Study On Interlaminar Fracture Toughness In Glued-Laminated Timber

Because of its environmental and economic benefits, glulam is widely used in construction sectors. However, the fracture behavior of glulam is more complex than that of wood. There are two types of fracture that can occur in glulam members:intralaminar fracture and interlaminar fracture. Intralaminar fracture is in essence the fracture in wood, while interlaminar fracture occurs at the interface of two adjacent laminae. Wood is an orthotropic material, the fracture toughness properties depend on different growth ring orientations, thus it is necessary to do the interlaminar fracture analysis of glulam concerning the difference of growth ring orientations, and then provides some guidance to production process and performance study of glulam. The relative studies were carried out as follows:(1) Experimental investigation on the interlaminar fracture toughness of glulamThe double cantilever beam (DCB) tests for Mode I fracture and three-point bending end-notched flexure (3ENF) tests for Mode Ⅱ fracture were performed to obtain the interlaminar fracture toughness GⅠC and GⅡC of glulam with focusing on the influence of growth ring orientations, the GⅠC and Guc were calculated by elementary beam theory after according to the critical loads PⅠC and PⅡC obtained in the tests. DCB tests results showed GⅠC increase at a certain degree as the increase of the difference between two adjacent laminae.3ENF tests results showed GⅡC increase slightly as the increase of the difference between two adjacent laminae.The three-point bending tests of a single-edge-notched specimen (3SENB) for Mode I fracture were performed with focusing on the influence of growth ring orientations, the tests results showed PⅠC decrease slightly as the increase of the difference between two adjacent laminae.(2) Determination of the interlaminar fracture toughness GⅠC and GⅡC of glulam in finite element analysisThe finite element analysis of DCB tests 3ENF tests were performed with focusing on the influence of growth ring orientations, the ABAQUS software uses the "contour integral" method to calculate J-integral, which could be interpreted as being equivalent to the energy release rate G under linear elastic material assumption. According to the critical loads PⅠC and PⅡC, the GⅠC and GⅡC calculated by finite element method (FEM) were found be good agreement with the tests results. The same finite element analysis above-mentioned were also conducted on 3SENB tests to obtain the GⅠC, the numerical results showed that the effect of growth ring orientations of two adjacent laminae on GⅠC was slight.The numerical results of GⅠC obtained from DCB were smaller than that obtained from 3SENB tests, which might be caused by the wood properties discreteness, the difference of specimens’ size and form, the crack producing error, the error of critical loads determination, and so on.(3) Determination of the interlaminar fracture toughness KⅠC and KⅡC of glulam in finite element analysisThe finite element analysis of DCB tests 3ENF tests were performed with focusing on the influence of growth ring orientations, the KⅠC and KⅡC were calculated using the stress extrapolation. DCB tests numerical results showed KⅠC increase at a certain degree as the increase of the difference between two adjacent laminae.3ENF tests numerical results showed KⅡC increase slightly as the increase of the difference between two adjacent laminae.The same finite element analysis above-mentioned were also conducted on 3SENB tests to obtain the kⅠC, the numerical results showed that the effect of growth ring orientations of two adjacent laminae on KⅠC was slight.The numerical results of KⅠC obtained from DCB were smaller than that obtained from 3SENB tests,, which might be caused by the wood properties discreteness, the difference of specimens size and form, the crack producing error, the error of critical loads determination, the error of the stress extrapolation, and so on.