Study On LVL Bending Creep Properties And Its Failure Mechanisms

The study on the creep deformation property of wood and wood composite material is anadvanced subject for wood composite materials science, which decides to increase deformationof material with increase of time under long-term load. In recent years, with the severesupply-demanding imbalance of wood, especially the structure wood, so the development andapplication of man-made structure wood is universally attached importance to at home andabroad. Parallel Laminated Wood, that is, LVL has excellent physical property, so it hasreplaced large diameter wood and been widely used in large-scale architectural truss, structurebeams of wood and concrete template. But under long-term load, because of the additionaldeformation in creep deformation, the structural stability and durability will be influenced andeven structure will suddenly collapse. With creep deformation property of LVL being studiedand rule of creep deformation distortion being obtained, then theory foundation will beprovided for engineering structure design and then damage owing to structural creepdeformation distortion will be prevented by pre-taking measures of increasing the adjustiveequipment of structure, which will have important significance to application of LVL inarchitectural structural field.The essential physical property and damage form of birch and basswood's LVL arestudied in the experiment. The result shows that LVL of both wood have good physicalproperty, but birch LVL is better than basswood LVL on all aspects; the main form of thebending damage as major structural damage has three main forms in rupture damage, crazingdamage and mixed damage; and the main damaging form of structural damage caused by sheardamage is unilateral cracking destruction.On the basis of essential assumption, theological property of LVL is analyzed bystickiness and elasticity theory, and being based on this analysis, Burgers model of bendingcreep in the different stress level for birch and basswood's LVL is founded. Themedium-and-short-term creep test on birch and basswood LVL for thirty days to forty-five daysis done, which will adopt form of bending creep, and stress level of which will choose 40%,50%,60% of the ultimate strength. The result shows that in limitative test time, this model canaccurately describe short-term creep distortion of LVL.For the foundation of LVL creep model under long load, Burgers model will be improved.And the stickiness distortion part of improved model is figured in exponential way, which willincrease fitting precision and be used for forecast of creep distortion under long load.Regel element is added to improved model, and makes up of Burgers-Regel model whichcan simulate entire stages of creep distortion until creep damage. The cusp catastrophe in catastrophe theory is inducted to analyze mechanism of creep destruction, and analysis pointsout that the occurrence of creep destruction is a course from gradual change to catastrophe. Thecontrol variable of potential decides creep distortion of LVL finally to stabilization, or causethe destruction of material or structure.