关键词: 分离式Hopkinson杆, 桦木, 压缩动力学特性, 应变率, 动态压缩屈服强度, 动态压缩韧性模量

Abstract:

【Objective】In the refiner grinding dissociation crushing stage, dynamic compression characteristics of wood raw materials were investigated by dynamic compression loading test. 【Method】Dynamic compression experiments along radial, tangential and axial loading directions of birch wood (average moisture content and density was 12.65% and 0.50 g·cm^-3, respectively) were implemented by SHPB experimental device, and the strain rates were approximately 400, 800, 1 200 s^-1. The separation behavior during dynamic compression loading processes, dynamic stress-strain curves and the corresponding mechanical properties of wood were obtained in different loading directions and at different strain rates condition. 【Result】The comparative analysis results of wood damage patterns indicated that: 1) When the strain rate was 400 s^-1, plastic deformation mainly occurred in specimens, irrespective of radial, tangential or axial loading direction. 2) When the strain rate was 800 s^-1, radial-loading specimens were separated into several large pieces, and there were some small pieces, like "matchstick", stripped out from the specimens. Severe plastic deformation was found along the loading direction in tangential-loading specimens. Furthermore, penetrating cracks along the top and bottom surfaces of the specimens were observed in tangential-loading specimens, which were dissociated into three large pieces. As for axial-loading specimens, crushing phenomenon and a large number of penetrating cracks occurred in loading surface, and some small pieces were stripped from the specimens. 3) When the strain rate was 1 200 s^-1, radial-loading specimens were dissociated into a large number of small "matchstick" pieces, and the size was obviously smaller than that at 800 s^-1strain rate. As for the tangential-loading specimens, plastic deformation degree was similar to the situation when strain rate was 800 s^-1. However, the top surfaces of the specimens were separated into several small sheet pieces, and some big cracks throughout the loading direction of specimens can be found. Axial-loading specimens were separated into a number of small stubby shape pieces with wrinkles. The Comparative analysis results of wood stress-strain curve showed that: 1) The stress-strain curve can be divided into elastic stage and weak linear strengthening stage by the yield point. 2) For radial-loading specimens, when the strain rates were approximately 400, 800, 1 200 s^-1, the yield strength and toughness modulus of wood were 4.56, 10.49, 14.22 MPa and 2.88, 8.32, 20.70 kJ·cm^-3,respectively. Furthermore, when the strain rate increased from 400 s^-1to 1 200 s^-1, the yield stress and toughness modulus increased by 2.11 times and 6.19 times, respectively. 3) For tangential-loading specimens, when the strain rates were about 400, 800, 1 200 s^-1, the yield strength and toughness modulus of wood were 5.87, 7.90, 9.65 MPa and 2.53, 7.41, 12.92 kJ·cm^-3, respectively. Furthermore, when the strain rate increased from 400 s^-1to 1 200 s^-1, the yield stress and toughness modulus increased by 0.64 times and 4.10 times, respectively. 4) For axial-loading specimens, when the strain rates were approximately 400, 800, 1 200 s^-1, the yield strength and toughness modulus of wood were 22.90, 71.41, 96.37 MPa and 18.79, 67.74, 114.32 kJ·cm^-3, respectively. Furthermore, when the strain rate increased from 400 s^-1 to 1 200 s^-1, the yield stress and toughness modulus increased by 3.21 times and 5.08 times, respectively. 【Conclusion】The results showed that dissociation degree of birch increased with the increasing of strain rate. The radial-loading specimens were the most easy to seperate and the axial-loading specimens were the most hard to dissociate. Dynamic compressive yield strength and dynamic compression toughness modulus of Birch exhibited high strain rate sensitivity. Therefore, Birch is a strain rate sensitive material.

Key words: split Hopkinson bars, birch wood, dynamic compression characteristics, strain rate, dynamic compression yield strength, dynamic compression toughness modulus