Study On Crack Tip Determination Method And Fracture Toughness Of Parallel Strand Bamboo(PSB) Mode ? Fracture

The determination method of crack tip and the real-time variation trend of load-crack propagation length in the whole process of fracture are always one of the important problems to be solved in the field of material fracture analysis.At present,the methods of crack tip determination and load-crack propagation length curve acquisition are mostly based on theoretical derivation,lacking of the comparison of real test results.Therefore,taking PSB as the research object and based on the Digital Image Correlation?DIC?and Open CV,PSB mode II fracture ENF test,which introduced the high-speed camera system,was conducted.And at last,the method for determining the crack tip and fracture toughness of PSB mode II fracture ENF specimens under normal temperature and humidity conditions were studied.The conclusions are as follows:?1?The destruction morphology of PSB mode ? ENF test can be divided into:continuous cracks,discontinuous cracks,flexural failure and end breaks;In addition,the crack propagation process can be divided into:microcrack generation,crack subcritical expansion,crack progressive expansion and crack propagation;?2?The size of the specimen had some influence on the critical fracture load value P and the P-? curve:under the condition of different initial crack length a0:Width B=20mm,Pu and mid-span displacement ? were decreased with the increase of internal crack B=55mm,62.9mm?a0?135mm,the longer a0 was,the smaller Pu and ? were;B=55mm,135mm?a0?171.5mm,Pu decreased slightly with the increase of a0,and ?was opposite.It was shown that the closer the load point to the mid-span,the more obvious the pseudo-toughening effect,and the ability of the specimen to resist deformation had an increasing tendency.Compared with the state where the plane strain accounts for a relatively large amount,the slope of the curve in the softening stage of the P-? curve was more obvious under the plane stress and plane strain combined state;?3?Both the DIC method and the Open CV method can be used for determining the crack tip.In contrast,the final crack propagation length calculated based on the Open CV method was slightly lower than the average final crack propagation length calculated based on the DIC method.The crack tip determination method based on the DIC method was more suitable for the determination of the crack tip of the remodeled bamboo ? fracture;?4?The overall R curve showed a trend of increasing and then flattening.With the increase of load,the energy at the crack tip gradually accumulated,the plastic deformation at the crack tip began to deepen,the FPZ in the fracture process area began to expand,and the strain energy release rate G? increased with the increase of crack.As the load continues to increase,the stress concentration area at the crack tip further expanded,the plastic area at the crack tip continued to increase,and the crack gradually expanded to a critical length.Subsequently,the crack expanded unstably,and the stress concentration was released at the fracture site.The stress concentration area moved with the crack tip,and with the sound of clear fracture,the strain energy release rate reached G_?C,and the curve became smooth.?5?B had a slight influence on G_?C and fracture toughness K_?C.Different from B,G_?C and K_?C generally showed a trend of decreasing with the increase of B.In addition,a0,30mm<2h?40mm and 175mm?L?225mm had little influence on G_?C and K_?C The G_?C and K_?C of PSB were?G_?C=6.3ON/mm?K_?C=202.18N/mm^3/2?...