Objective The effects of high-intensity microwave treatment on the flexural modulus of elasticity, bending strength, bending plastic work were investigated, and the damage process and failure mode of treated lumber during static bending load were explored.
Method The radiata pine lumber with moisture contents ranging from 50% to 70% was treated by high-intensity microwave at three microwave energy density levels, i.e., 60, 80 and 100 kW·h/m3, respectively. Both the treated and untreated lumber specimens were tested for three-point bending modulus of elasticity, bending strength and bending plastic work under the real-time monitoring of the acoustic emission (AE) system, and the damage evolution characteristics of different loading stages were compared by analyzing the fracture failure section morphology and AE parameters.
Result The average values of modulus of elasticity and bending strength of untreated lumber specimens were 5 520 and 61.7 MPa, respectively. The treated lumber specimens presented less than 10% change in their average bending strength and modulus of elasticity compared with untreated ones. However, high-energy microwave treatment significantly increased the bending plastic work of wood. Compared with the untreated lumber specimens, the bending plastic work of wood treated at 60 and 100 kW·h/m3 increased by 12% and 16%, respectively. Lumber treated with the energy density of 80 kWh/m3 showed the highest bending plastic work value, which was 22% higher than that of the untreated lumber specimens. The parameter analysis of AE showed that, with the increase of microwave energy density, the first occurrence time of AE signal of the treated materials was gradually advanced, and the duration of the first stable damage growth stage, the cumulative ringing count growth rate, as well as amplitude and energy activity of the plastic deformation stage increased gradually. During the process of bending test, the treated lumber specimens presented a higher damage growth rate and stress recombination efficiency, which indicated that there were more buckles and collapse damage occurring in the cell wall, Therefore, the cracks in the treated lumber specimens expanded faster, which weakened the stress concentration effect and increased the bending plastic work of wood to a certain extent. The analysis on the tested specimens’ fracture morphology verified the AE test results: the cross-sectional morphology of the tensile region, neutral layer, and compression region of the treated materials were rougher than that of the untreated ones.
Conclusion Appropriate high-intensity microwave treatment could significantly increase the bending plastic work of wood while only slightly changing the modulus of elasticity and bending strength of wood. This suggests new potential applications of high-intensity microwave-treated lumber. The research methods and results can effectively show the damage evolution characteristics of high-intensity microwave-treated lumber, and will provide a reference to the related research on the damage evolution of wooden materials.