Study On Mixed-mode Interlaminar Fracture Characteristics Of Glued-laminated Timber

As an environment-friendly material with great mechanical performance,glulam(glued-laminated timber)has been widely used in the comtemprorary civil engerneering.The study on the fracture characteristics of glulam was concentrated on the pure mode I and II fracture modes,while the study on the mixed-mode I/II is not adequate,especially on the interlaminar fracture.Wood can be regard as an orthotropic material,thus the mixed-mode interlaminar fracture characteristics may be affected by the different growth ring orientations of the adjacent laminae.Mixed mode bending tests,double cantilever beam and end-notched flexure tests were conducted on glulam to investigate the initial interlaminar fracture toughness of Scot pine.Based on the test results,ABAQUS was used to calculate the critical stress intensity factor Kc and energy release rate Gc,which was then compared with the counterparts calculated from the test results.The mixed-mode I/II interlaminar fracture test results was summarized as follows.The crack propagates along the bonding suface initiall,then it extends to the laminae,growing along the fiber near the bonding suface.The combination compliance method wsa used to calculate the mode I and mode II fracture toughness.The mode II frcture toughness increase obviously as the growth ring orientation increases,while the mode I fracture toughness of the DCB and MMB specimen changes slightly.According to the test results,the liner mixed-mode fracture criterion is a little conservative,and the elliptic curve can fit the test result well.By aid of the finite element analysis results,the critical stress intensity fractor Kc of MMB,DCB,and ENF specimens was determined according to displacement extrapolation method.With the increasement of the growth ring orientation,the variation of mode II fracture toughness is much more apparent than the mode I fracture toughness.The mixed-mode I/II fracture criterion fitted by the finite element analysis results reveal that the liner mixed-mode fracture criterion can not be approprite to describe the mixed-mode interlaminar fracture characteristics of glulam.The quadratic curve is more suitable to represent the mixed-mode fracture criterion.Vitural crack closure technic was used to obatain the energy release rate based on the finite element analysis.The energy release rate Gc of pure mode I and the contribution of mode I in the mixed-mode fracture is quite agreement to the test results,but the value of pure mode II and the contribution of mode II in the mixed-mode fracture is smaller than the test results.However,the variation tendency of both mode I and mode II fracture toughness is similar.The critical energy release rate of pure mode II fracture increase apparently as the growth riing orientation angle representation of the lamina.Same as the test results,the finite element analysis results also indicate that the quadratic mixed-mode fracture criterion is more suitable than the liner one for the Mixed-mode Interlaminar Fracture Characterion of Glued-Laminated Timber.