Optimization of finger joint parameters and nondestructive testing of bending properties of radiata pine laminates

  Objective  By the research of finger joint technology of radiated pine laminates, the parameters in finger joint process were optimized and the mechanical properties were improved. Based on the stress wave method, the bending properties of finger joint laminates were studied nondestructively and the strength stability of finger joint laminates was enhanced.

  Method  The influence of three factors on the laminate mechanical properties was studied with ending pressure determined by pre-experiment process, four milling cutters selected with different finger lengths, three different chimerism degree and finally compared with the standard. The dynamic elastic modulus of the specimens was measured by stress wave method, and the relationship between the two non-destructive testing parameters of density and dynamic elastic modulus and the two bending property parameters of static elastic modulus and bending strength were analyzed. And the bending properties of the laminates were evaluated by testing the density and dynamic modulus of elasticity.

  Result  The finger joint parameters had great effect on the bending strength. The finger length and chimerism degree had a degree of effect on the bending strength, while the ending pressure had little effect; within the scope of this experiment, the mean value of bending strength of radiata pine laminates with better finger joint parameters could reach 29.80 MPa, while the standard value of bending strength was 20.64 MPa and the static modulus of elasticity was 8 533 MPa; the density and dynamic elastic modulus of the specimens were significantly correlated with the dynamic elastic modulus and bending strength, and the prediction model using dynamic elastic modulus to evaluate the bending properties of the laminates was significantly better than the density-based prediction model.

  Conclusion  Based on the end-pressure-displacement curve theory, the optimal combination of finger joint parameters for structural glued laminated timber of radiated pine was obtained as follows: finger length 15 mm, tooth tip width 0.8 mm, tooth base width 0.7 mm, tooth root width 3.3 mm, inclination ratio 1/12.90, chimerism degree 0.1 mm, and end pressure 11−17 MPa. The dynamic elastic modulus measurement method based on stress wave can evaluate the bending properties of finger-jointed radiata pine laminates well.