Issue 35

T. Haiyan, Frattura ed Integrità Strutturale, 35 (2016) 472-480; DOI: 10.3221/IGF-ESIS.35.53 475 K IIC and K IIIC , unless investigate every condition when crack and fiber direction have any fixed angle and test applicability of linear elastic fracture mechanics under all condition, but the operation is inconvenient and even impossible [14]. However, if the initial direction of crack is consistent with fiber and principal axis of orthotropy is coincident with crack surface direction and crack growth direction, all derivation with linear elastic fracture mechanics theory mentioned above can be eliminated. That is because many experiments have verified that, crack expands in initial direction which is consistent with direction of fiber; displacement is no longer compound; composite parameters of materials are constants under the condition of fixed crack and fiber direction (a=0), thus stress distribution of crack tip is only a function with regard to r and θ. The above three situations suggest that, theory of linear elastic fracture mechanics is applicable for crack whose direction is consistent with fiber. Most cracks and defects formed in the growing period of tree and processing process are in the direction of fiber of wood, as the resistance to expansion of crack is the minimum in the direction of fiber. As TL crack growth is quite similar to radial shake and meanwhile RL crack growth is similar to ring shake, theory of linear elastic fracture mechanics is thought to be applicable for parallel-to-grain growth of crack of wood. Studying and measuring toughness which is a representation of resistance to parallel-to-grain cracking of wood is of important practical value for design of wooden structure and processing technique optimization. P ROCESS OF DAMAGE CRACK ON WOOD rack mechanics is a subject involving macro crack growth rule and quantitative analysis [15], but mechanical effects of inevitable microdefects which have existed before macro cracks are not included. In wood, a large amount of original microdefects such as pit, crack on cell wall and interface damage will gradually evolve or emerge into macro crack under load. Wood damage refers to mechanical property degradation induced by progressive decrease of internal cohesion resulting from microdefects formed under the effect of load or environment. It is an irreversible and energy-consuming process of internal microstructure. Macro cracks form when damage variable reaches extreme value. Damage evolution is the premise for formation of cracks and moreover crack growth expands the damage; therefore, damage and crack of wooden materials reflects a whole physical process from deformation to damage. Materials, equipments and methods This test is to explore the effect of defects on acoustic emission in the process of bending taking Picea jezoensis as test material. Picea jezoensis is made into two groups of specimens, i.e., standard group (wood without crack and in a size of 300(L) × 20(T) ×20(R) (mm)) and crack group (wood in a size of 300(L) × 30(T) × 20(R) (mm)). Wood in crack group is cut a 10 mm deep sharp crack along tangential direction to make a 20×20 (mm) net section on crack tip. Both groups include 30 specimens, 60 in total. Three-point bending load along tangential direction is used. Equipments used in the test include microcomputer controlled material testing machine, AE-4 acoustic emission equipment [16] and R1 acoustic emission sensor. Compared to other non-destructive testing technologies, acoustic emission technique has a distinctive character, that is, detected objects involve in the detection process actively. Based on the received acoustic wave and external conditions inducing acoustic wave, we can understand both the status of defects and formation of defects as well as growth tendency under practical condition [17]. Therefore, acoustic emission technology can be used in monitoring damage accumulation of materials in the process of deformation and failure, identifying failure mechanism and confirming damage site. Experimental results and analysis It is difficult to identify and distinguish acoustic emission signals derived from different damage mode in different stages of bending of wood. That is because, wood as a composite biomaterial with multiple unit structures usually has multiple kinds of deformation and damage which can change energy in the same stage in the process zone around crack tip. Therefore, we design a double cantilever beam on parallel-to-grain cracking and a compression test (Fig. 3). Mode I crack is found in the former test and the latter test only results in cell wall bending and collapse damage. Experimental results suggest that, parallel-to-grain cracking only leads to low amplitude and low energy acoustic emission event, whereas acoustic emission signal produced by bending and collapse even has lower energy. We analyzed and summarized a large amount of acoustic signals from different wood samples and found that, peak amplitude Vmax in acoustic emission parameters and root mean square (RMS) of effective voltage can be used to identify different damage type, when sensor is put in a place less than 10 m away from damage source. RMS is more effective and C