Experimental Research On Mode ? Fracture Of Parallel Strand Bamboo:Detection Of Crack Tip And Measurement Of Fracture Toughness

Parallel strand bamboo?PSB?is a kind of bamboo-based composite material made of bamboo pieces which are disassembled and hot-pressed.It has a great prospect in engineering application due to its advantages in energy saving,lightweight,high strength,and abundance of raw materials.Due to its fabrication process,PSB has the basic characteristics of working with crack,and the mode I fracture is the most dangerous.Strain energy release rate G_I can be measured to characterize the fracture toughness of materials.Accurate measurement of crack growth length with respect to load is important for calculation of strain energy release rate.Therefore,this paper designed a double cantilever beam?DCB?test and high-speed camera was applied to synchronously collect digital images of the specimen surface and load values during the entire process of crack propagation.The full-field strain analysis was carried out by digital image correlation calculation?DIC?to determine the crack tip position in each picture and obtain the real-time crack propagation length.In this paper,the effects of initial crack length,height and width on fracture toughness were studied,and conclusions are as following:?1?Mode ? crack of PSB?DCB specimen?expands along the direction of the initial crack.Subcracks often occurred,but there was no visible stratified fracture in the specimen,that is,there was only one fracture surface.The load-displacement curves of mode I fracture of PSB are divided into two types.The load-displacement curve type is affected by the initial crack length.Due to the fiber bridging effect of PSB and generation and expansion of subcracks,the load-displacement curve shows a relatively gentle load decline section,load platform and even a rise with the micro amplitude,which leads to an increase in the strain energy release rate.?2?The longer the initial crack length is,the smaller the critical load is and the larger the critical displacement is.The critical load is positively correlated with the height and width of the specimen.The higher the specimen height,the smaller the critical displacement,and the thicker/wider the specimen,the greater the critical displacement.?3?By means of digital image correlation calculation?DIC-2D system?,the full-field strain of DCB specimen is analyzed to detect the crack tip,and thus real-time crack length can be calculated.According to the load-crack length curve,the load decreases almost linearly with the crack length,and the higher and thicker the specimen,the faster the load decreases after the crack expands for a period of time.?4?For specimens of different sizes,the crack length adjustment parameter in the modified beam theory method varies a lot from 6mm to 155mm,with no obvious regularity.The compliance coefficient fluctuates between 2 and 3,and the compliance correction coefficient values are concentrated between 20 and 40.The strain energy release rate calculated by beam theory method is relatively higher,but by modifying crack length,calibrating compliance through several tests and modified calibration compliance method,the calculation value of G_IC are basically the same.In this paper,strain energy release rate of PSB is obtained based on MCC.?5?The initial crack length and height which is between 20mm and 60mm have no obvious effect on the critical strain energy release rate.When the width of the specimen increases from20mm to 160mm,the critical strain energy release first decreases and then tends to be stable.When the strain energy release rate reaches the stable value 1.2 N/mm,the critical width value is 60mm.?6?In this article the reference value of the G_Iini takes 0.4N/mm,the reference value of G_ICC takes 1.2N/mm,the initial stress intensity factor K_Iini takes 70N/mm^3/2,and the critical stress intensity factor K_IC takes 133N/mm^3/2.PSB has better fracture toughness than the original bamboo?0.3N/mm?1.1 N/mm?,LVL?0.47 N/mm?,solid wood?0.49 N/mm?and glulam?0.63N/mm?.In this paper,it is suggested that the width of DCB specimens should be greater than 60mm to obtain a stable critical strain energy release rate,the thickness of specimens should between20mm and 40mm,and the length should be greater than 400mm to observe the phenomenon of fiber bridging and reach a stable value of strain energy release rate.