,crack propagated and the load-displacement shown a decreasing trend.(2)Based on the fracture mechanics,the single linear cohesive zone model and the bilinear cohesive zone model were used to analyse the mode-? intralaminar and interlaminar fracture behaviour of LVB material,respectively.The governing equations of CZM for mode-? intralaminar and interlaminar fracture ware established.The relationship between specimen compliance and crack length,the load-crack length curve and the load-displacement curve were obtained theoretically.Then,the parameters of CZM were obtained using an inverse method.The equations for solving the length of the plastic zone and the fracture toughness were also determined.The effectiveness of the CZM proposed in this study was verified by comparing the theoretical results with the experimental ones.(3)The determination method for the nominal crack tip using high-speed observation technology was proposed.The load-crack length relationship and the fracture toughness could be obtained using this technology.The effectiveness and advantage of this technology was verified by comparing with the experimental results.(4)The influence of initial crack length and specimen geometries on fracture toughness was investigated.It was demonstrated that the initial crack length and the specimen geometries had no obvious effect on the fracture toughness of mode-? intralaminar fracture,since the mode-? intralaminar fracture presented linear behaviour.For mode-? interlaminar fracture,with the increasing specimen height,the plastic zone at the crack tip developed gradually,and the fracture toughness increased.When the plastic zone developed completely,the toughness tended to a fixed value.As the specimen width increased,the crack tip transited from the plane stress condition to the plane strain condition,and the fracture toughness decreased to a fixed value.(5)The numerical simulation of mode-? fracture behaviour was conducted by using the cohesive zone model(C ZM)and the virtual crack closure technology(VCCT).The validity of the two methods for the fracture simulation was verified by comparing with the experimental results.The effect of specimen width on mode-? interlaminar fracture toughness was further investigated using CZM.It was demonstrated that the mode-? interlaminar fracture toughness decreased with the increase of specimen width.When the width was more than 55 mm,the crack tip was dominated by the plane strain condition and the fracture toughness fixed at the value of0.5395 N/mm.In conclusion,the linear and nonlinear fracture analysis using cohesive zone model(CZM)was conducted theoretically.The equations for the calculation of fracture toughness were obtained.The double cantilever beam(DC B)test for mode-? fracture was carried out using the digital image correlation(DIC)and high-speed observation technology,and the influence of initial crack length and specimen geometries on fracture toughness was studied.Furthermore,the effect of specimen width on the stress field at the crack tip and fracture toughness was investigated using numerical simulation.The plane strain fracture toughness was obtained.The results obtained in this research will provide a reference for the experimental study and theoretical analysis for investigating mode-? fracture problems of other composites.